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

TL;DR

The CMS collaboration performs a search for exclusive and quasi-exclusive two-photon production of W^+W^- in $pp$ collisions at $\sqrt{s}=7$ TeV using the $\mu^{\pm}e^{\mp}$ final state. A data-driven strategy with zero-extra-track exclusivity and high-$p_T$ dilepton requirements isolates the signal, with proton-dissociation effects constrained by high-mass $\mu^+\mu^-$ control samples. Two events are observed, consistent with the SM expectation of $2.2\pm0.4$ signal plus $0.84\pm0.15$ background, yielding a 95% CL cross-section limit of 10.6 fb and stringent limits on anomalous quartic gauge couplings with a dipole form factor ($\Lambda_{cutoff}=500$ GeV). The results provide the most sensitive constraints to date on AQGCs in this channel and demonstrate the efficacy of data-driven background control using exclusive dilepton processes. If no form factor is assumed, the limits on the AQGC parameters would be even more stringent but would be subject to unitarity concerns at high energies.

Abstract

A search for exclusive or quasi-exclusive W(+)W(-) production by photon-photon interactions, pp to p(*)W(+)W(-)p(*), at sqrt(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 mu(+/-)e(-/+) vertex with no additional associated charged tracks and dilepton transverse momentum pt(mu(+/-)e(-/+)) > 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 pt > 100 GeV. Model-independent upper limits are computed and compared to predictions involving anomalous quartic gauge couplings. The limits on the parameters a[W,(0,C)]/Lambda^2 with a dipole form factor and an energy cutoff Lambda(cutoff) = 500 GeV are of the order of 10E-4.

Figures (12)

• Figure 1: Quartic gauge coupling (a) and $t$- (b) and $u$-channel (c) $\mathrm{W}$-boson exchange diagrams contributing to the $\gamma\gamma\to\mathrm{W^+}\mathrm{W^-}$ process at leading order in the SM.
• Figure 2: Transverse momentum distribution of lepton pairs for the $\gamma\gamma\to \mathrm{W^+}\mathrm{W^-}$ process at generator level in the SM (shaded histogram), and for several values of anomalous quartic gauge coupling parameters $a^{\mathrm{W}}_{0}$ and $a^{\mathrm{W}}_{C}$ (open histograms). In this plot $\Lambda_{\text{cutoff}}=500\,\text{Ge\spaceV}\xspace$ is the energy cutoff scale in the dipole form factor.
• Figure 3: Invariant mass distribution of the muon pairs for the elastic selection with no additional track on the dimuon vertex. The dashed lines indicate the ${Z}$-peak region. The hatched bands indicate the statistical uncertainty in the simulation.
• Figure 4: Kinematic distributions for the elastic selection, for the ${Z}$ region only ($70\,\text{Ge\spaceV}\xspace{<m({\mu^+}{\mu^-})<}106\,\text{Ge\spaceV}\xspace$, left panel) and with the ${Z}$ region removed (right panel). The acoplanarity (above) and $p_{\mathrm{T}}\xspace$ of ${\mu^+}{\mu^-}$ pairs with zero extra tracks (below) are shown. The hatched bands indicate the statistical uncertainty in the simulation.
• Figure 5: Invariant mass distribution of the muon pairs for the dissociation selection. The dashed lines indicate the ${Z}$-peak region. The hatched bands indicate the statistical uncertainty in the simulation.