The Kaon
The kaon is the lightest particle that can carry strangeness, and that simple fact makes it the only particle in the standard model with a property it can lose but not regain in the same form. There are four of them — K⁺, K⁻, K⁰, K̄⁰ — two charged and two neutral, each the antiparticle of the other. The charged ones decay by the weak interaction and live long enough to leave a visible track in a cloud chamber. The neutral ones do something the charged ones cannot: they oscillate.
Strangeness was discovered in 1947, when a British team at the Bristol laboratory found that certain cosmic-ray events produced particles that behaved like mesons but decayed far more slowly than any meson had any right to decay. The explanation was a new quantum number, strangeness S, conserved by the strong force but not by the weak. The kaon got S = +1 or S = −1 depending on whether it was a particle or antiparticle. The name came from the Greek kappa because it was the first member of the strange particle family, and the convention stuck.
The K⁺ is an up–antistrange quark bound by gluons: mass 493.7 MeV, lifetime 1.24×10⁻⁸ s. It decays mostly to muon + neutrino or to three pions. The K⁻ is its mirror: up antiquark bound to strange quark. The two are straightforward. They are what you expect a particle and antiparticle to be.
The neutral ones are not.
The K⁰ contains a down quark and an antistrange quark. The K̄⁰ contains an antidown and a strange quark. They are distinct particles, exactly as the charged K⁺ and K⁻ are distinct. But the weak interaction does not conserve strangeness, and it does not care about the distinction between down and antidown when strangeness is on the table. The result is that a K⁰ produced in a strong interaction — a π⁻ proton collision that conserves strangeness, which always produces a K⁰ together with a Λ — will, as it propagates, turn into a K̄⁰ and back again, a full oscillation every few centimetres at relativistic speed.
This is not a small effect. It is the defining feature of the neutral kaon system, and it is what makes kaons interesting to particle physicists who do not have a reason to be interested in them for any other reason.
The K⁰ and K̄⁰ are flavor eigenstates. They are produced and detected this way. But they do not propagate this way. The states that propagate — the ones with definite mass and lifetime — are linear combinations of the flavor states. To first approximation these are K_S (short) and K_L (long), named for their lifetimes: K_S decays in about 0.9×10⁻¹⁰ s, K_L in about 5.1×10⁻⁸ s. K_S is essentially a CP-even state; K_L is essentially CP-odd. In a world where CP is exactly conserved, K_L would never decay to two pions, because two pions have CP = +1 and K_L has CP = −1.
It was this expectation — the rule that K_L → ππ is forbidden by CP conservation — that James Cronin and Val Fitch broke in 1964, and in doing so revealed that the universe is not perfectly symmetric between matter and antimatter. That is the experiment. The kaon is the thing that was broken.
Without kaons there is no CP violation in the standard model that is accessible to experiment, and without CP violation there is no natural explanation for why the universe is made of matter rather than antimatter. Kaons are small and short-lived and decay into particles nobody can see without equipment that costs millions of dollars. They are also, arguably, the most consequential particles ever discovered.
The kaon teaches you that a particle and its antiparticle are not just mirror images of each other. They are partners in a system that does not respect the mirror. The mirror is there. You can see it. The kaon steps through it anyway.
▚ trolla · kaon is field/trolla/the-kaon-mixing · cp-violation is stories/trolla/the-cp-violation · baryogenesis is meta/trolla/the-baryogenesis