Observation of proton-tagged, central (semi)exclusive production of high-mass lepton pairs in pp collisions at 13 TeV with the CMS-TOTEM precision proton spectrometer

Abstract

The process pp$\to {\ell }^{+}{\ell }^{-}$p${}^{(\ast )}$, with ${\ell }^{+}{\ell }^{-}$ a muon or an electron pair produced at midrapidity with mass larger than 110 GeV, has been observed for the first time at the LHC in pp collisions at $\sqrt{s}=$ 13 TeV. One of the two scattered protons is measured in the CMS-TOTEM precision proton spectrometer (CT-PPS), which operated for the first time in 2016. The second proton either remains intact or is excited and then dissociates into a low-mass state p${}^{\ast }$, which is undetected. The measurement is based on an integrated luminosity of 9.4 fb${}^{-1}$ collected during standard, high-luminosity LHC operation. A total of 12 ${\mu }^{+}{\mu }^{-}$ and 8 e${}^{+}$e${}^{-}$ pairs with $m({\ell }^{+}{\ell }^{-})>$ 110 GeV, and matching forward proton kinematics, are observed, with expected backgrounds of 1.49 $\pm$ 0.07 (stat) $\pm$ 0.53 (syst) and 2.36 $\pm$ 0.09 (stat) $\pm$ 0.47 (syst), respectively. This corresponds to an excess of more than five standard deviations over the expected background. The present result constitutes the first observation of proton-tagged $\gamma \gamma$ collisions at the electroweak scale. This measurement also demonstrates that CT-PPS performs according to the design specifications.

Replaced with published version. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/PPS-17-001 (CMS Public Pages)

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