Tuesday, 15 December 2015

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....

81 comments:

Anonymous said...

Is it theoretically possible that somethign exists at 750 GeV, which can only be produced indirectly as a decay product of something heavier but not directly by proton proton collisions ?

Jester said...

yes, perfectly possible

Tommaso Dorigo said...

Hi Jester, the background must grow as sqrt 's does. So the same number of signal events at 13 TeV are expected to be less significant...

Jester said...

Is it obvious the diphoton background is also gluon-gluon produced? Eyeballing the spectrum, there seems to be less background in the 13 TeV data compared to 8 TeV.

mfb said...

The speakers discussed the possibility of heavier particles decaying to 750 GeV particle + X, but both collaborations didn't see anything surprising in the events in the signal region (MET, additional jets, diphoton variables, ...). Quite hard to have a heavier particle decay hiding so well.

Gluon gluon and quark quark are both relevant for diphoton production, so the increase in cross-section is somewhere in between.

Brian Amadio said...

There are almost certainly different cuts applied at 13 TeV than at 8 TeV, so I'm not sure you can just compare event counts to get an idea of the change in background.

Jester said...

Mfb, difficult but not impossible - the models are being cooked by theorists as we speak. But I agree that, at the moment, Occam's razor points to a direct production of a 750 GeV particle.
Brian, I agree it's complicated and I don't know a precise answer yet. However, just from the fact that the excess is not excluded by run-1, I would expect that it must be due to a favorable signal-over-background in run-2.

Anonymous said...

Jester,
what is the meaning of the colours in your plot ?
What exactly are you fitting ?
Thanks!

Jester said...

The green is 1 and 2 sigma regions. I'm fitting directly to the excess events in the ATLAS and CMS data in the bins around 750 GeV . It's of course very rough, but it should give a an approximate picture of the results obtained from a more careful analysis.

Daniel Stolarski said...

Are the ATLAS slides posted somewhere?

Anonymous said...

But how do you explain the 40 GeV width?

mfb said...

The results are consistent with a narrow width. A larger width fits a bit better for ATLAS, but that can easily be the limited statistics.

@Jester: I didn't think of models with that statement. Purely from an experimental point of view: you would need some heavy particle that decays to 750 GeV boson + X where m(X)+750 GeV is very close to the heavy particle mass (to avoid diphoton pT), and X is invisible. Possible, but I don't see the motivation to assume some weird mass coincidence here.

kbot3000 said...

Please keep in mind that the significances you quote are local. If you take into account the look elsewhere effect it is a lot lower. That is to say - both ATLAS and CMS search over a very wide mass range and quoting the local signficance (not taking all the bins) is not a very meaningful number unless you already had a reason to only consider these few bins.

RBS said...

Somebody somewhere in HEP has been really nice this year?

Jester said...

kbot3000: the global significance after combining ATLAS and CMS is large, my guess is that it will be almost 4 sigma. It's not very likely to get 2 huge fluctuations in the same place in 2 independent experiments.

Mfb, I agree there's currently no compelling hint for a larger width, though this of course may change with more data.


andrew said...

Are any composite particle decays being considered seriously?

Ervin Goldfain said...

Jester, you say:

"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."

Here are my questions:

1) What particular "larger structure" do you have in mind?

2) Wouldn't the 750 GeV boson complicate rather than clarify the gauge hierarchy problem?

3) How come we're not seeing the effect of this event in the other channels?

Chris Austin said...

In the big picture paragraph:

"125 GeV dilepton" - do you mean 125 GeV diphoton?

"decay to W boson pairs which should be the dominant mode for a Higgs-like scalar" - isn't this only for a Higgs that couples to SU(2), (so isn't a singlet), and has a significant VEV? Do the measured couplings of the 125 GeV Higgs still allow much of a VEV for another Higgs doublet?

Lucy M said...

It's something to get excited about. Without wanting to take away from that too much, what if anything is suggested by they checked Run 1 for the excess and drew a blank? What sort of resolutions have been verified for mismatches of this kind, in the past?

Alex Small said...

Between now and the next big data release, is there anything much to do? Are there other experiments where a 750 GeV particle decaying to two photons might leave a signature? Is part of the goal of announcing this to get people looking in other places where a hypothesized new particle might leave its signature?

Jester said...

Alex: there's certainly some room for improvement of the 13 TeV analyses, for searching for the 750 GeV thing in other channels, and combining with the run-1 data. But the decisive push will come from next year's data.

