Azimuthal asymmetries of back-to-back π±−(π0, η, π±) pairs in e+e− annihilation

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Abstract

This work reports the first observation of azimuthal asymmetries around the thrust axis in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation of pairs of back-to-back charged pions in one hemisphere, and ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ mesons in the opposite hemisphere. These results are complemented by a new analysis of pairs of back-to-back charged pions. The ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ asymmetries rise with the relative momentum $z$ of the detected hadrons as well as with the transverse momentum with respect to the thrust axis. These asymmetries are sensitive to the Collins fragmentation function ${H}_{1}^{\ensuremath{\perp}}$ and provide complementary information to previous measurements with charged pions and kaons in the final state. In particular, the $\ensuremath{\eta}$ final states will provide additional information on the flavor structure of ${H}_{1}^{\ensuremath{\perp}}$. This is the first measurement of the explicit transverse-momentum dependence of the Collins fragmentation function from Belle data. It uses a dataset of $980.4\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ collected by the Belle experiment at or near a center-of-mass energy of 10.58 GeV.

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