Study of the fragmentation of b quarks into B mesons at the Z peak

TL;DR

This study measures b-quark fragmentation into B mesons at the Z peak using ~4 million hadronic Z decays from the ALEPH detector. It reconstructs B-meson energies through semi-exclusive B→ℓνD^{(*)} decays and employs both model-dependent fits (Peterson, Kartvelishvili, Collins) and a model-independent unfolding to extract the B-meson energy spectra for weakly decaying and leading B mesons. The results yield ⟨x_B^{wd}⟩=0.7163±0.0061(stat)±0.0056(syst) and ⟨x_B^{L}⟩=0.7361±0.0063(stat)±0.0063(syst); Kartvelishvili provides a slightly better description than Peterson, while Collins is clearly disfavoured. The findings are consistent with prior measurements from L3, OPAL, and SLD, and the work refines our understanding of b-quark hadronisation in e+e− annihilation.

Abstract

The fragmentation of b quarks into B mesons is studied with four million hadronic Z decays collected by the ALEPH experiment during the years 1991-1995. A semi-exclusive reconstruction of B->l nu D(*) decays is performed, by combining lepton candidates with fully reconstructed D(*) mesons while the neutrino energy is estimated from the missing energy of the event. The mean value of xewd, the energy of the weakly-decaying B meson normalised to the beam energy, is found to be mxewd = 0.716 +- 0.006 (stat) +- 0.006 (syst) using a model-independent method; the corresponding value for the energy of the leading B meson is mxel = 0.736 +- 0.006 (stat) +- 0.006 (syst). The reconstructed spectra are compared with different fragmentation models.

Figures (5)

• Figure 1: Reconstructed $D$ mass peaks in the five channels. In the second channel the second peak at lower mass comes from the decay channel $D^0\to K \pi\pi\pi \pi^0$ and is excluded from the fit.
• Figure 2: In the five channels, $x_B^{\mathrm{obs}}$ spectra, after non-$b\bar{b}$ background subtraction and before acceptance corrections.
• Figure 3: Acceptance corrections $\epsilon_i^{\mathrm {wd}}(c)$ for $x_B^{\mathrm {wd}}$. The absolute scale is chosen as to conserve the total number of selected events for each channel. Only the distributions for channels 1 and 4 are shown, since they represent the extreme behaviours.
• Figure 4: Scaled energy of the leading and weakly-decaying $B$ meson, as reconstructed from data. The best-fit distributions for the Peterson model, the Kartvelishvili model, and the Collins model are superimposed. For the data, the bin-to-bin errors are highly correlated, as shown in the error matrices in the Appendix.
• Figure 5: Scaled energy of the weakly-decaying $B$ meson, as reconstructed from data. The inner error bars represent statistical errors, the larger ones the total uncertainties. The best-fit distributions for the Peterson model and the Kartvelishvili model are superimposed. For the data, the bin-to-bin errors are highly correlated, as shown in the error matrices in the Appendix.