The Sigma BaryonSigma 重子
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. sigma 是一种包含奇夸克的重子。三种不同 sigma 的夸克组成如上所示。这三种粒子具有相似的质量，并被称作同位旋三重态。唯一比 sigma 质量小且含有奇夸克的重子是中性 lambda 重子。中性 sigma 可以衰变成 lambda 而不违反 strangeness 的守恒，因此它通过电磁相互作用迅速衰变。sigma-zero 和 lambda-zero 有相同的夸克组成，因此 sigma-zero 可以被认为是 lambda-zero 的电磁激发态。带电的 sigma 粒子没有不涉及奇夸克转变的衰变路径，因此它们的衰变过程要慢得多，只能通过弱相互作用进行。
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. 根据粒子数据手册，西格马正粒子的衰变分支比为：通过pπ⁰路径为51.57%，通过nπ⁺路径为48.31%。这种近似相等的情况让我感到很惊讶——中子路径看起来要困难得多。
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Index Particle concepts 索引粒子概念 | ||||||||||||||||||||||||||||||||||||||
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