Low-Multiplicity Jets as Probes of GeV-Scale Light-Quark-Coupled Particles

We propose a search at the LHC for GeV-scale particles coupling predominantly to light quarks based on low-multiplicity jets. The search targets production in association with a hard photon and uses the feature that a light gauge-singlet can only decay into a small number of hadronic channels, yielding jets with anomalously low charged-track multiplicity and mass compared to QCD jets at the same transverse momentum. We determine the sensitivity to scalar and pseudoscalar couplings to up-quarks, and suggest a data-driven estimate that reduces the sensitivity to jet modeling uncertainties. This search extends the reach to hadronically-coupled particles into a previously inaccessible regime.