W-boson production at HERA

We discuss W boson production at HERA including NLO QCD corrections. The production of W bosons at ep colliders is mediated by photon, Z and W exchange between the electron/positron and the hadronic part of the process [1]. In practice, it is convenient to distinguish two regions, a small Q 2 one (photoproduction region), that can be treated using the Weizs¨acker-Williams photon spectrum convoluted with the cross section for γq → q ′ W , and a large Q 2 one (DIS region). While the treatment of the DIS region is straightforward [a typical contribution is shown in the third diagram of Fig. 1], the small Q 2 region requires the inclusion of the contribution of the hadronic component of the photon, in which the photon, behaving like a hadron, produces the W in the collision with the proton via the standard Drell-Yan mechanism [first diagram of Fig. 1]. This component is, in fact, the dominant one for this process, and full NLO corrections should include both the NLO correction to the Drell-Yan process [2], and the leading hard photon contribution [a typical contribution is shown in the second diagram of Fig. 1], suitably subtracted for collinear singularities.