750 GeV Bump Update

Article: Search for resonant production of high-mass photon pairs in proton-proton collisions at sqrt(s) = 8 and 13 TeV
Authors: CMS Collaboration
Reference: arXiv:1606.04093 (Submitted to Phys. Rev. Lett)

Following the discovery of the Higgs boson at the LHC in 2012, high-energy physicists asked the same question that they have asked for years: “what’s next?” This time, however, the answer to that question was not nearly as obvious as it had been in the past. When the top quark was discovered at Fermilab in 1995, the answer was clear “the Higgs is next.” And when the W and Z bosons were discovered at CERN in 1983, physicists were saying “the top quark is right around the corner.” However, because the Higgs is the last piece of the puzzle that is the Standard Model, there is no clear answer to the question “what’s next?” At the moment, the honest answer to this question is “we aren’t quite sure.”

The Higgs completes the Standard Model, which would be fantastic news were it not for the fact that there remain unambiguous indications of physics beyond the Standard Model. Among these is dark matter, which makes up roughly one-quarter of the energy content of the universe. Neutrino mass, the Hierarchy Problem, and the matter-antimatter asymmetry in the universe are among other favorite arguments in favor of new physics. The salient point is clear: the Standard Model, though newly-completed, is not a complete description of nature, so we must press on.

Background-only p-values for a new scalar particle in the CMS diphoton data. The dip at 750 GeV may be early evidence for a new particle.
Background-only p-values for a new scalar particle in the CMS diphoton data. The dip at 750 GeV may be early evidence for a new particle.

Near the end of Run I of the LHC (2013) and the beginning of Run II (2015), the focus was on searches for new physics. While searches for supersymmetry and the direct production of dark matter drew a considerable deal of focus, towards the end of 2015, a small excess – or, as physicists commonly refer to them, a bump – began to materialize in decays to two photons seen by the CMS Collaboration. This observation was made all the more exciting by the fact that ATLAS observed an analogous bump in the same channel with roughly the same significance. The paper in question here, published June 2016, presents a combination of the 2012 (8 TeV) and 2015 (13 TeV) CMS data; it represents the most recent public CMS result on the so-called “di-photon resonance”. (See also Roberto’s recent ParticleBite.)

This analysis searches for events with two photons, a relatively clean signal. If there is a heavy particle which decays into two photons, then we expect to see an excess of events near the mass of this particle. In this case, CMS and ATLAS have observed an excess of events near 750 GeV in the di-photon channel. While some searches for new physics rely upon hard kinematic requirements or tailor their search to a certain signal model, the signal here is simple: look for an event with two photons and nothing else. However, because this is a model-independent search with loose selection requirements, great care must be taken to understand the background (events that mimic the signal) in order to observe an excess, should one exist. In this case, the background processes are direct production of two photons and events where one or more photon is actually a misidentified jet. For example, a neutral pion may be mistaken for a photon.

Part of the excitement from this excess is due to the fact that ATLAS and CMS both observed corresponding bump sin their datasets, a useful cross-check that the bump has a chance of being real. A bigger part of the excitement, however, are the physics implications of a new, heavy particle that decays into two photons. A particle decaying to two photons would likely be either spin-0 or spin-2 (in principle, it could be of spin-N where N is an integer and N ≥ 2). Models exist in which the aforementioned Higgs boson, h(125), is one of a family of Higgs particles, and these so-called “expanded Higgs sectors” predict heavy, spin-0 particles which would decay to two photons. Moreover, in models which there are extra spatial dimensions, we would expect to find a spin-2 resonance – a graviton – decaying to two photons. Both of these scenarios would be extremely exciting, if realized by experiment, which contributed to the buzz surrounding this signal.

So, where do we stand today? After considering the data from 2015 (at 13 TeV center-of-mass energy) and 2012 (at 8 TeV center-of-mass energy) together, CMS reports an excess with a local significance of 3.4-sigma. However, the global significance – which takes into account the “look-elsewhere effect” and is the figure of merit here – is a mere 1.6-sigma. While the outlook is not extremely encouraging, more data is needed to definitively rule on the status of the di-photon resonance. CMS and ATLAS should have just that, more data, in time for the International Conference on High Energy Physics (ICHEP) 2016 in early August. At that point, we should have sufficient data to determine the fate of the di-photon excess. For now, the di-photon bump serves as a reminder of the unpredictability of new physics signatures, and it might suggest the need for more model-independent searches for new physics, especially as the LHC continues to chip away at the available supersymmetry phase space without any discoveries.

References and Further Reading

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Connor Richards

Connor Richards is a member of the CMS Collaboration and a Gates Cambridge scholar doing Part III of the Maths Tripos. He received his Bachelors of Science from the University of California, Riverside, where he was a Chancellor's Research Fellow and a UC LEADS Fellow. After Cambridge, he will pursue his doctorate in Experimental High-Energy Physics on a Centennial Fellowship at Princeton University, and his research interests center on searches for new physics at the LHC. He is a scientist, tutor, public speaker, and an avid sports fan.

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7 Replies to “750 GeV Bump Update”

    1. We would be sad! Of course, we would like for an excess like this to stick around, but we have to accept what the LHC is telling us about nature, either way.

      If you are wondering something along the lines of “what would that mean?”, the answer is that it would appear the excess was just a background fluctuation that happened to be found – at roughly the same mass, in the same channel, and at roughly the same significance – by two different experiments.

      Now, as unlikely as that may sound, it is hardly unprecedented – in fact, something similar happened about a year ago. There was a great deal of buzz about the “di-boson excess” seen at the end of Run I of the LHC. CMS and ATLAS reported excesses in decays to two vector bosons (meaning WW, WZ, and ZZ) at a mass of roughly 2 TeV. The excesses were in the 2–3 sigma range, at the same mass, and in the same channels. When the LHC turned on again in 2015, we saw that this had likely just been a background fluctuation, as the excesses were nowhere to be found in the new dataset.

      In particle physics, we deal with extremely small probabilities on a daily basis, but we also deal in extremely large sample sizes. When the average person talks about something being “one in a million”, it is often hyperbole; they basically mean “that’s impossible”, because one million is a huge number in the context of everyday life. But when a particle physicist talks about an outcome being “one in a million”, they mean it: for every million attempts, you should expect that outcome to happen one time.

      So, while the odds of something like this being a complete coincidence may seem pretty slim at first, the reality is that CMS and ATLAS conduct an absolutely huge number of searches for new physics. The more searches performed, the more probable that “coincidences” like the one seen in the case of the di-boson excess become. Soon we should know definitively whether this was another “coincidence” or evidence of new physics.

  1. It is gone , stagnation for fifty years at least in particle physics ….. The SM is designed as such so SUSY is not needed to explain it .

  2. A great effort are done to explain the parameters of the SM , ….. I would like to ask : after thirty years of failure of finding explanation thru SUSY and M- theory plus the fairy tales of multiverse .. Megaverse .. Hyperverse .. Metaverse …,etc.
    Why we just not accept that the SM is a great structure that is designed to govern our universe ? And as such all its parameters are dictated by the metaphysics of that structure then the explanation of the SM would be the meta-requirements for its stability and logical structure with no further needed extra – explanations thru more undefined parameters which leads to an infinite regress which means no physical explanation can be found in principle !!??

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