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

TL;DR

This paper reports the first observation of proton-tagged, high-mass lepton pair production in proton-proton collisions at 13 TeV, using the CMS-TOTEM precision proton spectrometer, confirming the detector's performance and the process's occurrence at the electroweak scale.

Contribution

First observation of proton-tagged gamma-gamma collisions producing high-mass lepton pairs at the LHC with the CMS-TOTEM detector.

Findings

Observation of 12 muon pairs and 8 electron pairs with >5 sigma significance

Validation of the CMS-TOTEM spectrometer's performance in high-luminosity conditions

Confirmation of gamma-gamma production at the electroweak scale

Abstract

The process pp$\to\ell^+\ell^-$p$^{(*)}$, 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$^{*}$, 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.