The Upgrade of the CMS Outer Tracker

During the high luminosity phase of the LHC starting around 2025 (HL-LHC), the machine is expected to deliver an instantaneous luminosity of $5\cdot10^{34}$~cm$^{-2}$s$^{-1}$. A total of 3000~fb$^{-1}$ of data is foreseen to be delivered, hereby increasing the physics potential of the LHC experiments significantly. However, this fivefold increase in luminosity compared to the design luminosity of the LHC will lead to a higher track multiplicity in the silicon tracking detectors of the experiments, and to severe radiation levels. In order to maintain physics capability, CMS will build a completely new tracking detector comprising a pixel detector and an outer tracker. Furthermore, information from the outer tracker will be used in the first level trigger of CMS to ensure a sufficient trigger rejection. For this purpose, CMS will use so-called p$_{T}$ modules which will provide a momentum measurement at the module level. These modules consist of two back-to-back strip sensors for the outer layers, and a strip sensor and a macro-pixel sensor for the inner layers, respectively. This paper will introduce the concept of p$_T$ modules and give an overview of the ongoing research and development activities concerning radiation hard silicon sensor materials.