During summer and fall 2015, the ATLAS and CMS experiments at the LHC collected, for the first time ever, data from proton-proton collisions at a center-of-mass energy of 13 TeV. By the end of December, the results of a few analyses employing these data were already available, and they've been presented in a conference on December 15th. One of the most intriguing results, that drew attention of many physicists, came from the searches of new heavy particles (with a mass of at least 200 GeV) that decay into two photons.
In fact, both the ATLAS and CMS collaborations observed an excess of events over the expected background, corresponding to a resonance with a invariant mass of 750 GeV and a roughly estimated width of 45 GeV.
The local statistical significance of the excess is 3.9σ for ATLAS and 2.6σ for CMS, but when the look elsewhere effect* is taken into account individually for each experiment, the global significance is just 2.3σ for ATLAS and 1.2σ for CMS. Although statistically this hint is very weak (5σ are needed to claim a discovery!), the exciting possibility that beyond-Standard-Model physics has finally been found has called the attention of a very large fraction of the community.
Many physicists started to investigate the nature of this hypothetical new particle (their results summed up to a very large number of scientific papers in a very short time!): here are a few examples of the most basic questions they've been trying to answer.
Q: No resonance has been observed in the previous searches, performed with proton-proton collisions at 7 and 8 TeV. Is this in contradiction to the new results?
A: There is of course some tension, but this is actually acceptable: according to the ATLAS collaboration, a resonance at 750 GeV with a 45 GeV width would be consistent with the 8 TeV run at the 1.4σ level.
Q: What else do we know about the excess?
A: The collaborations mention the lack of "unusual" activity in the diphoton events, which could mean no other signal that is easily tagged, like large missing transverse energy or leptons.
Q: What would be the spin of the hypothetical new particle?
A: We can already make a theoretical guess: since it has been observed decaying into two photons (that have spin 1), quantum mechanics tells that the new particle must have integer spin. Nonehteless, the Landau-Yang's theorem implies that a spin-1 resonance cannot decay to two massless spin-1 particles. The most natural possibility, then, is that the new guy has spin zero, just like the Higgs boson. As an alternative, it could have spin two, although this hypothesis is somewhat more exotic.
Q: What would be its electric charge?
A: Since the electric charge is conserved in the decay process, and the photons are neutral, then the new particle itself shall be neutral.
Q: But how does a spin-0 neutral particle decay into two photons?
A: This is indeed strange: neutral particles usually no dot couple to photons! However, we have a similar example in nature: the Higgs boson does decay into two photons, thanks to the mediation of a charged fermion: the top quark. Analogously, if the new signal has been produced by a new neutral boson, then we would conclude that a new heavy fermion must be there too! In fact, the well-known top quark cannot be chosen as a mediator candidate: in that case we should have seen the new boson decaying into top-antitop pairs. Finally, the new fermion shall be vector-like, i.e. its left-handed and right-handed components shall have the same quantum numbers.
Q: How is the resonance produced at the LHC?
A: Most probably in gluon-fusion, just like the Higgs boson. Nonetheless, this would imply that the new particle should also be able to decay into two gluons, and thus it should be witnessed as a resonance in the di-jet final state. This is still an open possibility, as such results are still not available. Very importantly, analogously to the interaction with photons, the coupling to gluons should also be mediated by a new heavy, colored fermion. As an alternative production channel, vector boson fusion could indeed provide a large enough production cross section.
Q: If there is really a new particle behind the di-photon excess, should it be possible to observe it in some other decay channel too?
A: In principle, whenever a particle can decay into two photons, it must be able to do it also into two gauge bosons, i.e. into ZZ or W+W- pair or into a Z and a photon, and this should happen quite frequently. This was indeed the case of the Higgs boson, that was first discovered in the ZZ channel. No such signal was observed in the searches at 8 TeV, but the results for the 13 TeV run are not available yet, which leaves the door still open.
Q: What can the hypothetical new particle be? Does it fit in any of the theoretical models that have been proposed so far?
A: A very large number of possible explanations has been suggested in the last month, that could explain the origin of this would-be new particle. Among these there are some very intriguing possiblities that, nonetheless, cannot be tested in any way yet. Indeed, a neutral scalar boson arises easily in a very large variety of frameworks, of which we just mention here a very inexhaustive subset: it can be a Higgs partner in a 2-Higgs doublets model, in a composite Higgs model or in Supersymmetry. It can be the dilaton, a component of the Dark Matter, an axion-line particle, it can arise in left-right symmetric models, in models for the generation of fermion masses, or even in String Theory. Also, if it has spin 2, it may be a Kaluza-Klein graviton of the Randall-Sundrum extra dimensions model.
In conclusion, there are still many open questions that need to be answered before any conclusion is drawn from this shy excess. Some of these questions may be already addressed this coming summer, while others will take more time to be clarified. In particular, a confirmation or a disproof of this signal will most likely be given in the next fall, after the two experiments will have been able to collect and analyse more data.
Text by Ilaria Brivio and Pedro Machado