Charge symmetry breaking in the A = 4 hypernuclei

Charge symmetry breaking (CSB) in the Λ-nucleon strong interaction generates a charge dependence of Λ separation energies in mirror hypernuclei, which in the case of the A=4 mirror hypernuclei 0⁺ ground states is sizable, ΔB_Λ ^J=0≡B_Λ ^J=0(HeΛ4)−B_Λ ^J=0(HΛ4)=230±90 keV, and of opposite sign to that induced by the Coulomb repulsion in light hypernuclei. Recent ab initio calculations of the (HΛ4, HeΛ4) mirror hypernuclei 0_g.s. ⁺ and 1_exc ⁺ levels have demonstrated that a Λ−Σ⁰ mixing CSB model due to Dalitz and von Hippel (1964) is capable of reproducing this large value of ΔB_Λ ^J=0. These calculations are discussed here with emphasis placed on the leading-order chiral EFT hyperon–nucleon Bonn–Jülich strong-interaction potential model used and the no-core shell-model calculational scheme applied. The role of one-pion exchange in producing sizable CSB level splittings in the A=4 mirror hypernuclei is discussed.