February 22, 2010

A Continued History of Particle Physics

Our atmosphere is constantly bombarded by high energy particles, also known as ions. These cosmic ray showers include muons, which are heavy, not-very-stable electrons.

Neutrinos were discovered because Beta-decay violates energy conservation. For instance, a neutron decays into a proton and an electron, but the products don't have enough mass, and their energy isn't enough greater to compensate. Momentum, as well, is not conserved by this reaction. Pauli hypothesized that there is a particle that we aren't seeing. This particle is electrically neutral (so undiscovered) and very light, with few interactions with other stuff. Fermi decided to call it the neutrino. In the 1950s, the search for the neutrino began.

Lots of fancy stuff happened, and it was found that there are neutrinos and anti-neutrinos, even though the particles are neutral. Reactions with neutrinos were measured outside of a nuclear reactor, because these produce copious quantities of beta-decay. Even with this increased decay, there were still only 2-3 reactions observed in a given two hour period. This low rate was in part because anti-neutrinos were not reacting as predicted.

To explain this, a new law had to be created, as Feynman declared that if you aren't seeing it, it's not happening. This new law was lepton conservation, and lepton numbers are negative for electrons, muons, and neutrinos, but neutral for everything else. Anti-electrons, anti-muons, and anti-neutrinos have opposite lepton numbers. No, I don't know why this is, but it does explain that the anti-neutrino reaction didn't occur because lepton number wasn't conserved.

Then, the strange conservation principle was added. Strange particles, including kaons, were found in the late 1940s. These particles were strange because their production is mediated quickly by the strong force, but their decay occurs slowly and is mediated by the weak force. These mesons were found by scientists at the leading accelerators of the time, at Columbia, Berkeley, and the USSR. Then baryons, heavier particles, were found later as accelerators grew. Protons are the only known stable baryons. The discovery of baryons led in turn to the baryon number, a value that prevents proton decay. Meson number, however, does absolutely nothing. Strangeness explained the strange particle behavior.

Then I went to the bathroom. When I got back, we had embarked on the eightfold way, which is like Mendeleev's table, where you can use it to predict elements, but is much more Buddhist-y. Or, in my words, "if you do a lot of crazy mathematical shit, you end up with cool geometric patterns." The fact that particles were categorizable like this indicated an internal structure, in the same way that the periodic table implied protons, neutrons, and electrons. These substructures are called quarks. They come in different flavors and colors, but only sometimes taste good. They also have partial charges, but they can't exist in isolation, so don't defy quantization.

Baryons have three quarks. Antibaryons have 3 anti-quarks. Mesons have a quark and an anti-quark. All particles have colors that add up to white. My favorite quark is an anti-red strange quark. A particle and its antiparticle can chill out for a short period time without mutual annihilation, and their interactions are probabilistic in nature. Another particle was found, called the J/Psi particle, because no one could agree what to call it.

The End.