Azimuthal asymmetries of back-to-back $\pi^\pm-(\pi^0,\eta,\pi^\pm)$ pairs in $e^+e^-$ annihilation

TL;DR

This paper presents the first measurement of azimuthal asymmetries in back-to-back pion and meson pairs in electron-positron annihilation, revealing insights into the Collins fragmentation function and its flavor structure.

Contribution

It provides the first explicit measurement of the transverse-momentum dependence of the Collins fragmentation function using Belle data.

Findings

Azimuthal asymmetries observed in pion and meson pairs increase with momentum fractions and transverse momentum.

Results offer new information on the flavor dependence of the Collins fragmentation function.

First measurement of the transverse-momentum dependence of $H_1^{ot}$ from $e^+e^-$ data.

Abstract

This work reports the first observation of azimuthal asymmetries around the thrust axis in $e^+e^-$ annihilation of pairs of back-to-back charged pions in one hemisphere, and $\pi^0$ and $\eta$ mesons in the opposite hemisphere. These results are complemented by a new analysis of pairs of back-to-back charged pions. The $\pi^0$ and $\eta$ 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 $H_1^{\perp}$ and provide complementary information to previous measurements with charged pions and kaons in the final state. In particular, the $\eta$ final states will provide additional information on the flavor structure of $H_1^{\perp}$. 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~fb$^{-1}$ collected by the Belle experiment at or near a center-of-mass energy of 10.58 GeV.