Measurements of Wgamma and Zgamma production in pp collisions at sqrt{s}= 7 TeV with the ATLAS detector at the LHC

TL;DR

ATLAS performs a comprehensive study of Wγ and Zγ production in pp collisions at 7 TeV with 4.6 fb⁻¹, delivering integrated and differential fiducial cross sections unfolded to particle level. The analysis combines precise object reconstruction, data-driven background control, and unfolding to compare to NLO SM predictions (via MCFM) and LO/SHERPA/ALPGEN generators, highlighting good agreement for Zγ/νν̄γ and tensions in inclusive Wγ at high ET due to missing higher-multiplicity QCD contributions. Additionally, the paper constrains anomalous WWγ, ZZγ, and Zγγ couplings and searches for narrow technicolor resonances, setting the first Wγ resonance limits and competitive Zγ limits, with no evidence of new physics observed. Overall, the work strengthens SM tests in Vγ production at the LHC and provides stringent new bounds on beyond-Standard-Model scenarios with high-energy photons.

Abstract

The integrated and differential fiducial cross sections for the production of a W or Z boson in association with a high-energy photon are measured using pp collisions at sqrt{s} = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6 fb^{-1} collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the W and Z bosons (W(e nu,mu nu) and Z(e+ e-, mu+ mu-, nu nubar)) with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWgamma, ZZgamma and Zgammagamma triple-gauge-boson couplings and to search for the production of vector resonances decaying to Zgamma and Wgamma. No deviations from Standard Model predictions are observed and limits are placed on anomalous triple-gauge-boson couplings and on the production of new vector meson resonances.

Figures (13)

• 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. Diagrams of the signal contributions from the $W+q(g)$ processes when a photon emerges from the fragmentation of (e) a gluon and (f) a quark in the final state.
• Figure 2: Sketch of the two-dimensional plane defining the four regions used in the sideband method. Region A is the signal region. The nonisolated control regions (B and D) are defined for photons with $E^{\mathrm{iso}}_{\mathrm{T}}>7$Ge V. The "low quality photon identification" control regions (C and D) include photon candidates that fail the full photon shower-shape selection criteria, but pass a subset of them. For the data driven $W+$jets background estimation to the inclusive $W\gamma$ measurement, about 1000 $W+$jets candidates are selected in the nonisolated control regions, and about 2000 $W+$jets candidates are selected in the "low quality photon identification" control regions.
• Figure 3: Combined distributions for $\ell\nu\gamma$ candidate events in the electron and muon channels of (a) the photon transverse energy, (b) the missing transverse energy, (c) the jet multiplicity, and (d) the three-body transverse mass distribution as defined in Eq. (\ref{['equ:MT3']}). The selection criteria are defined in Sec. \ref{['sec:Event_Selection']}, in particular, the photon transverse energy is required to be $E_{\mathrm{T}}^{\gamma}>15$Ge V, except for panel (d) where it is required to be $E_{\mathrm{T}}^{\gamma}>40$Ge V. The distributions for the expected signals are taken from the ALPGEN MC simulation and scaled by a global factor ($\sim 1.5$) such that the total contribution from the predicted signal and background is precisely normalized to the data. The ratio of the number of candidates observed in the data to the number of expected candidates from signal and background processes is also shown. Only the statistical uncertainties on the data are shown for these ratios. As the expected signal is normalized to match the extracted number of signal events, the ratio provides a comparison only between the observed and predicted shapes of the distributions. The histograms are normalized by their bin width.
• Figure 4: Distribution for $\ell^{+}\ell^{-}\gamma$ candidate events combining the electron and muon channels of (a) the photon transverse energy, (b) the jet multiplicity, and (c) the three-body mass distribution. The selection criteria are defined in Sec. \ref{['sec:Event_Selection']}, in particular, the photon transverse energy is required to be $E_{\mathrm{T}}^{\gamma}>15$Ge V, except for panel (c) where it is required to be $E_{\mathrm{T}}^{\gamma}>40$Ge V. The distributions for the expected signals are taken from the SHERPA MC simulation and scaled by a global factor ($\sim 1.0$) such that the total contribution from the predicted signal and background is precisely normalized to the data. The ratio of the number of candidates observed in the data to the number of expected candidates from signal and background processes is also shown. Only the statistical uncertainties on the data are shown for these ratios. The histograms are normalized by their bin width.
• Figure 5: Distributions of inclusive $\nu\bar{\nu}\gamma$ candidate events of (a) the photon transverse energy, (b) the missing transverse energy $E_{\mathrm{T}}^{\mathrm{miss}}$, and (c) the jet multiplicity. The selection criteria are defined in Sec. \ref{['sec:Event_Selection']}. The distributions for the expected signals are taken from the SHERPA MC simulation and scaled by a global factor ($\sim 1.0$) such that the total contribution from the predicted signal and background is precisely normalized to the data. The ratio of the number of candidates observed in the data to the number of expected candidates from signal and background processes is also shown. Only the statistical uncertainties on the data are shown for these ratios. The histograms are normalized by their bin width.