Measurement of Charm and Beauty Photoproduction at HERA

using D* mu Correlations



Introduction

Measuring heavy quark production in ep collisions at HERA is an ideal testing ground for studying the strong force. At HERA heavy quarks are predominantly produced via photon-gluon fusion, which is illustrated in the diagram on the right hand side. This process is directly driven by the gluon density g(xg) in the proton, where xg is the relative momentum fraction of the gluon with respect to the total proton momentum. The large mass of the heavy quark provides a hard kinematical scale, which allows the calculation of cross sections by means of perturbative-Quantum-Chromo-Dynamics (pQCD). The heavy quark production cross section is largest for photoproduction, i.e. for photons with virtuality Q2 approximately 0. The light quarks u,d and s are produced much more copiously than c and b, and beauty production is suppressed by a factor of approximately 200 compared to charm. Charm and beauty measurements performed at HERA so far relied on the tagging of only one heavy quark in each event. While the charm measurements were mostly based on the reconstruction of D mesons, the beauty measurements used semi-leptonic decays or lifetime signatures or both. In this analysis both heavy quarks are tagged in a large fraction of events using a D* and a muon as signatures.

The correlations between the direction of the muon with respect to the D* and their electric charges are used to separate charm and beauty contributions. The measurement of the b cross section presented here extends to significantly lower centre-of-mass energies of the b-pair system than previous measurements at HERA. The simultaneous detection of the D* meson and the muon makes possible new tests of higher order QCD effects. For instance, in the photon-gluon rest frame the angle between the heavy quarks is 180o at leading order, but at next-to-leading order it can differ significantly from this value due to hard gluon radiation. Furthermore, the D*muon pair is expected to be sensitive to a possible transverse momentum kT of the gluons entering the quark pair production process.

Experimental technique

In this measurement only D* events decaying into a D0 and a slow pion, where the D0 decays than itself into a charged kaon and pion, are considered. Muons are required to be identified in the instrumented iron.

In order to separate charm and beauty contributions the charge and azimuthal angle correlations of the D* meson and muon are exploited. The azimuthal angle difference between the D* and the muon and their respective electric charges are used to define for 'correlation regions' I-IV. This is sketched in the upper left plot. The four regions are populated differently by charm and beauty events. The values indicated in this figure are obtained with PYTHIA without detector effects. Neglecting any transverse momenta of the photon and the gluon, the photon-gluon fusion process leads to a back-to-back configuration of the two quarks (see middle and right upper plot). Approximating the directions of the D* meson and the muon with those of the quark and antiquark and selecting opposite charges, charm pair events populate correlation region IV. In contrast, beauty events populate regions II, III and IV, depending on whether the muon originates from the same b quark as the D* or from the opposite anti-b (see upper right figure). If the muon originates from the same b quark as the D* meson, the events lie in region III. For muons coming from the anti-b opposite to the D* meson, the direct decay populates region II, while the cascade process populates region IV. Region IV hence receives contributions from both charm and beauty events and region I stays empty. The azimuthal angle correlations are smeared by fragmentation and semileptonic decay processes and by higher order QCD effects such as gluon radiation and any initial transverse momentum of the gluon. According to the PYTHIA Monte Carlo simulation, which takes into account these smearing effects, the relative population of the four region is as given by the numbers in the upper left figure. These numbers apply in the analysed kinematic region and do not include detector effects. Finally the charm and beauty contributions in the data are determined by performing a simultaneous likelihood fit of the mass difference of the D* and D0 candidate in each correlation regions.

Results

The total measured cross section for charm in the visible kinematic region is in agreement with the NLO QCD prediction, while the beauty cross section is higher than predicted. The kinematic region of the latter is characterised by lower b anti-b centre-of-mass energies than in the most previous analyses, which require high momentum jets.

Differential cross sections for D*muon production in the visible kinematic region are evaluated as a function of variables characterising the D* meson, the muon and the D*muon system. For theses distributions the complete data set, which contains the contributions from charm and beauty, are used. The lower left plot shows exemplarily the D*muon cross section as a function of the azimuthal angle difference between the D* meson and the muon. The QCD calculations including higher order effects (indicated in the left plot with FMNR) show general agreement with the shape of the data. Effects beyond the LO approximation are directly observed.

In addition differential cross sections for D*muon production are determined for a smaller sample, where both the heavy quarks are tagged by either a D* or a muon. For this purpose correlation region IV is used, which is dominated by charm. The dilution of the correlation of quantities characterising the quark pair and the measured D*muon pair as obtained for the complete data sample due to the events from correlation region II in which both D* and muon originate from the same b quark is avoided in this smaller sample. The upper right plot shows exemplarily the D*muon cross section of the quark-antiquark tag sample as a function of the transverse momentum of the D*muon pair, which is related to a possible tranverse momentum of the initial gluon kT. In the kinematic region studied, effects due to kT factorisation, as implemented in CASCADE, are found to be small compared to the experimental errors.

What did we learn

The total measured cross section for charm in the visible kinematic region is in agreement with the NLO QCD prediction, while the beauty cross section is higher than predicted. The kinematic region of the latter is characterised by lower b anti-b centre-of-mass energies than in the most previous analyses, which require high momentum jets. Comparisons of the shapes of the measured differential distributions with the QCD calculations including higher order effects show general agreement. Effects beyond the LO approximation are directly observed.