We calculate the polarization of prompt $J/\psi$ and $\Upsilon$(1S) production using the color evaporation model at leading order. We present the polarization parameter $\lambda_\vartheta$ as a function of center of mass energy and rapidity in $p+p$ collisions. We also compare the $x_F$ dependence to experimental results in $p$+Cu and $\pi$+W collisions, and predict the $x_F$ dependence in $p$+Pb collisions at fixed-target energies. At energies far above the $Q\overline{ Q}$ production threshold, we find the prompt $J/\psi$ and $\Upsilon$(1S) production to be longitudinally polarized with $\lambda_\vartheta^{ J/\psi}=-0.51^{ +0.05}_{ -0.16}$ and $\lambda_\vartheta^{ \Upsilon \rm{ (1S)}}=-0.69^{ +0.03}_{ -0.02}$. Both prompt $J/\psi$ and prompt $\Upsilon$(1S) are also longitudinally polarized at central rapidity, becoming transversely polarized at the most forward rapidities.
Categories: hep-ph