Why Quarks are Hidden

If quarks are ``real'' particles and not just a cute mnemonic metaphor for some esoteric mathematics, we ought to be able to ``see'' one in a bubble chamber or other device ``watching'' a high energy scattering event. Unfortunately, this can never be. The reason is intriguing.

  
Figure: Left: in first order --- two quarks exchange a single gluon at close range. Right: if the two quarks get too far apart, the original gluon gets an chance to branch into several guons, strengthening the attractive force.

The `` superstrong'' force between quarks is transmitted by the exchange of gluons [a nice descriptive name, for once!] which are massless, like photons, but have one trick up their sleeves that photons don't: they can ``branch'' (one gluon coupling to two gluons, and so on) if given enough room. Thus, while the electromagnetic force drops off as , the superstrong or force actually increases with increasing distance between the two quarks! Once the distance gets big enough --- as in a high-energy collision --- the energy stored in the gluon field is so intense that quark-antiquark pairs are created out of the vacuum between the quarks and the original quark grabs the new antiquark to become a meson, while the new quark takes the place of the old one in the hadron that has collided.

Thus, try as we might, we can never create a free quark. We can never ``see'' these ubiquitous particles that make up everything around us except leptons. This is very frustrating and for years led many particle physicists to insist that quarks were just figments of theorists' imaginations. But of course the paradigm works too well to be abandoned and the skeptics have by now pretty much given up.