One of the major uncertainties comes from the estimation of the various contributions to the cross-section which depends on Monte Carlo techniques. This problem has been addressed in a different way in the ZEUS analysis [23]. Here, the mass spectrum, MX2, is measured as a function of W and Q2, as shown in Fig. 15 for a representative interval, where the measured mass is reconstructed in the calorimeter and corrected for energy loss but not for detector acceptance, resulting in the turnover at large MX2. The diffractive data are observed as a low mass shoulder at low MX2. which becomes increasingly apparent at higher W. Also shown in the figure are the estimates of the non-diffractive contribution based on a direct fit to the data, discussed below.
The probability of producing a gap is exponentially suppressed as a function of the rapidity gap, and hence as a function of ), for non-diffractive interactions. The slope of this exponential is directly related to the height of the plateau distribution of multiplicity in the region of rapidity where the subtraction is made. The data can thus be fitted to functions of the form , in the region where the detector acceptance is uniform, where b, C and D are determined from the fits. Here, D represents a first-order estimate of the diffractive contribution which is approximately flat in ). The parameter which determines the background is b. In general the measured value of b is incompatible with that of the ARIADNE Monte Carlo. This result in itself is interesting, since the fact that ARIADNE approximately reproduces the observed forward ET ( multiplicity) flow but does not reproduce the measured value of b suggests that significantly different correlations of the multiplicities are present in non-diffractive DIS compared to the Monte Carlo expectations. This method enables a diffractive cross-section to be determined directly from the data at the expense of being limited in the range of large masses that can analysed.
Finally, the advent of the leading proton spectrometers (LPS) at HERA is especially important in these diffractive measurements, since internal cross-checks of the measurements as a function of t, M2, W2 and Q2 can be performed and underlying assumptions can be studied experimentally. Only in these measurements can we positively identify the diffracted proton and hence substantially reduce uncertainties on the non-diffractive and double dissociation backgrounds. This is illustrated in Fig. 16, where the xL (where xL = p'/p) distribution includes a clear diffractive peak for . It should be noted, however, that the contribution from other Reggeon exchanges cannot be neglected until (in fact the result at lower xL can be simply interpreted via reggeon (approximated by pion) exchange, as discussed below.) However, new experimental uncertainties are introduced due to the need for precise understanding of the beam optics and relative alignment of the detectors. Reduced statistical precision also results due to the limited geometrical acceptance of the detectors ( 6%).
Photoproduction Results:
ZEUS has measured the photon dissociation t distribution using the LPS,
as shown in Fig. 17.
An exponential fit to the data yields a b-slope
parameter,
GeV-2. A comparison of the data
with lower W data from Chapin et al. shows that the result is consistent
with shrinkage, as previously
discussed in relation to exclusive
production.
H1 results on the photon dissociation cross-sections as a function of
MX2 in two W intervals are shown in Fig. 18.
Regge theory predicts the form of
the cross-section as a function of MX and W, as discussed with respect
to proton dissociation. The cross-section is therefore fitted to the form
Deep inelastic structure of diffraction:
A new era for diffraction was opened with the study of the dissociation
of virtual photons.
Here, the photon can be considered as probing the structure of the
exchanged colourless object mediating the interaction.
The deep inelastic structure of colour singlet exchange is therefore being
studied.
In the presentation of the results, the formalism changes [24],
reflecting an assumed underlying partonic description,
and two orthogonal variables are determined
In addition to the structure of the pomeron, corresponding to large xL, it is also possible to study the structure of the reggeons that contribute at lower xL. H1 has analysed the leading proton data at lower xL ( 0.7<xL<0.9) and employed the formalism noted above to measure the structure of the exchange for reasonably forward protons, as shown in Fig. 19 [25]. The data are consistent with a flat dependence in all intervals of and Q2 i.e. . This is consistent with a factorisable ansatz of where measures the flux of reggeons in the proton and is the probed structure of these reggeons. The exponent of is identified as , where measures the effective dependence ( dependence at fixed MX2 and Q2) of the cross-section, integrated over t. The data are consistent with , corresponding to . These data involve colour singlet exchange and need to be explained in terms of QCD, but they are clearly not of a diffractive nature.
The area of interest for diffraction is in the behaviour of small values of , where is now identified as . Here, the data fall approximately as (equivalent to a flat cross-section with increasing W) and therefore the data are plotted as in Fig. 20. In the H1 case, the measurement is presented with no explicit subtraction for the non-diffractive contribution and quoted for limited masses of the dissociated proton system (MN < 1.6 GeV). The measurement relies upon a good understanding of the various contributions to the cross-section in and around the measured region: the control plots in Fig. 14 illustrate how well this is achieved by combining the different Monte Carlo contributions.
Fits of the form are performed where the normalisation constants are allowed to differ in each interval. The fits are motivated by the factorisable ansatz of where measures the flux of pomerons in the proton and is the probed structure of the pomeron. The exponent of