The God Particle

by Leon Lederman

  We started the run late in 1960 and were immediately plagued by background "noise" created by neutrons and other debris from the target sneaking around our formidable forty feet of steel, crudding up our spark chambers, and skewing our results. Even if only one particle in a billion got through, it created problems. Leave it to background to know that one chance in a billion is the legal definition of a miracle. We struggled for weeks plugging cracks anywhere neutrons could sneak in. We searched diligently for electrical ducts under the floor. (Mel Schwartz, exploring, crawled into one, got stuck, and had to be hauled out by several strong technicians.) Every thin area was plugged with blocks of rusty steel from the ex-battleship. At one point, the director of Brookhaven's brand-new accelerator drew the line: "You'll pile those dirty blocks near my new machine over my dead body," he thundered. We didn't take him up on his offer as this would have made an unsightly lump in the shielding. So we compromised—only slightly. By late November the background was reduced to manageable proportions.

  Here is what we were doing.

  The protons from the AGS smashed into a target, producing about three pions on average for each collision. We produced about 1011 (100 billion) collisions per second. Assorted neutrons, protons, occasional antiprotons, and other debris were also generated. The debris that headed our way crossed a space of about fifty feet before smashing into our impenetrable steel wall. In that distance some 10 percent of the pions decayed so we had something like a few tens of billions of neutrinos. A much smaller number headed in the right direction, toward our forty-foot-thick steel wall. On the other side of the wall, about a foot away, our detector the spark chamber lay waiting. We estimated that if we were lucky we'd see one neutrino collision in our aluminum spark chamber per week! In that week the target would spray about 500 million billion (5 × 1017) particles in our general direction. This is why we had to reduce background so severely.

  We expected two kinds of neutrino collisions: (1) a neutrino hits an aluminum nucleus, which results in a muon and an excited nucleus, or (2) a neutrino hits a nucleus, which results in an electron and an excited nucleus. Forget about the nuclei. What's important is that we expected muons and electrons to emerge from the collision in equal numbers, accompanied by occasional pions and other debris from the excited nucleus.

  Virtue triumphed and in an eight-month exposure we observed fifty-six neutrino collisions, of which perhaps five were spurious. Sounds easy, but I will never never forget that first neutrino event. We had developed a roll of film, the result of a week of data taking. Most of the frames were empty or showed some obvious cosmic ray tracks. But suddenly, there it was: a spectacular collision with a long, long muon track speeding away. That first event was the mini-Eureka moment, the flash of certainty, after so much effort, that the experiment would work.

  Our first task was to prove that these were indeed neutrino events since this was the first experiment of its kind ever. We pooled all of our experience and took turns playing devil's advocate in trying to pick holes in our own conclusion. But the data were in fact rock solid, and it was time to go public. We felt secure enough to present the results to our colleagues. You should have heard Schwartz's talk to a jammed Brookhaven auditorium. Like a lawyer he ruled out, one by one, all possible alternatives. There were smiles and tears in the audience. Mel's mother had to be helped out, sobbing uncontrollably.

  There were three (always three) major consequences of the experiment. Remember that Pauli first posited the existence of the neutrino to explain the missing energy in beta decay, in which an electron is ejected from the nucleus. Pauli's neutrinos were always associated with electrons. In almost all of our events, however, the product of the neutrino collision was a muon. Our neutrinos refused to produce electrons. Why?

  We had to conclude that the neutrinos we were using had a new specific property of "muon-ness." Since these neutrinos were born with a muon in the decay of pions, somehow "muon" was imprinted on them.

  To prove this to the audience of genetically conditioned skeptics, we had to know and show that our apparatus did not more readily see muons, and that it therefore—by stupid design—was incapable of detecting electrons. Galileo's telescope problem all over again. Fortunately, we were able to demonstrate to our critics that we had built electron-detection capability into our equipment and had indeed verified this in test beams of electrons.

