Kaons and other strange mesons

1* The neutral Kaons K⁰_s and K⁰_L represent symmetric and antisymmetric mixtures of the quark combinations down-antistrange and antidown-strange.

中性K介子K⁰ₛ和K⁰ᴸ是下夸克与反strange夸克组合的对称混合和反对称混合。

The charged kaons are mesons which have a quark composition of up-antistrange for the positive kaon and antiup-strange for the negative kaon. They decay in about 10^-8 seconds by the processes:

带电的K介子是具有夸克组分的介子：正K介子由上夸克和反 strangeness 构成，负K介子则由反上夸克和strange 构成。它们通过以下过程衰变，寿命约为10^-8秒：

Decay times in this range indicate decay by the weak interaction. None of the decay products has a strange quark, so this decay violates conservation of strangeness and cannot proceed by the strong interaction.

这一范围内的衰变时间表明是通过弱相互作用衰变。衰变产物中没有奇夸克，因此这种衰变违反了 strangeness 的守恒，因此不能通过强相互作用进行。

Describing the neutral kaons is much more complex. There are two versions of this particle with the same mass but different decay lifetimes!

One is called K-zero-short 一种称为K-zero-short
The other is called K-zero-long. 另一种称为K-zero-long。

These two particles are considered to be combinations of down-antistrange and antidown-strange quarks. These particles decay into pions by

描述中性K介子是相当复杂的。这种粒子有两种版本，它们的质量相同但衰变寿命不同！这两种粒子被认为是下夸克与反奇异夸克以及反下夸克与奇异夸克的组合。这些粒子通过衰变成π介子的方式进行衰变。

The quark mixing which leads to these combinations involves the exchange of two W bosons.

导致这些组合的夸克混合涉及两个W玻色子的交换。

The neutral kaons are important historically for their part in advancing our understanding of quark processes. When first discovered in the late 1940s they were labeled the τ and θ mesons, and their decay was a great mystery, labeled the τ-θ puzzle. The particles were identical in mass, and the only thing which apparently differentiated them was their decay processes and the fact that the two different sets of decay products had different parity. Particle decays by the strong or electromagnetic interactions had been observed to conserve parity in their decays. Theoretical physicists T. D. Lee and C. N. Yang proposed in 1956 that parity need not be conserved in weak interaction decays. In 1957 Chien-Shiung Wu showed this violation of parity conservation in the beta decay of cobalt. Even with the violation of parity, it was thought that the combination of charge conjugation and parity would leave the system invariant (CP invariance). An experiment by Cronin and Fitch in 1964 showed that there was a small CP violation in the kaon decay, so the kaon has played a central role in the discussions of these symmetries since that time.

中性K介子在历史上对于理解夸克过程起到了重要作用。当它们于20世纪40年代末被首次发现时，被标记为τ和θ介子，其衰变过程是一个巨大的谜题，被称为τ-θ之谜。这些粒子在质量上是相同的，唯一明显区分它们的是它们的衰变过程以及两种不同衰变产物的宇称不同。由强相互作用或电磁相互作用引起的粒子衰变已被观察到在衰变过程中守恒宇称。理论物理学家T. D. Lee和C. N. Yang于1956年提出，在弱相互作用衰变中宇称可能不守恒。1957年，Chien-Shiung Wu通过钴的β衰变实验展示了宇称守恒的违反。即使存在宇称的违反，人们仍认为电荷共轭与宇称的组合会保持系统不变（CP守恒）。1964年Cronin和Fitch的实验显示，在K介子衰变中存在微小的CP违反现象，因此自那时起，K介子在这些对称性的讨论中扮演了核心角色。

When Rochester and Butler discovered the short-lived version of the neutral kaon in 1946, they did so by observing a characteristic "V" pattern in a cloud chamber. The decay they saw was

1946年，罗切斯特和巴特勒在云室中观察到一种寿命很短的中性K介子的短寿命版本，他们通过观察云室中出现的特征“V”形图案实现了这一发现。他们所观察到的衰变是

The neutral kaon did not leave a track in the cloud chamber, so the "V" track of pions revealed its presence upon decay.