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 = 8 TeV, 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 8 TeV results are combined with the previous 7 TeV results (obtained for 5.05 fb⁻¹ 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 3.4σ 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_0,C ^W (dimension-6) and f_M0,1,2,3 (dimension-8), the most stringent to date, are derived from the measured dilepton transverse momentum spectrum.