The Revolution That Shook Particle Physics
The discovery of a mysterious particle led to the Standard Model we know today.
Burton Richter ’52, PhD ’56, likes to describe his Nobel Prize–winning discovery with a nursery rhyme:
“Yesterday upon the stair, I met a man who wasn’t there. He wasn’t there again today. I wish that he would go away.”
It was 1974, just two years after Richter and his team at the Stanford Linear Accelerator Laboratory had finished building a new particle collider they called SPEAR. The $6 million machine accelerated electrons and positrons through a loop of magnets 80 meters in diameter and smashed them together into a shower of particle debris to analyze. Richter wasn’t looking for a new particle, but there it was—a tall, narrow peak in the data indicating a heavy particle with an unusually long lifetime. So long, in fact—seven zeptoseconds, or about 7,000 trillionths of a second—that it defied physicists’ understanding of fundamental particles.
At first, Richter and others thought something was wrong with the equipment, but they soon realized they had a discovery on their hands. “Nobody expected what we found. Nobody. It came as a complete surprise,” says Richter, who first got hooked on particle physics after working in MIT’s synchrotron laboratory during his first year in graduate school.
Perhaps just as surprising, a team on the opposite side of the country simultaneously found the same thing. At Brookhaven National Laboratory, MIT professor Sam Ting was leading an experiment that was, in a way, the opposite of Richter’s. Instead of annihilating electrons and positrons, Ting’s group was smashing protons into a fixed target of beryllium to produce heavy particles that would then decay into electrons and positrons. Coincidentally, the two groups had set their accelerators to the same energy range and stumbled upon the same unusual peak in their data.
Ulrich Becker, an associate professor at MIT at the time, played a central role in the Brookhaven discovery. Now a professor emeritus, he still has the original, hand-drawn graph in his office, showing that unexpected peak. Becker says that even though his group was hoping to find heavy particles, they too were shocked to find one with such a long lifetime. “We had no idea why the hell this was,” he says. “We were highly suspicious of it, but since it was so clear cut, there was very little room for doubt.”
In November of that year, Ting flew to Stanford for an administrative meeting and crossed paths with Richter.
“Sam said, ‘Burt, I got something exciting to tell you,’” Richter recalls, “and I said, ‘Sam, I got something exciting to tell you.’”
When the two researchers realized that their teams had made the same discovery, they organized a lab seminar and presented their results that day. Within a month, the groups published back-to-back papers in Physical Review Letters. Ting’s group dubbed the new particle “J,” and Richter’s group called it “Ψ” (psi).
What was so surprising about the particle was that it lasted about a thousand times longer than physicists would have predicted at the time, meaning there was something about it that physicists had never encountered before, at least not in the lab.
About 10 years earlier, theorists had introduced the concept of quarks—fundamental particles (and corresponding antiparticles) that bind together to form other particles, like protons and neutrons. By 1974, experimentalists had found evidence for three types of quarks—up, down, and strange. But a proposed fourth quark, charm, remained elusive.
In early 1975, Becker went to Germany to give a talk about the new particle. In attendance was Werner Heisenberg, the theoretical physicist famous for his work on quantum mechanics. At one point, Heisenberg interrupted Becker: “He said, ‘Whenever they don’t know what it is, they invent a new quark,’” Becker recalls. “I was really flabbergasted. I told him, ‘Look, Professor Heisenberg, I’m not arguing whether this is charm or not charm. I’m telling you it’s a particle which doesn’t go away.’ Dead silence. It got very cold in the room. Then Heisenberg said, ‘Accepted.’”
Physicists soon agreed that the particle, later called the J/Ψ, consisted of one charm quark and one anticharm quark.
Now referred to as the November revolution, the J/Ψ discovery took the stable of fundamental particles from a disordered menagerie to something structured and predictable, described by what physicists called the Standard Model. Ting and Richter won Nobel Prizes in 1976.
“What lots of people have been trying to do ever since is find what’s beyond the current Standard Model,” Richter says. “So far, it has stood impervious to all attacks.”
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