Just what is a Quark anyway?

Apart from being a character in Deep Space Nine, a "quark" is a very fundamental building block of nature. The best way to explain what they are is to tell how they came about.

It was the 1960's, and the people at SLINAC (Stanford Linear Accelerator) and CERN (Centre pour European Recherche Nuclaire) thought they had finally got the big picture of atoms, nuclei and stuff. But the machines doing this smashing suddenly produced some particles never before seen. They behaved differently from the others and were labelled "strange". They disappeared very quickly into energy according to E = mc**2. None were present in nature.

Where did they fit in? The whole kaboodle could be explained by a whole new way of thinking: that protons and neutrons themselves were made up of something even smaller. Indeed, scattering experiments (firing the particles at themselves) seemed to indicate, by the pattern alone, that there was something else inside. Electrons had nothing inside them. Scattering experiments with them told us they acted like true "point" particles.

But just what was inside the protons and neutrons? They settled on the name "quark" and the proton and neutron had three each. One had two "up" quarks and a "down" quark, and the other, two "down" and an "up". "Up" and "down" being arbitrary names having nothing to do with the normal meaning, they might as well have called them "Laurel and Hardy", or even worse,"Bill and Ted".

But what were the strange particles? They reasoned that these must have another "flavor" of quark inside (nothing to do with taste!), which they called "strange". They predicted other particles to turn up with other combinations of up, down and strange, and sure enough, as energies (accelerator energies) went up, these predicted particles turned up. Then they reasoned, "Why stop at three quarks?" - there could be more. They tentatively predicted a fourth, and they called it "charm" (!), and this would produce around 33% more new particles. The lowest energy of these was called the "Charmonium", and yes, it was found, along with all the debris. It was essentially a quark-antiquark pair, or "bare" charm.

How many more were there, and where would it stop? For reasons not appropriate to be given here, they figured out there were no more than 6 in total, and they called the last two "top" and "bottom".

These quarks were getting heavier and heavier and were very hard to produce, but the last one to be detected was bare bottom, in the mid-80's.

It has to be borne in mind that it was never the quarks they saw, just big particles with quarks inside. The question arose - why can't we get a quark on it's own. It would be pretty easy to detect, as each had either 1/3 or 2/3 of the charge on the electron. However, not one had been seen,

The reason they gave was astonishing. Up till then the forces of nature were gravity, electromagnetism, the weak force (only affects electrons and neutrinos), and the "strong" force, which keeps a nucleus together against the static electromagnetic repulsion of the protons.

They reasoned that inside the protons, the strong force was the real strong force, much stronger than had previously been thought, and it was this that was "holding" in the quarks, as if they were tied to it with elastic bands. ( see previous posts for more info). Try to pull a quark out, and the elastic gets more stretched, as it gathers energy from the "pulling", until SNAP it breaks in the middle, not with a loud noise, but a new quark-antiquark pair, matter from energy (E = mc**2). The quark goes back into the old particle, and the new antiquark makes another new particle with the quark just pulled out!

There was another problem with this scenario. It goes back to an old rule developed by Wolfgang Pauli - no two particles can have the same description and be at the same place.

But in the proton there were two "up" quarks!! They figured. that the strong force, like electricity, had the equivalent of "charge", but whereas electricity has two, plus or minus, the strong force had three, and they called these three "color" charges (nothing to do with rainbows). So, in a proton, an "up " must have a different color charge to the other "up".

Because of this "color field" they named the theory Quantum ChromoDynamics.

Believe it or not, that was an introduction. I have left much out.

There IS truth out there!