Abstract
Bose-Einstein Correlations (BEC) of three identical charged pions were studied in $4 \times 10^6$ hadronic ${\rm Z}^0$ decays recorded with the OPAL detector at LEP. The genuine three-pion correlations, corrected for the Coulomb effect, were separated from the known two-pion correlations by a new subtraction procedure. A significant genuine three-pion BEC enhancement near threshold was observed having an emitter source radius of $r_3 = 0.580 \pm 0.004 \mbox{ (stat.)} \pm 0.029 \mbox{ (syst.)} \mbox{ fm}$ and a strength of $\lambda_3 = 0.504 \pm 0.010 \mbox{ (stat.)}\pm 0.041 \mbox{ (syst.)}$ . The Coulomb correction was found to increase the $\lambda_3$ value by $\sim $ 9% and to reduce $r_3$ by $\sim $ 6%. The measured $\lambda_3$ corresponds to a value of $0.707 \pm 0.014 \mbox{ (stat.)} \pm 0.078 \mbox{ (syst.)}$ when one takes into account the three-pion sample purity. A relation between the two-pion and the three-pion source parameters is discussed.
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