Measure of $\pi$'s and $\Lambda$'s emitter radius via Bose-Einstein and Fermi-Dirac Statistics

Abstract

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 $\pi^{\pm} \pi^{\pm} \pi^{\pm}$ have been studied by OPAL with $4 \times 10^6$ 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 $\mathrm r_3 = 0.580 \pm 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 $0.74 \pm 0.09$. Next we summarise 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 M$_W$ measurements. Finally we describe the first attempts to evaluate the $\Lambda\Lambda$($\bar{\Lambda}\bar{\Lambda}$) emitter dimension via the Pauli exclusion principle with a combined result of $ \mathrm R_{\Lambda\Lambda} = 0.14 ^{+ 0.07}_{-0.03} ~~{\mathrm fm} taup