Diffractive dissociation of quasi–real photons at a photon–proton centre of mass energy of
W ≈ 200 GeV is studied with the ZEUS detector at HERA. The process under consideration
is γp → XN, where X is the diffractively dissociated photon system of mass MX and N is
either a proton or a nucleonic system with mass MN < 2 GeV. The cross section for this process
in the interval 3 < MX < 24 GeV relative to the total photoproduction cross section was
measured to be σ
partial
D /σtot = 6.2 ± 0.2(stat) ± 1.4(syst)%. After extrapolating this result to
the mass interval of m2
φ < M2
X < 0.05W2 and correcting it for proton dissociation, the fraction
of the total cross section attributed to single diffractive photon dissociation, γp → Xp, is found
to be σSD/σtot = 13.3 ± 0.5(stat) ± 3.6(syst)%. The mass spectrum of the dissociated photon
system in the interval 8 < MX < 24 GeV can be described by the triple pomeron (IP IP IP)
diagram with an effective pomeron intercept of αIP (0) = 1.12 ± 0.04(stat) ± 0.08(syst). The
cross section for photon dissociation in the range 3 < MX < 8 GeV is significantly higher than
that expected from the triple pomeron amplitude describing the region 8 < MX < 24 GeV.
Assuming that this discrepancy is due to a pomeron–pomeron–reggeon (IP IP IR) term, its contribution
to the diffractive cross section in the interval 3 < MX < 24 GeV is estimated to be
fIP IP IR = 26 ± 3(stat) ± 12(syst)%.