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J/ψ

The particle known as the J/ψ has a double name because of the history of its discovery. Particles are traditionally named by their discoverers. What happens when two separate experimenters announce their discovery on the same day? The solution was to keep both names.

The J/ψ particle is a meson made from a charmed quark together with an anticharmed quark. Its discovery was a turning point in the development of the Standard Model of particle physics. All previously known particles could be explained in terms of just three quark types or flavors: up, down, and strange. The down and strange quarks have the same charge. Observations put very stringent limits on transitions between these two.

A proposed theory of the weak interactions, involving the W and Z bosons, needed an additional type of quark to avoid a wrong prediction. With only three quarks, the theory predicted quark flavor-changing, Z -mediated processes at rates inconsistent with observation. Sheldon Glashow, John Illiopoulos, and Luciano Maiani showed that adding a fourth quark type, which Glashow had earlier dubbed charm, provided an additional contribution canceling the wrongly predicted rate. But in mid-1974 only a small part of the physics community took these ideas seriously. The discovery of the J/ψ particle and subsequent related measurements proved that the fourth quark existed, paving the way to the current Standard Model.

The group that named their discovery J worked at Brookhaven National Laboratory in New York, studying the production of electron-positron pairs in the collision of protons with nuclei. They plotted the rate of pair production as a function of the mass of the combined system. Any produced particle that can decay into an electron and a positron shows up as a peak in this plot, centered at the mass of the particle and with a width inversely related to its lifetime.

Starting in mid-September 1974, the group began to see that a new type of particle was being produced at a mass of about 3.1 GeV/c². The data showed a clear and very narrow peak, indicating a particle with an anomalously long lifetime for a meson of this mass. The experiment was led by MIT professor Samuel Ting, known for his cautious attention to detail. He would not let his group announce a new and somewhat anomalous effect without first making cross-checks. Over the next month and a half these checks steadily confirmed the effect. Ting revealed the result to very few people outside his group, and those he swore to secrecy. He continued to require further checks.

Meanwhile a second group, at Stanford Linear Accelerator Center (SLAC), was hot on the trail of the same particle. The SLAC experiment is essentially the reverse of the Brookhaven one. Starting with colliding electron and positron beams with equal and opposite momentum, it measured the rate of events that produce hadrons at each energy setting. The narrow peak found at Brookhaven translates into an increased rate for the SLAC experiment only if the accelerator is tuned to precisely the right energy.

The SLAC experiment had earlier scanned the rate as a function of energy in steps of about 0.1 GeV. Dr. Roy Schwitters, one of the physicists working on the experiment, noticed that in two measurements when the machine energy was nominally set at the same energy, 3.1 GeV, this rate was about 30 percent higher than in the other data at similar energies. This warranted checking. The SLAC group decided to study the energy region around 3.1 GeV in smaller energy increments. Perhaps the anomalous rate had occurred when the energy setting was slightly different from the intended one.

Immediately this approach yielded dramatic results. At 3.12 GeV they found that hadrons were produced at three times the normal rate, at 3.11 GeV it was almost a factor of seven. With great excitement collaborators came rushing to the site as they heard this news. They mapped out an extremely prominent and narrow peak in the rate, centered at 3.105 GeV, which was an indisputable indication of a new particle, which they chose to name. The group leader Burton Richter began to draft a paper describing the results. Word of the discovery spread around the world within a day.

Credit for a scientific discovery is based not on when the measurement is made, but on when it is formally announced in a paper submitted to a journal or conference. Ting was on his way to SLAC to attend an advisory group meeting at the time the SLAC group was making their discovery. That night he heard from his collaborators about the SLAC discovery. The time for caution was over. Overnight the Brookhaven group sent data plots to Ting. The next day, at SLAC, Schwitters presented the SLAC results and Ting the Brookhaven results in a joint public seminar. Both groups immediately submitted their results for publication. The papers appeared in the same issue of Physical Review Letters. Ting and Richter shared the 1976 Nobel Prize in Physics for this discovery.

See also:Charmonium; Quarks