Measurement of the $W\gamma$ and $Z\gamma$ inclusive cross sections in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ and limits on anomalous triple gauge boson couplings

Measurements of $W\gamma$ and $Z\gamma$ production in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ 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 $5.0{\mathrm{fb}}^{-1}$. The cross sections are measured for photon transverse momenta $p_{\mathrm{T}}^{\gamma }>15\mathrm{GeV}$, and for separations between photons and final-state charged leptons in the pseudorapidity-azimuthal plane of $\mathrm{\Delta }R(\ell ,\gamma )>0.7$ in $\ell \nu \gamma$ and $\ell \ell \gamma$ final states, where $\ell$ refers either to an electron or a muon. A dilepton invariant mass requirement of $m_{\ell \ell }>50\mathrm{GeV}$ 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.

Figure 1

The three lowest-order diagrams for $V\gamma$ production, with $V$ corresponding to both virtual and on-shell $\gamma$, $W$, and $Z$ bosons. The three diagrams reflect contributions from (a) initial-state radiation, (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 $15<p_{\mathrm{T}}^{\gamma }<20\mathrm{GeV}$ in data with signal and background templates in the (a) barrel and (b) end caps.

Figure 5

The $R_p$ ratio (described in text) as a function of the $p_{\mathrm{T}}$ of photon candidates for the barrel region of the ECAL in $\gamma +\text{jets}$ and multijet data. The difference in $R_p$ values for the two processes is attributed to the fact that jets in $\gamma +\text{jets}$ events are dominated by quark fragmentation, while jets in multijet events are dominated by gluon fragmentation.

Figure 6

Distributions in $p_{\mathrm{T}}^{\gamma }$ for $W\gamma$ candidate events in data, with signal and background MC simulation contributions to (a) $W\gamma \to e\nu \gamma$ and (b) $W\gamma \to \mu \nu \gamma$ channels shown for comparison.

Figure 7

Charge-signed rapidity difference $Q_{\ell }\times \mathrm{\Delta }\eta$ between the photon candidate and a lepton for $W\gamma$ candidates in data (filled circles) and expected SM signal and backgrounds (shaded regions) normalized to (a) data, and (b) background-subtracted data. The hatched bands illustrate the full uncertainty in the MC prediction.

Figure 8

Distributions in $p_{\mathrm{T}}^{\gamma }$ for $Z\gamma$ candidate events in data, with signal and background MC simulation contributions to the (a) $Z\gamma \to ee\gamma$ and (b) $Z\gamma \to \mu \mu \gamma$ channels shown for comparison.

Figure 9

Bias in the background contamination related to the background templates for ${\sigma }_{\eta \eta }$, as a function of $p_{\mathrm{T}}^{\gamma }$, for the (a) barrel and (b) end-cap regions of ECAL.

Figure 10

A summary of measured cross sections for three $p_{\mathrm{T}}^{\gamma }$ thresholds, compared to SM predictions for (a) $W\gamma$ and (b) $Z\gamma$ production.

Figure 11

Observed (solid curve) and expected (dashed curve) 95% C.L. exclusion contours for anomalous $WW\gamma$ couplings, with $\pm 1$ and $\pm 2$ standard deviation contours from uncertainties in the measurements indicated by light and dark shaded bands, respectively.

Figure 12

Observed (solid curves) and expected (dashed curves) 95% C.L. exclusion contours for anomalous (a) $Z\gamma \gamma$ and (b) $ZZ\gamma$ couplings, with $\pm 1$ and $\pm 2$ standard deviation contours indicated by light and dark shaded bands, respectively.

Figure 13

Distributions in the W-boson transverse invariant mass for $W\gamma$ candidate events in data, with the signal and background MC simulation contributions to the (a) $W\gamma \to e\nu \gamma$ and (b) $W\gamma \to \mu \nu \gamma$ channels shown for comparison. The contributions for the number of jets with $p_{\mathrm{T}}>30\mathrm{GeV}$ are given in (c) and (d), respectively. The separation in $R$ between the charged lepton and the photon is given in (e) for the electron channel, and that for muon channel is illustrated in (f).

Figure 14

Distributions in the dilepton invariant mass for $Z\gamma$ candidate events in data, with the signal and background MC simulation contributions to the (a) $Z\gamma \to ee\gamma$ and (b) $Z\gamma \to \mu \mu \gamma$ channels shown for comparison. The contributions for the dilepton-plus-photon invariant mass are given in (c) and (d). The smallest separation in $R$ between any charged leptons and the photon is given in (e) for the electron channel, and that for muon channel is illustrated in (f).

Figure 15

Distributions in the number of jets with $p_{\mathrm{T}}>30\mathrm{GeV}$ for $Z\gamma$ candidate events in data, with the signal and background MC simulation contributions to the (a) $Z\gamma \to ee\gamma$ and (b) $Z\gamma \to \mu \mu \gamma$ channels shown for comparison.