Measure of π ’ s and Λ ’ s emitter radius via Bose - Einstein and Fermi - Dirac Statistics

This report summarises the recent studies of the Bose-Einstein Correlations (BEC) carried out by the four LEP experiments using hadronic Z 0 decay events and e + e − reactions leading to the WW decay final states. The three identical charged pion systems π ± π ± π ± have been studied by OPAL with 4 × 10 6 hadronic Z 0 decays events. The genuine three-pion correlations corrected for Coulomb effects , were separated from the known two-pion correlations by a novel subtraction procedure using the data events themselves. From this analysis a value of r3 = 0.580 ± 0.029 fm was obtained. The recent L3 study of the two-dimensional two-pion BEC is also discussed. In this study, which utilised the longitudinal co-moving system, evidence is given for a non-spherical di-pion emitter shape where the ratio of the minor to major axes is found to be 0.74 ± 0.09. Next we sum-marise the experimental aspects of the BEC and colour reconnection phenomena in W-pair events at LEP and their contribution to the systematic error associated with the MW measurements. Finally we describe the first attempts to evaluate the ΛΛ(¯ Λ ¯ Λ) emitter dimension via the Pauli exclusion principle with a combined result of RΛΛ = 0.14 +0.07 −0.03 fm.