Measurements of $W\gamma$ and $Z\gamma$ production in $pp$ collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$ with the ATLAS detector at the LHC

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 $\sqrt{s}=7\mathrm{TeV}$. The analyses use a data sample with an integrated luminosity of $4.6{\mathrm{fb}}^{-1}$ 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 $Z(e^{+}{e}^{-},{\mu }^{+}{\mu }^{-},\nu \overline{\nu })$] 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 $WW\gamma$, $ZZ\gamma$, and $Z\gamma \gamma$ triple-gauge-boson couplings and to search for the production of vector resonances decaying to $Z\gamma$ and $W\gamma$. 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.

Figure 1

Feynman diagrams of $W\gamma$ and $Z\gamma$ production in (a) $u$-channel (b) $t$-channel and (c) final-state photon radiation 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 $E_{\mathrm{T}}^{\mathrm{iso}}>7\mathrm{GeV}$. 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 $W+\mathrm{\text{jets}}$ background estimation to the inclusive $W\gamma$ measurement, about 1000 $W+\mathrm{\text{jets}}$ candidates are selected in the nonisolated control regions, and about 2000 $W+\mathrm{\text{jets}}$ 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. (6). The selection criteria are defined in Sec. 5, in particular, the photon transverse energy is required to be $E_{\mathrm{T}}^{\gamma }>15\mathrm{GeV}$, except for panel (d) where it is required to be $E_{\mathrm{T}}^{\gamma }>40\mathrm{GeV}$. 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. 5, in particular, the photon transverse energy is required to be $E_{\mathrm{T}}^{\gamma }>15\mathrm{GeV}$, except for panel (c) where it is required to be $E_{\mathrm{T}}^{\gamma }>40\mathrm{GeV}$. The distributions for the expected signals are taken from the sherpa 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 $\nu \overline{\nu }\gamma$ 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. 5. The distributions for the expected signals are taken from the sherpa 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 6

Measured $E_{\mathrm{T}}^{\gamma }$ differential cross sections of (a) the $pp\to \ell \nu \gamma$ process and of (b) the $pp\to {\ell }^{+}{\ell }^{-}\gamma$ process, using combined electron and muon measurements in the inclusive ($N_{\mathrm{jet}}\ge 0$) and exclusive ($N_{\mathrm{jet}}=0$) extended fiducial regions. The lower plots show the ratio of the data to the predictions by different generators. The Monte Carlo uncertainties are shown only in the ratio plots. The cross-section predictions of the sherpa and alpgen generators have been scaled by a global factor to match the total number of events observed in data. The global factor is 1.5 for the alpgen $\ell \nu \gamma$ signal sample and 1.0 for the sherpa ${\ell }^{+}{\ell }^{-}\gamma$ signal sample. No global factor is applied for mcfm predictions.

Figure 7

The differential cross-section measurements as a function of the jet multiplicity for the $pp\to \ell \nu \gamma$ and $pp\to {\ell }^{+}{\ell }^{-}\gamma$ processes, for (a) $E_{\mathrm{T}}^{\gamma }>15\mathrm{GeV}$, $pp\to \ell \nu \gamma$, (b) $E_{\mathrm{T}}^{\gamma }>60\mathrm{GeV}$, $pp\to \ell \nu \gamma$, (c) $E_{\mathrm{T}}^{\gamma }>15\mathrm{GeV}$, $pp\to {\ell }^{+}{\ell }^{-}\gamma$, and (d) $E_{\mathrm{T}}^{\gamma }>60\mathrm{GeV}$, $pp\to {\ell }^{+}{\ell }^{-}\gamma$. The lower plots show the ratio of the data to the predictions by different generators. The mcfm prediction for inclusive (exclusive) $\ell \nu \gamma$ cross section with $p_{\mathrm{T}}^{\gamma }>60\mathrm{GeV}$ is $171\pm 23\mathrm{fb}$ ($80\pm 22\mathrm{fb}$). The corresponding predictions for $p_{\mathrm{T}}^{\gamma }>15\mathrm{GeV}$ are given in Table 7. mcfm does not provide the predictions for two and three jet bins for the $pp\to \ell \nu \gamma$ process, therefore only alpgen and sherpa predictions are shown in (a) and (b).

