Observation of Transverse $\Lambda/\bar{\Lambda}$ Hyperon Polarization in $e^+e^-$ Annihilation at Belle

TL;DR

This paper reports the first observation of transverse polarization of Lambda hyperons in electron-positron annihilation, revealing insights into hyperon spin dynamics and fragmentation processes.

Contribution

It provides the first measurement of transverse Lambda hyperon polarization in e+e- collisions, highlighting the role of polarizing fragmentation functions.

Findings

Significant transverse polarization observed

Polarization increases with hyperon energy fraction

Results consistent across Lambda and anti-Lambda hyperons

Abstract

We report the first observation of the spontaneous polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons transverse to the production plane in $e^+e^-$ annihilation, which is attributed to the effect arising from a polarizing fragmentation function. For inclusive $\Lambda/\bar{\Lambda}$ production, we also report results with subtracted feed-down contributions from $\Sigma^0$ and charm. This measurement uses a dataset of 800.4 fb$^{-1}$ collected by the Belle experiment at or near a center-of-mass energy of 10.58 GeV. We observe a significant polarization that rises with the fractional energy carried by the $\Lambda/\bar{\Lambda}$ hyperon.