Observation of Electroweak Production of Same-Sign $W$ Boson Pairs in the Two Jet and Two Same-Sign Lepton Final State in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

The first observation of electroweak production of same-sign $W$ boson pairs in proton-proton collisions is reported. The data sample corresponds to an integrated luminosity of $35.9{\mathrm{fb}}^{-1}$ collected at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. Events are selected by requiring exactly two leptons (electrons or muons) of the same charge, moderate missing transverse momentum, and two jets with a large rapidity separation and a large dijet mass. The observed significance of the signal is 5.5 standard deviations, where a significance of 5.7 standard deviations is expected based on the standard model. The ratio of measured event yields to that expected from the standard model at leading order is $0.90\pm 0.22$. A cross section measurement in a fiducial region is reported. Bounds are given on the structure of quartic vector boson interactions in the framework of dimension-8 effective field theory operators and on the production of doubly charged Higgs bosons.

Figure 1

Representative Feynman diagrams for single, triple, and quartic gauge couplings of the EW-induced same-sign $W$ boson pair production (left, middle-left, middle-right) and QCD-induced background (right).

Figure 2

Distributions of $m_{jj}$ (left) and $m_{\ell \ell }$ (right) in the signal region. The normalization of the EW $W^{\pm }{W}^{\pm }$ and background distributions corresponds to the result of the fit. The hatched bands include statistical and systematic uncertainties from the predicted yields. The histograms for other backgrounds include the contributions from QCD $WW$, $W\gamma$, wrong-sign events, double parton scattering, and triboson processes. The overflow is included in the last bin.

Figure 3

Expected and observed 95% C.L. upper limits on the cross section times branching fraction, ${\sigma }_{\mathrm{VBF}}(H^{\pm \pm })\mathcal{B}(H^{\pm \pm }\to W^{\pm }{W}^{\pm })$ as a function of doubly charged Higgs boson mass.