Transverse polarization of quark pairs created in string fragmentation
  Classical arguments predict that the quark and the antiquark of a pair created during string fragmentation are both transversely polarized in the direction of \hat z \times \vec q_T, where z is the direction of the pull exerted by the string on the antiquark and \vec q_T (-\vec q_T) is the transverse momentum of the quark (antiquark). The existence of this effect at the quantum-mechanical level is investigated by considering two analogous processes involving the tunnel effect in a strong field: (1) dissociation of the positronium atom (2) electron pair creation. In case (1) the positronium is taken in the 3P_0 state to simulate the vacuum quantum numbers J^PC = 0^++. Using the nonrelativistic WKB method, the final electron and positron are indeed found to be transversely polarized along \hat z \times \vec p_T. On the contrary, case (2), treated with the Dirac equation, shows no correlation between transverse polarization and transverse momentum both when the field is uniform and when it depends on z and t. The pair is nevertheless produced in a triplet spin state. The difference between these two results and their relevance to transverse spin asymmetry in inclusive reactions is discussed. 
X.Artru (IPN Lyon, France) and J.Czyzewski (Jagellonian Univ. Cracow,   Poland)