CMS-FSQ-16-005 ; CERN-EP-2017-310 | ||
Constraints on the double-parton scattering cross section from same-sign W boson pair production in proton-proton collisions at $ \sqrt{s} = $ 8 TeV | ||
CMS Collaboration | ||
6 December 2017 | ||
JHEP 02 (2018) 032 | ||
Abstract: A first search for same-sign WW production via double-parton scattering is performed based on proton-proton collision data at a center-of-mass energy of 8 TeV using dimuon and electron-muon final states. The search is based on the analysis of data corresponding to an integrated luminosity of 19.7 fb$^{-1}$. No significant excess of events is observed above the expected single-parton scattering yields. A 95% confidence level upper limit of 0.32 pb is set on the inclusive cross section for same-sign WW production via the double-parton scattering process. This upper limit is used to place a 95% confidence level lower limit of 12.2 mb on the effective double-parton cross section parameter, closely related to the transverse distribution of partons in the proton. This limit on the effective cross section is consistent with previous measurements as well as with Monte Carlo event generator predictions. | ||
Links: e-print arXiv:1712.02280 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Schematic diagrams corresponding to the production of a same-sign W boson pair via the DPS process (left) and via SPS processes (right). |
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Figure 1-a:
Schematic diagram corresponding to the production of a same-sign W boson pair via the DPS process. |
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Figure 1-b:
Schematic diagrams corresponding to the production of a same-sign W boson pair via SPS processes. |
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Figure 2:
Distributions of the $p_{\mathrm {T}_{2}}$ (top-left), $m_\mathrm {T}(\mu _{2} , {{p_{\mathrm {T}}} ^\text {miss}})$ (top-right), $\Delta \phi (\vec{p}_{\mathrm {T}_{1}}, \vec{p}_{\mathrm {T}_{2}})$ (bottom-left), and $\Delta \phi (\vec{p}_{\mathrm {T}_{2}}, {\vec{p}_{\mathrm {T}}^{\text {miss}}})$ (bottom-right) variables for the dimuon channel, after the same-sign WW selection criteria have been applied. The data are represented by the black dots and the shaded histograms represent the predicted signal and background processes normalized according to the estimated cross sections and the luminosity. For each individual distribution, the bottom panels show the ratio of the number of events observed in the data to that predicted by the simulation, along with the associated statistical uncertainty. The hatched bands in all cases represent the sum of the systematic and statistical uncertainties of the simulated samples, added in quadrature. |
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Figure 2-a:
Distribution of the $p_{\mathrm {T}_{2}}$ variable for the dimuon channel, after the same-sign WW selection criteria have been applied. The data are represented by the black dots and the shaded histograms represent the predicted signal and background processes normalized according to the estimated cross sections and the luminosity. The bottom panel shows the ratio of the number of events observed in the data to that predicted by the simulation, along with the associated statistical uncertainty. The hatched bands in all cases represent the sum of the systematic and statistical uncertainties of the simulated samples, added in quadrature. |
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Figure 2-b:
Distribution of the $m_\mathrm {T}(\mu _{2} , {{p_{\mathrm {T}}} ^\text {miss}})$ variable for the dimuon channel, after the same-sign WW selection criteria have been applied. The data are represented by the black dots and the shaded histograms represent the predicted signal and background processes normalized according to the estimated cross sections and the luminosity. The bottom panel shows the ratio of the number of events observed in the data to that predicted by the simulation, along with the associated statistical uncertainty. The hatched bands in all cases represent the sum of the systematic and statistical uncertainties of the simulated samples, added in quadrature. |
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Figure 2-c:
Distribution of the $\Delta \phi (\vec{p}_{\mathrm {T}_{1}}, \vec{p}_{\mathrm {T}_{2}})$ variable for the dimuon channel, after the same-sign WW selection criteria have been applied. The data are represented by the black dots and the shaded histograms represent the predicted signal and background processes normalized according to the estimated cross sections and the luminosity. The bottom panel shows the ratio of the number of events observed in the data to that predicted by the simulation, along with the associated statistical uncertainty. The hatched bands in all cases represent the sum of the systematic and statistical uncertainties of the simulated samples, added in quadrature. |
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Figure 2-d:
Distribution of the $\Delta \phi (\vec{p}_{\mathrm {T}_{2}}, {\vec{p}_{\mathrm {T}}^{\text {miss}}})$ variable for the dimuon channel, after the same-sign WW selection criteria have been applied. The data are represented by the black dots and the shaded histograms represent the predicted signal and background processes normalized according to the estimated cross sections and the luminosity. The bottom panel shows the ratio of the number of events observed in the data to that predicted by the simulation, along with the associated statistical uncertainty. The hatched bands in all cases represent the sum of the systematic and statistical uncertainties of the simulated samples, added in quadrature. |
