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

Final state interactions may give origin to non-zero values of the off-diagonal element of the helicity density matrix of vector mesons produced in annihilations, as confirmed by recent OPAL data on and 's. Predictions are given for of several mesons produced at large and small , i.e. collinear with the parent jet; the values obtained for and 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. Received: 15 May 1997 / Published online: 20 February 1998