Averages of b-hadron, c-hadron, and tau-lepton properties as of early 2012

TL;DR

HFAG delivers updated world averages for heavy-flavor properties by harmonizing results from multiple experiments through a rigorous, parameter-constrained averaging framework. The paper details the methodology for handling external parameter dependencies and correlated systematics, then presents updated averages for b-hadron production fractions, lifetimes, and mixing parameters across Υ(4S), Υ(5S), and high-energy environments. Key findings include near-equal B^+ and B^0 production at the Υ(4S), a fractional abundance of B_s states around 0.21–0.24 at the Υ(5S) with B_s* production dominating, and an ongoing assumption of universal fractions at high energy with observed p_T-dependent variations in baryon production. These averages feed into CKM determinations and CP-violation studies, underpinning precision flavor physics across LEP, Tevatron, and LHC data.

Abstract

This article reports world averages of measurements of b-hadron, c-hadron, and tau-lepton properties obtained by the Heavy Flavor Averaging Group (HFAG) using results available through the end of 2011. In some cases results available in the early part of 2012 are included. For the averaging, common input parameters used in the various analyses are adjusted (rescaled) to common values, and known correlations are taken into account. The averages include branching fractions, lifetimes, neutral meson mixing parameters, CP violation parameters, parameters of semileptonic decays and CKM matrix elements.

Figures (4)

• Figure 1: The left-hand plot (a) compares the 68% confidence-level contours of a hypothetical measurement's unconstrained (large ellipse) and constrained (filled ellipse) likelihoods, using the Gaussian constraint on $y_i$ represented by the horizontal band. The solid error bars represent the statistical uncertainties $\sigma(x)$ and $\sigma(y_i)$ of the unconstrained likelihood. The dashed error bar shows the statistical error on $x$ from a constrained simultaneous fit to $x$ and $y_i$. The right-hand plot (b) illustrates the method described in the text of performing fits to $x$ with $y_i$ fixed at different values. The dashed diagonal line between these fit results has the slope $\rho(x,y_i)\sigma(y_i)/\sigma(x)$ in the limit of a parabolic unconstrained likelihood. The result of the constrained simultaneous fit from (a) is shown as a dashed error bar on $x$.
• Figure 2: The upper plots (a) and (b) show examples of two individual measurements to be combined. The large ellipses represent their unconstrained likelihoods, and the filled ellipses represent their constrained likelihoods. Horizontal bands indicate the different assumptions about the value and uncertainty of $y_i$ used by each measurement. The error bars show the results of the approximate method described in the text for obtaining $x$ by performing fits with $y_i$ fixed to different values. The lower plots (c) and (d) illustrate the adjustments to accommodate updated and consistent knowledge of $y_i$ as described in the text. Open circles mark the central values of the unadjusted fits to $x$ with $y$ fixed; these determine the dashed line used to obtain the adjusted values.
• Figure 3: An illustration of the combination of two hypothetical measurements of $x$ using the method described in the text. The ellipses represent the unconstrained likelihoods of each measurement, and the horizontal band represents the latest knowledge about $y_i$ that is used to adjust the individual measurements. The filled small ellipse shows the result of the exact method using ${\cal L}_{\text{comb}}$, and the hollow small ellipse and dot show the result of the approximate method using $\chi^2_{\text{comb}}$.
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