Power to the Pentaquark: Research

According to the Standard Model, protons, neutrons, pi-mesons, and other related particles are composed of various combinations of quarks. In this theory, a particle consisting of five quarks—the pentaquark—is possible. There is an old saying in physics that “Anything not forbidden is required.” Perhaps with this in mind, physicists have been looking for the five-quark particle for 30 years. In 1997 this search received new direction with a prediction by three Russian physicists that the pentaquark consisting of two up quarks, two down quarks, and one anti-strange quark would have about 1.5 times the mass of the proton and could be detected with present-day particle physics technology.

The production of a pentaquark through the absorption of a gamma ray (wiggly line) by a nucleus. The pentaquark decays so rapidly that it cannot be observed, so its existence must be inferred from observations of the gamma rays and of the K+ mesons that are produced when the pentaquark decays (image courtesy of Physics News Graphics, American Institute of Physics).

The K meson detector at Jefferson Lab (JLab), one of the labs where the pentaquark was observed (photo courtesy of Greg Adams, Jefferson Lab).

The pentaquark is produced by bombarding light nuclei with high energy gamma rays, as shown in the diagram. The pentaquark itself cannot be detected, since it lives only about 10^-20 seconds before decaying into a neutron and a K+ meson (see upper right part of the diagram). Observations of the meson and the gamma rays established the presence of the pentaquark, and three additional experiments have confirmed this result.

This exotic new kind of particle has caused quite a stir in the physics world. As often happens, the discovery has raised interesting new questions, for it turns out that the very theory that led the experimentalists to their discovery does not predict all pentaquark properties correctly. A competing theory has already appeared, and since both theories predict as-yet-undiscovered particles, and with differing masses, future experiments may resolve this disagreement. In any case, it’s an exciting time in particle physics.