Chris: thx, dilepton->diphoton corrected.
What i meant in this paragraph is that the singlet has no conserved quantum numbers, much like the Higgs boson after electroweak symmetry breaking. So the two particles are allowed to mix with each other, and they often do in many specific models. If they do, then the singlet will inherit the large coupling to WW and ZZ from the Higgs.

mfb said...

Well, the run 1 result didn't have anything significant, but still slightly more events than expected. Run 1 is compatible both with the excess and with background only in both experiments. For CMS including it increases the significance of the excess a bit, ATLAS didn't show a combination but quoted very reasonable figures for the agreement.


The conferences notes are public now.
ATLAS: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/December2015-13TeV
CMS: http://cms-results.web.cern.ch/cms-results/public-results/preliminary-results/CMS/index.html

@Alex: Looks like the CMS analysis was designed for spin-2 particles, while the ATLAS analysis mainly looked for scalar particles. The different spins lead to different kinematics, so a different selection can give different results. A bit problematic to do this properly blinded now, but not impossible. I guess we can also see more plots and cross-checks for Moriond in March.
Other decay channels can be searched for an excess at 750 GeV.

Apart from that: yeah, more data. Restart is supposed end of April. Summer conferences, I guess.

Chris Austin said...

Sorry. In my comment:

couples to SU(2), (so isn't a singlet)

I meant:

couples to SU(2) at tree level, (so isn't a singlet)

In your reply, about the two particles mixing, and the singlet inheriting the large coupling to WW and ZZ from the Higgs, can you suggest a recent paper that gives an example of that, which isn't disfavoured by the new Higgs couplings, e.g. on page 45 of ATLAS-CONF-2015-044, (the 15 Sep 2015 joint paper with CMS), http://cds.cern.ch/record/2052552 ? I thought that because the best value of \kappa_V is actually slightly larger than 1, and the lowest \kappa_V on the 1 sigma contour is about 0.96, models where the 246 GeV SM Higgs VEV is significantly shared between the 125 GeV Higgs and some other Higgs are strongly disfavoured now. Also, if the 125 GeV Higgs mixes significantly with another Higgs, won't that generically lead to large FCNC's?

Jester said...

For almost up-to-date limits, you can look e.g. at my http://arxiv.org/abs/1502.01361
(but there's been many similar papers with very similar conclusions in the last 3 years).

Chris Austin said...

Thanks

Lucy M said...

I apologize for not reading the whole article before firing off this question. The blogger fully treats the matters and more.

Drew Day said...

Jester:

There was another publication with an excess which was hardly reported on at all. Can you take a look?

The excess is in the [80 - 100] GeV bins from this publication.

The 8 TeV version of the same paper mentions an excess (a 3 sigma excess...), but doesn't have any similar graph with a narrow, one- or two- bin excess. In fact, pages 19, 28, and 29 have similar plots to the above but the aforementioned region is singled out as a normalization region and is therefore excluded as a search region. Perhaps they are similarly treating the 80 - 110 GeV region as a normalization region in the 13 TeV paper, but they don't state this fact as clearly in the 13 TeV graph (though the normalization region is listed as 80 - 101 GeV elsewhere in the paper).


Can you investigate a little bit and let me know what's going on here? I'm not actually interested in the interpretation of the excess as supporting gluinos; I'm just curious why this bump is there.

Anonymous said...

Engelbert Boson, anyone?

Alex K said...



"Engelbert Boson, anyone?"


Super idea! We could denote it with an "H"... for Humperdinck

InConstruction said...

I have a few questions (as a non-physicist-physics aficionado):

1) On nature.com i read it is a 1500GeV particle as it decayed in to two 750GeV photons. Here I read it is a 750GeV particle. I am confused. Especially that in other blogs they say that SUSY is probably dead if this particle exist (pushing the gluino mass too high).

2) What it in the data tells you that it is a boson and not any other type of particle?

3) Why is it a scalar and not another gauge boson?

I apologise if the these are simple questions. I am a simple man ;)

Thanks!

Jester said...

1) The hypothetical particle is 750 GeV - the guy in nature must have got confused.
2) If it decays to 2 bosons (like photons) it must be a boson by general rules of quantum mechanics (more precisely, by angular momentum conservation).
3) Scalar (spin 0) is the simplest possibility. It could also be a massive graviton (spin 2) or higher spins. Vector boson (spin 1) is not possible due to a general though non-trivial arguments - the so-called Landau-Yang theorem.


felipe said...

