Searches for periodic signals in diphoton and dilelectron invariant mass spectra with the ATLAS detector at the Large Hadron Collider

This analysis presents a search for periodic signals in the dielectron and diphoton final states. The search uses the full Run-2 $pp$ collisions recorded by ATLAS with a center-of-mass energy of $\sqrt{13}$ TeV at the LHC corresponding to an integrated luminosity of 139 fb$^{-1}$. A functional form is used for dilepton and diphoton invariant mass distributions of the data to model the contribution from background processes, and generic signals are used to determine the significance of observed deviations from this background estimate in significance scans in the frequency vs mass space. Model-dependent 95\% CL for the two parameters $M_5$ and $k$ are provided for a benchmark Clockwork/Linear Dilaton (CW/LD) model. The $ee$ and $\gamma\gamma$ invariant mass spectrum are consistent with the Standard Model expectation when using pseudo-data as the model-independent approach shows no deviation from the SM expectation with for both the $ee$ and $\gamma\gamma$ channel. The $95\%$ CL exclusion limits are set on the parameter space of the CW/LD model. The $95\%$ expected CL exclusion limits were strongest at values of $M_5$ $\approx$ 8~TeV and $k$ $\approx$ 800 GeV for the $ee$ channel and $M_5$ $\approx$ 9~TeV and $k$ $\approx$ 750 GeV for the $\gamma\gamma$ channel. For the $\gamma\gamma$ channel the $95\%$ observed CL exclusion limit reaches values of $M_5$ $\approx$ 10~TeV and $k$ $\approx$ 600 GeV. The $95\%$ observed CL exclusion limits using pseudo-data generated from the background-only hypothesis for the $ee$ and $\gamma\gamma$ channels have similar values as the expected limit.