Dark Mesons at the LHC

Abstract

A new, strongly-coupled dark sector could be accessible to LHC searches now. These dark sectors consist of composites formed from constituents that are charged under the electroweak group and interact with the Higgs, but are neutral under Standard Model color. In these scenarios, the most promising target is the dark meson sector, consisting of dark vector-mesons as well as dark pions. In this paper we study dark meson production and decay at the LHC in theories that preserve a global SU(2) dark flavor symmetry. Dark pions can be pair-produced through resonant dark vector meson production, $p p\to\rho_D\to\pi_D\pi_D$, and decay in one of two distinct ways: gaugephobic, when $\pi_D\to f\bar{f}'$ generally dominates; or gaugephilic, when $\pi_D\to W+h,Z+h$ dominates once kinematically open. Unlike QCD, the decay $\pi^0_D\to\gamma\gamma$ is virtually absent due to the dark flavor symmetry. We recast a vast set of LHC searches to determine the current constraints on dark meson production and decay. When $m_{\rho_D}$ is slightly heavier than $2 m_{\pi_D}$ and $\rho_D^{\pm,0}$ kinetically mixes with the weak gauge bosons, the 8 TeV same-sign lepton search strategy sets the best bound, $m_{\pi_D}>500$ GeV. Yet, when only the $\rho^0_D$ kinetically mixes with hypercharge, we find the strongest LHC bound is $m_{\pi_D}>130$ GeV, that is only slightly better than what LEP II achieved. We find the relative insensitivity of LHC searches, especially at 13 TeV, can be blamed mainly on their penchant for high mass objects or large MET. Dedicated searches would undoubtedly yield substantially improved sensitivity. We provide a GitHub page to speed the implementation of these searches in future LHC analyses. Our findings provide a strong motivation for model-independent searches of the form $pp\to A\to B+C\to SM\, SM+SM\, SM$ where the theoretical prejudice is for SM to be a t,b,$\tau$ or W,Z,h.