Off-diagonal helicity density matrix elements for vector mesons produced at LEP

Abstract

Final state $q\bar q$ interactions may give origin to non-zero values of the off-diagonal element $\rho_{1,-1}$ of the helicity density matrix of vector mesons produced in $e^+ e^-$ annihilations, as confirmed by recent OPAL data on $\phi$ and $D^*$ 's. Predictions are given for $\rho_{1,-1}$ of several mesons produced at large $z$ and small $p_T$ , i.e. collinear with the parent jet; the values obtained for $\phi$ and $D^*$ are in agreement with data. The fragmentation functions of quarks and gluons into photons are studied beyond the Leading Logarithm approximation. We address the nature of the initial conditions of the evolution equation solutions and study problems related to factorization scheme invariance. The possibility of measuring these distributions in LEP experiments is discussed, and a comparison with existing data is made.