Friday 23 March 2012

New SUSY limits from ATLAS

There's been another dump of SUSY results on ATLAS public pages, with several search channels updated to 5 inverse femtobarns. The update includes the vanilla jets + missing energy search where ATLAS looks for events containing a number of energetic jets and displaying a large unbalanced transverse momentum. This is the most generic signal of supersymmetry when the lightest SUSY particle (LSP) is weakly interacting and escapes detection; ultimately, any such model should produce an excess in the jets+MET search. For example, a squark may decay to a quark and a neutralino LSP, or a gluino may decay to 2 quarks and a neutralino LSP, so a pair of squarks/gluinos typically produces a signal with 2/4 jets and missing energy (sometimes more, if colliding partons radiate additional jets, sometimes less, if a jet gets lost in a crack). In this note ATLAS focuses on events with the number of jets between 2 and 6. The money plot is this one:
It shows the parameter space of a simplified SUSY models where the only particles are 1st and 2nd generation squarks, a gluino and a massless neutralino. If the squarks are outside of the LHC reach the lower limit on the gluino mass is now above 900 GeV, whereas if the squarks and gluino have comparable masses the limits on both are approaching the impressive value of 1.5 TeV. ATLAS has been lucky with the background and therefore in most of the parameter space the limits are somewhat better than expected (in spite of an excess in one category with 4 jets). One can see that we're running out of the parameter space, as the plot legend barely fits the allowed region.

ATLAS also prepared 5 fb-1 updates of multijet and 1 lepton + jets SUSY searches. Those are less sensitive in a generic case, but for some specific SUSY spectra they could provide more stringent limits than the vanilla jets+MET search. No excess in either.

On the other hand, one should note that limits on the masses of the superpartners of the top quark still do not exist in a robust form. It is logically possible that the stop masses are relatively small, close to that of the top quark, while other colored superpartners are much heavier than TeV. This sort of spectrum is harder to achieve from the model-building perspective, but is favorable from the point of view of naturalness: light stops would cancel the dominant contribution to the Higgs mass from the induced in the Standard Model by the top quark. If that sort of spectrum is realized in nature then the limits discussed in the previous paragraphs do not apply, mostly due to the fact that the stop pair production cross section at the LHC is much smaller than that of gluinos and 1st generation squarks. In that case more elaborate experimental strategies are in order. Last February ATLAS presented limits on stops produced in decays of gluinos but, again, these limits are void if gluinos are beyond the current LHC reach. Now ATLAS has released one search targeting the direct stop production, however in the context of a more contrived SUSY model where the lightest neutralino spits off a Z boson while decaying to a gravitino (the event selection requires 2 leptons with the invariant mass in the Z window). In that particular model the limits on the stop mass are about 300 GeV. But, more generally, there are no model independent limits yet: for all we officially know, stops could be as light as the top quark...

Wednesday 14 March 2012

Higgs or BEEEH boson?

Most of you must have noticed a certain curious occurrence at the Moriond conference:
during the session featuring the latest experimental results on the Higgs boson searches the name Higgs did not appear at all. Instead, the speakers were discussing a mysterious SM scalar boson (?) or on a BEEEH boson (???), the latter name apparently inspired by a herd of sheep grazing outside the conference room. The most logical explanation is that the Moriond attendants have been collectively hypnotized and conditioned to say BEEEH every time they meant Higgs (I saw a similar trick at a hypnosis show in Las Vegas). That theory would also explain why only the strongest characters continued referring to the Higgs. The alternative explanation -- that someone at the conference had an idea to rename the Higgs boson into a BEEEH boson and talked to it so many otherwise reasonable scientists -- sounds utterly implausible. That's because the idea
  1. is obviously silly,
  2. betrays ignorance.
I guess there's no need to elaborate too long on why it's silly. In physics and mathematics communication heavily relies on established conventions. Like, we all agree the result of adding 2 and 2 should be called 4, although there is no deep reason for it; we could call 11 or 666 depending on the moon phase, and that would be perfectly consistent as long as we were careful to preserve the axioms of the addition group. However calling it 4 at all times makes our life easier and helps avoiding confusion. By similarly established conventions, spontaneous breaking of a local symmetry in a quantum field theory is referred to (somewhat unfairly) as the Higgs mechanism, while a spin-0 particle excitation often associated with that breaking is called (deservedly) the Higgs boson. This is how it's been for the last 40 years and this is how it'll remain.

