Reflections on charge symmetry breaking in hypernuclear physics

Abstract

Charge symmetry is broken in Quantum Chromodynamics (QCD) by the up-down quark mass difference and electro-magnetic interactions. In Λ hypernuclei, charge symmetry breaking (CSB) manifests itself in the charge dependence of Λ binding energies, BΛ. The full understanding of the sign, the size, and the spin-dependence of the CSB effect for isospin multiplets of different mass numbers remains one of the unresolved issues of hypernuclear physics. Experimentally, the only sizable CSB effect in hypernuclei was found in the A = 4 isospin mirror pair, making it exceptional among the hypernuclei. A renewed statistical analysis of six published BΛ values for HΛ4 is presented, adopting the Particle Data Group (PDG) procedures for error treatments including correlations. Its binding energy was estimated to be BΛ = 2.095 ± 0.043 MeV. The world database of hypernuclear binding energies, in particular for the A = 4 system and the problematic CΛ12 case, is critically reviewed. Obviously, precise binding and excitation energy data represent the fundamental building blocks to study CSB and many other aspects of hypernuclear physics.