Performance and operation of the CMS Phase 1 pixel detector

The silicon pixel detector of the CMS experiment at the LHC provides high precision tracking in the region closest to the proton-proton collision point. It operates in a particularly harsh environment characterized by high track multiplicity and heavy irradiation which poses severe challenges to the detector design. At the beginning of 2017, the original pixel detector was replaced with an upgraded 4-layer pixel system to maintain the excellent tracking performance of CMS in view of the increasing luminosity scenario (up to 2.5 x 10(34) cm(-2) s(-1) during Run 2 and 3). The CMS Phase 1 detector features a new readout chip to reduce the dynamic inefficiency at high rate as well as an additional innermost layer at a radius of 3 cm from the interaction point to improve the impact parameter resolution. This paper reviews the main design features of the CMS Phase 1 pixel detector and discuss its performance and the challenges in operation during the first year of data-taking in 2017. Furthermore, the paper reports about the consolidation work performed during the LHC year end technical stop and the status of the Phase 1 pixel detector and re-commissioning toward operation in 2018.