First measurement of charged current cross sections at HERA with longitudinally polarised positrons

Data taken with positrons of different longitudinal polarisation states in collision with unpolarised protons at HERA are used to measure the total cross sections of the charged current process, e+p→ν¯X, for negative four-momentum transfer squared Q2>400GeV2 and inelasticity y<0.9. Together with the corresponding cross section obtained from the previously published unpolarised data, the polarisation dependence of the charged current cross section is measured for the first time at high Q2 and found to be in agreement with the Standard Model prediction.