Two-dimensional analysis of Bose-Einstein correlations in hadronic Z decays at LEP

Bose-Einstein correlations are studied in pairs of charged pions from hadronic Z decays, collected by the ALEPH detector. The correlation function, measured using either the unlike-sign or the mixed reference sample, is studied in terms of the Lorentz-invariant four-momentum difference and its transverse, QT, and longitudinal, QL, components with respect to the longitudinal centre-of-mass system. Values for the correlation radii, RT and RL, are obtained from the fit of the Goldhaber parametrisation. The results indicate that the correlation radii values depend on the chosen kind of reference sample and on the two-jet purity. (orig.)