Abstract
Deep inelastic scattering and its diffractive component, e p → e ′ γ ∗ p → e ′ X N , have been studied at HERA with the ZEUS detector using an integrated luminosity of 52.4 pb −1. The M X method has been used to extract the diffractive contribution. A wide range in the centre-of-mass energy W (37–245 GeV), photon virtuality Q 2 (20–450 GeV 2) and mass M X (0.28–35 GeV) is covered. The diffractive cross section for 2 < M X < 15 GeV rises strongly with W, the rise becoming steeper as Q 2 increases. The data are also presented in terms of the diffractive structure function, F 2 D ( 3 ) , of the proton. For fixed Q 2 and fixed M X , x P F 2 D ( 3 ) shows a strong rise as x P → 0 , where x P is the fraction of the proton momentum carried by the pomeron. For Bjorken- x < 1 × 10 −3 , x P F 2 D ( 3 ) shows positive log Q 2 scaling violations, while for x ⩾ 5 × 10 −3 negative scaling violations are observed. The diffractive structure function is compatible with being leading twist. The data show that Regge factorisation is broken.
Highlights
The observation of events with a large rapidity gap in deep inelastic electron proton scattering (DIS) at HERA by the ZEUS experiment [1] has paved the way for a systematic study of diffraction at large centre-of-mass energies with a variable hard scale provided by the mass squared, −Q2, of the virtual photon
The H1 and ZEUS experiments at HERA have presented results on diffractive scattering in photoproduction and deep inelastic electron–proton scattering for many different final states
The hadronic system was reconstructed from energy-flow objects (EFO) [31,32] which combine the information from CAL and forward-plug calorimeter (FPC) clusters and from central tracking detector (CTD) tracks, and which were not assigned to the scattered positron
Summary
The observation of events with a large rapidity gap in deep inelastic electron (positron) proton scattering (DIS) at HERA by the ZEUS experiment [1] has paved the way for a systematic study of diffraction at large centre-of-mass energies with a variable hard scale provided by the mass squared, −Q2, of the virtual photon. Diffraction is defined by the property that the cross section does not decrease as a power of the centre-of-mass energy This can be interpreted as the exchange of a colourless system, the pomeron, which leads to the presence of a large rapidity gap between the proton and the rest of the final state, which is not exponentially suppressed. Results on the proton structure function F2 and on the diffractive cross section and structure function are presented for a wide range of centre-of-mass energies, photon virtualities −Q2 and of mass MX of the diffractively produced hadronic system, using the data from the ZEUS experiment collected in 1999 and 2000. The combined data from the FPC I and FPC II analyses provide a measurement of the Q2 dependence of diffraction over a range of two orders of magnitude
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