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 -like the pions of QCD -can be pair-produced through resonant dark vector meson production, pp → ρ D → π D π D , and decay in one of two distinct ways: "gaugephobic", when π D → ff generally dominates; or "gaugephilic", when π D → W + h, Z + h dominates once kinematically open. Unlike QCD, the decay π 0 D → γγ is virtually absent due to the dark flavor symmetry. We recast a vast set of existing LHC searches to determine the current constraints on (and future opportunities for) dark meson production and decay. When m ρ D is slightly heavier than 2m π D and ρ ±,0 D kinetically mixes with the weak gauge bosons, the 8 TeV same-sign lepton search strategy sets the best bound, m π D > 500 GeV. Yet, when only the ρ 0 D kinetically mixes with hypercharge, we find the strongest LHC bound is m π D > 130 GeV, that is only slightly better than what LEP II achieved two decades ago. 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 missing energy. 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 for dark meson production and decay provide a strong motivation for model-independent searches of the form pp → A → B + C → SM SM + SM SM where the theoretical prejudice is for SM to be a 3rd generation quark or lepton, W, Z, or h.

Effective theories of dark mesons with custodial symmetry

“…One might think a dark meson sector whose low energy effective theory is a set of scalars with electroweak quantum numbers has already been fully (or mostly) covered by the wide range of existing search strategies. As we show in our companion paper [115], this is not the case. There we find that a dark vector meson could be as light as about 300 GeV, something that, at first glance, seems hard to believe given the multi-TeV bounds on new Z bosons from LHC data.…”

“…Below the dark confinement scale, the effective theory description of the composites includes "dark baryons" and "dark mesons"; the latter breaking up into "dark vector mesons" and "dark pions". Despite the naming convention, we emphasize that the new states are certainly not "dark" to collider experiments [115].…”

“…In general, these states rapidly decay into the lightest dark pions. While this is certainly highly relevant for phenomenology [88,115], it the lightest dark pion decays that are the main concern for this paper.…”

Cosmological production of dark nuclei

“…This requires a suppressed symmetric component so naturally M N < 100 TeV. Direct searches at the LHC place a bound of around 1 TeV for the mass [16,17], while the singlets just need to be heavier than about 10 MeV to avoid bound on number of species.…”

“…When the constituents of DM have electro-weak charges, direct searches at LHC imply that the scale of new states must be larger than about 1 TeV [16,17]. 3 For this value of the mass the numerical density in eq.…”