750 GeV: the bigger picture

This Thursday the ATLAS and CMS experiments will present updated analyses of the 750 GeV diphoton excess. CMS will extend their data set by the diphoton events collected in the periods when the detector was running without the magnetic field (which is not essential for this particular study), so the amount of available data will slightly increase. We will then enter the Phase-II of the excitement protocol,  hopefully followed this summer by another 4-th-of-July-style discovery. To close the Phase-I, here's a long-promised post about the bigger picture. There's at least 750 distinct models that can accommodate the diphoton signal observed by ATLAS and CMS. However, a larger framework for physics beyond the Standard Model it which these phenomenological models can be embedded is a more tricky question. Here is a bunch of speculations.

Whenever a new fluctuation is spotted at the LHC one cannot avoid mentioning supersymmetry. However,  the 750 GeV resonance cannot be naturally interpreted in this framework, not the least because it cannot be identified as a superpartner of any known particles. The problem is that explaining the observed signal strength requires introducing new particles with large couplings, and the complete theory typically enters into a strong coupling regime at the energy scale of a few TeV. This is not the usual SUSY paradigm, with weakly coupled physics at the TeV scale followed by a desert up to the grand unification scale. Thus, even if the final answer may still turn out to be supersymmetric, it will not be the kind of SUSY we've been expecting all along. Weakly coupled supersymmetric explanations are still possible in somewhat more complicated scenarios with new very light sub-GeV particles and cascade decays, see e.g. this NMSSM model.

Each time you see a diphoton peak you want to cry Higgs, since this is how the 125 GeV Higgs boson was first spotted. Many theories predict an extended Higgs sector with multiple heavy scalar particles, but again such a framework is not the most natural one for interpreting the 750 GeV resonance. There are two main reasons. One is that different Higgs scalars typically mix, but the mixing angle in this case is severely constrained by Higgs precision studies and non-observation of 750 GeV diboson resonances in other channel. The other is that, for a 750 GeV Higgs scalar, the branching fraction into the diphoton final state is typically tiny (e.g., ~10^-7 for a Standard-Model-Higgs-like scalar) and a complicated model gymnastics is needed to enhance it. The possibility that the 750 GeV resonance is a heavy Higgs boson is by no means excluded, but I would be surprised if this were the case.  

It is more tempting to interpret the diphoton resonance as a bound state of new strong interactions with a confinement scale in the TeV range. We know that the Quantum Chromodynamics (QCD) theory, which describes the strong interactions of the Standard Model quarks, gives rise to many scalar mesons and higher-spin resonances at low energies. Such a behavior is characteristic for a large class of similar theories.  Furthermore,  if the new strong sector contains mediator particles  that carry color and electromagnetic charges, the production in gluon fusion and decay into photons is possible for the composite states, see e.g. here.  The problem is that, much as for QCD, one would expect not one but an entire battalion of resonances. One needs to understand how the remaining resonances predicted by typical strongly interacting models could have avoided detection so far.

One way this could happen is if the 750 GeV resonance is a scalar that, for symmetry reasons, is much lighter than most of the particles in the strong sector. Here again our QCD may offer us a clue, as it contains pseudo-scalar particles, the so-called pions,  which are a factor of 10 lighter than the typical mass scale of other resonances. In QCD, pions are Goldstone bosons of the chiral symmetry spontaneously broken by the vacuum quark condensate. In other words, the smallness of the pion mass is  protected by a symmetry, and general theorems worked out in the 60s  ensure the quantum stability of such an arrangement. The similar mechanism can be easily implemented in other strongly interacting theories,  and it is possible to realize the 750 GeV resonance as a new kind of pion, see e.g. here.   Even the mechanism for decaying into photons -- via chiral anomalies -- can be borrowed directly from QCD. However, the symmetry protecting the 750 GeV scalar could also be completely different that the ones we have seen so far. One example is the dilaton, that is   a Goldstone boson of a spontaneously broken conformal symmetry, see e.g. here. This is a theoretically interesting possibility, since approximate conformal symmetry often arises as a feature of strongly interacting theories. All in all, the 750 GeV particle may well be  a pion or dilaton harbinger of new strong interactions at a TeV scale. One can then further speculate that the Higgs boson also originates from that sector, but that is a separate story that may or may not be true.

Another larger framework worth mentioning here is that of extra dimensions. In the modern view, theories with the new 4th dimension of space are merely an effective description of strongly interacting sectors discussed above. For example, the famous Randall-Sundrum model, with the Standard Model living in a section of a 5D AdS5 space, is a weakly coupled dual description of strongly coupled theories with a conformal symmetry and a large N gauge symmetry. These models thus offer a calculable way to embed the 750 GeV resonance in a strongly interacting theory. For example, the dilaton can be effectively described in the Randall-Sundrum model as the radion - a scalar particle corresponding to fluctuations of the size of the 5th dimension, see e.g. here. Moreover, the Randall-Sundrum framework  provides a simple way to realize the 750 GeV particle as a spin-2 resonance. Indeed, the model always contains massive Kaluza-Klein  excitations of the graviton, whose couplings to matter can be much stronger than that of the massless graviton. This possibility have been relatively less explored so far, see e.g.  here,  but that may change next week...

Clearly, it is impossible to say anything conclusive at this point. More data in multiple decay channels is absolutely necessary  for a more concrete picture to emerge. For me personally, a confirmation of the 750 GeV excess would be a strong hint for new strong interactions at a few TeV scale. And if this is indeed the case,  one may seriously think that our  40-years-long brooding about the hierarchy problem has not been completely misguided...

750 and what next

A week has passed since the LHC jamboree, but the excitement about the 750 GeV diphoton excess has not abated. So far, the scenario from 2011 repeats itself. A significant but not definitive signal is spotted in the early data set by the ATLAS and CMS experiments. This announcement is wrapped in multiple layers of caution and skepticism by experimentalists, but is universally embraced by theorists. What is unprecedented is the scale of theorist's response, which took a form of a hep-ph tsunami.    I still need time to digest this feast, and pick up interesting bits among general citation fishing.  So today I won't write about the specific models in which the 750 GeV particle could fit: I promise a post on that after the New Year (anyway, the short story is that, oh my god, it could be just anybody). Instead, I want to write about one point that was elucidated by the early papers,  namely that the diphoton resonance signal is unlikely to be on its own, and there should be accompanying signals in other channels. In the best case scenario, confirmation of the diphoton signal may come by analyzing the existing data in other channels collected this year or in run-1.

First of all, there should be a dijet signal. Since the new particle is almost certainly produced via gluon collisions,  it must be able to decay to gluons as well by time-reversing the production process. This would show up at the LHC as a pair of energetic jets with the invariant mass of 750 GeV. Moreover, in simplest models the 750 GeV particle decays to gluons most of the times. The precise dijet rate is very model-dependent, and in some models it  is too small to ever be observed, but typical scenarios predict order 1-10 picobarn dijet cross-sections. This would mean that thousands of such events have been produced in the LHC run-1 and this year in run-2. The plot on the right shows one example of a parameter space (green) overlaid with contours of dijet cross section (red lines) and limits from dijet resonance searches in run-1 with 8 TeV proton collisions (red area). Dijet resonance searches are routine at the LHC, however experimenters usually focus on the high-energy end of the spectrum, far above 1 TeV invariant mass. In fact, the 750 GeV region is not covered at all by the recent LHC searches at 13 TeV proton collision energy.

