Abstract
We perform numerical comparison of the fragmentation mechanism of charmonium production (g,grightarrow c,{bar{c}} followed by crightarrow psi ,c) with the full leading order calculation (g,grightarrow psi ,c,{bar{c}} at {{mathcal {O}}}(alpha _s^4)). We conclude that the non-fragmentation contributions remain important up to J/psi transverse momenta about as large as 40 GeV, thus making questionable the applicability of the fragmentation approximation at smaller transverse momenta.
Highlights
1 Introduction we wish to corroborate these results, to add more details and discussions of the related physical issues, and to examine the case of kt -factorization
We consider an O(αs2) subprocess g g → c cand convolute it with an O(αs2) fragmentation function c → ψ c, where ψ is meant to be either J/ψ or ψ(2S)
We perform a full O(αs4) calculation for the process g g → ψ c cand see to what extent does the ‘full result’
Summary
In pp collisions the leading order process of J/ψ cc production is glue–glue fusion, g + g → J/ψ + c + c. We will perform two calculations in parallel, using the odinary (collinear) and the kt -factorization approaches. The latter can be treated as an effective Next-to-Leading order (NLO) calculation, since a significant part of higher-order radiative corrections is taken into account in the form of kT - dependent (unintegrated) gluon densities. If the initial gluons are off-shell, we adopt the kt -factorization prescription [11,12] and take the spin density matrix in the form μ g. Calculation of the traces and of all Feynman diagrams was done with the algebraic system form [13]. For collinear calculations we omit the integration over k1T and k2T and use the MSTW leading-order set [22] for the ordinary gluon distribution functions. The multidimensional integration in (6) has been performed by means of the Monte-Carlo technique, using the routine VEGAS [23]
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