 Radiative Sigma-to-Lambda transition

The neutral Sigma baryon and Lambda baryon both have valence content $uds$, but different spin-isospin couplings. An electromagnetic transition changes those internal couplings without changing any quark flavour. In the simple ground-state quark model, the nonzero magnetic transition is associated with the up- and down-quark electromagnetic currents; the strange-quark spin operator alone cannot change the $ud$ pair from spin one to spin zero. Four-momentum conservation gives photon energy $E_{\gamma }=(M_{\Sigma }^2-M_{\Lambda }^2)/(2M_{\Sigma })$ in natural units.