Abstract
The observation of forward proton scattering in association with lepton pairs (e^{+}e^{-}+p or μ^{+}μ^{-}+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 (μμ+p) final state are selected, allowing the background-only hypothesis to be rejected with a significance exceeding 5 standard deviations in each channel. Proton-tagging techniques are introduced for cross-section measurements in the fiducial detector acceptance, corresponding to σ_{ee+p}=11.0±2.6(stat)±1.2(syst)±0.3(lumi) and σ_{μμ+p}=7.2±1.6(stat)±0.9(syst)±0.2(lumi) fb in the dielectron and dimuon channel, respectively.
Highlights
Electromagnetic fields sourced by protons at the Large Hadron Collider (LHC) are sufficiently intense to exceed the Schwinger limit of 1018 V m−1 [1,2,3] and produce lepton pairs via photon fusion, γγ → lþl−, where l denotes electrons or muons [4,5,6,7]
This process occurs in a wide range of astrophysical phenomena, such as cosmic gamma rays [8,9] and neutron stars [10,11]
A hallmark prediction of photon fusion processes at the LHC is the forward scattering of incident protons
Summary
The expected composition of the signal, the simulated samples are normalized to data with sides A and C combined and fit separately in the ee and μμ channels. Nobs (Nbkg) is the number of observed data (expected background) events passing event selection, and Ccent (CAFP) is an overall correction factor accounting for the centraldetector (AFP) efficiency. The integrated luminosity, L 1⁄4 14.6 fb−1, is measured using the LUCID-2 detector [95] and the uncertainty is determined to be 2.4% [96] In this tighter region, Nobs is found to be 19 (23) for the ee (μμ) channel and Nbkg 1⁄4 1.7 Æ 0.3ð2.3 Æ 0.5Þ. The 5% uncertainty in this correction is evaluated as the difference between either applying this data-driven method to simulated signal samples or imposing the N0tr.a5ckmsm 1⁄4 0 requirement on these samples. The CAFP factor is defined by the product εtrack · εsmear
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