  Another background effect came from cosmic radiation, which at sea level consists of muons. A cosmic-ray muon coming in from the back of our detector and stopping in the middle could be mistaken by lesser physicists as a muon from neutrinos going out, which is what we were looking for. We had installed a "block" against this, but how could we be sure it worked?

  The key was to keep the detector going whenever the machine was shut down—which was about 50 percent of the time. When the accelerator was off, any muons that showed up would be uninvited cosmic rays. But none appeared; cosmic rays were unable to get past our block.

  I mention all these technical details to show you that experimentation is not so easy and that the interpretation of an experiment is a subde affair. Heisenberg once commented to a colleague outside the entrance to a swimming pool, "These people go in and out all very nicely dressed. Do you conclude from this that they swim dressed?"

  The conclusion we—and most others—drew from the experiment was that there are (at least) two neutrinos in nature—one associated with electrons (the plain vanilla Pauli neutrinos) and one associated with muons. So we call them electron neutrinos (plain) and muon neutrinos, the kind we produced in our experiment. The distinction is now known as "flavor," in the whimsical lingo of the standard model, and people began to draw a little table:

  electron neutrino muon neutrino

  electron muon

  or in physics shorthand:

  νe νμ

  e μ

  The electron is placed under its cousin, the electron neutrino (indicated by the subscript), and the muon under its muon neutrino cousin. Let's recall that before this experiment we knew of three leptons—e, ν, and μ—which were not subject to the strong force. Now there were four: e, νe, μ, and νμ. The experiment was forever called the experiment of the Two Neutrinos, which ignorant people think is an Italian dance team. This turns out to be the button upon which the standard model overcoat is sewn. Note that we have two "families" of leptons, pointlike particles, arranged vertically. The electron and electron neutrino are the first family, which is found everywhere in our universe. The second family consists of the muon and the muon neutrino. Muons are not found readily today in the universe, but must be manufactured in accelerators or in other high-energy collisions, such as those produced by cosmic rays. When the universe was young and hot, these particles were abundant. When the muon, a heavy brother of the electron, was first discovered, 1.1. Rabi asked, "Who ordered that?" The two-neutrino experiment provided one of the early clues to the answer.

  Oh yes. The fact that two different neutrinos existed solved the crisis of the missing mu-e-gamma reaction. To review, a muon should decay into an electron and a photon, but no one was able to detect this reaction, though many tried. There should be a sequence of processes: a muon should first decay into an electron and two neutrinos—a regular neutrino and an antineutrino. These two neutrinos, being matter and antimatter, then annihilate, producing the photon. But nobody was seeing these photons. The reason why was now obvious. Clearly, the positive muon decays into a positron and two neutrinos, but these are an electron neutrino and an antimuon neutrino. These neutrinos don't annihilate each other because they're from different families. They simply stay neutrinos, and no photon is produced, thus no mu-e-gamma reaction.

  The second consequence of the Murder Inc. experiment was the creation of a new tool for physics: hot and cold running neutrino beams. These appeared, in due course, at CERN, Fermilab, Brookhaven, and Serpuhkov (USSR). Remember, previous to the AGS experiment, we weren't totally sure neutrinos existed. Now we had beams of them
on demand.

  Some of you might have noticed that I'm avoiding an issue here. What happened to Crisis No. 1, the fact that our equation for the weak force doesn't work at high energies? Indeed, our 1961 experiment demonstrated that the collision rate was increasing with energy. By the 1980s, the accelerator labs mentioned above—using more intense beams at higher energies and detectors weighing hundreds of tons—were collecting millions of neutrino events at the rate of several per minute (a lot better than our 1961 yield of one or two a week). Even so, the high-energy crisis of weak interactions was not solved, though it was greatly illuminated. The rate of neutrino collisions did increase with higher energy, as the low-energy theory predicted. However, the fear that the collision rate would become impossibly large was alleviated by the discovery of the W particle in 1982. This was part of the new physics that modified the theory and led to a gentler and kinder behavior. This postponed the crisis to which, yes, we will return.

