Why Do They Live So Long?

If a heavy particle is free to decay into lighter particles, then why isn't the universe filled with only the lightest particles? Why, for instance, doesn't an electron (mass 0.511 MeV) decay into photons (zero mass), with the excess mass appearing as kinetic energy? Well, to begin with, the electron has ``spin '' ( an intrinsic angular momentum of ), while a photon has ``spin 1'' (). There is no way to combine several spin 1 objects to make a spin object, so angular momentum conservation forbids an electron to decay into photons. What else? Well, the electron is charged, and the photons aren't! So what? Well, electric charge Q is a conserved quantity - not only is the total amount of charge in the universe constant, but the net charge in any reaction must also remain unchanged at every step.

OK, the electron is stable. But why can't the proton decay into a positron (the antiparticle of an electron), which has the same charge and the same spin as the proton? It could also give off two photons with opposite spins, satisfying all the criteria mentioned so far. Well, protons must have some special property that we will call baryon number because only heavy particles like the proton have it. So far as we know, baryon number manifests itself only as a conserved quantity in the interactions of elementary particles. We define the baryon number of a proton to be 1 and that of electrons and photons to be zero. Baryon number is conserved just like electric charge, and this accounts for the stability of protons: the proton is the lightest baryon, so there is nothing for it to decay into!

The next lightest baryon is the neutron, and it does indeed decay (slowly) into a proton, an electron (to compensate for the charge of the proton) and an electron antineutrino to compensate for the electron number. Huh? What's `` electron number?'' It's yet another conserved quantity that the weak interaction governing neutron decay has to keep account of. We know it exists only because neutrons don't decay into just a proton and an electron. The electron neutrino is a sort of chargeless, massless version of an electron that has almost no interaction with matter at all --- a typical neutrino can pass through the Earth (and a lot more planets besides!) without much chance of touching anything!

How about muons? Everyone says these are ``sort of like heavy electrons,'' so why can't a muon decay into an electron and a photon? The muon does decay into an electron plus an electron antineutrino and a muon neutrino, but not into an electron and a photon. This is because the muon has another different conserved quantity called - you guessed it - muon number which is a different flavour from electron number. Naturally, the muon neutrino has muon number too, and is therefore unmistakeable for an electron neutrino. But only because it never appears where an electron neutrino might.

Is all this perfectly clear? No, I don't blame you. Just remember, whenever a particle refuses to decay into lighter particles for no apparent reason, it is presumed to be because of some new conserved quantity that one has and the others don't. The assignment of names to these ephemeral quantities which Nature seems to hold in such reverence is pretty much arbitrary, so their ``discoverers'' get to think up names they think are mnemonic, allusive or just cute. There are some examples that are a little embarrassing.

For instance, while discovering hordes of new short-lived heavy particles in the 1950's, people ran across a heavy, spinless, uncharged particle called the neutral kaon which decayed (as expected) into lighter pions but very slowly, suggesting that kaons must have some new property which the strong interaction (that should make kaons decay very rapidly into pions) could not ``violate'' but the weak interaction could. This new quantity, conserved in strong interactions but not necessarily in weak interactions, was called `` strangeness'' for reasons that were obvious but hopelessly parochial. I hate this one, because it takes over a perfectly good English word that one might want to use in the same sentence.

It gets worse. But I have introduced far too many new particles and mentioned far too many jargony names without explaining what they are supposed to mean; I will come back to the literary tastes of particle physicists after I have outlined some of the currently used classification schemes.