$\mu \mu$ Correlations

Figure 39: Candidate of an event with beauty quarks which both decay into muons in the ZEUS detector.

The ZEUS experiment has measured the beauty cross section, using a sample of events in which the two muons are identified in the pseudo-rapidity range $-2.2<\eta<2.5$ [42]. The analysis provides almost full rapidity coverage and thus gives access to a direct measurement of the total beauty cross section. The beauty cross section is extracted from the like-sign and unlike-sign subsamples by adjusting the beauty contribution in the Monte Carlo simulation such that the normalization of the data is recovered. The normalization of the unlike-sign charm background is fixed to the results from the corresponding measurements. It is assumed that the background from light-quark events is the same in the like-sign and unlike-sign samples.

The results of the various measurements of beauty production at HERA are shown in fig.40. In this figure the ratios between the measured cross section and the predictions from the fixed order massive calculations in perturbative QCD are depicted. All the measurements are in reasonable agreement within the errors. Note that the kinematic region of the various measurements can differ drastically.

Figure: Summary of beauty measurements at HERA. The ratio of the cross sections to the predictions from the programs FMNR (HVQDIS) is shown. The open points and squares refer to the measurements with jets and muons (see fig.33), the stars correspond to the measurements using $D^*\mu$ correlations. The horizontal position of the photoproduction measurements, at $Q^2<1$ GeV$^2$, is chosen arbitrarily for better visibility.