Nuclear Physics A 848 (2010) 218–229 www.elsevier.com/locate/nuclphysa The role of spin in fragmentation production j/ψ(1S) and Υ(1S) Reza Sepahvand ∗ , Sareh Dadfar Department of Physics, Faculty of Science, Lorestan University, Khoramabad 6813717133, Iran Received 23 May 2010; received in revised form 25 July 2010; accepted 30 August 2010 Available online 8 September 2010 Abstract In this work we calculate the process-independent fragmentation functions for a heavy quark Q to frag- ment into vector Q ¯ Q( 3 S 1 ) and pseudoscalar Q ¯ Q( 1 S 0 ) heavy quarkonium to leading order perturbative QCD. We investigate the role of the spin orientation of the initial and final heavy quarks on the fragmen- tation functions. In the special case, we calculate fragmentation functions by using average over spin of initial state heavy quark and sum over spin of final states. The universal fragmentation probabilities for the c → j/ψ(1S) and b → Υ(1S) are about 10 −4 and 10 −5 , respectively. Moreover, the spin dependent observables such as V /(V + P) and V /P ratios are estimated.  2010 Elsevier B.V. All rights reserved. Keywords: Fragmentation functions; Spin; Heavy quarkonium 1. Introduction The production of heavy quarkonium states at high-energy colliders has attracted a lot of theo- retical [1–14] and experimental [15–21] interest. From the theoretical point of view, quarkonium production provides a very important test of perturbative QCD. Predictions for production of j/ψ and Υ heavy mesons at high-energy processes using the QCD calculations are accomplished in the color singlet [22–30] and the color octet [31–33] mechanisms. There are various channels to produce singlet or octet states, in which the direct quark and gluon fragmentations are thought to * Corresponding author. Tel.: +986612200079; fax: +986612200185. E-mail address: r.sepahvand@gmail.com (R. Sepahvand). 0375-9474/$ – see front matter  2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.08.017