Search for new light resonances decaying to jet pairs and produced in association with a photon or a jet in proton-proton collisions at $\sqrt{s}=13$~TeV with the ATLAS detector

This note describes a search for new resonances decaying to pairs of jets in 15.5 fb$^{-1}$ of proton-proton collisions with a centre-of-mass energy of $\sqrt{s}=13$~TeV recorded during 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The search requires the new resonance would be produced in association with a high-$p_{\textrm{T}}$ photon or jet, such as those radiated from the colliding partons, in order to efficiently trigger on events containing light resonances. The distribution of the invariant mass of the the pair of jets is examined for local excesses above a data-derived estimate of the smoothly falling background. No evidence of anomalous phenomena is observed in the data, which are used to exclude effective cross-sections of processes with Gaussian-shaped contributions to the observed dijet mass distribution and regions of the parameter space of a lepto-phobic axial-vector $Z^\prime$ benchmark model. These results extend limits on light dijet resonances obtained by ATLAS with 2015 data.