Transversity and $\Lambda$ polarization at COMPASS

TL;DR

This paper reports on measurements of $ ext{Lambda}$ hyperon polarization in SIDIS at COMPASS, finding results compatible with zero, which informs models of transversity and hyperon fragmentation functions.

Contribution

The study provides the first measurements of $ ext{Lambda}$ transversity-induced polarization in SIDIS, testing theoretical models and constraining transversity and fragmentation functions.

Findings

Results are compatible with zero polarization across all phase space regions.

Data constrains the ratio of polarized to unpolarized $ ext{Lambda}$ fragmentation functions.

Results inform models of $s$ quark transversity and hyperon polarization mechanisms.

Abstract

The study of $\Lambda$ polarization in Semi-Inclusive Deep Inelastic Scattering (SIDIS) with a transversely polarized target has often been suggested as a privileged way to access transversity, as the initial quark polarization can be transmitted to the hyperon and then observed via its weak decay. COMPASS measured the $\Lambda$ transversity-induced polarization as a function of Bjorken $x$, of the fraction $z$ of the longitudinal momentum of the fragmenting quark carried by the $\Lambda$ and of the $\Lambda$ transverse momentum $P_T$. The results are compatible with zero in all regions of phase space. Under the hypothesis that transversity is a valence object, the previous knowledge of the transversity function allows to get information on the ratio of polarized and unpolarized $z-$integrated $\Lambda$ fragmentation functions. On the other hand, if the $\Lambda$ polarization is entirely due to the $s$ quark, or if a quark-diquark model is applied, information can be obtained on the $s$ quark transversity function.