Saturday, 13 June 2015

On the LHC diboson excess

The ATLAS diboson resonance search showing a 3.4 sigma excess near 2 TeV has stirred some interest. This is understandable: 3 sigma does not grow on trees, and moreover CMS also reported anomalies in related analyses. Therefore it is worth looking at these searches in a bit more detail in order to gauge how excited we should be.

The ATLAS one is actually a dijet search: it focuses on events with two very energetic jets of hadrons.  More often than not, W and Z boson decay to quarks. When a TeV-scale  resonance decays to electroweak bosons, the latter, by energy conservation,  have to move with large velocities. As a consequence, the 2 quarks from W or Z boson decays will be very collimated and will be seen as a single jet in the detector.  Therefore, ATLAS looks for dijet events where 1) the mass of each jet is close to that of W (80±13 GeV) or Z (91±13 GeV), and  2) the invariant mass of the dijet pair is above 1 TeV.  Furthermore, they look into the substructure of the jets, so as to identify the ones that look consistent with W or Z decays. After all this work, most of the events still originate from ordinary QCD production of quarks and gluons, which gives a smooth background falling with the dijet invariant mass.  If LHC collisions lead to a production of  a new particle that decays to WW, WZ, or ZZ final states, it should show as a bump on top of the QCD background. ATLAS observes is this:

There is a bump near 2 TeV, which  could indicate the existence of a particle decaying to WW and/or WZ and/or ZZ. One important thing to be aware of is that this search cannot distinguish well between the above 3  diboson states. The difference between W and Z masses is only 10 GeV, and the jet mass windows used in the search for W and Z  partly overlap. In fact, 20% of the events fall into all 3 diboson categories.   For all we know, the excess could be in just one final state, say WZ, and simply feed into the other two due to the overlapping selection criteria.

Given the number of searches that ATLAS and CMS have made, 3 sigma fluctuations of the background should happen a few times in the LHC run-1 just by sheer chance.  The interest in the ATLAS  excess is however amplified by the fact that diboson searches in CMS also show anomalies (albeit smaller) just below 2 TeV. This can be clearly seen on this plot with limits on the Randall-Sundrum graviton excitation, which is one  particular model leading to diboson resonances. As W and Z bosons sometimes decay to, respectively, one and two charged leptons, diboson resonances can be searched for not only via dijets but also in final states with one or two leptons.  One can see that, in CMS, the ZZ dilepton search (blue line), the WW/ZZ dijet search (green line), and the WW/WZ one-lepton (red line)  search all report a small (between 1 and 2 sigma) excess around 1.8 TeV.  To make things even more interesting,  the CMS search for WH resonances return 3 events  clustering at 1.8 TeV where the standard model background is very small (see Tommaso's post). Could the ATLAS and CMS events be due to the same exotic physics?

Unfortunately, building a model explaining all the diboson data is not easy. Enough to say that the ATLAS excess has been out for a week and there's isn't yet any serious ambulance chasing paper on arXiv. One challenge is the event rate. To fit the excess, the resonance should be produced with a cross section of order 10 femtobarns. This requires the new particle to couple quite strongly to light quarks (or gluons), at least as strong as the W and Z bosons. At the same time, it should remain a narrow resonance decaying dominantly to dibosons. Furthermore, in concrete models, a sizable coupling to electroweak gauge bosons will get you in trouble with electroweak precision tests.

However, there is yet a bigger problem, which can be also  seen in the plot above. Although the excesses in CMS occur roughly at the same mass, they are not compatible when it comes to the cross section. And so the limits in the single-lepton search are not consistent with the new particle interpretation of the excess in dijet  and  the dilepton searches, at least in the context of the Randall-Sundrum graviton model. Moreover, the limits from the CMS one-lepton search are grossly inconsistent with the diboson interpretation of the ATLAS excess! In order to believe that the ATLAS 3 sigma excess is real one has to move to much more baroque models. One possibility is that  the dijets observed by ATLAS do not originate from  electroweak bosons, but rather from an exotic particle with a similar mass. Another possibility is that the resonance decays only to a pair of Z bosons and not to W bosons, in which case the CMS limits are weaker; but I'm not sure if there exist consistent models with this property.  

My conclusion...  For sure this is something to observe in the early run-2. If this is real, it should clearly show in both experiments already this year.  However, due to the inconsistencies between different search channels and the theoretical challenges, there's little reason to get excited yet.

