2024 Articles

Improving topological cluster reconstruction using calorimeter cell timing in ATLAS

Al Khoury, Konie; Angerami, Aaron; Brooijmans, Gustaaf H.; Busch, Elena Laura; Cole, Brian A.

Clusters of topologically connected calorimeter cells around cells with large absolute signal-to-noise ratio (𝘵𝘰𝘱𝘰-𝘤𝘭𝘶𝘴𝘵𝘦𝘳𝘴) are the basis for calorimeter signal reconstruction in the ATLAS experiment. Topological cell clustering has proven performant in LHC Runs 1 and 2. It is, however, susceptible to 𝘰𝘶𝘵-𝘰𝘧-𝘵𝘪𝘮𝘦 𝘱𝘪𝘭𝘦-𝘶𝘱 of signals from soft collisions outside the 25 ns proton-bunch-crossing window associated with the event’s hard collision.

To reduce this effect, a calorimeter-cell timing criterion was added to the signal-to-noise ratio requirement in the clustering algorithm. Multiple versions of this criterion were tested by reconstructing hadronic signals in simulated events and Run 2 ATLAS data. The preferred version is found to reduce the out-of-time pile-up jet multiplicity by ∼50% for jet 𝑝_T ∼ 20 GeV and by ∼80% for jet 𝑝_T 􏰁 50 GeV, while not disrupting the recon- struction of hadronic signals of interest, and improving the jet energy resolution by up to 5% for 20 < 𝑝_T < 30 GeV. Pile-up is also suppressed for other physics objects based on topo-clusters (electrons, photons, 𝜏-leptons), reducing the overall event size on disk by about 6% in early Run 3 pile-up conditions. Offline reconstruction for Run 3 includes the timing requirement.