Observation of Spin-Dependent Charge Symmetry Breaking in $\Lambda N$ Interaction: Gamma-Ray Spectroscopy of $^4_{\Lambda }$He

TL;DR

This study measures the energy difference in hypernucleus $^4_ ext{Lambda}$He to observe charge symmetry breaking in $ ext{Lambda}N$ interactions, revealing spin-dependent effects through gamma-ray spectroscopy.

Contribution

First direct measurement of charge symmetry breaking in $ ext{Lambda}N$ interaction using gamma-ray spectroscopy of $^4_ ext{Lambda}$He.

Findings

Charge symmetry breaking observed in $ ext{Lambda}N$ interaction.

CSB effect is larger in the $0^+$ state than in the $1^+$ state.

Demonstrates spin dependence of $ ext{Lambda}N$ CSB interaction.

Abstract

The energy spacing between the ground-state spin doublet of $^4_\Lambda $He(1$^+$,0$^+$) was determined to be $1406 \pm 2 \pm 2$ keV, by measuring $\gamma$ rays for the $1^+ \to 0^+$ transition with a high efficiency germanium detector array in coincidence with the $^4$He$(K^-,\pi^-)$ $^4_\Lambda $He reaction at J-PARC. In comparison to the corresponding energy spacing in the mirror hypernucleus $^4_\Lambda $H, the present result clearly indicates the existence of charge symmetry breaking (CSB) in $\Lambda N$ interaction. It is also found that the CSB effect is large in the $0^+$ ground state but is by one order of magnitude smaller in the $1^+$ excited state, demonstrating that the $\Lambda N$ CSB interaction has spin dependence.