The next important conclusion is that there should be matching signals in other diboson channels at the 750 GeV invariant mass. For the 125 GeV Higgs boson, the signal was originally discovered  in  both the γγ and  the ZZ final states, while in  the WW channel the signal is currently similarly strong. If the 750 GeV particle were anything like the Higgs, the resonance should actually first show in the ZZ and WW final states (due to the large coupling to longitudinal polarizations of vector bosons which is a characteristic feature of Higgs-like particles).  From the non-observation of anything interesting in run-1 one can conclude that there must be little Higgsiness in the 750 GeV particle, less than 10%.  Nevertheless, even if the particle has nothing to do with the Higgs (for example, if it's a pseudo-scalar), it should still decay to diboson final states once in a while. This is because a neutral scalar cannot couple directly to photons, and the coupling has to  arise at the quantum level through some other new electrically charged particles, see the diagram above. The latter couple not only to photons but also to Z bosons, and sometimes to W bosons too.  While the details of the branching fractions are highly dependent, diboson signals  with comparable rates as the diphoton one are  generically predicted.  In this respect, the decays of the 750 GeV particle to one photon and one Z boson emerge as a new interesting battleground.  For the 125 GeV Higgs boson, decays to Zγ have not been observed yet, but in the heavier mass range the sensitivity is apparently better.  ATLAS made a search for high-mass Zγ resonances in the run-1 data,  and their limits already put non-trivial constraint on some models explaining the 750 GeV excess. Amusingly, the ATLAS Zγ search has a 1 sigma excess at 730 GeV...   CMS has no search in this mass range at all, and both experiments are yet to analyze the run-2 data in this channel.  So, in principle,  it is well possible that we learn something interesting even before the new round of collisions starts at the LHC.

Another generic prediction is that there should be vector-like quarks or other new colored particles just behind the corner. As mentioned above, such particles are necessary to generate an effective coupling of the 750 GeV particle to photons and gluons. In order for those couplings to be large enough to explain the observed signal,  at least one of the new states should have mass below ~1.5 TeV. Limits on vector-like quarks depend on what they decay to,  but the typical sensitivity in run-1 is around 800 GeV. In run-2, CMS already presented a search for a charge 5/3 quark decaying to a top quark and a W boson, and they were able to improve the run-1 limits on the new quark's mass from 800 GeV up to 950 GeV. Limits on other type of new quarks should follow shortly.

On a bit more speculative side, ATLAS claims that the best fit to the data is obtained if the 750 GeV resonance is wider than the experimental resolution. While the statistical significance of this statement is not very high, it would have profound consequences if confirmed. Large width is possible only if the 750 GeV particle decays to other final states than photons and gluons. An exciting possibility is that the large width is due to decays to a new hidden sector with new light particles very weakly or not at all coupled to the Standard Model. If these particles do not leave any trace in the detector then the signal is the same monojet signature as that of dark matter: an energetic jet emitted before the collision without matching activity on the other side of the detector. In fact, dark matter searches in run-1 practically exclude the  possibility that the large width can be accounted for uniquely by invisible decays (see comments #2 and #13 below).  However, if the new particles in the hidden sector couple weakly to the known particles, they can decay back to our sector, possibly after some delay, leading to complicated exotic signals in the detector. This is the so-called hidden valley scenario that my fellow blogger has been promoting for some time. If the 750 GeV particle is confirmed to have a large width, the motivation for this kind of new physics will become very strong. Many of the possible signals that one can imagine in this context are yet to be searched for.    

Dijets, dibosons, monojets, vector-like quarks, hidden valley...  experimentalists will have hands full this winter.  A negative result in any of these searches would not strongly disfavor the diphoton signal, but would provide important clues for model building. A positive signal would break all hell loose, assuming it hasn't yet. So, we are waiting eagerly for further results from the LHC,  which should show up  around the time of the Moriond conference in March. Watch out for rumors on blogs and Twitter ;)

750 ways to leave your lover

A new paper last week straightens out the story of the diphoton background in ATLAS. Some confusion was created because theorists misinterpreted the procedures described in the ATLAS conference note, which could lead to a different estimate of the significance of the 750 GeV excess. However, once the correct phenomenological and statistical approach is adopted, the significance quoted by ATLAS can be reproduced, up to small differences due to incomplete information available in public documents. Anyway, now that this is all behind, we can safely continue being excited at least until summer.  Today I want to discuss different interpretations of the diphoton bump observed by ATLAS. I will take a purely phenomenological point of view, leaving for the next time  the question of a bigger picture that the resonance may fit into.

Phenomenologically, the most straightforward interpretation is the so-called everyone's model: a 750 GeV singlet scalar particle produced in gluon fusion and decaying to photons via loops of new vector-like quarks. This simple construction perfectly explains all publicly available data, and can be easily embedded in more sophisticated models. Nevertheless, many more possibilities were pointed out in the 750 papers so far, and here I review a few that I find most interesting.

Spin Zero or More?  
For a particle decaying to two photons, there is not that many possibilities: the resonance has to be a boson and, according to young Landau's theorem, it cannot have spin 1. This leaves at the table spin 0, 2, or higher. Spin-2 is an interesting hypothesis, as this kind of excitations is predicted in popular models like the Randall-Sundrum one. Higher-than-two spins are disfavored theoretically. When more data is collected, the spin of the 750 GeV resonance can be tested by looking at the angular distribution of the photons. The rumor is that the data so far somewhat favor spin-2 over spin-0, although the statistics is certainly insufficient for any serious conclusions.  Concerning the parity, it is practically impossible to determine it by studying the diphoton final state, and both the scalar and the pseudoscalar option are equally viable at present. Discrimination may be possible in the future, but  only if multi-body decay modes of the resonance are discovered. If the true final state is more complicated than two photons (see below), then the 750 GeV resonance may have  any spin, including spin-1 and spin-1/2.

Narrow or Wide? 
The total width is an inverse of particle's lifetime (in our funny units). From the experimental point of view, the width larger than detector's  energy resolution  will show up as a smearing of the resonance due to the uncertainty principle. Currently, the ATLAS run-2 data prefer the width 10 times larger than the experimental resolution  (which is about 5 GeV in this energy ballpark), although the preference is not very strong in the statistical sense. On the other hand, from the theoretical point of view, it is much easier to construct models where the 750 GeV resonance is a narrow particle. Therefore, confirmation of the large width would have profound consequences, as it would significantly narrow down the scope of viable models.  The most exciting interpretation would then be that the resonance is a portal to a dark sector containing new light particles very weakly coupled to ordinary matter.    

