Angular distributions of massive quarks and leptons close to threshold

• Published in: Physics letters. B Vol. 359; no. 3; pp. 355 - 361
• Main Authors: Brodsky, S.J., Hoang, A.H., Kühn, J.H., Teubner, T.
• Format: Journal Article
• Language: English
• Published: United States Elsevier B.V 12.10.1995
• ISSN: 0370-2693; 1873-2445
• DOI: 10.1016/0370-2693(95)01070-7

Predictions for the angular distribution of massive quarks and leptons are presented, including QCD and QED corrections. Recent results for the fermionic part of the two-loop corrections to the electromagnetic form factors are combined with the BLM scale fixing prescription. Two distinctly different scales arise as arguments of α s ( μ 2) near threshold: the relative momentum of the quarks governing the soft gluon exchange responsible for the Coulomb potential, and a large momentum scale approximately equal to twice the quark mass for the corrections induced by transverse gluons. Numerical predictions for charmed, bottom, and top quarks are given. One obtains a direct determination of α V( Q 2), the coupling in the heavy quark potential, which can be compared with lattice gauge theory predictions. The corresponding QED results for τ pair production allow for a measurement of the magnetic moment of the τ and could be tested at a future τ-charm factory.