ATLAS Collaboration
A search for new high-mass phenomena in events with a high-transverse-momentum photon and a jet is performed using data collected by the ATLAS experiment from proton-proton collisions at the LHC, with a centre-of-mass energy of 13 TeV. The analysis is based on a total integrated luminosity of 140 fb$^{-1}$. The $γ$$-$jet invariant mass spectrum measured in data is examined for localised excesses above the Standard Model background. No significant deviation from the Standard Model prediction is observed, allowing stringent limits to be set on the production cross sections of specific new physics scenarios. To investigate quark compositeness, excited quark models are studied for different couplings ($f$) and flavours: light $q_{\mathrm{L}}^{*}$ $(u^{*}/d^{*})$ and heavy ($b^{*}$, $c^{*}$). For $f=1$, masses up to 6.0 TeV ($q_{\mathrm{L}}^{*}$), 3.5 TeV ($c^{*}$), and 2.6 TeV ($b^{*}$) are excluded. These results provide the first direct constraints on $c^{*}$ production in the $γ$$-$ jet final state at the LHC. Additionally, the analysis excludes quantum black hole mass thresholds up to 5.3 (7.5) TeV in the Randall-Sundrum (Arkani-Hamed-Dimopoulos-Dvali) models at a 95% confidence level, further constraining these theoretical frameworks. Constraints on the visible cross-section are also obtained using generic Gaussian-shaped resonance production.