Transverse and longitudinal Bose-Einstein correlations in hadronic Z0 decays

Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z\(^0\) hadronic decays are studied as a function of the three components of the momentum difference, transverse (“out” and “side”) and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r\(_{t_{side}}\), and longitudinal, r\(_l\), dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y\(_{cut}\) = 0.04, are r\(_{t_{side}}\) = (0.809 \(\pm\) 0.009 (stat) \(^{+0.019}_{-0.032}\) (syst)) fm, r\(_l\) = (0.989 \(\pm\) 0.011 (stat) \(^{+0.030}_{-0.015}\) ({\it syst})) fm and r\(_l\)/r\(_{t_{side}}\) = 1.222 \(\pm\) 0.027 (stat) \(^{+0.075}_{-0.012}\) (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.