CMS-SMP-17-004

CMS-SMP-17-004 ; CERN-EP-2017-232
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 TeV
CMS Collaboration
18 September 2017
Phys. Rev. Lett. 120 (2018) 081801
Abstract: 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 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-eight effective field theory operators and on the production of doubly charged Higgs bosons.
Links: e-print arXiv:1709.05822 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	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).
png pdf	Figure 1-a: 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).
png pdf	Figure 1-b: 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).
png pdf	Figure 1-c: 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).
png pdf	Figure 1-d: 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).
png pdf	Figure 2: Distributions of $m_{jj}$ (left) and $ {m_{\ell \ell}}$ (right) in the signal region. The normalization of the EW $\mathrm{W} ^\pm \mathrm{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.
png pdf	Figure 2-a: Distributions of $m_{jj}$ (left) and $ {m_{\ell \ell}}$ (right) in the signal region. The normalization of the EW $\mathrm{W} ^\pm \mathrm{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.
png pdf	Figure 2-b: Distributions of $m_{jj}$ (left) and $ {m_{\ell \ell}}$ (right) in the signal region. The normalization of the EW $\mathrm{W} ^\pm \mathrm{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.
png pdf	Figure 3: Expected and observed 95% CL upper limits on the cross section times branching fraction, $\sigma _{\mathrm {VBF}}(\mathrm{H} ^{\pm \pm}) \mathrm {{\mathcal {B} }}(\mathrm{H} ^{\pm \pm} \to \mathrm{W} ^{\pm}\mathrm{W} ^{\pm})$ (left) and on $s_{\mathrm{H}}$ in the Georgi-Machacek model (right) as a function of doubly charged Higgs boson mass. The blue area in the upper-right corner covers the region where the model is not applicable [39].
png pdf	Figure 3-a: Expected and observed 95% CL upper limits on the cross section times branching fraction, $\sigma _{\mathrm {VBF}}(\mathrm{H} ^{\pm \pm}) \mathrm {{\mathcal {B} }}(\mathrm{H} ^{\pm \pm} \to \mathrm{W} ^{\pm}\mathrm{W} ^{\pm})$ (left) and on $s_{\mathrm{H}}$ in the Georgi-Machacek model (right) as a function of doubly charged Higgs boson mass. The blue area in the upper-right corner covers the region where the model is not applicable [39].
png pdf	Figure 3-b: Expected and observed 95% CL upper limits on the cross section times branching fraction, $\sigma _{\mathrm {VBF}}(\mathrm{H} ^{\pm \pm}) \mathrm {{\mathcal {B} }}(\mathrm{H} ^{\pm \pm} \to \mathrm{W} ^{\pm}\mathrm{W} ^{\pm})$ (left) and on $s_{\mathrm{H}}$ in the Georgi-Machacek model (right) as a function of doubly charged Higgs boson mass. The blue area in the upper-right corner covers the region where the model is not applicable [39].

Tables
png pdf	Table 1: Estimated signal and background yields after the selection. The statistical uncertainties are reported for all six channels, while the sums are reported with the statistical and systematic uncertainties added in quadrature. The processes contributing to less than 1% of the total background are not listed, but included in the total background yield.
png pdf	Table 2: Observed and expected 95% CL limits on the coefficients for higher-order (dimension-eight) operators in the effective field theory Lagrangian. The last column summarizes the previously observed limits obtained by the CMS Collaboration together with the appropriate reference.

Summary

In summary, we present the first observation of electroweak production of same-sign W boson pairs in proton-proton collisions. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected at $\sqrt{s}= $ 13 TeV with the CMS detector. Events are selected by requiring exactly two leptons of the same charge, moderate $p_{\mathrm{T}}^{\text{miss}}$, and two jets with large rapidity separation and large dijet mass. The two dominant sources of background events after the event selection requirements have been applied are nonprompt leptons and the $\mathrm{WZ} \to 3\ell\nu$ process. The observed significance is 5.5 standard deviations, where a significance of 5.7 standard deviations is expected based on the SM. 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 consistent with SM predictions. Bounds on the structure of quartic vector boson interactions are improved by a factor of up to six compared to previous results. Upper limits are given on the production cross section times branching fraction of doubly charged Higgs bosons.

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