Thanks to Chris for digging out the CMS plot.

19 comments:

Cédric Bardot said...

Thank you Jester for nourishing our "fol" science enthusiasm that consists in finding all the flukes that show up in the broad spectrum of publicized data and then painfully learning a few bits of physics information :-)

mfb said...

At least the masses are compatible between ATLAS and CMS, given the few percent uncertainty on the jet energy calibration.
Well, we'll see more with 1/fb. Such a heavy object should profit massively from the higher energy.

~2/pb collected, LHC at 1% design luminosity right now. Well, there is progress.

Drew Day said...

Jester:

You may wish to remind readers that the LHCC has revised down the estimated luminosity to be delivered this year by a factor of 2 (slide 29): the estimate is down to 5 fb^-1 now. This is due to about four lost weeks due to a magnet grounding problem during powering tests (and other problems during commissioning).

This fact, in isolation, may not be very interesting. However, coupled with the schedules of conferences at which ATLAS or CMS would present (post re-calibration) results, it may push more detailed analyses into next year.

Cheers,
-Drew

Jester said...

Thanks. Yes, I'm aware that the current projection is 5±5 fb-1. But even with 5 fb-1 we should see conclusively whether the thing is real. If the number drops by another factor of 2 then the statement is no longer true.

Anonymous said...

I love the background parametrizations these collaborations use, here it is
constant*(1-x)^(something)*x^(another thing) and x=mjets/E
totally ad hoc... I wonder why people wrote such sophisticated Monte Carlo simulations to end up with a background "model" that a sophomore can write (the word model is Atlas's, the quotes are mine). Who can estimate what systematic error this brings in? Then the significance "x-sigmas" becomes quite random. Similar to Higgs to two photons a couple of years back.

I noticed a work claiming already that this is the technirho. The authors will have to explain the referee a few things.

Thanks for the quick updates
felipe

mfb said...

5 to 10/fb according to the presentation. Even 5 would be sufficient to improve the limits on most particles above 1 TeV.
And, well, a lower luminosity 2015 makes 2016 more interesting ;) - http://resonaances.blogspot.de/2015/01/do-or-die-year.html

Anonymous said...

Here's another one from today: http://arxiv.org/abs/1506.03931
And the Technirho one from friday: http://arxiv.org/abs/1506.03751

Jester said...

neither is serious

RBS said...

Jester, it looks like the analysis of Run 1 isn't yet complete as it's providing us with this kind of nice surprises after doom and gloom of the last year. To what extent would we be looking forward to new findings in the Run1 records as opposed the new data from Run 2?

Jester said...

It's unlikely there are big surprises hiding in the run-1 data. Although it cannot be completely excluded that new physics manifests itself through signatures that have not been studied so far, all eyes are now to the run-2.

Anonymous said...

wait, wait, wait. what about higgs to muon tau from Atlas? regards LFV

Jester said...

Inconclusive.

Ervin Goldfain said...

"neither is serious"...

Let's keep in mind that the large number of null searches (flavor physics, electron dipole moment, EW precision data and so on) tightly constrains the form of any new physics near the low TeV scale. Evidence for BSM phenomena must be on a solid ground to be deemed conclusive.

mfb said...

3 sigma is never conclusive, given the large number of searches multiple 3 sigma effects are expected. Even a single 5 sigma observation won't be considered as conclusive I guess, only a verification from another experiment makes that solid enough to be a serious hint for new physics. And even then the analyses have to be checked in every detail. Some thing like monte carlo descriptions could have the same bug in all experiments.

Anonymous said...

"inconclusive"
apparently i've missed newest atlas results on h ->mu tau. could someone paste a link? regards LFV

Jester said...

This study is not public yet.

Anonymous said...

Jester why do you say those aren't serious studies? Not a rhetorical question. What do you think of Bogdan's paper and this one 1506.07511v1 ?

Jester said...

Bogdan's paper is probably the first interesting one. The early ones were not because they didn't even discuss how the required large production cross section is obtained.

mfb said...

Regarding H->mu tau: lepton flavor violation is mentioned in the abstract at this EPS talk, and the speaker is from ATLAS: https://indico.cern.ch/event/356420/session/4/contribution/598
As far as I heard CMS works on the other two modes (e tau and e mu) as well.