Improving topological cluster reconstruction using calorimeter cell timing in ATLAS

Clusters of topologically connected calorimeter cells around cells with large absolute signal-to-noise ratio (\textit{topo-clusters}) are at the base of calorimeter signal reconstruction in the ATLAS experiment. Topological cell clustering has proven to be a performant calorimeter signal definition throughout the LHC Runs 1 and 2. It is, however, susceptible to the piling up effects of particles originating from soft collisions outside the 25 ns bunch-window in which the collision of two protons from the LHC has been triggered (\textit{out-of-time pile-up}). A selection criterion on calorimeter cells' timing has been added to the signal-to-noise ratio requirement in the topological cell clustering algorithm, with the aim of reducing the effects from out-of-time pile-up. The impact of multiple versions of this selection on the reconstruction of hadronic signals is evaluated on both Monte Carlo simulation and Run 2 ATLAS data. The preferred version rejects calorimeter cells with signal-to-noise ratio less than -4 or between 4 and 20, with times incompatible with the triggered bunch collision time. The improved topological clustering is found to reduce the out-of-time pile-up jet multiplicity by $\sim50\%$ at jet $p_{T}\sim20\textrm{ GeV}$ and by $\sim80\%$ above jet $p_{T} \sim 50\textrm{ GeV}$, while not disrupting the reconstruction of hadronic signals of interest. The jet energy resolution is also found to improve by up to 5\% for $20\textrm{ GeV} < p_{T} < 30 \textrm{ GeV}$. The timing selection also has a pile-up-suppression effect on other physics objects based on topo-clusters (electron, photon, $\tau$-leptons), reducing the overall ATLAS event size on disk by about $6\%$ in early Run 3 pile-up conditions. The timing requirement has been included in the offline ATLAS reconstruction for Run 3.