Evidence for a Transverse Single-Spin Asymmetry in Leptoproduction of $\pi^{+} \pi^{-}$ Pairs

A single-spin asymmetry was measured in the azimuthal distribution of $\pi^{+} \pi^{-}$ pairs produced in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target. For the first time, evidence is found for a correlation between the transverse target polarization and the azimuthal orientation of the plane containing the two pions.The corresponding single-spin asymmetry is expected to be related to the product of the little-known quark transversity distribution function and an unknown naive-T-odd chiral-odd dihadron fragmentation function