25 Days of Particles: Day 8

Up Quark

Classification: fermion, quark
Fundamental: yes
Family: first
Mass: 1.7-3.3 MeV
Interactions: Electromagnetic (charge +2/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: stable

So we have now met several prominent members of the particle zoo, and there are several more that had also been discovered by the 1950s. The total was up to dozens of particles, all of which appeared to be fundamental, all with unique sets of quantum numbers. They could be classified by their quantum numbers, but all in all it was turning into a bit of a headache.

Between 1961 and 1964, several physicists independently proposed methods of classifying many of the known particles as clusters of smaller particles. These smaller particles had the properties of spin and electric charge, and the properties of particles like the proton or the kaon was the sum of the properties of its constituents. This enabled the dozens of known particles to be classified much more simply, and even enabled their (very) different masses to be related to one another via the masses of the constituents.

Murray Gell-Mann named this smaller particles quarks. The pronunciation was inspired by the quacking sounds of ducks, and the spelling came from a James Joyce's book, "Finnegans Wake."

This theory was not well liked. These quark things had weird properties like fractional charge, which had never been seen. The quark things themselves had never been seen in all the smashing of particles that had been going on in over fifty years of experiments. If there was stuff inside the proton or neutron, physicists would like to see it.

Quarks started giving more direct hints of existence in 1968. In experiments very similar to what Rutherford used to discover the nucleus, physicists at the Stanford Linear Accelerator Center were firing particles at protons and found that sometimes the particles recoiled like they had hit something hard within the proton. With experimental evidence to support the idea and mounting theoretical arguments stating that quarks made sense, the quark model of hadrons became accepted.

So, now to define a little terminology a bit more precisely:
hadron - any particle made of quarks
baryon - a particle made up of three quarks
meson - a particle made up of two quarks
lepton - a fundamental particle that doesn't interact via the strong force
quark - a fundamental particle that does interact via the strong force
fermion - a particle with half-integer spin
boson - a particle with integer spin

The lightest of the quarks is the up quark. He's a fermion with electric charge of +2/3 and a mass of between 1.7 and 3.3 MeV. We don't know his mass precisely, because like all the quarks we've never actually seen him directly. The quarks are a shy bunch that way.

Well, shy with the rest of the world. The quarks get along famously with each other. In fact, you could say that is the problem with them--they're clique-ish.