How many resonances?  
One resonance is enough, but a family of resonances tightly packed around 750 GeV may also explain the data. As a bonus, this could explain the seemingly large width without opening new dangerous decay channels. It is quite natural for particles to come in multiplets with similar masses: our pion is an example where the small mass splitting π± and π0 arises due to electromagnetic quantum corrections. For Higgs-like multiplets the small splitting may naturally arise after electroweak symmetry breaking, and  the familiar 2-Higgs doublet model offers a simple realization. If the mass splitting of the multiplet is larger than the experimental resolution, this possibility can tested by precisely measuring the profile of the resonance and searching for a departure from the Breit-Wigner shape. On the other side of the spectrum is the idea is that there is no resonance at all at 750 GeV, but rather at another mass, and the bump at 750 GeV appears due to some kinematical accidents.
   
Who made it? 

The most plausible production process is definitely the gluon-gluon fusion. Production in collisions of light quark and antiquarks is also theoretically sound, however it leads to a more acute tension between run-2 and run-1 data. Indeed, even for the gluon fusion, the production cross section of a 750 GeV resonance in 13 TeV proton collisions is only 5 times larger than at 8 TeV. Given the larger amount of data collected in run-1, we would expect a similar excess there, contrary to observations. For a resonance produced from u-ubar or d-dbar the analogous ratio is only 2.5 (see the table), leading to much more  tension. The ratio climbs back to 5 if the initial state contains the heavier quarks: strange, charm, or bottom (which can also be found sometimes inside a proton), however I haven't seen yet a neat model that makes use of that. Another possibility is to produce the resonance via photon-photon collisions. This way one could cook up a truly minimal and very predictive model where the resonance couples only to photons of all the Standard Model particles. However, in this case, the ratio between 13 and 8 TeV cross section is very unfavorable, merely a factor of 2, and the run-1 vs run-2 tension comes back with more force. More options open up when associated production (e.g. with t-tbar, or in vector boson fusion) is considered. The problem with these ideas is that, according to what was revealed during the talk last December, there isn't any additional energetic particles in the diphoton events. Similar problems are facing models where the 750 GeV resonance appears as a decay product of a heavier resonance, although in this case some clever engineering or fine-tuning may help to hide the additional particles from experimentalist's eyes.

Two-body or more?
While a simple two-body decay of the resonance into two photons is a perfectly plausible explanation of all existing data, a number of interesting alternatives have been suggested. For example, the decay could be 3-body, with another soft visible or invisible  particle accompanying two photons. If the masses of all particles involved are chosen appropriately, the invariant mass spectrum of the diphoton remains sharply peaked. At the same time, a broadening of the diphoton energy due to the 3-body kinematics may explain why the resonance appears wide in ATLAS. Another possibility is a cascade decay into 4 photons. If the  intermediate particles are very light, then the pairs of photons from their decay are very collimated and may look like a single photon in the detector.
   
 ♬ The problem is all inside your head ♬  and the possibilities are endless. The situation is completely different than during the process of discovering the  Higgs boson, where one strongly favored hypothesis was tested against more exotic ideas. Of course, the first and foremost question is whether the excess is really new physics, or just a nasty statistical fluctuation. But if that is confirmed, the next crucial task for experimentalists will be to establish the nature of the resonance and get model builders on the right track.  ♬ The answer is easy if you take it logically ♬ 

All ideas discussed above appeared in recent articles by various authors addressing the 750 GeV excess. If I were to include all references the post would be just one giant hyperlink, so you need to browse the literature yourself to find the original references.

Diphoton update

Today at the Moriond conference ATLAS and CMS updated their diphoton resonance searches. There's been a rumor of an ATLAS analysis with looser cuts on the photons where the significance of the 750 GeV excess grows to a whopping 4.7 sigma. The rumor had it that the this analysis would be made public today, so the expectations were high. However, the loose-cuts analysis was not approved in time by the collaboration, and the fireworks display was cancelled.  In any case,  there was some good news today, and some useful info for model builders was provided.











Let's start with ATLAS. For the 13 TeV results, they now have two analyses: one called spin-0 and one called spin-2. Naively, the cuts in the latter are not optimized not for a spin-2 resonance but rather for a high-mass resonance  (where there's currently no significant excess), so the spin-2 label should not be treated too seriously in this case. Both analyses show a similar excess at 750 GeV: 3.9 and 3.6 sigma respectively for a wide resonance. Moreover, ATLAS provides additional information about the diphoton events, such as the angular distribution of the photons, the number of accompanying jets, the amount of missing energy, etc. This may be very useful for theorists entertaining less trivial models, for example when the 750 GeV resonance is produced  from a decay of a heavier parent particle. Finally, ATLAS shows a re-analysis of the diphoton events collected at 8 TeV center-of-energy of the LHC. The former run-1 analysis was a bit sloppy in the interesting mass range; for example, no limits at all were given for a 750 GeV scalar hypothesis.  Now the run-1 data have been cleaned up and analyzed using the same methods as in run-2. Excitingly, there's a 2 sigma excess in the spin-0 analysis in run-1, roughly compatible with what one would expect given the observed run-2 excess!   No significant excess is seen for the spin-2 analysis, and the tension between the run-1 and run-2 data is quite severe in this case. Unfortunately, ATLAS does not quote the combined significance and the best fit cross section for the 750 GeV resonance.

For CMS, the big news is that the amount of 13 TeV data at their disposal has increased by 20%. Using MacGyver skills, they managed to make sense of the chunk of data collected when the CMS magnet was off due to a technical problem. Apparently it was worth it, as new diphoton events have been found in the 750 GeV ballpark. Thanks to that, and a better calibration,  the significance of the diphoton excess in run-2  actually increases up to 2.9 sigma!  Furthermore, much like ATLAS, CMS updated their run-1 diphoton analyses and combined them with the run-2 ones.  Again, the combination increases the significance of the 750 GeV excess. The combined significance quoted by CMS is 3.4 sigma,  similar for spin-0 and spin-2 analyses. Unlike in ATLAS, the best fit is for a narrow resonance, which is the more preferred option from the theoretical point of view.

In summary, the diphoton excess survived the first test.  After adding more data and improving the analysis techniques the significance slightly increases rather than decreases, as expected for a real particle.  The signal is now a bit more solid: both experiments have a similar amount of diphoton data and they both claim a similar significance of the  750 GeV bump.  It may be a good moment to rename the ATLAS diphoton excess as the LHC diphoton excess :)  So far, the story of 2012 is repeating itself: the initial hints of a new resonance solidify into a consistent picture. Are we going to have another huge discovery this summer?

A new boson at 750 GeV?

ATLAS and CMS presented today a summary of the first LHC results obtained from proton collisions with 13 TeV center-of-mass energy. The most exciting news was of course the 3.6 sigma bump at 750 GeV in the ATLAS diphoton spectrum, roughly coinciding with a 2.6 sigma excess in CMS. When there's an experimental hint of new physics signal there is always this set of questions we must ask:

0. WTF ?
0. Do we understand the background?
1. What is the statistical significance of  the signal?
2. Is the signal consistent with other data sets?
3. Is there a theoretical framework to describe it?
4. Does it fit in a bigger scheme of new physics?

Let us go through these questions one by one.

The background.  There's several boring ways to make photon pairs at the LHC, but they are expected to produce a  spectrum smoothly decreasing with the invariant mass of the pair. This expectation was borne out in run-1, where the 125 GeV Higgs resonance could be clearly seen on top of a nicely smooth background, with no breaks or big wiggles. So it is unlikely that some Standard Model processes (other than a statistical fluctuation) may produce a bump such as the one seen by ATLAS.

