Search for Low-Mass Quark-Antiquark Resonances Produced in Association with a Photon at 
s=13  TeV
A search for narrow low-mass resonances decaying to quark-antiquark pairs is presented. The search is based on proton-proton collisions events collected at 13 TeV by the CMS detector at the CERN LHC. The data sample corresponds to an integrated luminosity of 35.9 fb−1, recorded in 2016. The search considers the case where the resonance has high transverse momentum due to initial-state radiation of a photon. To study this process, the decay products of the resonance are reconstructed as a single large-radius jet with two-pronged substructure. The signal is identified as a localized excess in the jet invariant mass spectrum. No evidence for such a resonance is observed in the mass range 10 to 125 GeV. Upper limits at the 95% confidence level are set on the coupling strength of vector resonances decaying to quark pairs. The results obtained with this novel technique provide the first direct constraints on quark-antiquark resonance masses below 50 GeV obtained at a hadron collider. Submitted to Physical Review Letters c © 2019 CERN for the benefit of the CMS Collaboration. CC-BY-4.0 license ∗See Appendix A for the list of collaboration members ar X iv :1 90 5. 10 33 1v 1 [ he pex ] 2 4 M ay 2 01 9