Measurement of Wgamma and Zgamma production in proton-proton collisions at sqrt(s)=7 TeV with the ATLAS Detector

TL;DR

The paper reports a first ATLAS measurement of Wγ and Zγ production in 7 TeV pp collisions using ~35 pb⁻¹, detailing event selection with high-pT leptons and photons and robust background data-driven estimates. It combines detailed efficiency calibrations (triggers, lepton/photon ID, isolation) with NLO SM predictions to extract fiducial and production cross sections, and evaluates the Wγ/Zγ ratio as a sensitive probe of triple gauge couplings. The results are consistent with Standard Model expectations at O(α αs), providing important validation of electroweak processes and constraining potential new physics in gauge interactions at the LHC energy frontier.

Abstract

We present studies of W and Z bosons with associated high energy photons produced in pp collisions at sqrt(s)=7 TeV. The analysis uses 35 pb-1 of data collected by the ATLAS experiment in 2010. The event selection requires W and Z bosons decaying into high pT leptons (electrons or muons) and a photon with ET>15 GeV separated from the lepton(s) by a distance Delta_R(l,gamma)>0.7 in eta-phi space. A total of 95 (97) pp->e nu gamma + X (pp->mu nu gamma + X) and 25 (23) pp->e+ e- gamma + X (pp->mu+ mu- gamma + X) event candidates are selected. The kinematic distributions of the leptons and photons and the production cross sections are measured. The data are found to agree with Standard Model predictions that include next-to-leading-order O(alpha alpha_s) contributions.

Figures (8)

• Figure 1: Feynman diagrams of $W\gamma$ and $Z\gamma$ production in (a) u-channel (b) t-channel and (c) final state photon radiation (FSR) from the $W$ and $Z$ boson decay process. (d) Feynman diagram of $W\gamma$ production in the s-channel.
• Figure 2: Diagrams of the signal contributions from the $W+q(g)$ processes when a photon emerges from the fragmentation of the final state parton.
• Figure 3: Distributions for the combined electron and muon decay channels of the photon transverse energy (a), $\Delta R$ between lepton and photon (b), two body transverse mass ($m_{\mathrm{T}}(l,\nu)$) (c) and three body transverse mass ($m_{\mathrm{T}}(l,\nu,\gamma)$) (d) of the $W\gamma$ candidate events. MC predictions for signal and backgrounds are also shown.
• Figure 4: (a) Three body invariant mass $m_{l^{+}l^{-}\gamma}$ distribution for $Z\gamma$ data candidate events. MC predictions for signal and backgrounds are also shown. (b) Two-dimensional plots of $m_{l^{+}l^{-}\gamma}$ vs $m_{l^{+}l^{-}}$ for $Z\gamma$ data candidate events. The MC signal prediction is also shown. Both the electron and muon decay channels are included.
• Figure 5: Sketch of the two-dimensional plane defining the 4 regions used in the sideband method. Region A is the signal region. The non-isolated control regions (B and D) are defined for photons with $E^{\mathrm{iso}}_{\mathrm{T}}>6$Ge V. The "low quality photon identification" control regions (C and D) include photons passing all the identification criteria except the strip layer discriminating variable requirements (see Section \ref{['sec:reco']}).