Evidence for exclusive gamma gamma to W+W- production and constraints on anomalous quartic gauge couplings in pp collisions at sqrt(s) = 7 and 8 TeV

TL;DR

This paper reports evidence for exclusive gamma gamma to W+W- production in proton-proton collisions at 7 and 8 TeV, and sets new limits on anomalous quartic gauge couplings, consistent with the Standard Model.

Contribution

It provides the first combined analysis of 7 and 8 TeV data showing evidence for gamma gamma to W+W- production and establishes the most stringent limits on anomalous gauge couplings to date.

Findings

Observed 13 events at 8 TeV with expected background of 3.9

Combined data shows 3.4 sigma excess over background

Set new upper limits on anomalous quartic gauge couplings

Abstract

A search for exclusive or quasi-exclusive gamma gamma to W+W- production, via pp to p(*) W+W- p(*) to p(*) mu+/- e-/+ at sqrt(s) = 8 TeV, is reported using data corresponding to an integrated luminosity of 19.7 inverse femtobarns. Events are selected by requiring the presence of an electron-muon pair with large transverse momentum pt(mu+/- e-/+) > 30 GeV, and no associated charged particles detected from the same vertex. The 8 TeV results are combined with the previous 7 TeV results (obtained for 5.05 inverse femtobarns of data). In the signal region, 13 (2) events are observed over an expected background of 3.9 +/- 0.6 (0.84 +/-0.15) events for 8 (7) TeV, resulting in a combined excess of 3.4 standard deviations over the background-only hypothesis. The observed yields and kinematic distributions are compatible with the standard model prediction for exclusive and quasi-exclusive gamma gamma to W+W- production. Upper limits on the anomalous quartic gauge coupling operators a[W;0,C] (dimension-6) and f[M0,1,2,3] (dimension-8), the most stringent to date, are derived from the measured dilepton transverse momentum spectrum.