Abstract
We calculate the polarization of prompt $J/\psi$ production in the improved color evaporation model at leading order employing the $k_T$-factorization approach. In this paper, we present the polarization parameter $\lambda_\vartheta$ of prompt $J/\psi$ as a function of transverse momentum in $p+p$ and $p$ + A collisions to compare with data in the helicity, Collins-Soper and Gottfried-Jackson frames. We also present calculations of the charmonium production cross sections as a function of rapidity and transverse momentum. This is the first $p_T$-dependent calculation of charmonium polarization in the improved color evaporation model. We find agreement with both charmonium cross sections and polarization measurements.
Highlights
The production mechanism of quarkonium remains uncertain even more than 40 years after the discovery of the J=ψ
The traditional color evaporation model (CEM) can describe the unpolarized yield of charm and J=ψ production at both leading order (LO) and next-to-leading order (NLO) assuming collinear factorization [23,39]
The improved CEM (ICEM) can describe the ψð2SÞ to J=ψ ratio at NLO while, in the traditional CEM, this ratio is independent of pT [22]
Summary
The production mechanism of quarkonium remains uncertain even more than 40 years after the discovery of the J=ψ. The LDMEs, which weight the contributions from each color and spin state, are fit to the data above some minimum transverse momentum, pT These LDMEs, which are conjectured to be universal, fail to describe the yields and polarization simultaneously for pT cuts less than twice the mass of the quarkonium state [4,5]. This paper serves as a continuation of the previous work by presenting a pT-dependent LO ICEM calculation of the polarization in prompt J=ψ production using the kT-factorization approach This is a pT-dependent result because the transverse momenta of the incoming gluons and their off-shell properties are not neglected in the kTfactorization approach. FQ is a universal factor for the directly produced quarkonium state Q and is independent of the projectile, target, and energy In this approach, the cross section is dpT d4σ dydsdφ.
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