Two-dimensional analysis of Bose-Einstein Correlations in Hadronic Z Decays at LEP

An analysis of Bose-Einstein correlations in pairs of charged pions in hadronic Z decays, collected by ALEPH in 1994, is presented. The correlation function, which is measured using the unlike-sign and the mixed reference sample, is studied in terms of the Lorentz-invariant four-momentum difference and its transverse, $Q_{\textrm{T}}$, and longitudinal, $Q_{\textrm{L}}$, components with respect to the longitudinal centre-of-mass system. The values for the correlation radii are obtained from the fit of the Goldhaber parameterisation. The results indicate that the correlation radii values depend on the chosen kind of reference sample and on the two-jet purity. In any case a difference between $R_{\textrm{T}}$ and $R_{\textrm{L}}$, yielding $R_{\textrm{T}}/R_{\textrm{L}}<1$, is found, which is in agreement with the predictions of the Lund string model.