Search for high-mass dilepton resonances in $pp$ collisions at $\sqrt{s}=13.6$ TeV combined with 13 TeV results using the ATLAS detector

A search for spin-1 $ee$ and $μμ$ resonances in the high-mass range is presented. Two proton-proton ($pp$) collision datasets recorded by the ATLAS detector at CERN's Large Hadron Collider are utilised. A search is first performed using a dataset recorded at a center-of-mass energy of 13.6 TeV between 2022 and 2024, corresponding to an integrated luminosity of 165 fb$^{-1}$. The results of this search are subsequently statistically combined with results using 140 fb$^{-1}$ of data recorded between 2015 and 2018 at 13 TeV. The dilepton invariant mass spectrum is fitted with a functional form to model the contribution from Standard Model background processes. A generic spin-1 signal shape is used to determine the significance of observed deviations from the background expectation. No significant deviation is observed, therefore upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for a spin-1 resonance with various width hypotheses, individually for $ee$, $μμ$ final states and for their combination. From the combined 13 TeV and 13.6 TeV results, mass limits are extracted for various benchmark model bosons, including a $Z^{\prime}_{\text{SSM}}$ (5.5 TeV), $Z^{\prime}_χ$ (5.1 TeV) and $Z^{\prime}_ψ$ (4.8 TeV). These results represent the most stringent limits to date on high-mass dilepton resonances on the considered $Z'$ models.