CMS-PAS-SMP-24-019

CMS-PAS-SMP-24-019
Measurement of the photon-fusion production cross section of a pair of W bosons
CMS Collaboration
2025-07-18

Abstract: A measurement of the cross section of W boson pair production in photon fusion ($ \gamma\gamma\rightarrow\mathrm{WW} $) is presented, using an integrated luminosity of 138 fb$ ^{-1} $ of proton-proton collision data collected in 2016-2018 at a center-of-mass energy of $ \sqrt{s}= $ 13 TeV. Events are selected in the final state with one electron, one muon, and a low multiplicity of tracks associated with the electron-muon production vertex. The total cross section of the $ \gamma\gamma\rightarrow\mathrm{WW} $ production is measured to be 659$ ^{+82}_{-78} $ fb, and a fiducial cross section of 4.1 $ \pm $ 0.5 fb is measured in a phase space defined with selection criteria close to those of the analysis. In addition, constraints on dimension-8 operators from an effective field theory framework are set.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Normalized simulated $ N_\text{tracks} $ (left) and acoplanarity (right) distributions for photon- and quark-induced processes. The acoplanarity distribution is shown for events with $ N_\text{tracks}= $ 0. The VV$ \to2\ell2\nu $ process includes vector boson pair production with two leptons (electrons, muons, or $ \tau $ leptons) and two neutrinos in the final state.
png pdf	Figure 1-a: Normalized simulated $ N_\text{tracks} $ (left) and acoplanarity (right) distributions for photon- and quark-induced processes. The acoplanarity distribution is shown for events with $ N_\text{tracks}= $ 0. The VV$ \to2\ell2\nu $ process includes vector boson pair production with two leptons (electrons, muons, or $ \tau $ leptons) and two neutrinos in the final state.
png pdf	Figure 1-b: Normalized simulated $ N_\text{tracks} $ (left) and acoplanarity (right) distributions for photon- and quark-induced processes. The acoplanarity distribution is shown for events with $ N_\text{tracks}= $ 0. The VV$ \to2\ell2\nu $ process includes vector boson pair production with two leptons (electrons, muons, or $ \tau $ leptons) and two neutrinos in the final state.
png pdf	Figure 2: Observed and predicted $ p_{\mathrm{T}}^{\mathrm{e}\mu} $ distributions for events with $ N_\text{tracks}= $ 0 (left) or 1 $ \leq N_\text{tracks}\leq $ 5, using the data from the full Run 2 data set. The distributions are shown after the maximum likelihood fit to the data ("postfit distributions"). The last bin includes the overflow.
png pdf	Figure 2-a: Observed and predicted $ p_{\mathrm{T}}^{\mathrm{e}\mu} $ distributions for events with $ N_\text{tracks}= $ 0 (left) or 1 $ \leq N_\text{tracks}\leq $ 5, using the data from the full Run 2 data set. The distributions are shown after the maximum likelihood fit to the data ("postfit distributions"). The last bin includes the overflow.
png pdf	Figure 2-b: Observed and predicted $ p_{\mathrm{T}}^{\mathrm{e}\mu} $ distributions for events with $ N_\text{tracks}= $ 0 (left) or 1 $ \leq N_\text{tracks}\leq $ 5, using the data from the full Run 2 data set. The distributions are shown after the maximum likelihood fit to the data ("postfit distributions"). The last bin includes the overflow.
png pdf	Figure 3: Distributions of $ p_{\mathrm{T}}^{\mathrm{e}\mu} $ for the difference between the observed events and the backgrounds (black markers), SM prefit $ {\gamma\gamma\to\mathrm{W}\mathrm{W}} $ prediction (red line), and $ {\gamma\gamma\to\mathrm{W}\mathrm{W}} $ predictions for two benchmark points with nonzero EFT coefficients (blue and orange lines). The difference between the predictions for the SM and nonzero EFT coefficients is visible only in the overflow bin. The background uncertainty is shown with a gray band.
png pdf	Figure 4: Summary of observed limits on $ f_\textrm{M} $ Wilson coefficients for this analysis and previous results by the CMS Collaboration. The limits are arranged from most sensitive to least sensitive in descending order, with the current result highlighted in red. In some cases, limits on the Wilson coefficients are shown under the assumption of unitarity bounds, as indicated in the right-most column.
png pdf	Figure 5: Summary of observed limits on $ f_\textrm{T} $ Wilson coefficients for this analysis and previous results by the CMS Collaboration. The limits are arranged from most sensitive to least sensitive in descending order, with the current result highlighted in red. In some cases, limits on the Wilson coefficients are shown under the assumption of unitarity bounds, as indicated in the right-most column.

Tables
png pdf	Table 1: Selection criteria in the $ \mathrm{e}\mu $ SR and CR, and in the $ \mu\mu $ final state. The $ p_{\mathrm{T}} $ and pseudorapidity ranges correspond to different sets of triggers in different data-taking periods. The leading (subleading) lepton in the $ \mathrm{e}\mu $ SR and CR must have $ p_{\mathrm{T}} > $ 24 (15) GeV.
png pdf	Table 2: Systematic uncertainties and their relative effects in the inclusive cross section measurement. An uncertainty of 2% in the signal cross section is considered in the EFT interpretation.
png pdf	Table 3: Selection criteria of the fiducial region. The two $ p_{\mathrm{T}} $ values for the electron and muon correspond to the requirements for the leading and subleading leptons.
png pdf	Table 4: Observed and predicted event yields in the SR and CR. The signal and background yields are the result of the global fit including all sources of uncertainties.
png pdf	Table 5: 95% CL intervals on EFT operators varied one at a time while fixing the other ones to zero.

Summary

This note describes a measurement of the $ {\gamma\gamma\to\mathrm{W}\mathrm{W}} $ production cross section in proton-proton collisions at a center-of-mass energy of 13 TeV. The signal is selected in the $ \mathrm{e}\mu $ final state using exclusivity criteria related to the track multiplicity associated with the dilepton vertex. Using 138 fb$ ^{-1} $ of proton-proton data collected by the CMS detector during the Run 2 of the LHC, the inclusive cross section of the $ {\gamma\gamma\to\mathrm{W}\mathrm{W}} $ process is measured to be 659$ ^{+82}_{-78} $ fb, and the fiducial cross section is 4.1 $ \pm $ 0.5 fb in a phase space close to that of the analysis. Stringent constraints are also set on dimension-8 operators in an effective field theory approach.

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