A convenient smoke screen to retire from the 2TeV WW diboson battlefield (where the two experiments never set step on officially anyway)?

John said...

Thanks for the update Jester.

Anonymous said...

gold rush started: http://astrumia.web.cern.ch/astrumia/InstantPaper.html

InConstruction said...

Thanks for your reply. Meanwhile, the article in Nature has been corrected

A follow-up question, if I may: how is this bump related to the idea that supersymmetry is "in danger" (lack of a better word)? Or is it the run in itself an indication that there is no supersymmetric particle?

Jester said...

It isn't related: the 750 GeV bump may or may not fit supersymmetric models, and we gain no additional insight in this regard.
However, other results presented during the same seminar (gluino searches, etc.) have pushed the limits on supersymmetric particles to even higher masses, which makes supersymmetric theories even less attractive than before. Not Even Wrong has a brief summary: http://www.math.columbia.edu/~woit/wordpress/?p=8189

Anonymous said...

@felipe: Both experiments looked at that range again in the presentations yesterday. No excess in run 2, but not enough data to fully rule out the run 1 result. In other words: forget 2 TeV dibosons.

Anonymous said...

At these energies shouldn't one be writing B_mu_nu in the operator and not A_mu_nu? And hence if it's new physics should it not decay to ZZ as well?

Anonymous said...

1) How many fb-1 more we need to confirm or reject discovery of a 750 GeV particle?

2) If it is confirmed, and if we assume it to be a super-partner, is there any way to tell which fermion it might associated, or is it model-dependent?

Alex K said...

There were I think 9 papers on the arXiv concerned with interpreting the alleged 750 GeV boson, some of them quite long. As far as I can tell the conclusions so far are (and much of it is already in Jester's blog post)

- a heavy Higgs state from a simple two Higgs doublet model can't explain it (but maybe a second almost degenerate pseudoscalar state could fake a large width)

- the MSSM alone can't explain it

- people lean towards strongly interacting to explain the large width

- maybe unsurprisingly, one can have the new boson as a portal to dark matter

felipe said...

Probably a controlled filtration. Can six people coordinate a decent 30+ page paper with tables and figures in 3-4 hours? Chapeau.

Curious: Atlas sees no light Higgs in two photons. If we did not have the 8TeV data the headline today would be "hints of the Higgs" (of course this would create trouble with precision LEP legacy)

Jester said...

Anon 16:03, that's correct: the scalar couples to BB, so there will also be decays to ZZ and Z \gamma with a similar rate as for the diphoton. However, the current limits on ZZ resonances in this mass range are much weaker, of order 0.5 pb, so these other decays are not observable at the moment.

Anonymous said...

The BB operator predicts Z gamma a factor of 3 below its 95% CL bound. Z gamma is more sensitive than ZZ

Anonymous said...


It really strange. I am not able to see any higgs at 125 in ATLAS data in the gg and 4l channels.
CMS has still blinded data on this. Might just be a matter of bad luck or still too rudimentary analyses,
but the Z peak seems fine. Am I missing something or really the old higgs does not show up in the presented results?

Jester said...

Thanks Anon, i wasn't aware that Z \gamma is so sensitive, this may indeed be a very important test.

mfb said...

@Anonymous 20:57: Higgs is there. ATLAS sees a weak signal for both Higgs to diphotons and 4 leptons. Just ~1.5 standard deviations above background-only, but also just <=1 standard deviations below theory predictions. There is no reason to expect the Higgs cross-section to not increase, so this is simply a ~1 sigma fluctuation.

Anonymous said...

1512.04928 Rays of light from the LHC

I see what they did there ...

Unknown said...

Why is WTF x'ed out? Sounds exactly what I. I. Rabi would say if he were alive today.

Alex K said...

Jester,

Can you off the top of your head say whether such a thing could be accomodated as a resonance in composite higgs of any known variety?

Jester said...

From top of my head, i don't recall any model that was making that specific prediction (new light scalar state seen in diphoton at around 750 GeV). However, "composite pion prime" or "composite dilaton" is one of the first things you think of after WTF. I'm sure people will think of these models more carefully now.

Anonymous said...

Hi Jester -- thanks for the note. Would you happen to know where we can get the ATLAS and CMS graphs of ZZ and Z\gamma channels for the 8 TeV 20 fb^-1 data?? These graphs may also have some useful info in interpreting the diphoton excess at 750 GeV.

