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Description
A search for low-mass resonances decaying into a quark-antiquark pair in the mass range of 10 to 150 GeV is presented. This search uses data from the LHC in proton-proton collisions at √s = 13 TeV, collected by the CMS detector at the CERN LHC in 2016–2018, corresponding to an integrated luminosity of 138 fb−1. The analysis strategy makes use of an initial state photon recoiling against the resonance. As a result the resonance is produced with high transverse momentum, and the quark-antiquark decay products are reconstructed as a single large-radius jet with an internal two-pronged structure. A machine learning algorithm is used to distinguish such jets from single-prong jets. The results are interpreted in the context of a theoretical spin-1 vector boson, a leptophobic Z′, which is a dark-matter mediator candidate. No significant excess is observed when searching for this signature above the Standard Model background in the jet mass spectrum. Upper limits at the 95% confidence level are set on the coupling strength of the Z′ decaying to quark-antiquark pairs. This represents the most sensitive search of low mass resonances to quark-antiquark pairs to date in this mass range.