Sunday, 20 December 2015

Friday wrap-up: diphoton excess, no diboson, no gluinos...

What a week! We have already seen some 40-odd papers submitted to hep-ph in the last few days on the "recent observed diphoton resonance" [1]. Well I certainly wouldn't go that far but ATLAS and CMS have each seen an excess of events in the diphoton spectrum at around 750 GeV, which is amazing since apparently they weren't even searching for it [2], and anyway beside the point because they also discovered a gluino [3]. Sloppy science writing aside, what do we know?...

• The CMS and ATLAS Run II physics results presentations can be found here. Of course, all results presented are preliminary. The result that has hep-ph buzzing, though, is a little bump atop the falling diphoton invariant mass background (conference notes here and here). [See Jester, Motl, Strassler (here and here), PhysicsMatt, or Eilam Gross for some physicist perspectives. Else in popular media I thought the NY Times article was fairly balanced, but then I am a phenomenologist]. You can eyeball the bumps in question below (credit to Strassler for this image):

But what about the numbers? The rumours were as accurate as one could reasonably ask: assuming a narrow width resonance, CMS observed a 2.6σ local (1.2σ global) excess at 760 GeV [increases to 3.0σ local (1.7σ global) at 750 GeV when combined with the 8 TeV data], and; ATLAS observed 3.6σ local (2.0σ global) at 750 GeV [have not yet combined with 8 TeV, but if they did it appears the significance would fall]. Allowing the width to float to larger values, the CMS result goes down to 2.0σ local, whereas ATLAS observes a best fit 45 GeV (6%) width at 3.9σ local (2.3σ with multivariate look-elsewhere). The relevant slides are below:

It is a tantalizing excess. Sensibly, what one would like to know is the global significance of the fully combined (CMS+ATLAS 8+13 TeV) datasets. It is non-trivial to get an exact number (see here or here), but one can at least make a good bet that it's greater than about $\sim \sqrt{1.7^2+2.0^2}\approx 2.6\sigma$, perhaps in the vicinity of $\sim 3\sigma$. [I would imagine the demand for a joint analysis is high enough to be a priority for the collaborations (or they might try to avoid feeding the hep-ph sharks?), so maybe we will have that number by Moriond]. This being a (very rough!) ~1/300 chance then, and given the hundreds of plots CMS and ATLAS produce, it is very possible that this is just a statistical fluctuation. Nonetheless, this excess is being taken fairly seriously, and will be exercising our scrolling finger on hep-ph for the foreseeable future while we grapple with the sensible question: if it is real, then what could it be and what does it imply? The answer to this question may have implications for the experimental program of the LHC over the next few years (at least), and so phenomenologists are already relentlessly hard at work...

So let's try to answer that question: what could it be? Well, there is no evidence for any extra activity in the excess events, so it appears consistent with a simple $gg\to X\to \gamma\gamma$ resonance. If taken as a resonance, the events translate to a cross-section $\sigma(pp\to X)\times Br(X\to \gamma\gamma)$ of $\sim 2$/fb ($\sim 6$/fb) in the narrow (wide) width scenario. Let us try to build a model with these properties. The simplest thing is to add a scalar singlet to the standard model. To couple it to gluons and photons let's borrow the Higgs' trick and couple it to some coloured/charged fermion(s) which then induce the couplings via a loop. Let's try Yukawa coupling it to a vector-like up-type quark first, write down the effective couplings, and calculate the Yukawa necessary; we find that it has to be huge ($\sim 5$ or so). And there's a potential problem, since the singlet will want to decay most of the time to the up-type quark. That's okay! We will just make it heavy enough (> 375 GeV) so that it's not allowed. Now we're done, and this solution is "already well-known" [4]. We can add more vector-like fermions to quell the large Yukawa(s) somewhat and/or dial the $gg$ and $\gamma\gamma$ couplings independently. If we take the large width seriously, we still have to add extra decay channels, and then dial up the production and/or branching to photons to compensate. The obvious options are a dark sector or some other standard model states, which we have to hide from previous searches. We could also try constraining ourselves inside some more predictive (restrictive) model.

Of course there are several papers on just the above, the implication being that you need more than just the singlet scalar, which is obviously quite interesting. The immediate implications for the LHC are: look for anything at 750 GeV in $jj, Z\gamma, ZZ$ (in roughly descending order of promise) as soon as is possible.

But this is just a minimal model. It could also be a scalar/pseudoscalar/bound state connected to compositeness/extended gauge group/extra dimensions/hidden valley/SUSY/dark matter/naturalness, and you can be sure there are already arXiv submissions on all of these. On that, it seems to me that arXiv isn't quite the ideal platform for all this. It would be nice instead to have all the various proposals in the same place, with the same formatting, in no-nonsense form, all grouped by some general properties. Then the interested phenomenologist/experimentalist could go and browse a list of, for example: (1) candidate; (2) production; (3) couplings; (4) decays; (5) associated activity; (6) additional particles; (7) additional predictions. Of course this will inevitably be done anyway by some authors in a review, but it seems like the same could be achieved much more efficiently with a community-run wiki or similar, as long as there were some moderators willing to dedicate their time to such a project... any thoughts on this from readers?

In my book there's not much more to say except we need more data, to tell (1) if this is real, or (2) what it is. Looking forward to more excellent work from our experimental colleagues in the new year.

• Now onto other matters from the presentation. First the diboson excess from Run I. Before the meeting a couple of useful papers appeared on the arXiv: a third-party CMS+ATLAS statistical combination, and; a thorough summary and literature survey. Now we know both CMS and ATLAS see nothing significant in Run II data (although they do not have sensitivity to conclusively probe the parameter space of interest):

• Also in Run II data, the on-Z excess is not seen by CMS, but still persists at ATLAS...

• As well, lots of gluino searches in different final states but nothing seen, and limits improve to roughly 1.2--1.8 TeV in the simplified models considered (but of course there are always compressed places to hide!).
• CMS have not unblinded any of their Higgs analyses, but ATLAS reported results in γγ and ZZ: they were expecting 3.4σ observation and saw instead 1.4σ. Obviously the Higgs has packed up, moved to 750 GeV, and remembered its earlier proclivity for photons (this hypothesis will be robustly tested in upcoming LHC analyses).
• Moving on to other news, LUX has released new limits on spin-independent dark matter nucleon scattering. See the press release and/or this blog post from Sally Shaw for a summary. They're almost observing solar neutrinos!

• "NuPhys2015: Prospects in Neutrino Physics" was on this week (indico).
• Strumia's insta-paper archive.
• Quanta: "A Fight For the Soul of Science," on the recent meeting at the intersection of the philosophy of science and theoretical physics.
• Quanta: "Landmark Algorithm Breaks 30-Year Impasse."
• In audio/video media:

[1] arXiv: "The recent observed diphoton resonance around 750 GeV at the LHC..."
[2] Nature News: "... the 750 GeV boson is not one of the particles that LHC physicists have been searching for..."
[3] Tech Times: "Physicists Have Discovered Evidence Of A Gluino Particle, The Cousin Of The Higgs Boson."
[4] arXiv: "It is already well-known that a real singlet scalar ϕ with Yukawa couplings ϕXX to vector-like fermions X with mass mX>mϕ/2 can easily explain the observed signal, provided X carries both SM color and electric charge."