CMS-PAS-SMP-17-004

CMS-PAS-SMP-17-004
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 13 TeV
CMS Collaboration
May 2017

Abstract: The observation of electroweak production of same-sign W boson pairs in proton-proton collisions at 13 TeV is reported. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected with the CMS detector. Events are selected by requiring exactly two leptons of the same charge, moderate missing momentum, and two jets with large rapidity separation and large dijet mass. The observed significance is 5.5 standard deviations, where a significance of 5.7 standard deviations is expected based on the standard model. A cross section measurement in a fiducial region is reported. Bounds on the structure of quartic vector-boson interactions are given in the framework of dimension-eight effective field theory operators, together with upper limits on the production of doubly charged Higgs bosons.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 120 (2018) 081801. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagrams for the electroweak and QCD induced same-sign W boson pair production.
png pdf	Figure 1-a: Representative Feynman diagram for the electroweak induced same-sign W boson pair production.
png pdf	Figure 1-b: Representative Feynman diagram for the electroweak induced same-sign W boson pair production.
png pdf	Figure 1-c: Representative Feynman diagram for the electroweak induced same-sign W boson pair production.
png pdf	Figure 1-d: Representative Feynman diagram for the QCD induced same-sign W boson pair production.
png pdf	Figure 2: Distributions of $m_{jj}$ (left) and $ {m_{ {\ell } {\ell }}}$ (right) in the signal region. The normalization of the predicted signal and background distributions corresponds to the result of the fit. The hatched bars include statistical and systematic uncertainties. For illustration, the doubly charged Higgs boson signal normalized to a cross section of 0.1 pb (left) and the distribution with aQGCs are shown. The histograms for other backgrounds include the contributions from QCD WW, W$ \gamma $, wrong-sign events, DPS, and VVV processes.
png pdf	Figure 2-a: Distribution of $m_{jj}$ in the signal region. The normalization of the predicted signal and background distributions corresponds to the result of the fit. The hatched bars include statistical and systematic uncertainties. For illustration, doubly charged Higgs boson signals normalized to a cross section of 0.1 pb are shown. The histograms for other backgrounds include the contributions from QCD WW, W$ \gamma $, wrong-sign events, DPS, and VVV processes.
png pdf	Figure 2-b: Distribution of $ {m_{ {\ell } {\ell }}}$ in the signal region. The normalization of the predicted signal and background distributions corresponds to the result of the fit. The hatched bars include statistical and systematic uncertainties. For illustration, distributions with aQGCs are shown. The histograms for other backgrounds include the contributions from QCD WW, W$ \gamma $, wrong-sign events, DPS, and VVV processes.
png pdf	Figure 3: Expected and observed 95% CL upper limits on the cross section times branching fraction, $\sigma _{VBF}( {\mathrm {H}}^{\pm \pm }) \times \mathrm {{\mathcal{B}}}( {\mathrm {H}}^{\pm \pm } \to \mathrm{ W } ^{\pm }\mathrm{ W } ^{\pm })$ (left) and on $s_H$ in the Georgi-Machacek model (right) as a function of doubly charged Higgs boson mass. The blue area covers the theoretically not allowed parameter space [32].
png pdf	Figure 3-a: Expected and observed 95% CL upper limits on the cross section times branching fraction, $\sigma _{VBF}( {\mathrm {H}}^{\pm \pm }) \times \mathrm {{\mathcal{B}}}( {\mathrm {H}}^{\pm \pm } \to \mathrm{ W } ^{\pm }\mathrm{ W } ^{\pm })$ as a function of doubly charged Higgs boson mass.
png pdf	Figure 3-b: Expected and observed 95% CL upper limits on $s_H$ in the Georgi-Machacek model as a function of doubly charged Higgs boson mass. The blue area covers the theoretically not allowed parameter space [32].

Tables
png pdf	Table 1: Signal and background yields after the full selection. Only statistical uncertainties are reported. Background processes contributing to less then 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 BSM higher order (dimension-eight) operators in the EFT Lagrangian. The last column is summarizing the LHC Run-I observed limits obtained by CMS.

Summary

In summary, we present a first observation of electroweak production of same-sign W boson pairs in proton-proton collisions at 13 TeV. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected with the CMS detector. Events are selected by requiring exactly two leptons of the same charge, moderate missing transverse energy, and two jets with large rapidity separation and large dijet mass. The two dominating sources of background events after the event selection are non-prompt leptons and the WZ $ \to 3\ell\nu$. The event yield of the signal process is extracted using a 2-dimensional fit of $m_{jj}$ and $m_{\ell\ell}$ variables. The observed significance is 5.5 standard deviations, where a significance of 5.7 standard deviations is expected based on the standard model. A cross section measurement in a fiducial region is reported. No evidence for anomalous quartic gauge couplings is observed, and stringent bounds on the structure of quartic vector-boson interactions are given in the framework of dimension-eight effective field theory operators, together with upper limits on the production of doubly charged Higgs bosons.

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