J/psi polarization at hadron colliders in nonrelativistic QCD
  With nonrelativistic QCD factorization, we present a full next-to-leading order computation of the polarization observable for $J/psi$ production at hadron colliders including all important Fock states, i.e. $^3S_1[1,8]$, $^1S_0[8]$, and $^3P_J[8]$. We find the $^3P_J[8]$ channel contributes a positive longitudinal component and a negative transverse component. So the $J/psi$ polarization puzzle may be understood as the transverse components canceling between $^3S_1[8]$ and $^3P_J[8]$ channels, which results in mainly the unpolarized (even slightly longitudinally polarized) $J/psi$. This may give a possible solution to the long-standing $J/psi$ polarization puzzle. Predictions for $J/psi$ polarization at the LHC are also presented. 
Kuang-Ta Chao, Yan-Qing Ma, Hua-Sheng Shao, Kai Wang, Yu-Jie Zhang
hep-ph hep-ex