Production of Z0 bosons in elastic and quasi-elastic ep collisions at HERA

The production of Z0 bosons in the reaction ep →eZ0 p(∗), where p(∗) stands for a proton or a lowmass nucleon resonance, has been studied in ep collisions at HERA using the ZEUS detector. The analysis is based on a data sample collected between 1996 and 2007, amounting to 496 pb−1 of integrated luminosity. The Z0 was measured in the hadronic decay mode. The elasticity of the events was ensured by a cut on ηmax <3.0, where ηmax is the maximum pseudorapidity of energy deposits in the calorimeter defined with respect to the proton beam direction. A signal was observed at the Z0 mass. The cross section of the reaction ep → eZ0 p(∗) was measured to be σ(ep → eZ0 p(∗)) = 0.13 ± 0.06(stat.) ± 0.01(syst.) pb, in agreement with the Standard Model prediction of 0.16 pb. This is the first measurement of Z0 production in ep collisions.