Inclusive Charm Production in Upsilon(nS) Decay

TL;DR

This paper calculates the inclusive charm production rates in Upsilon(nS) decays using NRQCD, revealing branching fractions around 7-9% and analyzing the momentum distributions of charm quarks and hadrons.

Contribution

It provides the first leading-order NRQCD calculation of charm production in Upsilon decays, including momentum distributions and fragmentation effects.

Findings

Branching fractions for Upsilon(nS) to charm are 7-9%.

Color-singlet annihilation into gamma* and cc-bar gg dominate charm production.

Charm hadron momentum distributions are softer due to fragmentation.

Abstract

Based on the NRQCD factorization formalism, we calculate the inclusive charm production rate in Upsilon(nS) decay at leading order in the strong coupling constant alpha_s and the relative velocity v of the b quark in the quarkonium rest frame. The branching fractions for Upsilon(nS) to charm for n=1, 2, and 3 are all around 7-9%. About 60% of the branching fraction into charm is from annihilation of the color-singlet bb-bar pair into gamma^* -> cc-bar. Most of the remaining branching fraction is from annihilation of the color-singlet bb-bar pair decaying into cc-bar gg. We also compute the momentum distributions of the charm quark and charmed hadrons in the decays. The virtual-photon contribution to the charm quark momentum distribution is concentrated at the end point while the cc-bar gg contribution is broad. The momentum distributions for charmed hadrons are obtained by convolving the charm-quark momentum distribution with charm fragmentation functions. This makes the momentum distributions for charmed hadrons softer than that for the charm quark. This effect is particularly significant in the virtual-photon contribution.