Measurement of W gamma and Z gamma production cross sections in pp collisions at sqrt(s) = 7 TeV and limits on anomalous triple gauge couplings with the ATLAS detector

TL;DR

This ATLAS study tests the Standard Model electroweak sector by measuring Wγ and Zγ production in pp collisions at 7 TeV using 1.02 fb^-1 of data, analyzing leptonic decays with isolated photons across inclusive and exclusive samples. It combines data-driven background estimation with MC simulations to extract fiducial and extended-fiducial cross sections, compares them to NLO SM predictions, and demonstrates good agreement for exclusive selections while highlighting jet-multiplicity effects in inclusive high-ETγ events. The analysis further constrains anomalous triple gauge couplings (WWγ and ZVγ) via high-ETγ photon spectra, employing a form-factor approach and Bayesian limits across multiple Λ scenarios, finding no evidence of new physics. Collectively, the results reinforce SM predictions, quantify uncertainties in diboson production at LHC energies, and provide stringent limits on possible new physics in the gauge-boson sector.

Abstract

This Letter presents measurements of l nu gamma and l^+ l^- gamma (l=e, mu) production in 1.02 fb^-1 of pp collision data recorded at sqrt(s) = 7 TeV with the ATLAS detector at the LHC during 2011. Events dominated by W gamma and Z gamma production with leptonic decays of the W and Z bosons are selected, and their production cross sections and kinematic properties are measured in several ranges of the photon transverse energy. The results are compared to Standard Model predictions and are used to determine limits on anomalous W-W-gamma and Z-Z-gamma/Z-gamma-gamma couplings.

Figures (4)

• Figure 1: Distributions of the photon transverse energy for the combined electron and muon decay channels in (a) $W\gamma$ candidate events and (b) $Z\gamma$ candidate events, with no requirements on the recoil system. The selection criteria are defined in Section \ref{['sec:Event_Selection']}. The distributions for the expected signals are taken from the MC simulation and normalised to the extracted number of signal events shown in Table \ref{['tab:wgbg']} and Table \ref{['tab:zgbgsummary']}. The ratio between the number of candidates observed in the data and the number of expected candidates from the signal MC simulation and from the background processes is also shown.
• Figure 2: Distributions of the jet multiplicity for the combined electron and muon decay channels in (a) $W\gamma$ candidate events with $E_{\mathrm{T}}^\gamma>15$Ge V, (b) $W\gamma$ candidate events with $E_{\mathrm{T}}^\gamma>60$Ge V, (c) $W\gamma$ candidate events with $E_{\mathrm{T}}^\gamma>100$Ge V, (d) $Z\gamma$ candidate events with $E_{\mathrm{T}}^\gamma>15$Ge V, and (e) $Z\gamma$ candidate events with $E_{\mathrm{T}}^\gamma>60$Ge V. The selection criteria are defined in Section \ref{['sec:Event_Selection']}. Distributions for expected signal contribution are taken from signal MC simulation and normalized to the extracted number of signal events as shown in Table \ref{['tab:wgbg']} and Table \ref{['tab:zgbgsummary']}. The ratio between the number of candidates observed in the data and the number of expected candidates from the signal MC simulation and from the background processes is also shown.
• Figure 3: The measured cross section for (a) $W\gamma$ production, (b) $Z\gamma$ production as a function of the photon transverse energy, in the extended fiducial region as defined in Table \ref{['tab:fiducialcut']}, together with the SM model prediction. The lower plots show the ratio between the data and the prediction of the MCFM generator.
• Figure 4: The 95% CL intervals for anomalous couplings from ATLAS, D0 D0paper2, CDF CDFpaper, CMS CMSpaper and LEP LEP for (a),(b) the neutral aTGCs $h_{3}^{\gamma}$, $h_{3}^{Z}$, $h_{4}^{\gamma}$, $h_{4}^{Z}$ as obtained from $Z\gamma$ events, and (c) the charged aTGCs $\Delta\kappa_{\gamma}$, $\lambda_{\gamma}$. Integrated luminosities and new physics scale parameter $\Lambda$ are shown. The ATLAS, CMS and Tevatron results for the charged aTGCs are measured from $W\gamma$ production. The LEP charged aTGC results are obtained from $WW$ production, which is sensitive also to the $WWZ$ couplings and hence required some assumptions about the relations between the $WW\gamma$ and $WWZ$ aTGCs LEPLEP1LEP2LEP3. The sensitivity of the LEP data to neutral aTGCs is much smaller than that of the hadron colliders; therefore the LEP results have not been included in (a) and (b).