J/ψpolarization in photo-production up-to the next-to-leading order of QCD

Abstract

We investigate the J/psi polarization in photoproduction at the Hadron Electron Ring Accelerator(HERA) up to the next-to-leading order of QCD. The results show that the transverse momentum p_t and energy fraction z distributions of J/psi production do not agree with the observed ones very well. The theoretical uncertainties for the z distributions of the J/psi polarization parameters with respect to various choices of the renormalization and factorization scales are too large to give an accurate prediction. The uncertainties for the p_t distributions of these parameters are small when p_t>3GeV and the obtained p_t distributions can not describe the experimental data even in this region.

Figures (3)

• Figure 1: The energy fraction $z$ distribution of $J/\psi$ photoproduction with $P_t>$1GeV (left panel). The transverse momentum $p_t$ distribution of $J/\psi$ with $0.4<z<0.9$ (right panel). The dotted lines in each one are the LO results. The upper bound of the shaded band is obtained with $\mu_r=\mu_f=\frac{1}{2}\sqrt{(2m_c)^2+p_t^2}$ and $m_c$=1.4GeV, and the lower one with $\mu_r=\mu_f=2\sqrt{(2m_c)^2+p_t^2}$ and $m_c$=1.6GeV. The experimental data (filled circles with bars) are taken from Ref. Chekanov:2002at.
• Figure 2: The energy fraction $z$ distributions of polarization parameters $\lambda,\nu,$ and $\mu$ with $p_t>1$GeV, and transverse momentum $p_t$ distributions with $0.4<z<1$. Dashed lines are the LO results and the dot-dashed lines are the NLO results. Solid lines present the results with $\mu_r=\mu_f=8m_c$. The experimental data (filled circles with bars) are taken from Ref. Jungst:2008ip.
• Figure 3: The polarization parameter $\alpha$ distributions as functions of $P_t$ and $z$ in the helicity basis. Dashed lines are the results at LO and the dot-dashed lines are the ones at NLO. The results with $\mu_r=\mu_f=8m_c$ are presented by the solid lines.