Measurement of the $H \to \gamma \gamma$ fiducial cross-section in $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

The Higgs boson production cross-section in the di-photon decay channel is measured using $31.4 \ \mathrm{fb}^{-1}$ of $pp$ collision data collected with the ATLAS detector at a centre-of-mass energy of $\sqrt{s}=13.6 \ \mathrm{TeV}$. In order to reduce the model dependence, the measurement is restricted to a particle-level phase space that matches closely the reconstruction-level photon kinematic selection and is corrected for detector effects. The measured cross-section is $\sigma_\mathrm{fid}(pp \to H \to \gamma \gamma) = 76^{+14}_{-13} \ \mathrm{fb}$, in agreement with the Standard Model prediction of $67.5 \pm 3.4 \ \mathrm{fb}$. Assuming the Standard Model acceptance and di-photon branching fraction, the fiducial measurement is extrapolated to the full phase space yielding a total cross-section of $\sigma(pp \to H) = 67^{+12}_{-11} \ \mathrm{pb}$ at $13.6 \ \mathrm{TeV}$, to be compared with the SM prediction of $59.8 \pm 2.6$~pb.