Speaker
Description
Distinguishing quark-initiated jets from gluon-initiated jets is a powerful handle for multiboson measurements at the LHC: in vector-boson scattering (VBS) and vector-boson fusion (VBF) topologies, quark/gluon tagging can enhance signal purity by identifying the characteristic quark-initiated forward jets and suppressing QCD-initiated backgrounds such as V+jets; additionally, the differing quark/gluon composition of the leading associated jet in gluon–gluon fusion (e.g. H+jet, predominantly gluon-initiated) versus electroweak processes (e.g. Z+jet, predominantly quark-initiated) offers a complementary discriminant for cross-section measurements and coupling extractions. This talk presents DeParT, a transformer-based quark/gluon tagger developed by ATLAS that exploits the full information from jet constituents reconstructed in both the tracker and calorimeter systems, operating across an extended kinematic phase space with $p_{\mathrm{T}} > 20$ GeV and $|\eta| < 4.5$. The tagger calibration is performed using dijet events from the LHC Run 2 and Run 3 proton–proton collision data, deploying the jet topics method — a data-driven technique that reduces reliance on Monte Carlo modelling, yielding up to 20% smaller systematic uncertainties compared to traditional approaches in some kinematic regions. The resulting efficiency scale factors enable robust application of the tagger across the ATLAS physics programme, with particular relevance for precision multiboson measurements and searches for new physics in VBS/VBF final states.