750 GeV: the bigger picture

This Thursday the ATLAS and CMS experiments will present updated analyses of the 750 GeV diphoton excess. CMS will extend their data set by the diphoton events collected in the periods when the detector was running without the magnetic field (which is not essential for this particular study), so the amount of available data will slightly increase. We will then enter the Phase-II of the excitement protocol,  hopefully followed this summer by another 4-th-of-July-style discovery. To close the Phase-I, here's a long-promised post about the bigger picture. There's at least 750 distinct models that can accommodate the diphoton signal observed by ATLAS and CMS. However, a larger framework for physics beyond the Standard Model it which these phenomenological models can be embedded is a more tricky question. Here is a bunch of speculations.

Whenever a new fluctuation is spotted at the LHC one cannot avoid mentioning supersymmetry. However,  the 750 GeV resonance cannot be naturally interpreted in this framework, not the least because it cannot be identified as a superpartner of any known particles. The problem is that explaining the observed signal strength requires introducing new particles with large couplings, and the complete theory typically enters into a strong coupling regime at the energy scale of a few TeV. This is not the usual SUSY paradigm, with weakly coupled physics at the TeV scale followed by a desert up to the grand unification scale. Thus, even if the final answer may still turn out to be supersymmetric, it will not be the kind of SUSY we've been expecting all along. Weakly coupled supersymmetric explanations are still possible in somewhat more complicated scenarios with new very light sub-GeV particles and cascade decays, see e.g. this NMSSM model.

Each time you see a diphoton peak you want to cry Higgs, since this is how the 125 GeV Higgs boson was first spotted. Many theories predict an extended Higgs sector with multiple heavy scalar particles, but again such a framework is not the most natural one for interpreting the 750 GeV resonance. There are two main reasons. One is that different Higgs scalars typically mix, but the mixing angle in this case is severely constrained by Higgs precision studies and non-observation of 750 GeV diboson resonances in other channel. The other is that, for a 750 GeV Higgs scalar, the branching fraction into the diphoton final state is typically tiny (e.g., ~10^-7 for a Standard-Model-Higgs-like scalar) and a complicated model gymnastics is needed to enhance it. The possibility that the 750 GeV resonance is a heavy Higgs boson is by no means excluded, but I would be surprised if this were the case.  

It is more tempting to interpret the diphoton resonance as a bound state of new strong interactions with a confinement scale in the TeV range. We know that the Quantum Chromodynamics (QCD) theory, which describes the strong interactions of the Standard Model quarks, gives rise to many scalar mesons and higher-spin resonances at low energies. Such a behavior is characteristic for a large class of similar theories.  Furthermore,  if the new strong sector contains mediator particles  that carry color and electromagnetic charges, the production in gluon fusion and decay into photons is possible for the composite states, see e.g. here.  The problem is that, much as for QCD, one would expect not one but an entire battalion of resonances. One needs to understand how the remaining resonances predicted by typical strongly interacting models could have avoided detection so far.

One way this could happen is if the 750 GeV resonance is a scalar that, for symmetry reasons, is much lighter than most of the particles in the strong sector. Here again our QCD may offer us a clue, as it contains pseudo-scalar particles, the so-called pions,  which are a factor of 10 lighter than the typical mass scale of other resonances. In QCD, pions are Goldstone bosons of the chiral symmetry spontaneously broken by the vacuum quark condensate. In other words, the smallness of the pion mass is  protected by a symmetry, and general theorems worked out in the 60s  ensure the quantum stability of such an arrangement. The similar mechanism can be easily implemented in other strongly interacting theories,  and it is possible to realize the 750 GeV resonance as a new kind of pion, see e.g. here.   Even the mechanism for decaying into photons -- via chiral anomalies -- can be borrowed directly from QCD. However, the symmetry protecting the 750 GeV scalar could also be completely different that the ones we have seen so far. One example is the dilaton, that is   a Goldstone boson of a spontaneously broken conformal symmetry, see e.g. here. This is a theoretically interesting possibility, since approximate conformal symmetry often arises as a feature of strongly interacting theories. All in all, the 750 GeV particle may well be  a pion or dilaton harbinger of new strong interactions at a TeV scale. One can then further speculate that the Higgs boson also originates from that sector, but that is a separate story that may or may not be true.

Another larger framework worth mentioning here is that of extra dimensions. In the modern view, theories with the new 4th dimension of space are merely an effective description of strongly interacting sectors discussed above. For example, the famous Randall-Sundrum model, with the Standard Model living in a section of a 5D AdS5 space, is a weakly coupled dual description of strongly coupled theories with a conformal symmetry and a large N gauge symmetry. These models thus offer a calculable way to embed the 750 GeV resonance in a strongly interacting theory. For example, the dilaton can be effectively described in the Randall-Sundrum model as the radion - a scalar particle corresponding to fluctuations of the size of the 5th dimension, see e.g. here. Moreover, the Randall-Sundrum framework  provides a simple way to realize the 750 GeV particle as a spin-2 resonance. Indeed, the model always contains massive Kaluza-Klein  excitations of the graviton, whose couplings to matter can be much stronger than that of the massless graviton. This possibility have been relatively less explored so far, see e.g.  here,  but that may change next week...

Clearly, it is impossible to say anything conclusive at this point. More data in multiple decay channels is absolutely necessary  for a more concrete picture to emerge. For me personally, a confirmation of the 750 GeV excess would be a strong hint for new strong interactions at a few TeV scale. And if this is indeed the case,  one may seriously think that our  40-years-long brooding about the hierarchy problem has not been completely misguided...