Evidence for exclusive γγ → W⁺W^- production and constraints on anomalous quartic gauge couplings in pp collisions at √s = 7 and 8 TeV

A search for exclusive or quasi-exclusive γγ → W⁺W^- production, via pp → p^(∗)W+W^-p^(∗) → p^(∗)μ^±e^±p^(∗) at √s = 8TeV, is reported using data corresponding to an integrated luminosity of 19.7 fb^-1. Events are selected by requiring the presence of an electron-muon pair with large transverse momentum p_T(μ^±e^±) > 30 GeV, and no associated charged particles detected from the same vertex. The 8TeV results are combined with the previous 7TeV results (obtained for 5.05 fb^-1of 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 3.4σ over the background-only hypothesis. The observed yields and kinematic distributions are compatible with the standard model prediction for exclusive and quasi-exclusive γγ → 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.