CMS-SMP-18-015

CMS-SMP-18-015 ; CERN-EP-2019-167
Evidence for WW production from double-parton interactions in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
13 September 2019
Eur. Phys. J. C 80 (2020) 41
Abstract: A search for WW production from double-parton scattering processes using same-charge electron-muon and dimuon events is reported, based on proton-proton collision data collected at a center-of-mass energy of 13 TeV. The analyzed data set corresponds to an integrated luminosity of 77.4 fb$^{-1}$, collected using the CMS detector at the LHC in 2016 and 2017. Multivariate classifiers are used to discriminate between the signal and the dominant background processes. A maximum likelihood fit is performed to extract the signal cross section. This leads to the first evidence for WW production via double-parton scattering, with a significance of 3.9 standard deviations. The measured inclusive cross section is 1.41 $\pm$ 0.28 (stat) $\pm$ 0.28 (syst) pb.
Links: e-print arXiv:1909.06265 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Schematic diagrams corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via the DPS process (left) and via SHS processes (middle and right), with both W bosons further decaying leptonically.
png pdf	Figure 1-a: Schematic diagram corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via the DPS process, with both W bosons further decaying leptonically.
png pdf	Figure 1-b: Schematic diagram corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via a SHS process, with both W bosons further decaying leptonically.
png pdf	Figure 1-c: Schematic diagram corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via a SHS process, with both W bosons further decaying leptonically.
png pdf	Figure 2: Distribution of the final BDT classifier output for e$\mu$ (upper) and $\mu\mu$ (lower) final states, in the positive (left) and negative (right) charge configurations. Observed data are shown in black markers while the backgrounds and signal are shown in colored histograms with their postfit yields. The SHS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ and WWW contributions are grouped in the "Rare'' background category. The bottom panels show the ratio of data to the sum of all background contributions in the black markers along with the signal shown using a red line. The band represents the postfit background uncertainty, which includes both the statistical and systematic components.
png pdf	Figure 2-a: Distribution of the final BDT classifier output for the e$\mu$ final state, in the positive charge configuration. Observed data are shown in black markers while the backgrounds and signal are shown in colored histograms with their postfit yields. The SHS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ and WWW contributions are grouped in the "Rare'' background category. The bottom panel shows the ratio of data to the sum of all background contributions in the black markers along with the signal shown using a red line. The band represents the postfit background uncertainty, which includes both the statistical and systematic components.
png pdf	Figure 2-b: Distribution of the final BDT classifier output for the e$\mu$ final state, in the negative charge configuration. Observed data are shown in black markers while the backgrounds and signal are shown in colored histograms with their postfit yields. The SHS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ and WWW contributions are grouped in the "Rare'' background category. The bottom panel shows the ratio of data to the sum of all background contributions in the black markers along with the signal shown using a red line. The band represents the postfit background uncertainty, which includes both the statistical and systematic components.
png pdf	Figure 2-c: Distribution of the final BDT classifier output for the $\mu\mu$ final state, in the positive charge configuration. Observed data are shown in black markers while the backgrounds and signal are shown in colored histograms with their postfit yields. The SHS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ and WWW contributions are grouped in the "Rare'' background category. The bottom panel shows the ratio of data to the sum of all background contributions in the black markers along with the signal shown using a red line. The band represents the postfit background uncertainty, which includes both the statistical and systematic components.
png pdf	Figure 2-d: Distribution of the final BDT classifier output for the $\mu\mu$ final state, in the negative charge configuration. Observed data are shown in black markers while the backgrounds and signal are shown in colored histograms with their postfit yields. The SHS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ and WWW contributions are grouped in the "Rare'' background category. The bottom panel shows the ratio of data to the sum of all background contributions in the black markers along with the signal shown using a red line. The band represents the postfit background uncertainty, which includes both the statistical and systematic components.
png pdf	Figure 3: Observed cross section values for inclusive DPS WW production from the two lepton charge configurations and their combination. These values are obtained from the extrapolation of the observed DPS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ cross section to the inclusive WW case. The statistical and systematic uncertainties are shown using shaded bands. The predictions from PYTHIA and from the factorization approach are represented with the red dotted and green dashed lines, respectively.

Tables
png pdf	Table 1: Event selection criteria.
png pdf	Table 2: Postfit background and signal yields and their uncertainties, and the observed event counts in the four charge and flavor combinations. The uncertainties include both statistical and systematic components. The SHS $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ and WWW contributions are grouped as the "Rare'' background.
png pdf	Table 3: Results obtained from the maximum likelihood fit to the final classifier distribution.

Summary

A study of WW production from double-parton scattering (DPS) processes in proton-proton collisions at $\sqrt{s} = $ 13 TeV has been reported. The analyzed data set corresponds to an integrated luminosity of 77.4 fb$^{-1}$, collected using the CMS detector in 2016 and 2017 at the LHC. The WW candidates are selected in same-charge electron-muon or dimuon events with moderate missing transverse momentum and low jet multiplicity. Multivariate classifiers based on boosted decision trees are used to discriminate between the signal and the dominant background processes. A maximum likelihood fit is performed to extract the signal cross section, which is compared to the predictions from simulation and from an approximate factorization approach. A measurement of the DPS WW cross section is achieved for the first time, and a cross section of 1.41 $\pm$ 0.28 (stat) $\pm$ 0.28 (syst) pb is extracted with an observed significance of 3.9 standard deviations. This cross section leads to an effective cross section parameter of $\sigma_{\text{eff}}=$ 12.7$^{+ 5.0}_{- 2.9}$ mb. This result is the first evidence for DPS WW production.

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