The ignorance count deserves a bit longer explanation, as it is related to a misunderstanding that is not so uncommon. The main point is that one should clearly distinguish the Higgs mechanism from the Higgs boson; the two are often intricately related but formally they are distinct concepts, in particular the former may well exist without the latter.

The Higgs mechanism, or spontaneous gauge symmetry breaking, occurs when a Lorentz-invariant Lagrangian is also invariant under a local symmetry group with the corresponding set of gauge bosons, however some or all of these gauge bosons are massive. Such a theory still obeys a local symmetry, albeit non-linearly realized. We can describe it in a gauge invariant way with the help of a set of unphysical scalar particles called the Goldstone bosons who have a derivative mixing with the massive gauge bosons. For the public, we say that each massive gauge boson eats a Goldstone boson so as to acquire mass and another internal degree of freedom associated with it. For example, the W and Z boson must eat a triplet of Goldstone bosons. The ensemble of these phenomena is referred to as the Higgs mechanism, although more properly it should be called the Anderson-Nambu mechanism (who grasped the general idea, inspired by the related phenomenon of superconductivity in condensed matter physics), or the Brout-Englert-Higgs mechanism (who first understood its workings in the context of Lorentz invariant quantum field theories). However, the name of Higgs somehow stuck, probably because it's cute, or maybe because we all hate Anderson for cutting the throat of the SSC.

One important point is that a confirmation of the Higgs mechanism is not what the LHC is now after. Indeed, the fact that the fundamental interactions obey to a very good precision the local SU(2)xU(1) symmetry which is spontaneously broken to the electromagnetic U(1) was firmly established by the LEP experiment back in the 90s.

The LHC is now after the Higgs boson, which is something else. It turns out that in spontaneously broken gauge theories certain amplitudes, in particular those of the massive gauge bosons, grow with the center-of-mass energy. As a consequence, the theory cannot remain perturbative up to an arbitrarily high energy scale. In the Standard Model without the Higgs boson the loss of perturbativity would happen already at 1 TeV. Thus, there must be something that regulates the high-energy behavior of the W and Z scattering amplitudes. Out of several possibilities, the simplest one is to introduce an isospin-0 scalar resonance with the coupling to the W and Z bosons proportional to their masses. Voila the Higgs boson. It's not a unique possibility, but it's the one that is clearly favored by current experimental data.

Now, the Higgs boson first appeared in the paper of, surprise, Higgs in 1964, while it was completely missed in the earlier paper of Brout and Englert, and swept under the carpet in the paper by Guralnik, Hagen, and Kibble. In fact, the importance of that degree of freedom was not realized until a few years later, thanks to the papers of Higgs and Kibble, and ultimately thanks to papa Weinberg who incorporated it in the Standard Model in 1967.

In summary, if you hear someone speaking about the Brout-Englert-Higgs mechanism, that's fine, he's just trying to be fancy (and has a grudge against condensed matter). However, if you hear him talking about the BEEEH boson, that not only sounds funny but is also a good indicator that he has little idea about the subject.

Update: I see that I should clarify that this post is not about who should get the Nobel prize; that's a longer discussion. I just think that both esthetic reasons and historical truth dictate that we should continue to call the particle the Higgs boson.

Thursday 8 March 2012

Daya Bay observes theta13 at 5 sigma!

Too many news these days, so just a brief note on something that deserves a long article. The
Daya Bay experiment just announced the measurement of one of the last unknown fundamental parameters in the Standard Model (understood as the old Standard Model extended by the neutrino mass operators). The parameter is called the theta13 mixing angle and, roughly speaking, controls the oscillation probability of electron neutrinos. One way it could manifest itself is via appearance of electron neutrinos in a beam of muon neutrinos sent over several hundred kilometers. Another possible manifestation is via oscillation of electron neutrinos into the other flavors over a distance of a few hundred meters. More precisely, the survival probability of an electron neutrino with the energy E at the distance L from the source is given bywhere Δm31 is approximately equal to the "atmospheric mass difference" known to be of order 0.05 eV.