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Figure 3:
Distributions of the $p_{\mathrm {T}_{1}}$ (top-left), $p_{\mathrm {T}_{2}}$ (top-right), $\Delta \phi (\vec{p}_{\mathrm {T}_{2}}, {\vec{p}_{\mathrm {T}}^{\text {miss}}})$ (bottom-left), and $\Delta \phi (\vec{p}_{\mathrm {T}_{12}}, {\vec{p}_{\mathrm {T}}^{\text {miss}}})$ (bottom-right) variables for the electron-muon channel, after the same-sign WW selection criteria have been applied. Symbols and patterns are the same as in Fig. 2. |
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Figure 3-a:
Distribution of the $p_{\mathrm {T}_{1}}$ variable for the electron-muon channel, after the same-sign WW selection criteria have been applied. Symbols and patterns are the same as in Fig. 2. |
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Figure 3-b:
Distribution of the $p_{\mathrm {T}_{2}}$ variable for the electron-muon channel, after the same-sign WW selection criteria have been applied. Symbols and patterns are the same as in Fig. 2. |
png pdf |
Figure 3-c:
Distribution of the $\Delta \phi (\vec{p}_{\mathrm {T}_{2}}, {\vec{p}_{\mathrm {T}}^{\text {miss}}})$ variable for the electron-muon channel, after the same-sign WW selection criteria have been applied. Symbols and patterns are the same as in Fig. 2. |
png pdf |
Figure 3-d:
Distribution of the $\Delta \phi (\vec{p}_{\mathrm {T}_{12}}, {\vec{p}_{\mathrm {T}}^{\text {miss}}})$ variable for the electron-muon channel, after the same-sign WW selection criteria have been applied. Symbols and patterns are the same as in Fig. 2. |
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Figure 4:
Distribution of the BDT discriminant, for the dimuon channel (left) and for the electron-muon channel (right). The data are represented by the black dots and the shaded histograms represent the pre-fit signal and post-fit background processes. The bottom panels show the ratio of data to the sum of all signal and background contributions. The hatched bands represent the post-fit uncertainty, which includes both the statistical and systematic components. |
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Figure 4-a:
Distribution of the BDT discriminant, for the dimuon channel. The data are represented by the black dots and the shaded histograms represent the pre-fit signal and post-fit background processes. The bottom panel shows the ratio of data to the sum of all signal and background contributions. The hatched bands represent the post-fit uncertainty, which includes both the statistical and systematic components. |
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Figure 4-b:
Distribution of the BDT discriminant, for the electron-muon channel. The data are represented by the black dots and the shaded histograms represent the pre-fit signal and post-fit background processes. The bottom panel shows the ratio of data to the sum of all signal and background contributions. The hatched bands represent the post-fit uncertainty, which includes both the statistical and systematic components. |
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Figure 5:
Expected and observed 95% CL upper limits on the same-sign $\sigma ^\mathrm {DPS}_{\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}}$ for the dimuon and electron-muon final states, along with their combination. The predicted values of $\sigma ^\mathrm {DPS}_{\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}}$ from PYTHIA 8 and from the factorization approach [21] are also shown. |
Tables | |
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Table 1:
Event selection criteria for same-sign W boson pair production in dimuon and electron-muon channels. |
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Table 2:
Control regions enriched with misidentified leptons used to extract the lepton misidentification rate. Region 1 is used for the dimuon channel. Region 2, with the additional requirement of least one b-tagged jet, is used in the electron-muon channel to reduce semileptonically decaying $ {\mathrm{t} {}\mathrm{\bar{t}}} $ events. |
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Table 3:
Expected and observed 95% CL limits on the cross section for inclusive same-sign WW production via DPS for the dimuon and electron-muon channels along with their combination. |
Summary |
A first search for same-sign W boson pair production via double-parton scattering (DPS) in pp collisions at a center-of-mass energy of 8 TeV has been presented. The analyzed data were collected by the CMS detector at the LHC during 2012 and correspond to an integrated luminosity of 19.7 fb$^{-1}$. The results presented here are based on the analysis of events containing two same-sign W bosons decaying into either same-sign muon-muon or electron-muon pairs. Several kinematic observables have been studied to identify those that can better discriminate between DPS and the single-parton scattering (SPS) backgrounds. These observables with discriminating power are used as an input to a multivariate analysis based on boosted decision trees. No excess over the expected contributions from SPS processes is observed. A 95% confidence level (CL) upper limit of 0.32 pb is placed on the inclusive cross section for same-sign WW production via DPS. A corresponding 95% CL lower limit of 12.2 mb on the effective double-parton cross section is also derived, compatible with previous measurements as well as with Monte Carlo event generator expectations. |
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Compact Muon Solenoid LHC, CERN |