At HERA, single W production has a cross section of about 1pb, which is accessible with current luminosity (Z is an order of magnitude smaller). Measurement of W production is particularly interesting because it includes a diagram with W-W-gamma coupling, and thus sensitive to an anomalous triple gauge boson coupling which is beyond-SM physics. The cleanest way to search for W is via its leptonic decay, and this topology (lepton + hadronic system + missing Pt) is also a signal for some exotic processes, e.g. R-parity violating SUSY. Therefore, to understand W production cross section is very important in HERA physics.
H1 Collaboration reprorted observation of events with a high energy isolated lepton and missing transverse momentum.
ZEUS Collaboration also reported observation of events with a lepton and missing transverse momentum.