Discovery of the J/psi Subatomic Particle

Date November, 1974

Before the discovery of the J/psi particle, the hadron class of subatomic particles was thought to be composed of three kinds of quarks: the up, down, and strange quarks. The J/psi particle’s relatively long life called for the addition of a fourth kind of quark, the charm quark. This discovery changed the way physicists see the structure of matter.

Also known as November Revolution

Locale Upton, New York; Menlo Park, California

Key Figures

• Samuel Ting (b. 1936), physicist at the Massachusetts Institute of Technology and CERN
• Burton Richter (b. 1931), physicist at Stanford University
• Giovanni Bellettini (fl. late twentieth century), director of the Laboratori Nazionali di Frascati in Italy

Summary of Event

Thanks to cosmic-ray research, the construction of accelerators, and the increased sophistication of detectors, physicists discovered many new subatomic particles during the first half of the twentieth century. The sheer number of particles led to the conclusion that they could not all be elemental, but an explanation of their supposed structure was elusive. In 1964, physicists Murray Gell-Mann and George Zweig independently put forth the idea that all hadrons—subatomic particles affected by the strong nuclear force—are made up of three varieties of a new particle, christened the “quark” by Gell-Mann. These varieties were the up, down, and strange quarks. It was left for experimenters to test the validity of the three-quark model.

In 1971, Dr. Samuel Ting began a study of the interactions of light and heavy photons. The results of his early experiments in this area lent credence to the three-quark model, since they showed that interactions of the quarks would account for the interactions between photons and other particles that Ting’s group had observed. Ting then proposed to determine how many heavy photons existed and what their properties were. To do this, he started an experiment on the 30 giga-electron-volt (GeV) proton accelerator at Brookhaven National Laboratory in Upton, New York. These experiments began in the spring of 1972.

The Ting group fired protons at a target of nine pieces of beryllium, each 1.78 millimeters thick, separated by 7.3 centimeters. In the early summer of 1974, after tuning their magnets to accept an effective mass of 2.5 to 4.0 GeV, Ting’s team saw an unexpected narrow peak in electron-positron pair production at a mass of 3.1 GeV. After following several time-consuming procedures designed to ensure the accuracy of their data, the group concluded that they had in fact observed a massive new particle with a relatively long lifetime. After some discussion among the team members, they decided to name their discovery the J particle.

Ting postponed the announcement of his results because he wanted to do further experiments to investigate some other unexpected data from his experiments. After being urged by his colleagues to publish his results quickly, he wrote a draft in early November describing the discovery of the J particle. On November 11, he telephoned his colleague, Giovanni Bellettini, the director of the Laboratori Nazionali di Frascati in Frascati, Italy, and informed him of his discovery. The Frascati group confirmed on November 15 that they were also able to detect the J particle’s signal.

Meanwhile, at the Stanford Linear Accelerator Center (SLAC), in Menlo Park, California, Burton Richter’s team was looking into, among other things, the unexplained production of hadrons at energies where they should be very rare. Coincidentally, one source of these unexpected hadron results were experiments at the Laboratori Nazionali di Frascati.

The experimenters at SLAC were using the newly constructed Stanford Positron Electron Asymmetric Ring (SPEAR) to collide electrons and positrons. The particles created by the mutual annihilation of the electrons and positrons would be detected by SPEAR’s innovative magnetic detector later named Mark I. SPEAR and Mark I recorded their first data in 1973, about the same time experimenters at Harvard’s Cambridge Electron Accelerator (CEA) had been able to replicate the unexplained production of hadrons seen at Frascati. Like the Frascati results, the events at CEA were too few to have any statistical weight or lead to any conclusions.

The Richter group focused their attention on the ratio of the rate of hadron production to muon pair production (R). It was thought that R should always be equal to the sum of the electric charges of all types of partons regardless of the beam energy. Later experiments showed that in fact, R doubled from a value of 2.5 at SPEAR’s lowest energy to 5.0 at its highest energy. It was then suggested that perhaps R actually should grow as the square of the collision energy. In order to test this idea, higher energies were needed. A decision was made to shut SPEAR down to upgrade its top energy. Just before shutting down, the team did some experiments at lower energies to examine some unexpected fluctuations of R at the 3 and 4 GeV levels. Analysis of the data showed that R was not a smooth function at all, but fluctuated, especially at 3.1 GeV.