Bernd said...

Would such a model give rise to additional CP violations? If yes, are such processes not ruled out by electron EDM measurements?

Anonymous said...

Dear Jester, do you have any idea why diphoton data from ATLAS is so noisy when compared to CMS ? Should we take seriously the almost 3 sigma arount 1.6 TeV ?

Jester said...

I do not see any noise, the data look smooth, and the 2 events at 1.6 TeV do not constitute a statistically significant excess

Jester said...

Bernd: for the moment, we know absolutely nothing about CP properties of that particle. If it has CP violating couplings, then there are of course indirect constraint, but for the moment no piece of data suggests we should worry about it.

Jester said...

Anon, in run-1 ZZ channels were studied by ATLAS (arXiv:1507.05930) and CMS (arXiv:1504.00936) but the limits on the cross section are very weak, O(100) fb at 750 GeV (whereas the 750 GeV resonance is well fit with O(10b) cross section). For the Z-gamma resonance searches in this mass range, there's an ATLAS analysis in arXiv:1407.8150. Indeed, their sensitivity is not so far from what may be expected if the diphoton signal is true. In the toy model one predicts 1-5 fb cross section in the Z-gamma channel, while the current limit is O(10fb).

Alex K said...

Hmmm.

Do you guys think there's a chance that we don't have to wait till Summer for new insights, maybe to see something in existing 13 TeV Z gamma data by optimizing this analysis? Maybe for a larger width?
How well do we expect CMS to do with the Z gamma final state...

Jester said...

That's right. If there's a matching excess in Z-gamma, WW, dijet, or monojets, then the case for the 750 GeV particle will grow stronger. This is in fact what happened with the Higgs boson. The initial hints in December 2011 were strengthened by additional analyses in winter 2012, such that by the time of the official discovery in July 2012 there was little doubt we were seeing a real signal.

RBS said...

Jester, for some time after Higgs celebrations HEP community was dreading "desert till Planck scale". If confirmed, would these developments dispel those fears perhaps pointing to some greater structures, hierarchies of symmetries or end up as some technical appendice to SM? What would be a more likely scenario at this point, in your view?

Jester said...

Too early to say, but if the 750 GeV particle is real then it is very likely to be a part of a larger structure. In any model constructed so far, you need additional particles to explain the signal, for example heavy vector-like quarks. My bet would be on new strong interactions at a few TeV scale, but many other possibilities exist.

Anonymous said...

Thanks Jester for the links to the 8 TeV Zgamma and ZZ papers.

Anonymous said...

hi jester -- just a question to clarify the section on "The Model" -- what is "v" -- 170 or 246 GeV (that is weak scale) or 750 GeV (new physics scale)?

Jester said...

v is always 246 GeV, except in a small area around Pisa :) But don't read too much into this figure, it was just a quick and dirty sketch. A more careful version is e.g. in Fig.5 of 1512.05777

RBS said...

Like, a hierarchy of interlocked SM-like structures in generations each one defining the landscape for the next one, all the way to Planck? Ok, that'd be really, really early to speculate

Alex K said...

I don't think I understand what you mean. I don't think we have reasons to think that there will be more structures that are very SM like at higher scales - beyond the generic idea that there might be more gauge groups that are broken at different scales, which is quite possible. But there is afaik no reason to think that this repeats in some kind of repetitive pattern. The only physics that I know of which gives you something similar is extra space dimensions. In that case you can have Kaluza Klein resonances which look like repeating broken copies of the SM gauge groups. The more likely scenario would be that there is one or few additional strong interactions providing the higgs, the 750 GeV resonance and then no further fundamental scalars until we're near some higher new physics scale wirh susy or quantum gravity of some sort. But he latter part of that is just daydreaming.

Anonymous said...

I would like to know the story about v being 246 GeV, except a small area around Pisa. I think I will learn something amusing...

Somdatta Bhattacharya said...

In the case of scalars coupling to both gluons and photons why don't dijets dominate over photons in the decay channels?

Jester said...

They typically do. However, experimental bounds on dijets in this mass range are much weaker, so there's no problem.

Alex K said...

You can always put the hypercharge on hyperdrive :)

mfb said...

For dijets, the (background) cross section is something like 3 pb/GeV at 750 GeV (within the acceptance of ATLAS/CMS). With ~5% resolution (optimistic), we have ~40 GeV width, so ~120 pb background. Assuming everything else is perfect, ATLAS would have seen ~400 million background events, CMS ~300 million. Even with a perfect (permille-level) background modeling, 1 standard deviation is ~600 events (~0.2 pb). That is 50 times the excess seen with diphotons (~10 events). Give or take a factor of 2.

