On the Interpretation of a Possible $\sim 750$ GeV Particle Decaying into ${\hat{I}}^3{\hat{I}}^3$

Abstract

We consider interpretations of the recent $\sim 3\ddot{I}ƒ$ reports by the CMS and ATLAS collaborations of a possible $X(\sim 750\mathrm{GeV})$ state decaying into ${\hat{I}}^3{\hat{I}}^3$ final states. We focus on the possibilities that this is a scalar or pseudoscalar electroweak isoscalar state produced by gluon-gluon fusion mediated by loops of heavy fermions. We consider several models for these fermions, including a single vector-like charge $2/3$ T quark, a doublet of vector-like quarks $(T,B)$, and a vector-like generation of quarks, with or without leptons that also contribute to the $X\to {\hat{I}}^3{\hat{I}}^3$ decay amplitude. We also consider the possibility that $X(750)$ is a dark matter mediator, with a neutral vector-like dark matter particle. These scenarios are compatible with the present and prospective direct limits on vector-like fermions from LHC Runs 1 and 2, as well as indirect constraints from electroweak precision measurements, and we show that the required Yukawa-like couplings between the $X$ particle and the heavy vector-like fermions are small enough to be perturbative so long as the $X$ particle has dominant decay modes into $gg$ and ${\hat{I}}^3{\hat{I}}^3$. The decays $X\to ZZ,Z{\hat{I}}^3$ and $W^{+}{W}^{-}$ are interesting prospective signatures that may help distinguish between different vector-like fermion scenarios.

36 pages, 7 figures. V2: typos corrected, references added, version submitted to journal. V3: Version accepted by JHEP