Color-Octet $Jpsi$ Production at Low $p_perp$.pdfVIP

a r X i v : h e p - p h / 9 6 0 3 2 6 6 v 1 1 1 M a r 1 9 9 6 NORDITA-96/18 P March 1996 hep-ph/9603266 Color-Octet J/ψ Production at Low p⊥ Wai-Keung Tang? Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 M. Va?nttinen? NORDITA, Blegdamsvej 17, DK-2100 Copenhagen (February 1, 2008) Abstract We study contributions from color-octet quarkonium formation mechanisms to J/ψ hadroproduction at low p⊥. We include transitions of color-octet cc? states into “direct” J/ψ and into χ1,2 which decay radiatively into a J/ψ. Together with earlier work, this calculation constitutes a complete analysis of p⊥-integrated J/ψ production at leading twist. We find that the leading-twist contribution is not sufficient to reproduce the observed production rates and polarization of the J/ψ and χ1,2. Hence there must exist other important quarkonium production mechanisms at low p⊥. Typeset using REVTEX 1 I. INTRODUCTION The production and decays of heavy quarkonia have been widely studied using perturba- tive QCD [1]. Due to the large masses of the c and b quarks, the production or annihilation of heavy-quark–antiquark pairs takes place at a much shorter timescale than the forma- tion of the bound state. This makes it possible to factorize the transition amplitudes. In hadroproduction reactions, also the initial-state hadronic structure is usually expected to be factorizable from the QQ? production dynamics in terms of single parton distributions; in other words, quarkonium production is usually taken to be a leading-twist process. In the simplest approach, one takes the hadroproduction cross section to be a fixed fraction of the integrated QQ? cross section below the open heavy flavour threshold. This is called the semilocal duality model or color evaporation model. This model reproduces successfully the dependence of the cross sections on the center-of-mass energy of the colli- sion and on the longitudinal momentum of the quarkonium [2]. However, many observ