Measurement of the diffractive cross section in deep inelastic scattering

Diffractive scattering of gamma*p --> X + N, where N is either a proton or a nucleonic system with M(N) < 4 GeV has been measured in deep inelastic scattering (DIS) at HERA. The cross section was determined by a novel method as a function of the gamma*p c.m, energy W between 60 and 245 GeV and of the mass M(X) of the system X up to 15 GeV at average Q(2) values of 14 and 31 GeV2. The diffractive cross section d sigma(diff)/dM(X) is, within errors, found to rise linearly with W. Parameterizing the W dependence by the form d sigma(diff)/dM(X) proportional to (W-2)((2<(alpha?(IP))over bar>-2)) the DIS data yield for the pomeron trajectory <(alpha(IP))over bar> = 1.23 +/- 0.02(stat) +/- 0.04(syst) averaged over t in the measured kinematic range assuming the longitudinal photon contribution to be zero. This value for the pomeron trajectory is substantially larger than <(alpha(IP))over bar> extracted from soft interactions. The value of <alpha(IP))over bar> measured in this analysis suggests that a substantial part of the diffractive DIS cross section originates from processes which can be described by perturbative QCD. From the measured diffractive cross sections the diffractive structure function of the proton F-2(D(3))(beta,Q(2),x(IP)) has been deterz mined, where beta is the momentum fraction of the struck quark in the pomeron. The form F-2(D(3)) = constant .(1/x(IP))(a) gives a good fit to the data in all beta and Q(2) intervals with a = 1.46 +/- 0.04(stat) +/- 0.08(syst).