  BRAZILIAN DEBT, SHORT SKIRTS, AND VICE VERSA

  The third consequence of the experiment was that Schwartz, Steinberger and Lederman were awarded the Nobel Prize in physics, but not until 1988, some twenty-seven years after the research had been done. Somewhere I heard of a reporter interviewing the young son of a new laureate: "Would you like to win a Nobel Prize like your father?" "No!" said the young man. "No? Why not?" "I want to win it alone."

  The Prize. I do have some comments. The Nobel is awesome to most of us in the field, probably because of the luster of the recipients, starting with Roentgen (1901) and going through so many of our heroes including Rutherford, Einstein, Bohr and Heisenberg. The Prize gives a colleague who wins it a certain aura. Even when your best friend, one with whom you have peed together in the woods, wins the Prize it somehow changes him in your eyes.

  I had known that at various times I had been nominated. I suppose I could have received the Prize for the "long-lived neutral kaon," which I discovered in 1956, for this was quite an unusual object, used today as a tool for studies of crucial CP symmetry. I could have gotten it for the pion-muon parity research (with C. S. Wu), but Stockholm chose to honor the theoretical instigators instead. Actually, that was a reasonable decision. Still, the byproduct discovery of polarized muons and their asymmetric decay has had extensive applications to condensed matter and atomic and molecular physics, so much so that international conferences on this subject are held regularly.

  As the years passed, October was always a nervous month, and when the Nobel names were announced, I would often be called by one or another of my loving offspring with a "How come...?" In fact, there are many physicists—and I'm sure this is true of candidates in chemistry and medicine as well as in the nonsciences—who will not get the Prize but whose accomplishments are equivalent to those of the people who have been recognized. Why? I don't know. It's partly luck, circumstances, the will of Allah.

  But I have been lucky and have never lacked recognition. For doing what I love to do, I was promoted to full professor at Columbia in 1958 and paid reasonably well. (Being a professor in an American university is the best job in Western civilization. You can do anything you want to do, even teach!) My research was vigorous, aided by some fifty-two graduate students over the years 1956–1979 (at which time I became Fermilab director). Most of the time the rewards came when I was too busy to anticipate them: election to the National Academy of Science (1964), the President's Medal of Science (Lyndon Johnson gave it to me in 1965), and other assorted medals and citations. In 1983 Martin Perl and I shared the Wolf Prize, given by the state of Israel, for discovering the third generation of quarks and leptons (the b quark and the tau lepton). Honorary degrees also came in, but that's a seller's market, since hundreds of universities are each seeking four or five people to honor every year. With all that, one begins to acquire a modicum of security and a calm attitude toward the Nobel.

  When the announcement finally came, in the form of a 6 A.M. phone call on October 10, 1988, it released a hidden store of uncontrolled mirth. My wife, Ellen, and I, after very respectfully acknowledging the news, laughed hysterically until the phone starting ringing and our lives started changing. When a reporter from the New York Times asked me what I was going to do with the prize money, I told him I couldn't decide between buying a string of racehorses or a castle in Spain, a quote he duly printed. Sure enough, a real estate agent called me the next week, telling me about a great deal on a chateau in Castille.

  Winning the Nobel Prize when you are already reasonably prominent has interesting side effects. I was director of Fermilab, which has 2,200 employees, and the staff basked in the publicity, taking the occasion as a sort of early Christmas present. A lab-wide meeting had to be repeated several times so everyone could listen to the Boss, who was already pretty funny, but who was suddenly considered on a par with Johnny Carson (and was being taken seriously by really important people). The Chicago Sun-Times shook me up by headlining NOBEL STRIKES HOME, and the New York Times put a picture of me, sticking my tongue out, on the front page—above the crease!

