Jet structure

The question of the constituent content of the pomeron can also be addressed via measurements of diffractively produced jets in the photoproduction data [33]. Jets are reconstructed at large W (130< W < 270 GeV) using the cone algorithm with cone radius 1 and E_T^jet > 8 GeV. The diffractive contribution is identified as a tail in the distribution of these events above the PYTHIA 5.7 [34] Monte Carlo expectation. In Figure 9(a) the measured cross section is compared to various model predictions as a function of the jet rapidity. Comparison with the non-diffractive contribution estimated from PYTHIA indicates a significant excess at lower values of . Here, standard photon and proton parton distributions are adopted and the overall scale, which agrees with the non-diffractive data normalisation, is set by E_T^jet. Also shown are the predicted diffractive cross sections from POMPYT using a hard (z(1-z)) quark, hard gluon or soft ((1-z)⁵) gluon where a Donnachie-Landshoff flux factor is adopted and the momentum sum rule is assumed to be satisfied in each case. Sampling low-energy (soft) gluons corresponds to a small cross section and can be discounted, whereas high-energy (hard) gluons and/or quarks can account for the cross section by changing the relative weights of each contribution. The distribution for these events, where is the reconstructed momentum fraction of the interacting photon, is peaked around 1, indicating that at these E_T^jet values a significant fraction of events is due to direct processes where the whole photon probes the pomeron constituents.

We now have two sets of data, the DIS data [24] probing the pomeron structure at a scale Q and the jet data probing at a scale of E_T^jet. Each probes the large z structure of the pomeron with the jet and DIS data, predominantly sampling the (hard) gluon and quark distributions, respectively. In Figure 9(b) the preferred momentum fraction carried by the (hard) gluon, c_g, is indicated by the overlapping region of the jet (dark band) and DIS (light band) fits to the data. Considering the systematics due to the non-diffractive background, modelled using the Monte Carlo models, a range of values consistent with can be estimated. The result depends on the assumption that the cross sections for both sets of data factorise with a universal flux, characterised by the same value of in this W range, but does not assume the momentum sum rule.

 
Figure 9: (a) Jet cross section as a function of jet rapidity for events with . (b) Momentum sum rule assuming a Donnachie-Landshoff flux, , versus the momentum fraction carried by the gluons in the pomeron, c_g. The dark (light) error bands correspond to statistical errors on the fits to the jet (DIS) data discussed in the text.

So far we have only considered the case of small-t diffraction with respect to the outgoing proton. Further insight into the diffractive exchange process can be obtained by measurements of the rapidity gap between jets. Here, a class of events is observed with little hadronic activity between the jets [35]. The jets have E_T^jet > 6 GeV and are separated by a pseudorapidity interval ( ) of up to 4 units. The scale of the momentum transfer, t, is not precisely defined but is of order (E_T^jet)². A gap is defined as the absence of particles with transverse energy greater than 300 MeV between the jets. The fraction of events containing a gap is then measured as a function of , as shown in Figure 10. The fit indicates the sum of an exponential behaviour, as expected for non-diffractive processes and discussed in relation to the diffractive DIS data, and a flat distribution expected for diffractive processes. At values of , an excess is seen with a constant fraction over the expectation for non-diffractive exchange at . This can be interpreted as evidence for large-t diffractive scattering. In fact, secondary interactions of the photon and proton remnant jets could fill in the gap and therefore the underlying process could play a more significant rôle. The size of this fraction is relatively large when compared to a similar analysis by D0 and CDF where a constant fraction at is observed [36, 37]. The relative probability may differ due to the higher W values of the Tevatron compared to HERA or, perhaps, due to differences in the underlying and interactions.

 
Figure 10: Gap-fraction, , as a function of the rapidity gap between the two jets compared with the result of a fit to an exponential plus a constant.