When SPEAR was brought back on line, the team was given one weekend to investigate the anomaly around 3.1 GeV. SPEAR would then be used in higher-energy experiments. Pursuing this anomaly, on the weekend of November 9-10, Richter’s team discovered that at slightly more than 3.1 GeV the production of hadrons increased by a factor of 70. With a completely different experimental technique, they had found the same narrow peak in their energy distribution at 3.1 GeV that Samuel Ting found. Without any knowledge of Ting’s results, they reached the same conclusion: They had discovered a massive new particle with a relatively long life. They named the new particle psi.

The Richter group planned to submit a paper describing their discovery the following Monday, November 11. Before they did, however, they would announce their findings at SLAC’s Program Advisory Committee (PAC), which was meeting that day to discuss proposals for SPEAR experiments. It happened that one of the members of PAC was Samuel Ting, who attended that meeting and planned to tell the committee of his discovery of the J particle. No doubt, Ting and Richter were each stunned to learn of the other’s discovery. Eventually, Ting, Richter, and Bellettini all published papers simultaneously in Physical Review Letters. Samuel Ting and Burton Richter shared the 1976 Nobel Prize in Physics for the discovery of the judiciously named J/psi particle.

Significance

Just as physicists trace the trails of particles back to the point of impact where they were separated or created, so too can many current ideas about the nature of matter trace their paths back to the discovery of the J/psi particle. Searches for particles suggested by the existence of the J/psi and the charmed quark were given new impetus by its discovery. Within a decade following the discovery, a total of six quarks (up, down, strange, charm, top, and bottom) and six leptons (electron, muon, Tau, and their respective neutrinos) were known, along with three kinds of gauge bosons that carry the forces between these particles. These are now considered the fundamental building blocks of all matter.

Within a year of the J/psi discovery, a neutral charmed particle made up of a charmed quark and a down antiquark, the D0 (D-zero), was discovered. The longer-than-expected lifetime of the D0 and subsequent charmed particles is strong evidence that the strong force weakens as energy increases. This knowledge opens up exciting possibilities for physicists studying the fundamental forces of nature. Even though the everyday matter of the world is made up mostly of up and down quarks, physicists can now speculate about an ultra-high-energy universe filled with charmed and strange quarks, atoms and molecules. At these energies, the strength of the strong force may be no greater than that of the electromagnetic force, which makes the elusive grand unified theory of nature (a “theory of everything” that promises to relate all the forces of nature) seem more attainable.

The story of the J/psi particle is the story of two independent research teams, thousands of miles apart, using different techniques to study different phenomena, both discovering the same tiny particle with an infinitesimal life span that has helped change and shape our fundamental understanding of nature.

Bibliography

Allday, Jonathan. Quarks, Leptons, and the Big Bang. Philadelphia: Institute of Physics Publishing, 1998. A look at particle physics, cosmology, and their relationship.

Close, Frank E., Michael Marten, and Christine Sutton. The Particle Explosion. New York: Oxford University Press, 1987. Excellent history of the exploration of subatomic particles in the twentieth century.

Fraser, Gordon, ed. The Particle Century, 1998. Philadelphia: Institute of Physics Publishing, 1998. Describes the discoveries of subatomic particles during the twentieth century, including an excellent description of the discovery of the J/psi particle contributed by Roy F. Schwitters, a member of the SLAC team.

Heyde, Kris L. G. Basic Ideas and Concepts in Nuclear Physics: An Introductory Approach. Philadelphia: Institute of Physics Publishing, 1999. A somewhat advanced “introduction” to nuclear physics.

Lundqvist, Stid, ed. Nobel Lectures, Physics, 1971-1980. River Edge, N.J.: World Scientific Publishing, 1992. Contains the Nobel lectures and autobiographies of Samuel Ting and Burton Richter.