Measurement of W-gamma and Z-gamma production in pp collisions at sqrt(s) = 7 TeV

Abstract

A measurement of W-gamma and Z-gamma production in proton-proton collisions at sqrt(s) = 7 TeV is presented. Results are based on a data sample recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 36 inverse picobarns. The electron and muon decay channels of the W and Z are used. The total cross sections are measured for photon transverse energy >10 GeV and spatial separation from charged leptons in the plane of pseudorapidity and azimuthal angle >0.7, and with an additional dilepton invariant mass requirement of > 50 GeV for the Z-gamma process. The following cross section times branching fraction values are found: sigma(pp to W-gamma+X) B(W to lepton neutrino) = 56.3 +/- 5.0 (stat.) +/- 5.0 (syst.) +/- 2.3 (lumi.) pb and sigma(pp to Z-gamma+X) B}(Z to lepton lepton) = 9.4 +/- 1.0 (stat.) +/- 0.6 (syst.) +/- 0.4 (lumi.) pb. These measurements are in agreement with standard model predictions. The first limits on anomalous WW gamma, ZZ gamma, and Z gamma gamma trilinear gauge couplings at sqrt(s) = 7 TeV are set.

Figures (7)

• Figure 1: Background from misidentified jets as a function of the photon candidate $E_\text{T}$, estimated from the ratio method, is shown with blue squares together with an alternative method that uses energy deposition shape templates (magenta circles), and MC simulation (green filled histogram) for (a) $\text{W} \gamma$ and (b) $\text{Z} \gamma$ channels. Uncertainties include both statistical and systematic sources.
• Figure 2: Transverse energy distribution for the photon candidates for W$\gamma$ production. Data are shown with black circles with error bars; expected signal plus background is shown as a black solid histogram; the contribution from misidentified jets is given as a hatched blue histogram, and the background from $\gamma+\text{jets}$, $\mathrm{t\bar{t}}$, and multiboson processes is given as a solid green histogram. A typical aTGC signal is given as a red dot-and-line histogram. The last bin includes overflows. Entries in wider bins are normalized to the ratio of 10 GeV and the bin width.
• Figure 3: The background-subtracted charge-signed rapidity difference for the combined electron and muon channels of W$\gamma$ production is shown for data (black circles with error bars) and SM simulation (blue hatched region). The results of the Kolmogorov-Smirnov test of the agreement between data and MC prediction is 57%, which indicates a reasonable agreement.
• Figure 4: The transverse energy distribution of photon candidates in the $\text{Z} \gamma$ channel in data is shown with black circles with error bars; the expected signal plus background is shown as a solid black histogram, while the contribution from misidentified jets is given as a hatched blue histogram. A typical aTGC signal is given as a red dot-and-line histogram. The last bin includes overflows. Entries in wider bins are normalized to the ratio of 10 GeV and the bin width.
• Figure 5: Distribution of the $\ell\ell\gamma$ invariant mass as a function of the dilepton invariant mass for selected Z$\gamma$ candidates in the electron (filled circles) and muon (open circles) final states. The data accumulation at $M_{\ell\ell\gamma} \simeq M_Z$ corresponds to FSR events, while the data at $M_{\ell\ell} \simeq M_Z$ correspond to ISR events.