There is no theoretical reason for theta13 to be zero, however it is known to be a bit smaller than the other two neutrino mixing angles (who are known quite precisely). Several experiments have been racing to measure it: T2K in Japan, Minos in the US, Double Chooz in France, RENO in South Korea, and Daya Bay in China. Recently, there has been a few experimental hints that the value is about 10 degrees, although none of the experiments could by itself present a 3 sigma evidence.

Now it seems the first prize has been snatched by the Chinese. Daya Bay looks for disappearance of electron antineutrinos produced in nuclear reactors (if an electron neutrino transforms into other flavors it cannot be detected by this experiment, so effectively it "disappears"). Comparing the observed flux in near (~500 m) detectors and a far (~1500m) detector they conclude that about 6% of the electron neutrinos disappear in between. Based on that they quote the value of the mixing angle
or theta13 ≈ 9 degrees in more familiar units. This result suggests that neutrinos are anarchists. Unlike the quark mixing angles that display a highly hierarchical structure, the neutrino mixing angles are of similar magnitude and apparently random. The deeper reason for either of these 2 facts is currently a mystery...

So the last thing we don't know about the Standard Model is the absolute scale of the neutrino masses, and the CP violating phase in the neutrino mixing matrix. We'll probably learn those too before the end of the century.

Wednesday 7 March 2012

Higgs: evidence growing stronger

The news of the day is the update on Higgs searches released by the LHC and Tevatron at the Moriond conference. The most important updates are:

  • The significance of the low mass Higgs excess at the Tevatron increased much more than expected, given a tiny amount of data added since the previous combination. As explained at more length in my previous post, most of the steam comes from the improved b-tagging efficiency in CDF, which greatly helps to pick the H → bb signal. All in all, the Tevatron now reports a ~2.5 sigma excess in the interesting mass region between 120 and 130 GeV, although the poor mass resolution does not allow them to clearly favor any particular mass in that range. The excess is somewhat larger than predicted by the Standard Model Higgs, with the best fit value around 1.5 standard cross sections, however the standard cross section is within the 1 sigma band.
  • It is also interesting to have a look at the separate H → bb search channel where for a few more months the Tevatron is still superior to the LHC. In that channel alone the excess is even larger than in the full combination, corresponding to a ~3 sigma local significance. You should not pay too much attention to the fact that the best fit cross section on the right peaks at 135 GeV. Due to the poor mass resolution, 120 and 135 GeV Higgs looks about the same to the Tevatron, however the heavier Higgs the smaller the predicted event rate so one needs a larger boost to explain the same number of observed event. This can be better seen on the CDF plot of the p-value on the left: the significance of the excess over the background is about 3 sigma for any Higgs mass hypothesis between 125 and 135 GeV. What is more interesting is that near 125 GeV Higgs the observed signal strength is noticeably higher than the one predicted by the Standard Model, with the best fit around twice the Standard Model cross section. And that is mildly exciting.
  • ATLAS updated more search channels with the full 5fb-1 dataset (in December they updated only the 2 most sensitive γγ and ZZ→4l channels). Even if neither of the new WW, bb and ττ channels taken separately has a sufficient sensitivity yet, combined they do have some impact. Thanks to that, they are now able to exclude at 95% confidence level the Higgs boson up to 122.5 GeV (except for the probably irrelevant blip at 118). Together with CMS, they are leaving only a small allowed window between 122.5 and 127.5 GeV. We're almost there! From now on if anybody mentions the look-elsewhere effect the correct response is "what elsewhere?" ;-)
  • Amusingly, the new ATLAS channels have not only shrunk the allowed Higgs mass range, but also decreased by almost 1 sigma the combined significance of the excess at 125 GeV. You can see an example in this table that shows the expected and observed number of events in different categories of the H → WW → 2l2v search channel. Some categories show a small excess, some a small deficit, but nothing jumps in your face although it might have. More data will tell if this is just bad luck on the part of ATLAS, or something interesting is going on...
  • Although CMS fired most of their guns in December (having updated most channels to the full 5fb-1 statistics back then) they keep making small improvements in the analysis. One noticeable improvement presented today is in the diphoton search channel, thanks to a new fancy multivariate analysis whose expected sensitivity (red) is much better than that of the previous cut-based analysis (blue). Thanks to that the diphoton signal around 125 GeV grew a bit stronger and now corresponds to a p-value of almost 3 sigma. At the same time they are able to exclude the Higgs mass in the windows of 117.5-­120.5 GeV and 128.5-­132.0 GeV.
  • CMS also presented a nice graphics showing how the 125 GeV Higgs signal splits between different search channels. We see that we're getting a bit too much signal in the diphoton channel, and a bit too little signal in the WW and ZZ channel, but otherwise there is a decent agreement with the expectations from the Standard Model Higgs. If ATLAS provide an analogous plot it would look similar, except they have a somewhat stronger signal in ZZ and weaker in WW. The LHC has does not yet have enough sensitivity to the Higgs decaying to fermions (unlike the Tevatron), and thus the b-bbar and tau-tau channels currently do not provide any meaningful insight.

