The Sld Collaboration

We have developed a new technique for inclusive reconstruction of the energy of B hadrons. The excellent efficiency and resolution of this technique allow us to make the most precise determination of the b-quark fragmentation function, using e + e − → Z 0 decays recorded in the SLD experiment at SLAC. We compared our measurement with the predictions of a number of fragmentation models. We excluded several of these models and measured the average scaled energy of weakly-decaying B hadrons to be x B = 0.714 ± 0.005 (stat) ± 0.007 (syst) ± 0.002 (model-dependence). In high-energy strong-interaction processes quarks and gluons are not observed directly, but appear as jets of colorless hadrons. This fundamental process of 'jet fragmentation' affects all high-energy physics measurements involving strongly-interacting particles, but is only poorly understood at a quantitative level. The fragmentation of heavy quarks is of particular experimental interest since many expected signatures of new heavy particles, such as Higgs and SUSY particles, involve decays to b quarks. It is hence vital to understand the production and properties of b jets. Here we present a significantly improved determination of the b-quark fragmentation function, D(x B) = (1/σ)dσ/dx B , measured using Z 0 → b ¯ b decays, where x B = E B /E beam represents the fraction of the b-quark energy retained by the weakly-decaying B hadron. In Quantum Chromodynamics (QCD) the b-quark mass serves as a cutoff for collinear gluon radiation. The distribution of b-quark energies prior to hadronisation can therefore be calculated perturbatively [1–5]. However, the additional effects which yield the experimentally-accessible distribution D(x B) are non-perturbative, and have been studied phenomenologically via a number of different approaches [1–12]. Measurements of D(x B) provide direct tests of these perturbative QCD and model predictions. D(x B) has been measured previously [13–16] by reconstructing the energies of B hadrons that decay semi-leptonically (B → lνDX) in Z 0 → b ¯ b events. In these studies the inclusive B selection efficiency was smaller than 1%, and much lower for E B < 20 GeV. The resulting low-statistics samples, and limited energy resolutions, yielded poor constraints on the shape of the distribution. We have developed a new technique for measuring E B using only kinematic information from charged tracks. Our 307 Mpixel CCD-based vertex detector, combined with the micron-sized SLC interaction point (IP), allows us to reconstruct B-decay vertices and the …