Study of exclusive two-photon production of W+W− in pp collisions at √s = 7 TeV and constraints on anomalous quartic gauge couplings

Abstract

A search for exclusive or quasi-exclusive W⁺W^- production by photon-photon interactions, pp→p^(∗)W⁺W^-p^(∗), at √s = 7 TeV is reported using data collected by the CMS detector with an integrated luminosity of 5.05 inverse femtobarns. Events are selected by requiring a μe vertex with no additional associated charged tracks and dilepton transverse momentum p_T (μe) > 30 GeV. Two events passing all selection requirements are observed in the data, compared to a standard model expectation of 2.2±0.4 signal events with 0.84±0.15 background. The tail of the dilepton pT distribution is studied for deviations from the standard model. No events are observed with p_T (μe) > 100 GeV. Model-independent upper limits are computed and compared to predictions involving anomalous quartic gauge couplings. The limits on the parameters aW0,C/Λ2 with a dipole form factor and an energy cutoff Λcutoff = 500 GeV are of the order of 10−4.

Figures

Figure 1. Transverse momentum distribution of lepton pairs for the γγ→WW process at generator level in the SM (shaded histogram), and for several values of AQGC parameter a0W/Λ2, while aCW/Λ2 is set to zero (open histograms). In this plots Λ = 500 GeV is the scale for new physics and p is set to 2.0. All distributions are scaled to a integrated luminosity of 5.05 fb-1.

Figure 2. Invariant mass distribution of the muon pairs for the elastic selection. The dotted lines indicates the Z peak region. The hatched bands indicate the statistical uncertainty on the simulation.

Figure 3. Kinematic distributions for the elastic selection, for the Z region only (70 GeV < m(μμ) < 106 GeV, left column) and with the Z region removed (right column). The acoplanarity of μμ pairs with 0 extra tracks (above), and pT of μμ pairs with 0 extra tracks (below) are shown. The hatched bands indicate the statistical uncertainty on the simulation.

Figure 4. Invariant mass distribution of the muon pairs for the dissociation selection. The dotted lines indicates the Z peak region. The hatched bands indicate the statistical uncertainty on the simulation.

Figure 5. Kinematic distributions for the proton dissociation selection, for the Z region only (70 GeV < m(μμ) < 106 GeV, left column) and with the Z region removed (right column). The pT of μμ pairs with 0 extra tracks are shown. The hatched bands indicate the statistical uncertainty on the simulation.

Figure 6. Number of additional tracks on the electron-muon primary vertex for events with pT(μe) > 30 GeV. The shaded bands indicate the statistical uncertainty on the background estimation.

Figure 7. .μe invariant mass (left) and acoplanarity (right) for events with 1-6 extra tracks on the μe vertex and pT(μe) > 30 GeV. The shaded bands indicate the statistical uncertainty on the background estimation.

Figure 8. μe invariant mass (left) and acoplanarity (right) for events with 0 extra tracks on the μe vertex and pT(μe) < 30 GeV. The shaded bands indicate the statistical uncertainty on the background estimation.

Figure 9. μe invariant mass (left), acoplanarity (center), and missing transverse energy (right) for events in the signal region with 0 extra tracks on the μe vertex and pT(μe) > 30 GeV. The backgrounds (solid histograms) are stacked with statistical uncertainties indicated by the shaded region, the signal histogram (open histogram) is stacked on top of the backgrounds.

Figure 10. Full pT distribution for events with 0 extra tracks (left) and multiplicity of extra tracks for events with pT(μe) > 100 GeV (right). The backgrounds (solid histograms) are stacked with statistical uncertainties indicated by the shaded region, the signal histogram (open histogram) is stacked on top of the backgrounds. The expected signal is shown for the SM γγ→W+W- signal (solid lines), and for two representative values of the anomalous couplings a0W/Λ2 and aCW/Λ2 (dotted and dashed lines).

Figure 11. Excluded values of the anomalous coupling parameters a0W/Λ2 and aCW/Λ2 with \Lambda= 500 GeV.The area outside the solid contour is excluded by this measurement at 95\%~CL, obtained for pT(μ, e) > 20 GeV, η(μ, e) < 2.4, pT(μe) > 100 GeV. The predicted cross sections are rescaled to include the contribution from proton dissociation.

Event displays. Displays of the two selected candidate events.

Event displays. Display of the first selected candidate event showing all tracks in the event (left), and only tracks from the e-mu primary vertex (right).

Event displays. Display of the second selected candidate event showing all tracks in the event (left), and only tracks from the e-mu primary vertex (right).