For more reports on today's Higgs updates, see Tommaso, Philip, Matt and again Matt.

Monday 5 March 2012

Higgs: teaser from CDF

The Moriond conference is now unfolding up on the slopes of Mont Blanc. The real action -- the Higgs update from the LHC and the Tevatron -- will happen only on Wednesday. However, as in any other 007 movie, there is a teaser before the opening credits. On the previous week another conference known as the Italian Moriond took place in the same room of the same hotel on the same ski slope. It included the talk by Homer Wolfe that uncovered many details of the upcoming CDF Higgs analysis, although stopping short before the final shootout, that is before giving away the new Higgs limits.

There hasn't been much expectation about the upcoming Tevatron Higgs update . That's because the previous Tevatron combination is based on up to 8.6 fb-1 of data and doesn't show much in the interesting mass range: only a broad 0.5 sigma excess near 125 GeV and the limits on the Higgs production rate at the level of twice the Standard Model cross section. Naively, adding 1 more crappy femtobarn should not change much, unless dramatic improvementd in the analysis. have occurred.

Now, CDF claims that they do have a dramatic improvement!

The most revamped analysis is the one where the Higgs boson is produced in association with a W or Z boson and decays into a pair of b quarks. Much as at the LHC, W/Z+H is not the main production channel at the Tevatron. However, the charged lepton and/or missing energy from W/Z decay give a chance to trigger on the event, and thus pick the H → b bbar signal from the overwhelming QCD background. Luckily enough, W/Z+H is also the last channel where the Tevatron is still highly competitive with the LHC (the latter has to fight a much larger QCD background in this channel, and only later this year it will reach a sensitivity comparable to that of the Tevatron). The news is that between now and last summer CDF has largely improved their b-tagging, with the efficiency gains reaching 40%. This is of double importance in low mass Higgs searches, as signal events contain two b-jets. Combining that with other smaller improvements, CDF expects to see twice as many signal events in the interesting mass range compared to the previous analysis. Correspondingly, the expected sensitivity for exclusion is now at about 1.4 Standard Model cross sections near 130 GeV, tantalizingly close to where the signal should pop up.

Actually, CDF went as far as to showing the distribution of the invariant mass of the b-jet pairs in the events that pass the latest selection in the W+H channel. In theory, that distribution should have a peak at the true value of the Higgs mass, although the peak will be badly smeared due to large uncertainties inherent in measuring the energy of b-jets. One thing that is clear from the plot is that the Tevatron won't teach us anything new about the Higgs boson mass: any excess is expected to span many mass bins and it may be hard to tell the 115 GeV signal from the 130 GeV one. Another thing that is visible, at least to a naive eye, is that the plot shows.... an excess of Higgs-like events in the interesting mass range. Without an insider analysis it's of course impossible to say what that really means in terms of the signal significance, but it's not impossible we will be pleasantly surprised on Wednesday...