Commissioning studies on the Phase I pixel detector of CMS in early 2017 proton-proton collisions at 13 TeV

Algorithms that identify jets originating from heavy (bottom or charmed) hadrons rely heavily on the reconstruction of charged particle tracks inside those jets. Recently the innermost part of the CMS tracking detector, also referred to as the Pixel detector, was extended with a fourth layer which is located closer to the beampipe. On top of that, thanks to a new readout chip, this upgrade allows to record at a higher rate to be able to cope with the increasing luminosity at the LHC. These changes have an impact on the track reconstruction and may therefore affect for example the measured values of the impact parameter (IP) of reconstructed tracks (and the uncertainty on this value), which is an important variable in the identification of heavy flavour jets. We present here the distributions of the IP value and significance (defined as the value divided by its uncertainty) as well as the CSVv2 b tagging discriminator in early 2017 proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the CMS detector. The data are taken with the Phase I Pixel detector in place and are compared to data taken in 2016 proton-proton collisions (also at 13 TeV) in which the pre-Phase I Pixel detector was still deployed. Despite the higher luminosities the new detector achieves already similar performances.