The stats.   The local significance is 3.6 sigma in ATLAS and 2.6 sigma in CMS.  Naively combining the two, we get a more than 4 sigma excess. It is a very large effect, but we have already seen this large fluctuations at the LHC that vanished into thin air (remember 145 GeV Higgs?). Next year's LHC data will be  crucial to confirm or exclude the signal.  In the meantime, we have a perfect right to be excited.

The consistency. For this discussion, the most important piece of information is the diphoton data collected in run-1 at 8 TeV center-of-mass energy.  Both ATLAS and CMS have a small 1 sigma excess around 750 GeV in the run-1 data, but there is no clear bump there.  If a new 750 GeV  particle is produced in gluon-gluon collisions,  then the gain in the signal cross section at 13 TeV compared to 8 TeV is roughly a factor of 5.  On the other hand, there was 6 times more data collected at 8 TeV by ATLAS (3.2 fb-1 vs 20 fb-1). This means that the number of signal events produced in ATLAS at 13 TeV should be about 75% of those at 8 TeV, and the ratio is even worse for CMS (who used only 2.6 fb-1).  However, the background may grow less fast than the signal, so the power of the 13 TeV and 8 TeV data is comparable.  All in all, there is some tension between the run-1 and run-2 data sets,  however a mild downward fluctuation of the signal at 8 TeV and/or a mild upward fluctuation at 13 TeV is enough to explain it.  One can also try to explain the lack of signal in run-1 by the fact that the 750 GeV particle is a decay product of a heavier resonance (in which case the cross-section gain can be much larger). More careful study with next year's data  will be needed to test for this possibility.

The model.  This is the easiest part :)  A resonance produced in gluon-gluon collisions and decaying to 2 photons?  We've seen that already... that's how the Higgs boson was first spotted.  So all we need to do is to borrow from the Standard Model. The simplest toy model for the resonance would be a new singlet scalar with mass of 750 GeV coupled to new heavy vector-like quarks that carry color and electric charges. Then quantum effects will produce, in analogy to what happens for the Higgs boson, an effective coupling of the new scalar to gluons and photons:

By a judicious choice of the effective couplings (which depend on masses, charges, and couplings of the vector-like quarks) one can easily fit the diphoton excess observed by ATLAS and CMS. This is shown as the green region in the plot.

 If the vector-like quark is a T', that is to say, it has the same color and electric charge as the Standard Model top quark, then the effective couplings must lie along the blue line. The exclusion limits from the run-1 data (mesh) cut through the best fit region, but do not disfavor the model completely. Variation of this minimal toy model will appear in a 100 papers this week.

The big picture.  Here sky is the limit. The situation is completely different than 3 years ago, where there was one strongly preferred (and ultimately true) interpretation of the 125 GeV diphoton and 4-lepton signals as the Higgs boson of the Standard Model. On the other hand,  scalars coupled to new quarks appear in countless model of new physics. We may be seeing the radial Higgs partner predicted by little Higgs or twin Higgs models, or the dilaton arising due to spontaneous conformal symmetry breaking, or a composite state bound by new strong interactions.  It could be a part of the extended Higgs sector in many different context, e.g. the heavy scalar or pseudo-scalar in the two Higgs doublet models.  For more spaced out possibilities, it could be the KK graviton of the Randall-Sundrum model, or it could fit some popular supersymmetric models such as the  NMSSM. All these scenarios face some challenges.  One is to explain why the branching ratio into two photons is large enough to be observed, and why the 750 GeV scalar is not seen in other decays channels, e.g. in decay to W boson pairs which should be the dominant mode for a Higgs-like scalar.  However, these challenges are nothing that an average theorist could not resolve by tomorrow morning.  Most likely, this particle would just be a small part of the larger structure, possibly having something to do with electroweak symmetry breaking and the hierarchy problem of the Standard Model.  If the signal is a real thing, then it may be the beginning of a new golden era in particle physics....

Gunpowder Plot: foiled

Just a week ago I hailed the new king, and already there was an assassination attempt. A new paper claims that the statistical significance of the 750 GeV diphoton excess is merely 2 sigma local. The  story is being widely discussed in the corridors and comment sections because we all like to watch things die...  The assassins used this plot:

The Standard Model prediction for the diphoton background at the LHC is difficult to calculate from first principles. Therefore,  the ATLAS collaboration assumes a theoretically motivated functional form for this background as a function of the diphoton invariant mass. The ansatz contains a number of free parameters, which are then fitted using the data in the entire analyzed range of invariant masses. This procedure leads to the prediction represented by the dashed line in the plot (but see later). The new paper assumes a slightly more complicated functional form with more free parameters, such that the slope of the background is allowed to change.  The authors argue that their more general  ansatz provides a better fit to the entire diphoton spectrum, and moreover predicts a larger background for the large invariant masses.  As a result, the significance of the 750 GeV excess decreases to an insignificant value of 2 sigma.
     
There are several problems with this claim.  First, I'm confused why the blue line is described as the ATLAS fit, since it is clearly different than the background curve in the money-plot provided by ATLAS (Fig. 1 in ATLAS-CONF-2015-081). The true ATLAS background is above the blue line, and much closer to the black line in the peak region (edit: it seems now that the background curve plotted by ATLAS corresponds to a1=0  and one more free parameter for an overall normalization, while the paper assumes fixed normalization). Second, I cannot reproduce the significance quoted in the paper. Taking the two ATLAS bins around 750 GeV, I find 3.2 sigma excess using the true ATLAS background, and 2.6 sigma using the black line (edit: this is because my  estimate is too simplistic, and the paper also takes into account the uncertainty on the background curve). Third, the postulated change of slope is difficult to justify theoretically. It would mean there is a new background component kicking in at ~500 GeV, but this does not seem to be the case in this analysis.

Finally, the problem with the black line is that it grossly overshoots the high mass tail,  which is visible even to a naked eye.  To be more quantitative, in the range 790-1590 GeV there are 17 diphoton events observed by ATLAS,  the true ATLAS backgrounds predicts 19 events, and the black line predicts 33 events. Therefore, the background shape proposed in the paper is inconsistent with the tail at the 3 sigma level! While the alternative background choice decreases the  significance at the 750 GeV peak, it simply moves (and amplifies) the tension to another place.

So, I think the plot is foiled and the  claim does not stand scrutiny.  The 750 GeV peak may well be just a statistical fluctuation that will go away when more data is collected, but it's unlikely to be a stupid error on the part of ATLAS. The king will live at least until summer.

After the hangover

The loss of the 750 GeV diphoton resonance is a big blow to the particle physics community. We are currently going through the 5 stages of grief, everyone at their own pace, as can be seen e.g. in this comments section. Nevertheless, it may already be a good moment to revisit the story one last time, so as  to understand what went wrong.

