Measurement of Beauty Production at HERA Using Events with Muons and Jets


Measuring beauty production in ep collisions at HERA is an ideal testing ground for studying the strong force. The dominant production mechanism is the boson gluon fusion process as illustrated in the diagram below. This process is directly driven by the gluon density in the proton. The large mass mb of the beauty quark of about 5 GeV provides a hard kinematical scale, which allows the calculation of cross sections by means of perturbative-Quantum-Chromo-Dynamics (pQCD). Besides the mass of the b quark there can be additional hard scales available such as the virtuality Q2 of the exchanged photon or the transverse momenta of the outgoing b-quarks. The measurement presented here allows to study the interplay of the hard scales over a wide kinematic range. It is performed both in the region of photoproduction in which the exchanged photon is quasi-real (Q2 is approximately zero) and in deep inelastic scattering (DIS), with photon virtualities 2 < Q2< 100 GeV2. In the covered phase space it would be expected that pQCD calulations performed in the "massive scheme" at Next-to-Leading Order (NLO) give reliable results. In this scheme, u, d and s quarks are the only active flavours in the proton and the photon, and charm and beauty are produced dynamically in the hard scattering, as depicted in the diagram below for the beauty case. At scales Q2>>m2b the so called "massless scheme" should be superior, where charm and beauty quarks are treated as active flavours in the proton and in the photon, in addition to the light flavours u, d and s. In this paper we compare the data with NLO predictions in the "massive scheme". Predictions in the "massless" scheme are not yet available for the exclusive final states which we use in our measurement.

Experimental technique

The total beauty production cross-section at HERA is two orders of magnitude smaller than that of charm and another magnitude smaller than that of the light quarks. In this analysis the beauty events are identified using muons from semileptonic b-decays, which provide a clean experimental signature. In addition to the muon the event selection requires the presence of at least one jet (two jets) in the DIS (photoproduction) sample. To separate beauty events from charm and light quark background, two observables are used, which, for the first time at HERA, exploit simultaneously the large mass and the long lifetime of the b-quark, respectively: 1. The relative transverse momentum ptrel of the muon with respect to the axis of the associated jet: For muons from b-decays the spectrum extends to much larger values than for the other sources. 2. The impact parameter delta of the muon, which is the distance of the track in the transverse plane with respect to the ep collision vertex. For muons from b-decays this takes larger values as compared to the other sources. The precision measurement of muon track impact parameters is made possible by the H1 Central Silicon Track detector (CST). The fractions of b-quark events in the data samples are determined from a fit to the two-dimensional distribution of ptrel and delta.


The figure below shows as an examplary result the beauty cross sections in the photoproduction kinematic regime as a function of the transverse momentum of the muon. The data are compared to predictions from the massive NLO calculation. The yellow bands estimate the uncertainties of this prediction. The data are also compared to predictions from the leading order Monte Carlo generator programs CASCADE (dotted line) and PYTHIA (dash-dotted lines).
Observation: The data are in all bins above the predictions, the excess being most prominent at the lowest momenta.

The figure below summarises the recent beauty production cross section measurements at HERA as a function of the photon virtuality Q2. The plot shows the ratio of measured cross section to the NLO QCD prediction. The results of this paper (full circles and squares) are compared with other recent beauty production measurements from ZEUS and H1. The dotted lines indicate the typical uncertainties of the theory calculations.
Observation: A wide range in Q2 is stretched by the measurements. The data are in all bins somewhat above the predictions, with no clear trend as a function of Q2.

So what did we learn?

The measurement of beauty production in ep collisions at HERA is an interesting playground for our understanding of the strong force. In this analysis we have studied the production over a wide kinematical range of the resolving power of the photon emitted by the electron - from the photoproduction regime with photon virtualities Q2 of about zero to the DIS regime with Q2 reaching up to 100 GeV2. While it is expected that the perturbative QCD calculations in the massive scheme performed at next-to-leading order would give reliable predictions of the cross-sections, the cross-sections which we measured for a final state with a muon and jets seem to generally be somewhat above the calculations, rather independent of the Q2. An excess is observed at small transverse momenta of the muons or equivalently at small transverse momenta of the produced b-quarks.