Diboson Excess from a New Strong Force

Abstract: We explore a "partial unification" model that could explain the diphoton event excess around $750 \, \rm GeV$ recently reported by the LHC experiments. A new strong gauge group is combined with the ordinary color and hypercharge gauge groups. The VEV responsible for the combination is of the order of the $SU(2)\times U(1)$ breaking scale, but the coupling of the new physics to standard model particles is suppressed by the strong interaction of the new gauge group. This simple extension of the standard model has a rich phenomenology, including composite particles of the new confining gauge interaction, a coloron and a $Z'$ which are rather weakly coupled to standard model particles, and massive vector bosons charged under both the ordinary color and hypercharge gauge groups and the new strong gauge group. The new scalar glueball could have mass of around $750 \, \rm GeV$, be produced by gluon fusion and decay into two photons, both through loops of the new massive vector bosons. The simplest version of the model has some issues: the massive vector bosons are stable and the coloron and the $Z'$ are strongly constrained by search data. An extension of the model to include additional fermions with the new gauge coupling, though not as simple and elegant, can address both issues and more. It allows the massive vector boson to decay into a colorless, neutral state that could be a candidate of the dark matter. And the coloron and $Z'$ can decay dominantly into the new fermions, completely changing the search bounds. In addition, $SU(N)$ fermions below the symmetry breaking scale make it more plausible that the lightest glueball is at $750$~GeV. Whatever becomes of the $750$~GeV diphoton excess, the model is an unusual example of how new physics at small scales could be hidden by strong interactions.