Exclusive photon-photon production of muon pairs in proton-proton collisions at sqrt(s) = 7 TeV

TL;DR

This CMS study measures exclusive two-photon muon-pair production in proton-proton collisions at √s = 7 TeV using 40 pb−1. It relies on a track-based exclusivity criterion and a maximum-likelihood fit to disambiguate elastic γγ → μ+μ− events from proton-dissociation and DY backgrounds, with signal modeling provided by the LPAIR generator. The measured cross section, σ(pp→pμμp) = 3.38^{+0.58}_{-0.55} (stat) ±0.16 (syst) ±0.14 (lumi) pb, agrees with QED predictions within uncertainties, and the data preserve the characteristic kinematic distributions expected for γγ fusion. The analysis includes a comprehensive treatment of pileup, muon-efficiency corrections, and background systematics, establishing this exclusive process as a potential luminosity-calibration channel and a benchmark for future exclusive measurements in high-pileup LHC running conditions.

Abstract

A measurement of the exclusive two-photon production of muon pairs in proton-proton collisions at sqrt(s)= 7 TeV, pp to p mu^+ mu^- p, is reported using data corresponding to an integrated luminosity of 40 inverse picobarns. For muon pairs with invariant mass greater than 11.5 GeV, transverse momentum pT(mu) > 4 GeV and pseudorapidity |eta(mu)| < 2.1, a fit to the dimuon pt(mu^+ mu^-) distribution results in a measured cross section of sigma(pp to p mu^+ mu^- p) = 3.38 [+0.58 -0.55] (stat.) +/- 0.16 (syst.) +/- 0.14 (lumi.) pb, consistent with the theoretical prediction evaluated with the event generator Lpair. The ratio to the predicted cross section is 0.83 [+0.14-0.13] (stat.) +/- 0.04 (syst.) +/- 0.03 (lumi.). The characteristic distributions of the muon pairs produced via photon-photon fusion, such as the muon acoplanarity, the muon pair invariant mass and transverse momentum agree with those from the theory.

Figures (10)

• Figure 1: Schematic diagrams for the exclusive and semi-exclusive two-photon production of muon pairs in pp collisions for the elastic (left), single dissociative (center), and double dissociative (right) cases. The three lines in the final state of the center and right plots indicate dissociation of the proton into a low-mass system $N$.
• Figure 2: Efficiency of the zero extra tracks selection vs. distance to closest track computed with the artificial vertex method in zero-bias data. The points correspond to events with 0, 1, 2, and 8 real vertices in the event. Events to the right of the vertical dashed line are selected. The vertical error bars are negligible.
• Figure 3: Efficiency of the zero extra tracks selection vs. distance to closest track computed for DY events in simulation. Events to the right of the vertical dashed line are selected. The vertical error bars are negligible.
• Figure 4: Distribution of $p_{\mathrm{T}}\xspace({\mu^+}{\mu^-})$ for the selected sample. Data are shown as points with statistical error bars. The histograms represent the simulated signal (yellow), single (light green) and double (dark green) proton dissociative backgrounds, and DY (red). The yields are determined from a fit using the distributions from simulation.
• Figure 5: One and two standard-deviation contours in the plane of fitted parameters for the proton-dissociation yield ratio vs. modification parameter $a$ (left), the data-theory signal ratio vs. modification parameter $a$ (center), and the data-theory signal ratio vs. proton-dissociation yield ratio (right). The contours represent 39.3% and 86.5% confidence regions, where the cross indicates the best-fit point.