The b Quark Fragmentation Function, From LEP to TeVatron

Abstract

The b quark fragmentation distribution has been measured, using data registered by the DELPHI experiment at the Z pole, in the years 1994-1995. The measurement made use of 176000 inclusively reconstructed B meson candidates. The uncertainties of this measurement are dominated by systematic effects, the principal ones being related to the energy calibration. The distribution has been established in a nine bin histogram. Its mean value has been found to be = 0.704±0.001(stat.)±0.008(syst.) Using this measurement, and other available analyses of the b-quark fragmentation distribution in e +e − collisions, the non-perturbative QCD component of the distribution has been extracted independently of any hadronic physics modeling. This distribution depends only on the way the perturbative QCD component has been defined. When the perturbative QCD component is taken from a parton shower Monte-Carlo, the non-perturbative QCD component is rather similar with those obtained from the Lund or Bowler models. When the perturbative QCD component is the result of an analytic NLL computation, the non-perturbative QCD component has to be extended in a non-physical region and thus cannot be described by any hadronic modeling. In the two examples, used to characterize these two situations, which are studied at present, it happens that the extracted non-perturbative QCD distribution has the same shape, being simply translated to higher-x values in the second approach, illustrating the ability of the analytic perturbative QCD approach to account for softer gluon radiation than with a parton shower generator. Using all the available analyses of the b-quark fragmentation distribution in e +e − collisions, together with the result from DELPHI presented in this thesis, a combined world average b fragmentation distribution has been obtained. Its mean value has been found to be = 0.714±0.002. An analysis of the B hadron production at CDF is ongoing. It makes use of ∼ 6000 B± candidates, from 333pb−1 of data recorded by the CDF experiment, fully reconstructed in the decay channel B± → J/ψK ±. Characteristics of B mesons and accompanying tracks have been examined in the perspective of understanding the effect of fragmentation. These studies, done in the framework of the PYTHIA event generator, also involve the contributions from different bb production mechanisms. Distributions from a fully reconstructed Monte Carlo sample have ¯ been compared to data, and the agreement has been found to be reasonable. The analysis is ongoing, and the goal is to fit the fragmentation function parameters and/or the relative contributions from different production mechanisms to improve the agreement between data and Monte Carlo. A measurement of the b quark production cross section has been obtained using the same data. The analysis is still under way, and therefore the result is preliminary. It has been found: σ(pp → bX, pT (B) > 4GeV/c, |η| 4GeV/c, |η| < 0.6) = 11.9±0.6(stat.) ±1.0(syst.)µb (6)