In the recent years, physics beyond the Standard Model has seen 2 other flops of comparable impact: the faster-than-light neutrinos in OPERA, and the CMB tensor fluctuations in BICEP.  Much as the diphoton signal, both of the above triggered a binge of theoretical explanations, followed by a massive hangover. There was one big difference, however: the OPERA and BICEP signals were due to embarrassing errors on the experiments' side. This doesn't seem to be the case for the diphoton bump at the LHC. Some may wonder whether the Standard Model background may have been slightly underestimated,  or whether one experiment may have been biased by the result of the other... But, most likely, the 750 GeV bump was just due to a random fluctuation of the background at this particular energy. Regrettably, the resulting mess cannot be blamed on experimentalists, who were in fact downplaying the anomaly in their official communications. This time it's the theorists who  have some explaining to do.

Why did theorists write 500 papers about a statistical fluctuation?  One reason is that it didn't look like one at first sight. Back in December 2015, the local significance of the diphoton  bump in ATLAS run-2 data was 3.9 sigma, which means the probability of such a fluctuation was 1 in 10000. Combining available run-1 and run-2 diphoton data in ATLAS and CMS, the local significance was increased to 4.4 sigma.  All in all, it was a very unusual excess, a 1-in-100000 occurrence! Of course, this number should be interpreted with care. The point is that the LHC experiments perform gazillion different measurements, thus they are bound to observe seemingly unlikely outcomes in a small fraction of them. This can be partly taken into account by calculating the global significance, which is the probability of finding a background fluctuation of the observed size anywhere in the diphoton spectrum. The global significance of the 750 GeV bump quoted by ATLAS was only about two sigma, the fact strongly emphasized by the collaboration.  However, that number can be misleading too.  One problem with the global significance is that, unlike for the local one, it cannot be  easily combined in the presence of separate measurements of the same observable. For the diphoton final state we  have ATLAS and CMS measurements in run-1 and run-2,  thus 4 independent datasets, and their robust concordance was crucial  in creating the excitement.  Note also that what is really relevant here is the probability of a fluctuation of a given size in any of the  LHC measurement, and that is not captured by the global significance.  For these reasons, I find it more transparent work with the local significance, remembering that it should not be interpreted as the probability that the Standard Model is incorrect. By these standards, a 4.4 sigma fluctuation in a combined ATLAS and CMS dataset is still a very significant effect which deserves a special attention. What we learned the hard way is that such large fluctuations do happen at the LHC...   This lesson will certainly be taken into account next time we encounter a significant anomaly.

Another reason why the 750 GeV bump was exciting is that the measurement is rather straightforward.  Indeed, at the LHC we often see anomalies in complicated final states or poorly controlled differential distributions, and we treat those with much skepticism.  But a resonance in the diphoton spectrum is almost the simplest and cleanest observable that one can imagine (only a dilepton or 4-lepton resonance would be cleaner). We already successfully discovered one particle this way - that's how the Higgs boson first showed up in 2011. Thus, we have good reasons to believe that the collaborations control this measurement very well.

Finally, the diphoton bump was so attractive because theoretical explanations were  plausible.  It was trivial to write down a model fitting the data, there was no need to stretch or fine-tune the parameters, and it was quite natural that the particle first showed in as a diphoton resonance and not in other final states. This is in stark contrast to other recent anomalies which typically require a great deal of gymnastics to fit into a consistent picture.   The only thing to give you a pause was the tension with the LHC run-1 diphoton data, but even that became  mild after the Moriond update this year.

So we got a huge signal of a new particle in a clean channel with plausible theoretic models to explain it...  that was a really bad luck.  My conclusion may not be shared by everyone but I don't think that the theory community committed major missteps  in this case.  Given that for 30 years we have been looking for a clue about the fundamental theory beyond the Standard Model, our reaction was not disproportionate once a seemingly reliable one had arrived.  Excitement is an inherent part of physics research. And so is disappointment, apparently.

There remains a question whether we really needed 500 papers...   Well, of course not: many of  them fill an important gap.  Yet many are an interesting read, and I personally learned a lot of exciting physics from them.  Actually, I suspect that the fraction of useless papers among the 500 is lower than for regular daily topics.  On a more sociological side, these papers exacerbate the problem with our citation culture (mass-grave references), which undermines the citation count as a means to evaluate the research impact.  But that is a wider issue which I don't know how to address at the moment.

Time to move on. The ICHEP conference is coming next week, with loads of brand new results based on up to 16 inverse femtobarns of 13 TeV LHC data.  Although the rumor is that there is no new exciting  anomaly at this point, it will be interesting to see how much room is left for new physics. The hope lingers on, at least until the end of this year.

In the comments section you're welcome to lash out on the entire BSM community - we made a wrong call so we deserve it. Please, however, avoid personal attacks (unless on me). Alternatively, you can also give us a hug :) 

Game of Thrones: 750 GeV edition

The 750 GeV diphoton resonance has made a big impact on theoretical particle physics. The number of papers on the topic is already legendary, and they keep coming at the rate of order 10 per week. Given that the Backović model is falsified, there's no longer a theoretical upper limit.  Does this mean we are not dealing with the classical ambulance chasing scenario? The answer may be known in the next days.

So who's leading this race?  What kind of question is that, you may shout, of course it's Strumia! And you would be wrong, independently of the metric.  For this contest, I will consider two different metrics: the King Beyond the Wall that counts the number of papers on the topic, and the Iron Throne that counts how many times these papers have been cited.

In the first category,  the contest is much more fierce than one might expect: it takes 8 papers to be the leader, and 7 papers may not be enough to even get on the podium!  Among the 3 authors with 7 papers the final classification is decided by trial by combat the citation count.  The result is (drums):

Citations, tja...   Social dynamics of our community encourages referencing all previous work on the topic, rather than just the relevant ones, which in this particular case triggered a period of inflation. One day soon citation numbers will mean as much as authorship in experimental particle physics. But for now the size of the h-factor is still an important measure of virility for theorists. If the citation count rather the number of papers is the main criterion, the iron throne is taken by a Targaryen contender (trumpets):

This explains why the resonance is usually denoted by the letter S.

Update 09.08.2016. Now that the 750 GeV excess is officially dead, one can give the final classification. The race for the iron throne was tight till the end, but there could only be one winner:

As you can see, in this race the long-term strategy and persistence proved to be more important than pulling off a few early victories.  In the other category there have also been  changes in the final stretch: the winner added 3 papers in the period between the un-official and official announcement of the demise of the 750 GeV resonance. The final standing are:

Congratulations for all the winners.  For all the rest, wish you more luck and persistence in the next edition,  provided it will take place.

Off we go

The LHC is back in action since last weekend, again colliding protons with 13 TeV energy. The weasels' conspiracy was foiled, and the perpetrators were exemplarily electrocuted. PhD students have been deployed around the LHC perimeter to counter any further sabotage attempts (stoats are known to have been in league with weasels in the past). The period that begins now may prove to be the most exciting time for particle physics in this century.  Or the most disappointing.

The beam intensity is still a factor of 10 below the nominal one, so  the harvest of last weekend is meager 40 inverse picobarns. But the number of proton bunches in the beam is quickly increasing, and once it reaches O(2000), the data will stream at a rate of a femtobarn per week or more. For the nearest future, the plan is to have a few inverse femtobarns on tape by mid-July, which would roughly double the current 13 TeV dataset. The first analyses of this chunk of data  should be presented around the time of the  ICHEP conference in early August. At that point we will know whether the 750 GeV particle is real. Celebrations will begin if the significance of the diphoton peak increases after adding the new data, even if the statistics is not enough to officially announce  a discovery. In the best of all worlds, we may also get a hint of a matching 750 GeV peak in another decay channel (ZZ, Z-photon, dilepton, t-tbar,...) which would help focus our model building. On the other hand, if the significance of the diphoton peak drops in August, there will be a massive hangover...