Figure 8

The inclusive normalized differential cross section of (a) the $pp\to \ell \nu \gamma$ process as a function of $m_{\mathrm{T}}^{W\gamma }$ and (b) the $pp\to {\ell }^{+}{\ell }^{-}\gamma$ process as a function of $m^{Z\gamma }$. The lower plots show the ratio of the data to the prediction by different generators.

Figure 9

The 95% C.L. intervals for anomalous couplings from ATLAS, D0 [5, 6], CDF [4], and LEP [54] 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 $\Delta {\kappa }_{\gamma }$, ${\lambda }_{\gamma }$. The integrated luminosities and new-physics scale parameter $\Lambda$ are shown. The ATLAS and D0 results are for the charged aTGCs measured from $W\gamma$ production. Except for the coupling under study, all other anomalous couplings are set to zero. The LEP charged aTGCs results were obtained from $WW$ production, which is also sensitive to the $WWZ$ couplings and therefore required some assumptions [${\lambda }_Z={\lambda }_{\gamma }$, $\Delta {\kappa }_{\gamma }=({cos}^2{\theta }_W/{sin}^2{\theta }_W)(\Delta g^Z-\Delta {\kappa }_Z)$] about the relations between the $WW\gamma$ and $WWZ$ aTGCs [54, 72, 73, 74] but did not require assumptions about the scale $\Lambda$. The combined aTGC results from the D0 experiment are obtained from $WW+WZ\to \ell \nu jj$, $WW+WZ\to \ell \nu {\ell }^{+}{\ell }^{-}$, $W\gamma \to \ell \nu \gamma$, and $WW\to \ell \nu \ell \nu$ events [75]. The LEP limits on neutral aTGC’s are much larger than those from hadron colliders and are not included in (b).

Figure 10

Observed 95% C.L. limits shown in the two-parameter planes for pairs of anomalous couplings (a) $h_3^Z$ and $h_4^Z$, (b) $h_3^{\gamma }$ and $h_4^{\gamma }$, and (c) $\Delta k_{\gamma }$ and ${\lambda }_{\gamma }$, corresponding to an infinite cutoff scale. The horizontal and vertical lines inside each contour correspond to the limits found in the one-parameter fit procedure, and the ellipses indicate the correlations between the one-parameter fits. Since all sensitivity of the measurement is contained in a single measurement of the $V\gamma$ cross sections in the high-$E_{\mathrm{T}}^{\gamma }$ regions, the likelihood ratio used to obtain the two-dimensional limits has one effective degree of freedom.

Figure 11

Three-body transverse mass for (a) the $e\nu \gamma$ and (b) $\mu \nu \gamma$ final states. The background-only fit to the data is shown. The significance quoted is defined as $\frac{(D-B)}{\sqrt{B}}$, where $D$ is the number of data events and $B$ the number of predicted events by the fit in the bin considered. The background distributions for the expected $\ell \nu \gamma$ events are taken from the MC simulation (generated with alpgen) and normalized to the extracted number of $\ell \nu \gamma$ events. The signal near the limit at $m_{a_{\mathsf{T}}}=700\mathrm{GeV}$ is also shown.

Figure 12

Three-body invariant mass for (a) the $e^{+}{e}^{-}\gamma$ and (b) ${\mu }^{+}{\mu }^{-}\gamma$ final states. The background-only fit to the data is shown. The significance quoted is defined as $\frac{(D-B)}{\sqrt{B}}$, where $D$ is the number of data events and $B$ the number of predicted events by the fit in the bin considered. The background distributions for the expected ${\ell }^{+}{\ell }^{-}\gamma$ events are taken from the MC simulation (generated with sherpa) and normalized to the extracted number of ${\ell }^{+}{\ell }^{-}\gamma$ events. The signal near the limit at $m_{{\omega }_{\mathsf{T}}}=490\mathrm{GeV}$ is also shown.

Figure 13

95% C.L. limits on narrow vector resonance production obtained using $\mathcal{L}=4.6{\mathrm{fb}}^{-1}$ of data for (a) the $pp\to {\ell }^{+}{\ell }^{-}\gamma$ final state and for (b) the $pp\to \ell \nu \gamma$ final state. The parametrization of LSTC [61, 62] used to benchmark the observed limit is obtained using $m_{{\rho }_{\mathsf{T}}}=m_W-m_{{\pi }_{\mathsf{T}}}$.