Measurement of the diffractive cross section in deep inelastic scattering using ZEUS 1994 data

The DIS diffractive cross section, d sigma(gamma*p-->XN)(diff)/dM(X), has been measured in the mass range M-X < 15 GeV for gamma*p c.m. energies 60 < W < 200 GeV and photon virtualities Q(2) = 7 to 140 GeV2. For fixed Q(2) and M-X, the diffractive cross section rises rapidly with W, d sigma(gamma*p-->XN)(diff) (M-X, W, Q(2))/dM(X) proportional to W-adiff with a(diff) = 0.507 +/- 0.034(stat)(-0.046)(+0.155) (syst) corresponding to a t-averaged pomeron trajectory of <(alpha)over bar>(P) = 1.127 +/- 0.009(stat)(-0.012)(+0.039) (syst) which is larger than <(alpha)over bar>(P) observed in hadron-hadron scattering The W dependence of the diffractive cross section is found to be the same as that of the total cross section for scattering of virtual photons on protons. The data are consistent with the assumption that the diffractive structure function F-2(D(3)) factorizes according to x(P)F(2)(D(3))(x(p), beta, Q(2)) = (x(0)/x(P))F-n(2)D(2) (beta, Q(2)). They are also consistent with QCD based models which incorporate factorization breaking. The rise of x(P)F(2)(D(3)) with decreasing x(P) and the weak dependence of F-2(D(2)) On Q(2) suggest a substantial contribution from partonic interactions.