By the end of October, when the 2016 proton collisions are scheduled to end, the LHC hopes to collect some 20 inverse femtobarns of data. This should already give us a rough feeling of new physics within the reach of the LHC. If a hint of another resonance is seen at that point, one will surely be able to confirm or refute it with the data collected in the following years.  If nothing is seen... then you should start telling yourself that condensed matter physics is also sort of fundamental,  or that systematic uncertainties in astrophysics are not so bad after all...  In any scenario, by December, when first analyses of the full  2016 dataset will be released,  we will know infinitely more than we do today.

So fasten your seat belts and get ready for a (hopefully) bumpy ride. Serious rumors should start showing up on blogs and twitter starting from July.

Do-or-die year

The year 2016 began as any other year... I mean the hangover situation in particle physics. We have a theory of fundamental interactions - the Standard Model - that we know is certainly not the final  theory because it cannot account for dark matter, matter-antimatter asymmetry, and cosmic inflation. At the same time, the Standard Model perfectly describes any experiment we have performed here on Earth (up to a few outliers that can be shrugged off as statistical fluctuations)... If you're having a déjà vu, maybe it's the Monty Python sketch, or maybe because this post begins exactly the same as one written a year ago.   Back then,  I was optimistically  assuming that the 2015 LHC operation would go better than the projections, and that some 15 inverse femtobarn of data would be collected by each experiment. That amount would have clarified our chances for a discovery in the LHC run-2, and determine whether or not we should dust off our No Future t-shirts. Instead, due to machine's hiccups during the summer, only about 4 fb-1 was delivered to each experiment. This added up to the magnet and calorimeters problems in the CMS experiment who managed to collect only 2.4 fb-1 of useful data, against 3.6 fb-1 in ATLAS. With that amount, the discriminating power is improved with respect to run-1 only for particles heavier than ~1 TeV.  As a consequence, the boundaries of our knowledge have changed only slightly compared to the status a year ago. At the end of the day, it's this year, and not the previous one, that is going to be decisive for the field.

So the tension is similar as last year at this time, however the mood is considerably better, see the plot. We have two intriguing hints of new physics that have a non-negligible chance to develop into a strong evidence: one is the B-meson anomalies discussed several times in this blog, and the other is the 750 GeV diphoton excess. Especially the latter stirs theorists' imagination,  even if some experimentalists deplore the fact (theorists writing papers inspired by experimental data? oh horror...).   A significant deviation from the Standard Model seen independently  by 2 different collaborations  in an experimentally clean channel happens for the first time in my life.  In my private poll, the chances for the B-meson anomalies to be new physics are estimated as 1%, while for the diphoton the chances are 10%. This adds up to a whopping 11% chance, the biggest ever, of finding new physics soon. Moreover, if the diphoton excess is really a new particle, we are basically guaranteed to find other phenomena beyond the Standard Model. Indeed, most models accommodating the 750 GeV excess require new colored states with O(1) TeV mass, which are then most naturally embedded in a theory with new strong interactions at a few TeV scale.  Not only would that give a boost to future LHC analyses, but it would also motivate building a higher-energy collider,  e.g.  a 30 TeV collider that could be constructed at a short time scale at CERN.

Anything may happen this year, for good or for worse. Cross your fingers and fasten your seat belts.

April Fools' 16: Was LIGO a hack?

This post is an April Fools' joke. LIGO's gravitational waves are for real. At least I hope so ;) 

We have had recently a few scientific embarrassments, where a big discovery announced with great fanfares was subsequently overturned by new evidence.  We still remember OPERA's faster than light neutrinos which turned out to be a loose cable, or BICEP's gravitational waves from inflation, which turned out to be galactic dust emission... It seems that another such embarrassment is coming our way: the recent LIGO's discovery of gravitational waves emitted in a black hole merger may share a similar fate. There are reasons to believe that the experiment was hacked, and the signal was injected by a prankster.

From the beginning, one reason to be skeptical about LIGO's discovery was that the signal  seemed too beautiful to be true. Indeed, the experimental curve looked as if taken out of a textbook on general relativity, with a clearly visible chirp signal from the inspiral phase, followed by a ringdown signal when the merged black hole relaxes to the Kerr state. The reason may be that it *is* taken out of a  textbook. This is at least what is strongly suggested by recent developments.

On EvilZone, a well-known hacker's forum, a hacker using a nickname Madhatter was boasting that it was possible to tamper with scientific instruments, including the LHC, the Fermi satellite, and the LIGO interferometer.  When challenged, he or she uploaded a piece of code that allows one to access LIGO computers. Apparently, the hacker took advantage the same backdoor that allows the selected members of the LIGO team to inject a fake signal in order to test the analysis chain.  This was brought to attention of the collaboration members, who  decided to test the code. To everyone's bewilderment, the effect was to reproduce exactly the same signal in the LIGO apparatus as the one observed in September last year!

Even though the traces of a hack cannot be discovered, there is little doubt now that there was a foul play involved. It is not clear what was the motif of the hacker: was it just a prank, or maybe an elaborate plan to discredit the scientists. What is even more worrying is that the same thing could happen in other experiments. The rumor is that the ATLAS and CMS collaborations are already checking whether the 750 GeV diphoton resonance signal could also be injected by a hacker.

Meanwhile at the LHC

The excitement about the CDF bump is subsiding so we can relax and look back at the LHC. A lot is going on there, although the best of the action will occur later this summer.

• As everyone knows, the luminosity collected by ATLAS and CMS just passed the 1 fb-1 benchmark which had originally been the goal for the entire 2011. The LHC management is running a policy of making careful projections which can later be spectacularly surpassed. We all agree it's better than the previous policy of bold projections and spectacular catastrophes.
• Unofficial predictions for the luminosity at the end of 2011 are Gaussian distributed with a peak around 5 fb-1. It means that, this year we'll probably learn whether the Standard Model Higgs exists or not! That's massive.
• Several results using a good chunk of this year's data, around 200 pb-1, have already emerged from ATLAS on the occasion of the PLHC conference in Perugia 2 weeks ago. The most interesting result is the t-tbar invariant mass spectrum. The most disappointing, too. If Tevatron's excess in the top quark forward-backward asymmetry is due to a 1-2 TeV heavy Kaluza-Klein gluon the t-tbar spectrum should display a peak or an excess at the tail. Alas, nothing there, as you can see.
• About the update on searches for Z' decaying to leptons, see Tommaso's blog. There's also a new search for W' in the muon + missing energy channel. In those cases also nothing, only vague rumors.
• Meanwhile SUSY searches are continuing at full steam. New analyses in the jets+met and jets+lepton+met channels are out. From the plot you can read that, for equal squark and gluino masses, the limit on the masses is above the magic threshold of 1 TeV. If the squarks are decoupled the limit on the gluino mass is slightly less stringent, about 750 GeV, and similalry the other way around. Children take about 6 years to realize Santa Claus does not exist, the LHC may be quicker than that.
• Theorists, on the other hand, are trying to understand the deeper meaning of the LHC limits on SUSY. The conclusion is that SUSY must be just behind the corner, just a little bit more, one last effort, and we'll see it. One should note that preference of the global fits for light superparticle masses is driven by one measurement: the long standing 3 sigma excess in the muon anomalous magnetic moment. Interestingly, a recent paper reevaluates the theoretical contributions to the muon g-2 and concludes there is no excess whatsoever. I am not in a position to judge whether the paper is correct, drop your comment if you are.
• Meanwhile, ATLAS and CMS keep posting papers on arXiv which use only the meager last year's harvest of 35pb-1. At this point it feels like offering ZX Spectrum in an Apple store.

