Quirky explanations for the diphoton excess

Abstract

We propose two simple quirk models to explain the recently reported 750 GeV diphoton excesses at ATLAS and CMS. It is already well-known that a real singlet scalar $\phi$ with Yukawa couplings $\phi \bar X X$ to vector-like fermions $X$ with mass $m_X > m_\phi/2$ can easily explain the observed signal, provided $X$ carries both SM color and electric charge. We instead consider first the possibility that the pair production of a fermion, charged under both SM gauge groups and a confining $SU(3)_v$ gauge group, is responsible. If pair produced it forms a quirky bound state, which promptly annihilates into gluons, photons, v-gluons and possibly SM fermions. This is an extremely minimal model to explain the excess, but is already in some tension with existing displaced searches, as well as dilepton and dijet resonance bounds. We therefore propose a hybrid Quirk-Scalar model, in which the fermion of the simple $\phi \bar X X$ toy model is charged under the additional $SU(3)_v$ confining gauge group. Constraints on the new heavy fermion $X$ are then significantly relaxed. The main additional signals of this model are possible dilepton, dijet and diphoton resonances at $\sim$ 2 TeV or more from quirk annihilation, and the production of v-glueballs through quirk annihilation and $\phi$ decay. The glueballs can give rise to spectacular signatures, including displaced vertices and events with leptons, photons and $Z$-bosons. If the Quirk-Scalar model is responsible for the 750 GeV excess it should be discovered in one of these channels with 20 or $300 \mathrm{fb}^{-1}$ of LHC run 2 data.