CMS-PAS-SMP-19-002

CMS-PAS-SMP-19-002
Measurement of the inclusive W $\gamma$ production cross section in proton-proton collisions at $\sqrt{s} =$ 13 TeV and constraints on effective field theory coefficients
CMS Collaboration
November 2020

Abstract: The cross section of inclusive W $\gamma$ production in proton-proton collisions at a center-of-mass energy of 13 TeV has been measured based on 137.1 fb $^{-1}$ of data collected using the CMS experiment at the LHC. Both the electron and the muon decay modes of the W boson are considered by performing a maximum likelihood fit to the lepton-photon mass distribution. The measured cross section is compared to theoretical predictions made at next-to-leading order in quantum chromodynamics. In addition, 95% confidence level limits on anomalous triple gauge couplings are reported within the framework of effective field theory.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Accepted by PRL. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagram for ${\mathrm{p}} {\mathrm{p}} \to \ell ^+\nu _{\ell}\gamma$ production with a TGC vertex.
png pdf	Figure 2: Distributions of the invariant mass of the lepton-photon system in the muon channel (left) and electron channel (right). The electron-induced photon estimate is scaled by a factor of 1.8 in the plot of $m_{\ell \gamma}$ . The uncertainty in the expected yield is the sum of the systematic uncertainties added in quadrature. W $\gamma$ refers to the MadGraph-5_aMC@NLO simulation of W $\gamma$ .
png pdf	Figure 2-a: Distribution of the invariant mass of the lepton-photon system in the muon channel. The electron-induced photon estimate is scaled by a factor of 1.8 in the plot of $m_{\ell \gamma}$ . The uncertainty in the expected yield is the sum of the systematic uncertainties added in quadrature. W $\gamma$ refers to the MadGraph-5_aMC@NLO simulation of W $\gamma$ .
png pdf	Figure 2-b: Distribution of the invariant mass of the lepton-photon system in the electron channel. The electron-induced photon estimate is scaled by a factor of 1.8 in the plot of $m_{\ell \gamma}$ . The uncertainty in the expected yield is the sum of the systematic uncertainties added in quadrature. W $\gamma$ refers to the MadGraph-5_aMC@NLO simulation of W $\gamma$ .
png pdf	Figure 3: The photon ${p_{\mathrm {T}}}$ distribution used for the extraction of limits on dimension-six EFT operators. The last bin includes overflow.

Tables
png pdf	Table 1: Expected and observed number of events. The electron-induced photon estimate is scaled by a factor of $1.8$ . The uncertainty is the sum of the systematic uncertainties added in quadrature. W $\gamma$ refers to the MadGraph-5_aMC@NLO simulation of W $\gamma$ .
png pdf	Table 2: Observed and expected 95% confidence level limits on five dimension-six operator coefficients. The units of the limits are TeV $^{-2}$ .

Summary

In conclusion, the inclusive cross section for

$\mathrm{ pp \to W \gamma }$ production has been measured for the first time at a center-of-mass energy of 13 TeV and with a precision of 4.7%. The measured cross section is

$\sigma =$ 15.58

$\pm$ 0.75 pb

$=$ 15.58

$\pm$ 0.05 (stat)

$\pm$ 0.73 (exp)

$\pm$ 0.15 (theory) pb, which is consistent with the MadGraph-5_aMC@NLO in QCD prediction of

$\sigma =$ 15.4

$\pm$ 1.2 (scale)

$\pm$ 0.1 (PDF) pb and below the POWHEG NLO in QCD with NLO competition prediction of

$\sigma =$ 22.4

$\pm$ 3.2 (scale)

$\pm$ 0.1 (PDF) pb. The cross sections separately measured in the electron channel and the muon channel are consistent with each other. The high photon

$p_{\mathrm{T}}$ part of the photon

$p_{\mathrm{T}}$ distribution was used to set 95% confidence level limits on dimension-six EFT parameters, including the most stringent limit to date on the coefficient of the lowest dimension CP-even operator that directly alters the WW

$\gamma$ TGC.

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