Renormalization Mass Scale and Scheme Dependence in the Perturbative Contribution to Inclusive Semileptonic $b$ Decays

Abstract

We examine the perturbative calculation of the inclusive semi-leptonic decay rate $\Gamma$ for the $b$-quark, using mass-independent renormalization. To finite order of perturbation theory the series for $\Gamma$ will depend on the unphysical renormalization scale parameter $\mu$ and on the particular choice of mass-independent renormalization scheme; these dependencies will only be removed after summing the series to all orders. In this paper we show that all explicit $\mu$-dependence of $\Gamma$, through powers of ln$(\mu )$, can be summed by using the renormalization group equation. We then find that this explicit $\mu$-dependence can be combined together with the implicit $\mu$-dependence of $\Gamma$ (through powers of both the running coupling $a(\mu )$ and the running $b$-quark mass $m(\mu )$) to yield a $\mu$-independent perturbative expansion for $\Gamma$ in terms of $a(\mu )$ and $m(\mu )$ both evaluated at a renormalization scheme independent mass scale $IM$ which is fixed in terms of either the "$\overline{MS}$ mass" ${\overline{m}}_b$ of the $b$ quark or its pole mass $m_{pole}$. At finite order the resulting perturbative expansion retains a degree of arbitrariness associated with the particular choice of mass-independent renormalization scheme. We use the coefficients $c_i$ and $g_i$ of the perturbative expansions of the renormalization group functions $\beta (a)$ and $\gamma (a)$, associated with $a(\mu )$ and $m(\mu )$ respectively, to characterize the remaining renormalization scheme arbitrariness of $\Gamma$. We further show that all terms in the expansion of $\Gamma$ can be written in terms of the $c_i$ and $g_i$ coefficients and a set of renormalization scheme independent parameters ${\tau }_i$.

Comments: 26 pages, 4 figures, typo corrected