The Field Guide to Particle Physics

Strangeness - as a property of particles - was an attempt to explain why some particles took a really long time to decay. By that measure, the charged Kaons are definitely strange.

Show Notes

The Field Guide to Particle Physics : Season 2
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Charge Kaons

Strangeness - as a property of particles - was an attempt to explain why some particles took a really long time to decay. By that measure, the charged Kaons are definitely strange.

Weighing in at 493 MeV, the charged kaons are heavy. Three times as heavy as the pions. And yet, given their mass, it’s surprising that they’re lifetime is also measured in nanoseconds. About 12 nano seconds, actually. Quite long for such heavy particles.

The charged kaons are composed of a strange quark and an antiup quark. Or an up quark and an anti-strange quark. Kind of like pions. Only STRANGE.

Kaons are strange in more than the technical sense. Their decays confused everyone for quite some time.

In the late 1940s, particle physicists discovered a few strange particles that all seemed to have about half the mass of the proton, but decayed very differently.

Something physicists originally called the tau+ (a historical name, which should not to be confused with the tau lepton) decayed into THREE pions. A pi+ and two neutral pions. Something originally called the theta+ also decayed into pions, but only one pi+ and ONE pi 0.

Confusingly, these taus and theta appeared to be IDENTICAL otherwise. They should have been the same, actually, except for those different decays. And up until that time, no particle had ever been seen decaying to BOTH two AND three pions.

How could the tau+ and the theta+ be the same particle? It would be as if you were BOTH totally left handed AND totally right handed. Like you were literally your reflection in your mirror, but only sometimes. These ideas are captured by the idea of PARITY, nothing more than a twist on the idea of left and right handedness.  Decaying to THREE pions suggested that the kaon parity was ODD. But decaying to TWO pions suggested it had even parity. Numbers can’t be BOTH EVEN AND ODD, how could particles?

This might seem like an abstruse problem to have, but to physicists at the time, conservation of parity seemed as vital as the conservation of angular momentum. We know better now, as kaon decay involves strange quarks which each via the weak nuclear force. The weak nuclear force - carried by W and Z-bosons - violates parity explicitly. Maximally, as it turns out.

The charged kaons decay to muons about 63% of the time. Those two pion decays? That’s just over 20%. The three pion decays? Just about 5.6%. There are crazy things too, like the so-called semileptonic decays which include BOTH pions AND electrons or muons

Things get complicated when the masses get large. But this is only the tip of the strangeness iceberg. There’s plenty more to come.

What is The Field Guide to Particle Physics?

This is your informal guide to the subatomic ecosystem we’re all immersed in. In this series, we explore the taxa of particle species and how they interact with one another. Our aim is give us all a better foundation for understanding our place in the universe.

The guide starts with a host of different particle species. We’ll talk about their masses, charges and interactions with other particles. We’ll talk about how they are created, how they decay, and what other particles they might be made of.