CMS-PAS-SMP-18-015

CMS-PAS-SMP-18-015
Evidence for WW production from double-parton interactions in proton-proton collisions at $\sqrt{s}$ = 13 TeV
CMS Collaboration
March 2019

Abstract: A study of WW production from double-parton scattering processes using same-charge dimuon and electron-muon events is carried out based on proton-proton collision data at a center-of-mass energy of 13 TeV. The analyzed data set corresponds to an integrated luminosity of 77 fb$^{-1}$, collected with the CMS detector during 2016 and 2017 at the CERN LHC. 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. The observed significance of the double-parton scattering WW process is 3.9 standard deviations, and the measured inclusive cross section is 1.41 $\pm$ 0.28 (stat) $\pm$ 0.28 (syst) pb.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, EPJC 80 (2020) 41. The superseded preliminary plots can be found here.

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 (center and right).
png pdf	Figure 1-a: Schematic diagrams corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via the DPS process (left) and via SHS processes (center and right).
png pdf	Figure 1-b: Schematic diagrams corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via the DPS process (left) and via SHS processes (center and right).
png pdf	Figure 1-c: Schematic diagrams corresponding to the production of $\mathrm{W} ^{\pm}\mathrm{W} ^{\pm}$ via the DPS process (left) and via SHS processes (center and right).
png pdf	Figure 2: Distribution of the final BDT classifier output for $\mathrm{e} \mu $ (top) and $\mu \mu $ (bottom) 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 WW and WWW contributions are grouped as the "Rare'' background. 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 shaded band on the ratio plot 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 $\mathrm{e} \mu $ (top) and $\mu \mu $ (bottom) 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 WW and WWW contributions are grouped as the "Rare'' background. 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 shaded band on the ratio plot 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 $\mathrm{e} \mu $ (top) and $\mu \mu $ (bottom) 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 WW and WWW contributions are grouped as the "Rare'' background. 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 shaded band on the ratio plot 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 $\mathrm{e} \mu $ (top) and $\mu \mu $ (bottom) 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 WW and WWW contributions are grouped as the "Rare'' background. 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 shaded band on the ratio plot 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 $\mathrm{e} \mu $ (top) and $\mu \mu $ (bottom) 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 WW and WWW contributions are grouped as the "Rare'' background. 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 shaded band on the ratio plot 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 systematical uncertainties are shown using shaded bands. The predictions from {pythia} {}8 and the factorization approach are represented using 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 uncertainties and observed event counts in the four charge and flavor combinations. The SHS WW 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 processes in proton-proton collisions at $\sqrt{s} = $ 13 TeV has been reported. The analyzed data set corresponds to an integrated luminosity of 77 fb$^{-1}$, collected with the CMS detector during 2016 and 2017 at the CERN LHC. The WW candidates are selected in events with same-charge dimuon or electron-muon pairs 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 then compared to the predictions from simulation and from an approximate factorization approach. A measurement of the double-parton scattering 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. Furthermore, the parameter $\sigma_{\mathrm{eff}}$ is extracted from this observed cross section and takes a value of 12.7$^{+5.0}_{-2.9} mb. This result presents the first experimental evidence of the DPS WW process.

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