Observation of $\Xi^{-}$ Hyperon Transverse Polarization in $\psi(3686)\rightarrow\Xi^{-}\bar\Xi^{+}$

TL;DR

This paper reports the first observation of transverse polarization of the $\Xi^{-}$ hyperon in $\psi(3686)$ decays, measuring the relative phase of form factors and decay parameters with high significance using BESIII data.

Contribution

It provides the first measurement of the relative phase of electric and magnetic form factors in $\psi(3686)$ decays to hyperon pairs, with detailed decay parameter analysis.

Findings

$\Xi^{-}$ transverse polarization observed with 7.3$\sigma$ significance.

Relative phase of form factors measured as $\Delta\Phi = (0.667 \pm ext{stat}\pm ext{syst})$ rad.

Decay parameters and phase differences consistent with previous results.

Abstract

Using a sample of $(448.1~\pm~2.9)$ $\times 10^{6}$ $\psi(3686)$ decays collected with the BESIII detector at BEPCII, we report an observation of $\Xi^{-}$ transverse polarization with a significance of $7.3 \sigma$ in the decay $\psi(3686)\rightarrow\Xi^{-}\bar\Xi^{+}$ ($\Xi^{-}\rightarrow\pi^{-}\Lambda$, $\bar\Xi^{+}\rightarrow\pi^{+}\bar\Lambda$, $\Lambda\to p\pi^{-}$, $\bar\Lambda\to\bar{p}\pi^{+}$). The relative phase of the electric and magnetic form factors is determined to be $\Delta\Phi = (0.667 \pm 0.111 \pm 0.058)$ rad. This is the first measurement of the relative phase for a $\psi(3686)$ decay into a pair of $\Xi^{-}\bar\Xi^{+}$ hyperons. The $\Xi^{-}$ decay parameters ($\alpha_{\Xi^{-}}$, $\phi_{\Xi^-}$) and their conjugates ($\alpha_{\bar\Xi^{+}}$, $\phi_{\bar{\Xi}^{+}}$), the angular-distribution parameter $\alpha_{\psi}$, and the strong-phase difference $\delta_{p}-\delta_{s}$ for $\Lambda$-$\pi^-$ scattering are measured to be consistent with previous BESIII results.