Direct determination of two-pion correlations for $\overline{p}p \to 2\pi^{+}2\pi^{-}$ annihilation at rest

We study $\pi\pi$ correlations in the exclusive reaction $\ppb \to 2\pi^+2\pi^-$ at rest measured by the CPLEAR experiment. Avoiding the introduction of an arbitrary reference sample, we analyse differential distributions for equal charge pion pairs removing the phase-space factor event by event. A peak at small relative momenta is most pronounced for large total momentum of the pair. The physical implications of bosonic symmetrization for the properties of the pion source, in particular its radius, are briefly discussed. The two extremes considered are the chaotic Hanbury-Brown--Twiss mechanism and the coherent Skyrmion model.