  Cross sections for e^+p charged current deep inelastic scattering at a
centre-of-mass energy of 318 GeV have been determined with an integrated
luminosity of 60.9pb^-1 collected with the ZEUS detector at HERA. The
differential cross sections dsigma/dQ^2, dsigma/dx and dsigma/dy for Q^2>200
GeV^2 are presented. In addition, d^2sigma/dxdQ^2 has been measured in the
kinematic range 280 GeV^2 < Q^2 < 17000 GeV^2 and 0.008 < x < 0.42. The
predictions of the Standard Model agree well with the measured cross sections.
The mass of the W boson propagator is determined to be M_W=78.9 +/- 2.0 (stat.)
+/- 1.8 (syst.) +2.0 -1.8 (PDF) GeV from a fit to dsigma/dQ^2. The chiral
structure of the Standard Model is also investigated in terms of the (1-y)^2
dependence of the the double-differential cross section. The structure-function
F_2^CC has been extracted by combining the measurements presented here with
previous ZEUS results from e^-p scattering, extending the measurement obtained
in a neutrino-nucleus scattering experiment to a significantly higher Q^2
region.