The actual limits are significantly worse, see this CMS paper for example: http://arxiv.org/pdf/1302.4794v2.pdf
Even at 1000 GeV, where the background is lower by a factor of 3, the upper limit is 0.6 to 1.6 pb depending on the production process.

Somdatta Bhattacharya said...

Jester said...
...However, experimental bounds on dijets in this mass range are much weaker, so there's no problem.

Shouldn't there be some explanation for that, why can they be weaker? If the scalars are produced copiously enough, it means the gluons couple substantially to them, which in turn means that the dijet channel ought to contribute strongly in the decay channels. What am I missing?

Somdatta Bhattacharya said...

OK I finally got the point about the bounds being weaker. Thanks mfb and jester.

Alex K said...

I recall seeing some papers considering photon fusion production of the hypothetical scalar. I forget what the conclusions for such models were, though. I imagine one needs quite large couplings...

Somdatta Bhattacharya said...

However, here's something I'm still puzzled by.

mfb has said...

"...Even with a perfect (permille-level) background modeling, 1 standard deviation is ~600 events (~0.2 pb). ...

...The actual limits are significantly worse, see this CMS paper for example: http://arxiv.org/pdf/1302.4794v2.pdf
Even at 1000 GeV, where the background is lower by a factor of 3, the upper limit is 0.6 to 1.6 pb depending on the production process."

The limits being worse necessarily means that more events, bounded from above by (0.6 or 1.6 pb), are seen than those predicted (0.2 pb) (modulo the fact that the former is at 1 TeV and the latter at 750 GeV).

Now in the paper, various resonances are ruled out for various mass-ranges. The understanding being that the predicted cross-sections of the processes considered for those resonances are greater than the experimental upper bounds in the corresponding mass ranges.

So are the cross-sections of those processes involving the putative scalar at hand lower? Who has calculated this? Also isn't it this that one really needs to figure out in order to see whether the bound is violated?

mfb said...

"The limits being worse necessarily means that more events, bounded from above by (0.6 or 1.6 pb), are seen than those predicted (0.2 pb) (modulo the fact that the former is at 1 TeV and the latter at 750 GeV)."
No, it just means the detectors are not perfect. The 0.2 pb standard deviation (or ~0.4 pb limit) would be possible with perfect detectors (apart from calorimeter resolution, which was also assumed to be quite good) and perfect background models.

To compare results to the diphoton excess, you would need specific models that predict relative branching ratios. But even the back-of-the-envelope estimate shows that you are not sensitive to any models that predict a factor lower than a few hundred.

Somdatta Bhattacharya said...

"To compare results to the diphoton excess, you would need specific models that predict relative branching ratios. But even the back-of-the-envelope estimate shows that you are not sensitive to any models that predict a factor lower than a few hundred."

OK good, I get your point, but in your back-of-the-envelope estimate you just seem to be basing your analysis on a single standard deviation from the background. That seems to be independent of any model-dependent analysis that might predict a factor lower than a few hundred. So what exactly is the meaning of what you are actually estimating?

Anonymous said...

Hello Jester,

What is your estimate at the moment for this anomaly being new physics ?

mfb said...

@Somdatta Bhattacharya: If you have a model that predicts a dijet branching ratio of (let's say) 100, you can take the observed diphoton excess and scale it to dijets. And then you'll see that the expected signal strength is a factor of 10 (rough estimate) below the expected exclusion limit for dijet peaks at 750 GeV, and a factor of 5 below the statistical uncertainty of the signal. Even if the signal is there, you cannot see it.

Looking at dijets is still interesting, but to have a chance to see a signal there you would need some model that predicts a ~1000 times larger branching fraction to dijets compared to diphotons.

Able Lawrence said...

Isn't it too early to completely exclude a 2TeV resonance. Wouldn't we have to wait longer since Atlas does see a week bump and the 13TeV run hasn't even managed to replicate Higgs

mfb said...

It is too early to exclude it, but there is no particular reason to expect anything for 2 TeV dibosons. The case was never strong given the limited decay channels where it has been seen and the limited significance, and the negative results in 2015 didn't make it stronger.

alex-michael Skott said...

Could a dark matter particle be the unseen decay as dark matter cannot or does not react with light except at that energy