Kenichi Hatakeyama - 2003


Aspects of the structure of the proton


Kenichi Hatakeyama investigated the internal structure of the proton. When I was a graduate student, the proton was still considered to be an elementary particle, true to its name "proton", which in Greek means "first". With the discovery of quarks as the fundamental constituents of matter, the proton was demoted to a composite particle, consisting mainly of three quarks. But this is not the whole story. The three so-called "valence" quarks are held together by the "strong force", which is transmitted by gluons. The strong force is very cunning. When the quarks are close together, it all but vanishes and the quarks feel a sense of freedom, whimsically called "asymptotic freedom". But if a quark tries to get away from the group, a gluon radiated by another quark pulls it back to ground zero with a force that increases with distance, like the force of a spring. There is no escape! This property leads to quark "confinement". Radiated gluons can hang around and enjoy the same asymptotic freedom as quarks, or may split into quark-antiquark pairs, which radiate more gluons, and so on, until an equilibrium state is reached in which a swarm of gluons and quarks, called "the sea", share with the valence quarks the energy of the proton. The resulting energy distribution is the "structure function" of the proton.

The structure function is, so to speak, the DNA of the proton, and its measurement provides a test of Quantum Chromo-Dynamics or QCD, our theory of the strong force. In QCD, quarks and gluons carry a type of charge, called "color", which comes in three varieties: red, green and blue. White is reserved for a color-neutral or colerless combination of quarks and gluons. Colored objects are never found free, they are always confined within colorless particles. In collisions between protons and other particles, if a proton quark is hit hard, it breaks loose, but as it gets away it fragments into a jet of normal particles while the rest of the proton disintegrates. By measuring the energy and direction of such jets over many collisions, the structure function of the proton has been measured and the theory confirmed, but confinement remains a mystery yet to be understood.

Ken measured the structure function of the antiproton, the proton's antiparticle, in a certain class of antiproton-proton collisions, called diffractive, in which the antiproton emerges from the collision intact, leaving behind, along with the jet from the struck quark, a portion of its energy in a colorless combination of quarks and gluons. In effect, Ken measured the structure function of this colorless bundle of energy, which, ``dressed in white'' like a ghost, escaped confinement. Since the antiproton retained its quantum numbers, which define its identity, the escaped energy must have no quantum numbers, no ID card, just like the vacuum. So, in this sense, Ken measured the structure function of the vacuum. In our biological analogy, he deciphered the DNA of the vacuum. His measurement, whose importance has gained international recognition, will undoubtedly be on the list of conditions that must be met on the road to understanding quark confinement.