Search for light dijet resonances with the ATLAS detector using a Trigger-Level Analysis in LHC pp collisions at $\sqrt{s}=13$~TeV

Searches for dijet resonances with sub-TeV masses using the ATLAS detector are statistically limited by the bandwidth available to inclusive single-jet triggers. Due to large Standard Model multijet backgrounds, these triggers must be prescaled to record full events at a manageable rate. However, one can avoid this limitation by recording only the subset of information in each event needed for such a search, allowing much higher trigger rates. This note describes a search for new physics in the dijet final state using this strategy, targeting low-mass dijet resonances between 450 GeV and 950 GeV. The analysed dataset has an integrated luminosity of 3.4 fb$^{-1}$ and was recorded at a center-of-mass energy of 13 TeV. No excesses are found, and limits are set on Dark Matter mediators with axial vector couplings to quarks and Dark Matter particles, and on generic Gaussian resonances.