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

TL;DR

CMS reports inclusive Wγ and Zγ cross sections in pp collisions at √s = 7 TeV using 5.0 fb⁻¹, testing SM triple gauge couplings and setting limits on anomalous couplings via pT^γ-dependent yields. The analysis employs leptonic decays, data-driven background estimation, and NLO MC predictions (MCFM) with SHERPA/MadGraph cross-checks, achieving cross sections in agreement with SM. No evidence for anomalous couplings is found; 95% CL limits are set on WWγ, ZZγ, and Zγγ vertices, with FSR and ISR contributions constrained by the radiation-amplitude-zero feature and cross-section measurements. The results strengthen SM validation at the LHC and tighten constraints on new electroweak interactions in the diboson sector.

Abstract

Measurements of W gamma and Z gamma production in proton-proton collisions at sqrt(s) = 7 TeV are used to extract limits on anomalous triple gauge couplings. The results are based on data recorded by the CMS experiment at the LHC that correspond to an integrated luminosity of 5.0 inverse femtobarns. The cross sections are measured for photon transverse momenta pt[gamma] > 15 GeV, and for separations between photons and final-state charged leptons in the pseudorapidity-azimuthal plane of Delta R[l, gamma] > 0.7 in l nu gamma and ll gamma final states, where l refers either to an electron or a muon. A dilepton invariant mass requirement of m[ll] > 50 GeV is imposed for the Z gamma process. No deviations are observed relative to predictions from the standard model, and limits are set on anomalous WW gamma, ZZ gamma, and Z gamma gamma triple gauge couplings.

Figures (15)

• Figure 1: The three lowest order diagrams for ${V}\xspace\gamma$ production, with ${V}$ corresponding to both virtual and on-shell $\gamma$, $\mathrm{W}$, and ${Z}$ bosons. The three diagrams reflect contributions from (a) initial-state and (b) final-state radiation and (c) TGC. The TGC diagram does not contribute at the lowest order to SM ${Z}\gamma$ production since photons do not couple to particles without electric charge.
• Figure 2: Efficiency of photon selection, as a function of (a) photon transverse momentum and (b) photon pseudorapidity.
• Figure 3: Ratio of efficiencies for selecting photons in data relative to MC simulation, obtained through the tag-and-probe method, and the ratio of electron to photon efficiencies, obtained at the MC generator level, with both sets of ratios given as a function of the transverse momentum of the photon.
• Figure 4: Fit to the $\sigma_{\eta\eta}$ distribution for photon candidates with $15 < p_{\mathrm{T}}\xspace^\gamma < 20\,\text{Ge\spaceV}\xspace$ in data with signal and background templates in the (a) barrel and (b) endcaps.
• Figure 5: The $R_p$ ratio (described in text) as a function of the $p_{\mathrm{T}}\xspace$ of photon candidates for the barrel region of the ECAL in $\gamma$+jets and multijet data. The difference in $R_p$ values for the two processes is attributed to the fact that jets in $\gamma$+jets events are dominated by quark fragmentation, while jets in multijet events are dominated by gluon fragmentation.