Observation and measurement of forward proton scattering in association with lepton pairs produced via the photon fusion mechanism at ATLAS

TL;DR

This paper reports the first observation of forward proton scattering associated with lepton pairs produced via photon fusion at the ATLAS experiment, using proton tagging to measure cross sections with high significance.

Contribution

It introduces proton-tagging techniques for measuring photon fusion processes in proton-proton collisions at the LHC, providing new experimental insights.

Findings

Observation with >5 sigma significance in both channels

Measured cross sections for dielectron and dimuon final states

Proton tagging enables precise cross-section determination

Abstract

The observation of forward proton scattering in association with lepton pairs ($e^+e^-+p$ or $\mu^+\mu^-+p$) produced via photon fusion is presented. The scattered proton is detected by the ATLAS Forward Proton spectrometer while the leptons are reconstructed by the central ATLAS detector. Proton-proton collision data recorded in 2017 at a center-of-mass energy of $\sqrt{s} = 13$ TeV are analyzed, corresponding to an integrated luminosity of 14.6 fb$^{-1}$. A total of 57 (123) candidates in the $ee+p$ ($\mu\mu+p$) final state are selected, allowing the background-only hypothesis to be rejected with a significance exceeding five standard deviations in each channel. Proton-tagging techniques are introduced for cross-section measurements in the fiducial detector acceptance, corresponding to $\sigma_{ee+p}$ = 11.0 $\pm$ 2.6 (stat.) $\pm$ 1.2 (syst.) $\pm$ 0.3 (lumi.) fb and $\sigma_{\mu\mu+p}$ = 7.2 $\pm$ 1.6 (stat.) $\pm$ 0.9 (syst.) $\pm$ 0.2 (lumi.) fb in the dielectron and dimuon channel, respectively.