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, $\rm pp \to pXp$, 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) since 2016; PPS started as a joint CMS and TOTEM project, and then evolved into a standard CMS subsystem. The present document outlines the physics interest of a new near-beam proton spectrometer at the HL-LHC, and explores its feasibility and expected performance. The document has been edited by the members of the PPS group and builds on their experience in the construction and operation of PPS. Discussion with the machine groups has 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 (in this document these locations always imply both sides, unless otherwise noted). The locations at 196, 220, and 234\,m can be instrumented with Roman Pot devices similar to the ones presently used. The 420\,m location requires a bypass cryostat (which has been developed for other locations in the LHC) and a movable detector vessel approaching the beam from between the two beam pipes. Acceptance studies indicate that having the beams cross in the vertical plane at the interaction point, as implemented after Long Shutdown 3, is vastly preferable over the present horizontal crossing. This gives access to centrally produced states X in the mass range 133\,GeV$-$2.7\,TeV with the stations at 196, 220, and 234\,m. The mass range becomes 43\,GeV$-$2.7\,TeV if the 420\,m station is included, which makes it possible to study central exclusive production of the 125\,GeV Higgs boson. This is a major improvement with respect to the current mass range of 350\,GeV$-$2\,TeV. The radiation background has also been studied. Radiation hardness is required for all components in the tunnel. Service work during short technical stops will not be possible. The irradiation dose rate will be very strongly peaked near the beam. Detectors should therefore be vertically shifted with a remotely controlled movement system to move the irradiation peak to a different spot in the detector, and thus mitigate the effects of radiation damage. A shift by 0.5\,mm every 20\,fb$^{-1}$ is sufficient. The high ``pileup'' level foreseen at the HL-LHC (where the mean number of interactions per bunch crossing, $\mu$, can be as large as 200) makes it necessary to measure the longitudinal coordinate of the vertex via time of flight; sensors with a single plane resolution of 50$-$60\,ps are adequate. For the tracking part, pixel sizes between 100 and 200 $\mu$m are sufficient. For the choice of the detector technologies, synergies with the developments currently ongoing for the Phase-2 upgrade of the central pixel system and the forward MIP timing detector should be carefully considered. A preliminary estimation suggests that the stations at 196, 220, and 234\,m could be built at a total cost between 1 and 2\,MCHF. The project can easily be staged, and the costs spread over time; 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.