  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 hard 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 would be 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 resonances decaying to quark pairs. The results
obtained with this photon trigger strategy provide the first direct constraints
on quark-antiquark resonance masses below 50 GeV obtained at a hadron collider.