  All of this fades, but what didn't fade was the public awe at the tide. At receptions all over the city I was introduced as the winner of the 1988 Nobel Peace Prize in physics. And when I wanted to do something rather spectacular perhaps foolhardy, to help the Chicago public schools, the Nobel holy water worked. People listened, doors opened, and suddenly we had a program for improving science education in inner-city schools. The Prize is an incredible ticket to help one effect socially redeeming activities. The other side of the coin is that no matter what you won the Prize for you become an instant expert in all things. Brazilian debt? Sure. Social Security? Yeah. "Tell me, Professor Lederman, what length will women's dresses be?" "As short as possible!" responds the laureate with lust in his heart. But what I do intend is to use the Prize shamelessly to help advance science education in the United States. For this task a second Prize would be helpful.

  The Strong Force

  The triumphs in working out the intricacies of the weak force were considerable. But there were still those hundreds of hadrons nagging us, a plethora of particles, all of which were subject to the strong force, the force that holds the nucleus together. The particles had a variety of properties: charge, mass, and spin are some we have mentioned.

  Pions, for example. There are three different pions closely spaced in mass, which, after being studied in a variety of collisions, were placed together in a family—the pion family, oddly enough. Their electric charges are plus one, minus one, and zero (neutral). All the hadrons, it turned out, came in family clusters. The kaons line up like this: . (The signs, +, −, and 0, indicate the electric charge. The bar atop the second neutral kaon indicates that it is an antiparticle. The sigma family portrait looks like this: Σ+, Σ0, Σ−. A more familiar group to you is the nucleon family: the neutron and proton, components of the atomic nucleus.

  The families consist of particles of similar mass and similar behavior in strong collisions. To express this idea more specifically, the term "isotopic spin," or isospin, was invented. Isospin is useful in that it allows us to look at the concept of "nucleon" as a single object coming in two isospin states: neutron or proton. Similarly "pion" comes in three isospin states: π+, π− π0. Another useful property of isospin is that in strong collisions it is a conserved quantity, like charge. A violent collision of a proton and an antiproton may produce forty-seven pions, eight baryons, and other stuff, but the total isotopic spin number remains constant.

  The point is that physicists were trying to make some sense out of these hadrons by sorting through as many properties as they could find. So there are lots of properties with whimsical names: strangeness number, baryon number hyperon number and so on. Why "number"? Because all these are quantum properties, hence quantum numbers. And quantum numbers obey conservation principles. This permitted theorists or out-of-experiment experimentalists to play with the hadrons, organize them, and, inspired perhaps by biologists, classify them into larger family st
ructures. Theorists were guided by rules of mathematical symmetry, following the belief that the fundamental equations would respect such deep symmetries.

  One particularly successful organization was devised in 1961 by the Cal Tech theorist Murray Gell-Mann, who called his scheme the Eightfold Way, after the teaching of the Buddha: "This is the noble Eightfold Way: namely, right views, right intention, right speech..." Gell-Mann correlated hadrons almost magically into coherent groups of eight and ten particles. The allusion to Buddhism was yet another excursion into whimsy, so common in physics, but various mystics seized upon the name as proof that the true order of the world is related to Eastern mysticism.

  I got into trouble in the late 1970s, when I was asked to write a little biography of myself for the Fermilab newsletter on the occasion of the discovery of the bottom quark. Not expecting anyone other than my coworkers in Batavia to read the piece, I entitled the story "An Unauthorized Autobiography" by Leon Lederman. To my horror the story was picked up and reprinted in the CERN newsletter and then in Science, the official journal of the American Association for the Advancement of Science, read by hundreds of thousands of scientists in the United States. The story included the following: "His [Lederman's] period of greatest creativity came in 1956 when he heard a lecture by Gell-Mann on the possible existence of neutral K mesons. He made two decisions: First, he hyphenated his name..."

  Anyway, by any other name, a theorist would smell as sweet, and Gell-Mann's Eightfold Way gave rise to charts of hadron particles that were reminiscent of the Mendeleev periodic table of the elements, though admittedly more arcane. Remember Mendeleev's chart with its columns of elements having similar chemical properties? This periodicity was a clue to the existence of an internal organization, to the shell structure of electrons, even before we knew about electrons. Something inside the atoms was repeating, making a pattern as the atoms increased in size. In retrospect, after the atom was understood, it should have been obvious.