Inclusive Lifetime Tag Analyses

Figure 34: Differential beauty photoproduction cross section as a function of the jet transverse momentum as determined from lifetime distributions [45]. Also shown are the NLO QCD expectation in the massive scheme [80] as well as the Monte Carlo calculations PYTHIA [108] and CASCADE [104].

The H1 Experiment has reported new beauty and charm measurements [45,43,46] in which the impact parameters of selected tracks coming from secondary decay vertices are used to identify beauty and charm events (see section 5.5.2). The track selection requires full silicon vertex detector information and imposes a transverse momentum cut $p_t>500$ MeV.

The dijet beauty photoproduction cross section is measured for events with two jets with $p_t > 11(8)$ GeV [45] in the central region of pseudo-rapidity. In fig.34 the differential cross section is presented as a function of the jet transverse momentum $p_t$. The data are found to be higher than predictions from the NLO QCD calculation in the massive scheme FMNR [80] and from the Monte Carlo programs PYTHIA [108] and CASCADE [104] by about a factor of 1.8. The result for the charm dijet cross section as obtained in the same analysis shows good agreement of the theory with the data.

Figure: a) The measured structure function $F_2^{b\bar{b}}$ shown as a function of $Q^2$ for different values of $x$. The inner error bars show the statistical error, the outer error bars represent the statistical and systematic errors added in quadrature. The data are compared with a NLO QCD prediction [164] and with a NNLO QCD prediction [60]. b) The contributions to the total cross section $f^{c\bar{c}}$ and $f^{b\bar{b}}$ (see text) shown as a function of $Q^2$ for different bins of $x$. The lines indicate the prediction from the NLO QCD fit in the variable flavour number scheme [164].

The beauty and charm structure functions have been determined by the H1 collaboration in the range $12 < Q^2 < 650$ GeV and $0.07 < y < 0.7$ (see fig.35). In this kinematic range, more than 80% of the charm events and more than 96% of the beauty events have a track within the detector acceptance. The extrapolation from the measured sample to the full phase space is therefore small, leading to small uncertainties due to model assumptions. This is the first measurement of $F_2^{b\bar{b}}$.

The measurements of $F_2^{b\bar{b}}$ are shown in fig.35. The results for $F_2^{c\bar{c}}$ from this and other analyses are shown in fig.24. Scaling violations are visible which increase towards lower values of $x$, indicating that towards low $x$ charm and beauty production is dominated by the boson-gluon fusion process. The beauty data are compared with a NLO QCD predictions from NLO QCD [164] and with a recent prediction in NNLO [60]. Both calculations are performed in the variable flavour number scheme. The predictions of the QCD calculations are compatible with the data.

In fig.35b the measurements are presented in the form of the fractional contribution to the total $ep$ cross section

$$f^{c\bar{c}} = \frac{{\rm d}^2 \sigma^{c\bar{c}}} {{\rm d} x {\rm d} Q^2} / \frac{ {\rm d}^2 \sigma}{ {\rm d} x {\rm d} Q^2 }.$$ (9)

The $b$ fraction $f^{b\bar{b}}$ is defined in the same manner. NLO QCD is found to give a good description of the data, as shown by comparison with the ZM-VFNS prediction from the H1 PDF 2000 fit.