Upgrade of the CMS Tracker

The performance of the Tracker of the CMS experiment, comprising of a pixel and a strip detector, has so far been excellent, as reflected in the wealth of beautiful physics results from CMS. However, the foreseen increases of both the instantaneous and the integrated luminosity by the LHC during the next ten years will necessitate a stepwise upgrade of the CMS tracking detector. \\ In the extended end-of-year shutdown 2016/17 the pixel detector will be exchanged. The new device is designed for an instantaneous luminosity of $2\cdot 10^{34}$\,cm$^{-2}$s$^{-1}$ and an integrated luminosity of 500\,fb$^{-1}$. The number of layers will be increased from three to four in the barrel part and from two to three in the end caps, thus providing four-hit coverage over the full pseudorapidity range. A smaller beampipe allows the reduction of the radius of the innermost layer, improving the tracking performance. Further improvements include a new readout chip, reduction of material, and the installation of more efficient cooling and powering systems. \\ Around 2022, in Long Shutdown 3, the whole tracker will need to be replaced, in order to be able to cope with an instantaneous luminosity of $5\cdot 10^{34}$\,cm$^{-2}$s$^{-1}$ and an integrated luminosity of 3000\,fb$^{-1}$, as expected for the High Luminosity LHC (HL-LHC). A new challenge is the requirement that tracker information should be usable in the Level 1 trigger decision. This has led to a dedicated module design with two closely spaced sensor layers whose hits are correlated in the module electronics. In this way an estimate on the transverse momentum can be made and high momentum tracks can be selected, significantly reducing the data volume to be processed. \\ This paper will motivate the design choices for the CMS pixel and outer tracker upgrades. The status and plans of these projects will be summarized and highlights of the R\&D will be described.