Abstract
| The upgrade program of the CMS experiment in view of the LHC High Luminosity phase (HL-LHC) includes a replacement of the silicon pixel tracker. This is necessary to guarantee the same tracking performance of the current detector under harsher operating conditions, including higher radiation fluences and hit rates. The first layer of the central (barrel) detector will be located at radial distance of 3 cm from the interaction point where the radiation fluence after 7 years of operation will be $1.9\times10^{16}$ n$_{eq}$/cm$^2$. Following an extensive R\ and D program, comprising both laboratory and test beam measurements, 3D sensors will be employed in this layer due to their higher radiation tolerance and lower power consumption after irradiation. The final design of the sensors will be presented, together with the most recent laboratory measurements. The main focus will be on the most recents test beam measurements, before and after irradiation up to $1.5\times10^{16}$ n$_{eq}$/cm$^2$, that proved a hit detection efficiency larger than 96\% at normal incidence with less than 2\% masked pixels, for applied bias voltages larger than 120 V. |