First Study of the Radiation-Amplitude Zero in $W\gamma$ Production and Limits on Anomalous $WW\gamma$ Couplings at $\sqrt{s}=1.96\mathrm{TeV}$

We present results from a study of $p\overline{p}\to W\gamma +X$ events utilizing data corresponding to $0.7{\mathrm{fb}}^{-1}$ of integrated luminosity at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the D0 detector at the Fermilab Tevatron Collider. We set limits on anomalous $WW\gamma$ couplings at the 95% C.L. The one-dimensional 95% C.L. limits are $0.49<{\kappa }_{\gamma }<1.51$ and $-0.12<{\lambda }_{\gamma }<0.13$. We make the first study of the charge-signed rapidity difference between the lepton and the photon and find it to be indicative of the standard model radiation-amplitude zero in the $W\gamma$ system.

Figure 1

The photon transverse energy spectra for the SM (solid line), an anomalous coupling (AC) point (dashed line), combined electron and muon channel data candidates (black points), and the background estimate (shaded histogram). Uncertainties are shown as error bars on the points, lines, and histograms. The last bin includes overflows.

Figure 2

The ellipse is the 95% C.L. limit contour in ${\kappa }_{\gamma }-{\lambda }_{\gamma }$ space. One-dimensional 95% C.L. limits are shown as the horizontal and vertical bars.

Figure 3

The background-subtracted charge-signed rapidity difference for the combined electron and muon channels. The black points and error bars represent background-subtracted data with its associated uncertainties (statistical and from the subtraction procedure), and the shaded areas are the systematic uncertainties on the SM prediction (including on efficiencies and acceptances). The solid line is the distribution from the SM. A ${\chi }^2$ test comparing the data and SM using the full covariance matrix yields 17 for 12 degrees of freedom, indicating good agreement.

Figure 4

Distributions of the $R$-test statistic for the SM ensemble (solid line) and the MUH ensemble (dashed line). The vertical line indicates the measured value from the data. The inset plot indicates the positions of the two bins used for the $R$ test as determined by a fit to the SM $Q_{\ell }\times \Delta \eta$ distribution (solid line). For comparison, a fit to the MUH $Q_{\ell }\times \Delta \eta$ distribution is shown as the dashed line.