The CMS Precision Proton Spectrometer at the HL-LHC -- Expression of Interest

The CMS Collaboration intends to pursue the study of central exclusive production (CEP) events, pp $\to$ pXp, at the High-Luminosity LHC (HL-LHC) by means of a new near-beam proton spectrometer. In CEP events, the state X is produced at central rapidities, and the scattered protons do not leave the beam pipe. The kinematics of X can be fully reconstructed from that of the protons, which gives access to final states otherwise not visible. CEP allows unique sensitivity to physics beyond the standard model, e.g. in the search for anomalous quartic gauge couplings, axion-like particles, and in general new resonances. CMS has been successfully operating the Precision Proton Spectrometer (PPS), initially a joint CMS and TOTEM project, since 2016. The present document outlines the physics interest of a new PPS at the HL-LHC, and explores its feasibility and expected performance. Discussions with the machine groups led to the identification of four locations suitable for the installation of movable proton detectors: at 196, 220, 234, and 420 m from the interaction point, on both sides. Acceptance studies indicate that having the beams cross in the vertical plane at the interaction point, as implemented after Long Shutdown 3, is preferable over the present horizontal crossing. This gives access to states X in the mass range 133 GeV$-$2.7 TeV with the stations at 196, 220, and 234 m. The lower mass limit becomes 43 GeV if the 420 m station is included, which makes it possible to study central exclusive production of the 125 GeV Higgs boson. This is a major improvement with respect to the current mass range of 350 GeV$-$2 TeV. The project and its costs can easily be staged; the 420 m station can be addressed in a second step. In summary, a new near-beam proton spectrometer for operation at the HL-LHC appears feasible. It would greatly extend the CMS physics reach with a limited investment.