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±0.06MeV of the A=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-σ⁰ mixing. Using a schematic strong-interaction ΛN↔σN coupling model developed by Akaishi and collaborators for s-shell Λ hypernuclei, we revisit the evaluation of CSB in the A=4 Λ hypernuclei and extend it to p-shell mirror Λ hypernuclei. The model yields values of δBΛ4(0g.s.+)~0.25MeV. Smaller size and mostly negative p-shell binding energy differences are calculated for the A=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.