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 HERAare used to measure the total cross sections of the charged currentprocess, e^+ p \to \bar{\nu}X, for negative four- momentum transfersquared Q^2 > 400 GeV^2 and inelasticity y<0.9. Together with the corresponding cross section obtained from the previously published unpolarised data, the polarisation dependence of the charged currentcross section is measured for the first time at high Q^2 and found to be in agreement with the Standard Model prediction.