Transverse $\Lambda$ Polarization in $e^+ e^-$ collisions

TL;DR

This paper explores how different measurement axes in $e^+ e^-$ collisions reveal distinct mechanisms for $ ext{Lambda}$ hyperon polarization, developing formal frameworks and predicting observable effects for future experiments.

Contribution

It introduces a unified analysis of $ ext{Lambda}$ polarization using both TMD and twist-3 formalisms, highlighting the impact of measurement axes on the underlying fragmentation functions.

Findings

TMD formalism describes polarization with respect to the thrust axis.

Twist-3 formalism applies when measuring relative to initial leptons.

Predictions for $ ext{Lambda}$ polarization at Belle and OPAL.

Abstract

In this paper we study transverse polarization of $\Lambda$ hyperons in single-inclusive leptonic annihilation. We show that when the transverse momentum of the $\Lambda$ baryon is measured with respect to the thrust axis, a transverse momentum dependent (TMD) factorization formalism is required and the polarization is generated by the TMD polarizing fragmentation function (TMD PFF), $D_{1T}^\perp$. However, when the transverse momentum of the $\Lambda$ baryon is measured with respect to the momentum of the initial leptons, a collinear twist-3 formalism is required and the polarization is generated by the intrinsic collinear twist-3 fragmentation function $D_{T}$. Thus while these measurements differ from one another only by a change in the measurement axis, they probe different distribution functions. Recently, Belle measured a significant polarization in single-inclusive $\Lambda$ baryon production as a function of the transverse momentum with respect to the thrust axis. However, this data can in principle be re-analyzed to measure the polarization as a function of the transverse momentum of the $\Lambda$ baryon with respect to the lepton pair. This observable could be the first significant probe of the function, $D_{T}$. In this paper, we first develop a TMD formalism for $\Lambda$ polarization; we then present a recent twist-3 formalism that was established to describe $\Lambda$ polarization. Using the TMD formalism, we demonstrate that the $\Lambda$ polarization at OPAL and Belle can be described using the twist-2 TMD factorization formalism. Finally, we make a theoretical prediction for this polarization in the collinear twist-3 formalism at Belle.