Evidence for proton-tagged, central semi-exclusive production of high-mass muon pairs at 13 TeV with the CMS-TOTEM Precision Proton Spectrometer

The process $pp \rightarrow p \mu^+\mu^- p^{(*)}$ has been observed at the LHC for dimuon masses larger than $110~\mathrm{GeV}$ in $pp$ collisions at $\sqrt{s}=13~\mathrm{TeV}$. Here $p^{(*)}$ indicates that the second proton is undetected, and either remains intact or dissociates into a low-mass state $p^{*}$. The scattered proton has been measured in the CMS-TOTEM Precision Proton Spectrometer (CT-PPS), which operated for the first time in 2016. The measurement is based on an integrated luminosity of approximately $10~\mathrm{fb}^{-1}$ collected in regular, high-luminosity fills. A total of 12 candidates with $m(\mu\mu) > 110~\mathrm{GeV}$, and matching forward proton kinematics, is observed. This corresponds to an excess of more than four standard deviations over the background. The spectrometer and its operation are described, along with the data and background estimation. The present results constitute the first evidence of this process at such masses. They also demonstrate that CT-PPS performs as expected.