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.