Evaluation of Timepix3 as a luminosity detector at LHC during 2018 pp collisions at √s=13 TeV

Four Timepix3 detectors were installed in the ATLAS experiment at different positions to study their capabilities to measure luminosity during pp -collisions at √s = 13 s = 13 TeV in 2018. While the detectors were operated independently of the ATLAS triggering and acquisition scheme, continuous (dead-time free) measurement together with an orbit clock trigger allowed for synchronization with the LHC. The detectors benefit from a fine segmentation, a pixel pitch of 55 μ μ m, and a per-pixel time resolution of 1.6 ns allowing for a high-quality track reconstruction and particle identification. One of the 500 μ μ m thick silicon sensors was equipped with a ⁶ 6 LiF neutron converter, extending the particle identification capabilities. The installed system was used to study luminosity in different time frames: long term (run-by-run), short term (within a single run) and instantaneous (for each bunch crossing). For the long- and short-term luminosity, partly-independent algorithms: cluster and thermal neutron counting are proposed. A comprehensive analysis of the signal from induced radioactivity and its consecutive removal is presented together with a study demonstrating that the activation-corrected Timepix3 luminosity measurement provides good linearity with respect to the pile-up parameter μ μ . For measurement of the instantaneous luminosity, a cluster classification scheme was employed to decompose the measured response to the colliding bunch signal. Selecting a subset of the cluster categories, the signal-to-background ratio was improved and the impact of delayed particles from previous bunches could be reduced. Comparisons performed to the primary bunch-by-bunch luminosity measurement of the ATLAS experiment, provided by the LUCID-2 Cherenkov detector, show a good agreement.