Direct J/psi and psi' Polarization and Cross Sections at the Tevatron

Abstract

Transverse polarization of ${}^3 S_1$ charmonium states, produced directly in $p\bar{p}$ collisions at asymptotically large transverse momentum p_t, has emerged as the most prominent test of color octet contributions and spin symmetry in quarkonium production. We present predictions for the polar angle distribution at moderate values of p_t \sim 4 - 20 GeV, covered by the Tevatron Run I data. We update the fits of NRQCD matrix elements and discuss their theoretical uncertainties. With our best fit values, no transverse polarization is expected at p_t \sim 5 GeV, but the angular distribution is predicted to change dramatically as p_t increases to 20 GeV.

Figures (3)

• Figure 1: Fit of color octet contributions to direct $J/\psi$ production data from CDF ($\sqrt{s}=1.8\,$TeV, pseudorapidity cut $|\eta|<0.6$). The theoretical curves are obtained with CTEQ4L parton distribution functions, the corresponding $\Lambda_4=235\,$MeV, factorization scale $\mu=(p_t^2+4 m_c^2)^{1/2}$ and $m_c=1.5\,$GeV. The fitted color octet matrix elements are given as the central values in Tab \ref{['tab1']}.
• Figure 2: Same as Fig. \ref{['jpsi']} for prompt $\psi'$ production.
• Figure 3: $\alpha$ as a function of $p_t$. Parameter specifications as for Fig. \ref{['jpsi']}. The band is obtained from Eq. (\ref{['xil']}) and the ${\cal O}(\alpha_s^4)$ fragmentation contributions from BEN1 as described in the text. The solid curve is obtained with $M_{3.5}^{\psi}(^1\!S_0^{(8)},^3\!P_0^{(8)})=0$ and shows the importance of this parameter for polarization. The band does not include theoretical uncertainties in the short-distance coefficients, Tab. \ref{['tab2']}.