# Kaon Photoproduction and the $\Lambda$ Decay Parameter $\alpha_-$

## TL;DR

This paper provides a new, independent estimate of the $oldsymbol{ ext{Lambda}}$ decay parameter $oldsymbol{ extalpha_-}$ using polarization data from kaon photoproduction, supporting recent findings that it is larger than the long-accepted value.

## Contribution

The study offers a novel, data-driven determination of $ extalpha_-$, challenging the longstanding PDG value and aligning with recent BESIII results.

## Key findings

- New estimate of $ extalpha_-$: 0.721(6)(5)
- Supports BESIII's larger $ extalpha_-$ value
- Improves data description with modern phenomenological model

## Abstract

The weak decay parameter $\alpha_-$ of the $\Lambda$ is an important quantity for the extraction of polarization observables in various experiments. Moreover, in combination with $\alpha_+$ from $\bar\Lambda$ decay it provides a measure for matter-antimatter asymmetry. The weak decay parameter also affects the decay parameters of the $\Xi$ and $\Omega$ baryons and, in general, any quantity in which the polarization of the $\Lambda$ is relevant. The recently reported value by the BESIII collaboration of $0.750(9)(4)$ is significantly larger than the previous PDG value of $0.642(13)$ that had been accepted and used for over 40 years. In this work we make an independent estimate of $\alpha_-$, using an extensive set of polarization data measured in kaon photoproduction in the baryon resonance region and constraints set by spin algebra. The obtained value is 0.721(6)(5). The result is corroborated by multiple statistical tests as well as a modern phenomenological model, showing that our new value yields the best description of the data in question. Our analysis supports the new BESIII finding that $\alpha_-$ is significantly larger than the previous PDG value. Any experimental quantity relying on the value of $\alpha_-$ should therefore be re-considered.