Leading proton spectrometer measurements

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 ultimately be performed and underlying assumptions can be questioned 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. However, new 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 geometrical acceptance of the detectors (  6%). In figure 11(a), various observed distributions are shown for 240 events selected from the DIS data, with  GeV². The momentum distribution clearly indicates a significant diffractive peak at E_p = 820 GeV above the non-diffractive background and the observed M and W distributions are well described by the NZ Monte Carlo [38]. The distribution of versus indicates a significant fraction of events at small which are difficult to access using the experimental techniques described earlier. The measurements are currently being analysed, but a preliminary result on the t-dependence is shown in Figure 11(b), measured in a relatively high observed mass interval,  GeV, at relatively low Q². The slope can be characterised by a single exponential fit with . This is somewhat high compared to the value of expected for a predominantly hard pomeron but lies within the range of expectations of . However, before drawing conclusions, we should perhaps wait for further results on the general dependences measured in the LPS.

 
Figure 11: (a) Observed momentum, M, W and versus distributions in the LPS. The observed M and W distributions are compared to the NZ Monte Carlo predictions. (b) Corrected t distribution in the quoted range.