Measurement of the $H \to \gamma \gamma$ and $H \to ZZ^* \to 4 \ell$ cross-sections in $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

The Higgs boson production cross-section is measured in the di-photon and the $ZZ^* \to 4 \ell$ decay channels using 31.4 and 29.0 fb$^{-1}$ of $pp$ collision data collected with the ATLAS detector at a centre-of-mass energy of $\sqrt{s}=13.6$ TeV. In order to reduce the model dependence, the measurement in each channel is restricted to a particle-level phase space that matches closely the channel's reconstruction-level kinematic selection, and it is corrected for detector effects. The measured so-called fiducial cross-sections are $\sigma_{\mathrm{fid,}\gamma\gamma} = 76^{+14}_{-13}$fb and $\sigma_{\mathrm{fid,}4\ell} = 2.80 \pm 0.74$fb, in agreement with the corresponding Standard Model predictions of $67.6 \pm 3.7$ fb and $3.67 \pm 0.19$ fb. Assuming Standard Model acceptances and branching fractions for the two channels, the fiducial measurements are extrapolated to the full phase space yielding total cross-sections of $\sigma(pp\to H) = 67^{+12}_{-11}$ pb and $46 \pm 12$ pb at $13.6$ TeV from the di-photon and $ZZ^* \to 4 \ell$ measurements respectively. The two measurements are combined into a total cross-section measurement of $\sigma(pp\to H) = 58.2\pm 8.7$ pb to be compared with the Standard Model prediction of $\sigma(pp\to H)_\mathrm{SM} = 59.9 \pm 2.6$ pb.