Warped Passages - Lisa Randall [85]
Even though physicists agreed that hadrons were made of quarks, nine years elapsed after the suggestion of quarks before hadron physics was explained in terms of the strong force. Paradoxically, the strong force was the last force to be understood, in part because of its enormous strength. We now know that the strong force is so large that the fundamental particles, such as quarks, that experience the strong force are always bound together and are difficult to isolate and therefore to study. Particles that experience the strong force are not free to roam unchaperoned.
There are three types of every quark variety. Physicists playfully label the different types with colors and sometimes call them red, green, and blue. And these colored quarks are always found with other quarks and antiquarks, bound together into color-neutral combinations. These are the combinations in which the strong force “charges” of the quarks and antiquarks cancel each other, analogously to the way colors cancel in white light.* There are two types of color-neutral combination. Stable hadronic configurations contain either a quark and an antiquark that team up with each other, or else three quarks (and no antiquarks) that bond among themselves. For example, a quark pairs with an antiquark in particles called pions, and three quarks bind together in the proton and the neutron.
The strong force “charge” cancels among the quarks in hadrons, much as the charge of the positively charged proton and the negatively charged electron cancel in an atom. But whereas you can readily ionize an atom, it is very difficult to pry apart the objects, such as the proton and neutron, that are bound extraordinarily tightly by the gluons of the strong force. Gluons would be more aptly named “crazygluons,”† since their bonds are so difficult to break.
We are now almost ready to return to the discovery of quarks that Athena’s revisionist tale metaphorically described. The proton and neutron consist of combinations of three quarks in which the charge associated with the strong force cancels out. The proton contains two up quarks and one down quark—different types of quark with different electric charge. Because the up quark has electric charge +2/3 and the down quark has charge -1/3, the proton has electric charge +1.
A neutron, on the other hand, contains one up and two down quarks, so it has zero (the sum of -1/3, -1/3, and +2/3) electric charge.
Quarks can be thought of as hard, pointlike objects in a big, mushy proton. Quarks are embedded in a proton or neutron, like a pea buried under a mattress. But as with our bouncing princess who bruises herself on the pea, an active experimenter can shoot in a high-energy electron that emits a photon, which bounces directly off the quark. This looks very different from a photon bouncing off a big fluffy object, just as Rutherford’s alpha particle bouncing off a hard nucleus looked very different from one bouncing off more diffuse positive charge.
The Friedman-Kendall-Taylor deep inelastic scattering experiment, conducted at the Stanford Linear Accelerator Center (SLAC), demonstrated the existence of quarks by registering this effect. The experiment showed how electrons behave when they scatter off protons, thereby providing the first experimental evidence that quarks really exist. For this discovery, Jerry Friedman and Henry Kendall (who were my colleagues at MIT) and Richard Taylor won the 1990 Nobel Prize for Physics.
When quarks are produced in high-energy collisions, they aren’t yet bound into hadrons, but that doesn’t mean they’re isolated—they will always have a retinue of other quarks and gluons