The CMS tracker upgrade for the High Luminosity LHC

The CMS detector features the world's largest silicon tracker, comprising a strip and a pixel detector. However, the CMS tracker is expected to reach the end of its lifetime after Run 3 of the LHC, and a new device will be installed during Long Shutdown 3. The aim is to at least maintain, and if possible to improve, the performance of the present device, under much harsher conditions in terms of radiation levels, hit rates and pileup. The new tracker will feature an Inner Tracker with silicon pixel modules and an Outer Tracker with strip and macro-pixel silicon modules. The Inner Tracker will extend the acceptance up to much higher pseudorapidities. More than 4000 hybrid pixel modules with a reduced cell size and with a readout chip based on the RD53 development will be installed. For the Outer Tracker an innovative detector concept was developed, allowing the contribution of tracker data to the first trigger level. For this the data volume that must be sent out at 40\,MHz for the trigger decision is reduced already on-module, as each module carries two silicon sensors on top of each other with a distance of a few millimetres, read out by the same front-end ASICs. In that way a rough estimate of the transverse momentum of each particle can be made by exploiting the strong CMS magnetic field of 3.8\,T. The module and detector designs are built around this idea. The detector concept and the expected performance will be presented, and important design choices along with the present status will be discussed.