The next big dump of LHC results is bound to happen for the EPS conference in Grenoble end of July. Expect numerous new physics analyses using 1.x inverse femtobarn of data. It'll rock!

Where were we?

Last time this blog was active, particle physics was entering a sharp curve. That the infamous 750 GeV resonance had petered out was not a big deal in itself - one expects these things to happen every now and then.  But the lack of any new physics at the LHC when it had already collected a significant chunk of data was a reason to worry. We know that we don't know everything yet about the fundamental interactions, and that there is a deeper layer of reality that needs to be uncovered (at least to explain dark matter, neutrino masses, baryogenesis, inflation, and physics at energies above the Planck scale). For a hundred years, increasing the energy of particle collisions has been the best way to increase our understanding of the basic constituents of nature. However, with nothing at the LHC and the next higher energy collider decades away, a feeling was growing that the progress might stall.

In this respect, nothing much has changed during the time when the blog was dormant, except that these sentiments are now firmly established. Crisis is no longer a whispered word, but it's openly discussed in corridors, on blogs, on arXiv, and in color magazines.  The clear message from the LHC is that the dominant paradigms about the physics at the weak scale were completely misguided. The Standard Model seems to be a perfect effective theory at least up to a few TeV, and there is no indication at what energy scale new particles have to show up. While everyone goes through the five stages of grief at their own pace, my impression is that most are already well past the denial. The open question is what should be the next steps to make sure that exploration of fundamental interactions will not halt. 

One possible reaction to a crisis is more of the same.  Historically, such an approach has often been efficient, for example it worked for a long time in the case of the Soviet economy. In our case one could easily go on with more models, more epicycles, more parameter space,  more speculations.  But the driving force for all these SusyWarpedCompositeStringBlackHairyHole enterprise has always been the (small but still) possibility of being vindicated by the LHC. Without serious prospects of experimental verification, model building is reduced to intellectual gymnastics that can hardly stir imagination.  Thus the business-as-usual is not an option in the long run: it couldn't elicit any enthusiasm among the physicists or the public,  it wouldn't attract new bright students, and thus it would be a straight path to irrelevance.

So, particle physics has to change. On the experimental side we will inevitably see, just for economical reasons, less focus on high-energy colliders and more on smaller experiments. Theoretical particle physics will also have to evolve to remain relevant.  Certainly, the emphasis needs to be shifted away from empty speculations in favor of more solid research. I don't pretend to know all the answers or have a clear vision of the optimal strategy, but I see three promising directions.

One is astrophysics where there are much better prospects of experimental progress.  The cosmos is a natural collider that is constantly testing fundamental interactions independently of current fashions or funding agencies.  This gives us an opportunity to learn more  about dark matter and neutrinos, and also about various hypothetical particles like axions or milli-charged matter. The most recent story of the 21cm absorption signal shows that there are still treasure troves of data waiting for us out there. Moreover, new observational windows keep opening up, as recently illustrated by the nascent gravitational wave astronomy. This avenue is of course a non-brainer, already explored since a long time by particle theorists, but I expect it will further gain in importance in the coming years. 

Another direction is precision physics. This, also, has been an integral part of particle physics research for quite some time, but it should grow in relevance. The point is that one can probe very heavy particles, often beyond the reach of present colliders,  by precisely measuring low-energy observables. In the most spectacular example, studying proton decay may give insight into new particles with masses of order 10^16 GeV - unlikely to be ever attainable directly. There is a whole array of observables that can probe new physics well beyond the direct LHC reach: a myriad of rare flavor processes, electric dipole moments of the electron and neutron, atomic parity violation, neutrino scattering,  and so on. This road may be long and tedious but it is bound to succeed: at some point some experiment somewhere must observe a phenomenon that does not fit into the Standard Model. If we're very lucky, it  may be that the anomalies currently observed by the LHCb in certain rare B-meson decays are already the first harbingers of a breakdown of the Standard Model at higher energies.

Finally, I should mention formal theoretical developments. The naturalness problem of the cosmological constant and of the Higgs mass may suggest some fundamental misunderstanding of quantum field theory on our part. Perhaps this should not be too surprising.  In many ways we have reached an amazing proficiency in QFT when applied to certain precision observables or even to LHC processes. Yet at the same time QFT is often used and taught in the same way as magic in Hogwarts: mechanically,  blindly following prescriptions from old dusty books, without a deeper understanding of the sense and meaning.  Recent years have seen a brisk development of alternative approaches: a revival of the old S-matrix techniques, new amplitude calculation methods based on recursion relations, but also complete reformulations of the QFT basics demoting the sacred cows like fields, Lagrangians, and gauge symmetry. Theory alone rarely leads to progress, but it may help to make more sense of the data we already have. Could better understanding or complete reformulating of QFT bring new answers to the old questions? I think that is  not impossible. 

All in all, there are good reasons to worry, but also tons of new data in store and lots of fascinating questions to answer.  How will the B-meson anomalies pan out? What shall we do after we hit the neutrino floor? Will the 21cm observations allow us to understand what dark matter is? Will China build a 100 TeV collider? Or maybe a radio telescope on the Moon instead?  Are experimentalists still needed now that we have machine learning? How will physics change with the centre of gravity moving to Asia?  I will tell you my take on such and other questions and  highlight old and new ideas that could help us understand the nature better.  Let's see how far I'll get this time ;)

Weekend Plot: Stealth stops exposed

This weekend we admire the new ATLAS limits on stops - hypothetical supersymmetric partners of the top quark:

For a stop promptly decaying to a top quark and an invisible neutralino, the new search excludes the mass range between m_top and 191 GeV. These numbers do not seem impressive at first sight, but let me explain why it's interesting.

