Ratios of $W^\pm\gamma$ and $Z\gamma$ Cross Sections: New Tools in Probing the Weak Boson Sector at the Tevatron

Abstract

The ratios ${\cal R}_{\gamma ,\ell}=B(Z\to\ell^+\ell^-)\cdot\sigma(Z\gamma) /\allowbreak B(W\to\ell\nu)\cdot\sigma(W^\pm\gamma)$, ${\cal R}_{\gamma , \nu}= B(Z\to\bar\nu\nu)\cdot\sigma(Z\gamma)/\allowbreak B(W\to\ell\nu)\cdot\sigma(W^ \pm\gamma)$, ${\cal R}_{W\gamma}=\sigma(W^\pm\gamma)/\allowbreak\sigma(W^\pm)$, and ${\cal R}_{Z\gamma}=\sigma(Z\gamma)/\allowbreak\sigma(Z)$ are studied as tools to probe the electroweak boson self-interactions. As a function of the minimum photon transverse momentum, ${\cal R}_{\gamma ,\ell}$ and ${\cal R}_{ \gamma ,\nu}$ are found to directly reflect the radiation zero present in $W^ \pm\gamma$ production in the Standard Model. All four ratios are sensitive to anomalous $WW\gamma$ and/or $ZZ\gamma/Z\gamma\gamma$ couplings. The sensitivity of the cross section ratios to the cuts imposed on the final state particles, as well as the systematic uncertainties resulting from different parametrizations of parton distribution functions, the choice of the factorization scale $Q^2$, and from higher order QCD corrections are explored. Taking into account these uncertainties, sensitivity limits for anomalous three gauge boson couplings, based on a measurement of the cross section ratios with an integrated luminosity of 25~pb$^{-1}$ at the Tevatron, are estimated.