Charge symmetry breaking in Λ hypernuclei revisited

The large charge symmetry breaking (CSB) implied by the Λ binding energy difference ΔBΛ4(0g.s.+)≡BΛ(HeΛ4)−BΛ(HΛ4)=0.35±0.06 MeV of the A=4A=4 mirror hypernuclei ground states, determined from emulsion studies, has defied theoretical attempts to reproduce it in terms of CSB in hyperon masses and in hyperon–nucleon interactions, including one pion exchange arising from Λ–Σ0Σ0 mixing. Using a schematic strong-interaction ΛN↔ΣNΛN↔ΣN coupling model developed by Akaishi and collaborators for s  -shell Λ hypernuclei, we revisit the evaluation of CSB in the A=4A=4 Λ hypernuclei and extend it to p  -shell mirror Λ hypernuclei. The model yields values of ΔBΛ4(0g.s.+)∼0.25 MeV. Smaller size and mostly negative p  -shell binding energy differences are calculated for the A=7–10A=7–10 mirror hypernuclei, in rough agreement with the few available data. CSB is found to reduce by almost 30 keV the 110 keV BΛ10 g.s. doublet splitting anticipated from the hyperon–nucleon strong-interaction spin dependence, thereby explaining the persistent experimental failure to observe the 2exc−→1g.s.−γ-ray transition.