Characteristics and Performance of RD53A Readout Chip Equipped with Small-pixel Silicon Sensors

The luminosity of the Large Hadron Collider (LHC) at CERN will be upgraded by a factor of seven to 7.5 $\times$ 10$^{\text{34}}$ cm${^\text{-2}}$s$^{\text{-1}}$, hence boosting the physics potential of its experiments. The increased luminosity leads to a higher rate of proton-proton interactions, in the order of 200 events per bunch crossing and a larger dose and fluence. The CMS experiment will upgrade many of its sub-detectors in order to cope with the new experimental conditions. The current tracking system of the CMS will be fully replaced. New pixel sensor designs with pitches of 50 $\mu$m $\times$ 50 $\mu$m and 100 $\mu$m $\times$ 25 $\mu$m for the phase II upgrade of the CMS Inner Tracker are characterised in order to identify the designs which meet the requirements of the High-Luminosity LHC. The sensors are read out by the new RD53A readout chip which has been developed by the RD53 collaboration, a joint effort of the CMS and ATLAS collaborations. As part of the upgrade programme, the RD53A user community evaluates and characterises the performance of the readout chip when equipped with various sensors types. The studies performed on the RD53A chip within the CMS Inner Tracker upgrade research and development programme are presented in this paper. The studies include charge calibration of the RD53A Linear Frontend using X-ray fluorescence, minimum achievable threshold, crosstalk and efficiency and resolution measurements using particle beams.