The Sigma Baryon

The sigma is a baryon which contains a strange quark. The quark composition of the three different sigmas is shown above. The three varieties have similar masses and are said to be an isospin triplet. The only baryon with a strange quark which is less massive than the sigma is the neutral lambda baryon. The neutral sigma can decay to the lambda without violating conservation of strangeness, so it proceeds rapidly by the electromagnetic interaction. The sigma-zero and lambda-zero have the same quark constituents so the sigma-zero can be considered to be an electromagnetic excited state of the lambda-zero. The charged sigmas have no decay path which does not involve the transmutation of the strange quark, so their decays are much slower, proceeding only by the weak interaction.

According to the Particle Data Book, the branching ratio for the decays of the sigma-plus is 51.57% for the pπ⁰ pathway and 48.31% for the nπ⁺ pathway. This near equivalence is really surprising to me - the neutron pathway looks a lot harder.