No sign of SUSY at the LHC could mean that she is dead, or that she is resting hiding. Indeed, the current experimental coverage has several blind spots where supersymmetric particles, in spite of being produced in large numbers, induce too subtle signals in a detector to be easily spotted. For example, based on the observed distribution of events with a top-antitop quark pair accompanied by large missing momentum, ATLAS and CMS put the lower limit on the stop mass at around 750 GeV. However, these searches are inefficient if the stop mass is close to that of the top quark, 175-200 GeV (more generally, for m_top+m_neutralino ≈ m_stop). In this so-called stealth stop region,  the momentum carried away by the neutralino is too small to distinguish stop production from the standard model process of top quark production. We need another trick to smoke out light stops. The ATLAS collaboration followed theorist's suggestion to use spin correlations. In the standard model, gluons couple  either to 2 left-handed or to 2 right-handed quarks. This leads to a certain amount of correlation between  the spins of the top and the antitop quark, which can be seen by looking at angular distributions of the decay products of  the top quarks. If, on the other hand, a pair of top quarks originates from a decay of spin-0 stops, the spins of the pair are not correlated. ATLAS measured spin correlation in top pair production; in practice, they measured the distribution of the azimuthal angle between the two charged leptons in the events where both top quarks decay leptonically. As usual, they found it in a good agreement with the standard model prediction. This allows them to deduce that there cannot be too many stops polluting the top quark sample, and place the limit of 20 picobarns on the stop production cross section at the LHC, see the black line on the plot. Given the theoretical uncertainties, that cross section corresponds to the stop mass somewhere between 191 GeV and 202 GeV.

So, the stealth stop window is not completely closed yet, but we're getting there.

Hail the XENON excess

Where were we...  It's been years since particle physics last made an exciting headline. The result announced today by the XENON collaboration is a welcome breath of fresh air. It's too early to say whether it heralds a real breakthrough, or whether it's another bubble to be burst. But it certainly gives food for thought for particle theorists, enough to keep hep-ph going for the next few months.

The XENON collaboration was operating a 1-ton xenon detector in an underground lab in Italy. Originally, this line of experiments was devised to search for hypothetical heavy particles constituting dark matter, so called WIMPs. For that they offer a basically background-free environment, where a signal of dark matter colliding with xenon nuclei would stand out like a lighthouse. However all WIMP searches so far have returned zero, null, and nada. Partly out of boredom and despair, the xenon-based collaborations began thinking out-of-the-box to find out what else their shiny instruments could be good for. One idea was to search for axions. These are hypothetical superlight and superweakly interacting particles, originally devised to plug a certain theoretical hole in the Standard Model of particle physics. If they exist, they should be copiously produced in the core of the Sun with energies of order a keV. This is too little to perceptibly knock an atomic nucleus, as xenon weighs over a hundred GeV. However, many variants of the axion scenario, in particular the popular DFSZ model, predicts axions interacting with electrons. Then a keV axion may occasionally hit the cloud of electrons orbiting xenon atoms, sending one to an excited level or ionizing the atom. These electron-recoil events can be identified principally by the ratio of ionization and scintillation signals, which is totally different than for WIMP-like nuclear recoils. This is no longer a background-free search, as radioactive isotopes present inside the detector may lead to the same signal. Therefore collaboration have to search for a peak of electron-recoil events at keV energies.     

This is what they saw in the XENON1t data

Energy spectrum of electron-recoil events measured by the XENON1T experiment. 

The expected background is approximately flat from 30 keV down to the detection threshold at 1 keV, below which it falls off abruptly. On the other hand, the data seem to show a signal component growing towards low energies, and possibly peaking at 1-2 keV. Concentrating on the 1-7 keV range (so with a bit of cherry-picking), 285 events is observed in the data compared to an expected 232 events from the background-only fit. In purely statistical terms, this is a 3.5 sigma excess.

Assuming it's new physics, what does this mean? XENON shows that there is a flux of light relativistic particles arriving into their detector.  The peak of the excess corresponds to the temperature in the core of the Sun (15 million kelvin = 1.3 keV), so our star is a natural source of these particles (but at this point XENON cannot prove they arrive from the Sun). Furthermore, the  particles must couple to electrons, because they can knock xenon's electrons off their orbits. Several theoretical models contain particles matching that description. Axions are the primary suspects, because today they are arguably the best motivated extension of the Standard Model. They are naturally light, because their mass is protected by built-in symmetries, and for the same reason their coupling to matter must be extremely suppressed.  For QCD axions the defining feature is their coupling to gluons, but in generic constructions one also finds the pseudoscalar-type interaction between the axion a and electrons e:

To explain the excess, one needs the coupling g to be of order 10^-12, which is totally natural in this context. But axions are by no means the only possibility. A related option is the dark photon, which differs from the axion by certain technicalities, in particular it has spin-1 instead of spin-0. The palette of viable models is certainly much broader, with  the details to be found soon on arXiv.           

A distinct avenue to explain the XENON excess is neutrinos. Here, the advantage is that we already know that neutrinos exist, and that the Sun emits some 10^38 of them every second. In fact, the background model used by XENON includes 220 neutrino-induced events in the 1-210 keV range.
However, in the standard picture, the interactions of neutrinos with electrons are too weak to explain the excess. To that end one has to either increase their flux (so fiddle with the solar model), or to increase their interaction strength with matter (so go beyond the Standard Model). For example, neutrinos could interact with electrons via a photon intermediary. While neutrinos do not have an electric charge, uncharged particles can still couple to photons via dipole or higher-multipole moments. It is possible that new physics (possibly the same that generates the neutrino masses) also pumps up the neutrino magnetic dipole moment. This can be described in a model-independent way by adding a non-renormalizable dimension-7 operator to the Standard Model, e.g.
   

To explain the XENON excess we need d of order 10^-6. That mean new physics responsible for the dipole moment must be just behind the corner, below 100 TeV or so.

How confident should we be that it's new physics? Experience has shown again and again that anomalies in new physics searches have, with a very large confidence, a mundane origin that does not involve exotic particles or interactions.  In this case, possible explanations are, in order of likelihood,  1) small contamination of the detector, 2) some other instrumental effect that the collaboration hasn't thought of, 3) the ghost of Roberto Peccei, 4) a genuine signal of new physics. In fact, the collaboration itself is hedging for the first option, as they cannot exclude the presence of a small amount of  tritium in the detector, which would produce a signal similar to the observed excess. Moreover, there are a few orange flags for the new physics interpretation:

1.  Simplest models explaining the excess are excluded by astrophysical observations. If axions can be produced in the Sun at the rate suggested by the XENON result, they can be produced at even larger rates in hotter stars, e.g. in red giants or white dwarfs. This would lead to excessive cooling of these stars, in conflict with observations. The upper limit on the axion-electron coupling g from red giants is 3*10^-13, which is an order of magnitude  less than what is needed for the XENON excess.  The neutrino magnetic moment explanations faces a similar difficulty. Of course, astrophysical limits reside in a different epistemological reality; it is not unheard of that they are relaxed by an order of magnitude or disappear completely. But certainly this is something to worry about.  
2.  At a more psychological level, a small excess over a large background near a detection threshold.... sounds familiar. We've seen that before in the case of the DAMA and CoGeNT dark matter experiments, at it didn't turn out well.     
3. The bump is at 1.5 keV, which is *twice* 750 eV.  

So, as usual, more data, time, and patience is needed to verify the new physics hypothesis. On the experimental side, the near future is very optimistic, with the XENONnT, LUX-ZEPLIN, and PandaX-4T experiments all jostling for position to confirm the excess and earn eternal glory. On the theoretical side, the big question is whether the stellar cooling constraints can be avoided, without too many epicycles. It would be also good to know whether the particle responsible for the XENON excess could be related to dark matter and/or to other existing anomalies, in particular to the B-meson ones. For answers, tune in to arXiv, from tomorrow on.