Measurement of the diffractive cross section in deep inelastic scattering

Diffractive scattering of γ^∗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 γ^∗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 Q² values of 14 and 31 GeV². The diffractive cross section dσ^{di f f} /dM_X is, within errors, found to rise linearly with W. Parameterizing the W dependence by the form dσ^{di f f} /dM_X ∞ (W²)^{(2αIP - 2)} the DIS data yield for the pomeron trajectory α_IP = 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 α_IP extracted from soft interactions. The value of α_IP 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 F₂ ^D(3) (β,Q², x_IP) has been determined, where β is the momentum fraction of the struck quark in the pomeron. The form F₂ ^D(3) = constant·(1/x_IP)^a gives a good fit to the data in all β and Q² intervals with a = 1.46 ± 0.04(stat) ± 0.08(syst).