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§ Phase x · The Particle Zoo · Chapter 01

Discovery sequence

From a glowing tube in Cambridge to a 27-kilometer ring under the French border, the list of fundamental constituents went from one to seventeen in 117 years. Each entry on the list arrived with its own story: a misreading of a cloud chamber photograph, a balance-sheet of missing energy in beta decay, a pair of physicists in California seeing the same bump on the same Wednesday. This is the order in which nature handed over her pieces.

The story begins on the evening of April 30, 1897, in the lecture theatre of the Royal Institution in London. The audience was used to elegant talks about heat and electricity. Joseph John Thomson, the 40-year-old Cavendish Professor at Cambridge, stood up to tell them something rather different. He had been firing electric discharges through glass tubes for three years, the kind of tube you have probably seen in a high-school physics museum: a sealed bulb with most of the air pumped out, an electrode at each end, and a faint greenish glow that streamed from the negative end (the cathode) toward the positive. He could deflect the glow with a magnet. He could deflect it with an electric field. By comparing the two deflections he could compute, like a sailor triangulating from two bearings, the mass-to-charge ratio of whatever was streaming through the tube. The number he got was a shock.

The mass-to-charge ratio came out about a thousand times smaller than that of the hydrogen ion, which had been the lightest charged thing anyone knew. Either the charge was a thousand times bigger than the hydrogen ion’s charge, or the mass was a thousand times smaller. Thomson, weighing the evidence, chose mass. He had measured a piece of matter lighter than the lightest atom. He called the carriers “corpuscles” (the name “electron,” coined by the Irish physicist George Johnstone Stoney in 1891 for the natural unit of electric charge, would slowly stick). He told the Royal Institution that an atom must contain a great many of these tiny things, embedded in some positive medium “like raisins in a plum pudding.” It is the first model of a divisible atom in human history. The chapter you are reading is the long, strange list of particles that followed.

That list grew quickly. In 1909, in a dim Manchester basement, two young researchers named Hans Geiger and Ernest Marsden, working under Ernest Rutherford, fired alpha particles at a sheet of gold foil one-thousandth of a millimeter thick. Most went straight through, as expected; the plum pudding was soft. But about one in eight thousand bounced almost straight back, as if it had hit a tiny iron pellet sitting in the foil. Rutherford later called it “the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.” In 1911 he wrote up the consequence: nearly all the atom’s mass and all of its positive charge sit in a point-like nucleus at the center, ten thousand times smaller than the atom itself. Eight years later, he watched alpha particles knock chunks out of nitrogen nuclei and identified the chunks as nuclei of hydrogen. The simplest atom had become a name for a particle. The proton, on the periodic table since 1919.

For the next thirteen years the count of fundamental particles was exactly two, electron and proton, and physicists tried to build the whole atom out of them. It didn’t quite work. The nuclear charge of a helium atom is 2 and its mass is 4; if you build it from protons alone you have to put four protons in for the mass and add two electrons in the nucleus to cancel two of the charges, an arrangement that came to be called the “proton-electron nuclear model” and that nobody could make consistent with the new quantum mechanics. The atomic-mass problem nagged at the field through the 1920s. The fix came on February 17, 1932, when James Chadwick, by then Rutherford’s deputy at the Cavendish, reported a new neutral particle with very nearly the mass of the proton. He had inferred it from the way an unidentified radiation knocked protons out of paraffin wax. He called it the neutron, and he was right. The nucleus, it turned out, contained two species: charged protons and chargeless neutrons in about equal numbers.

In the same year, on the other side of the Atlantic, a 27-year-old Caltech physicist named Carl Anderson built a cloud chamber, surrounded it with a magnet, and pointed it at the sky. The cosmic rays raining down on Pasadena left curling vapor trails in the chamber: the tighter the curl, the slower the particle; the direction of curl told you the sign of the charge. On August 2, 1932, a track came in from below and curled the wrong way. It was the mass of an electron but its charge was positive. Anderson called it a “positive electron” and then “positron,” and he had found the first piece of antimatter, the existence of which the British theorist Paul Dirac had predicted three years earlier from a square root in his relativistic wave equation. The list of fundamental particles was now four (electron, proton, neutron, positron), or really five if you counted the photon, which Einstein had floated in 1905 and which Compton’s 1923 scattering data had finally made undeniable.

While Chadwick and Anderson were finding particles in matter, a quieter problem was eating at the theorists. In beta decay, a radioactive nucleus emits an electron. If the only things in the process are the daughter nucleus and the electron, the electron should come out at a single, sharp energy fixed by the mass difference, the way a photon comes out at a single sharp energy in an atomic transition. But Lise Meitner and Otto Hahn’s careful measurements showed that beta electrons come out with a continuous spectrum, smeared from zero up to that maximum. Energy was apparently not being conserved. Niels Bohr was prepared to give up energy conservation in the nuclear realm; he had said as much in print. In December 1930, Wolfgang Pauli wrote his colleagues a polite, half-joking letter (he was due at a conference in Tübingen but had been double-booked at a ball in Zurich) proposing the alternative: a third, invisible, neutral particle carries off the missing energy. He called it the “neutron.” Enrico Fermi, who picked up the idea and built it into the first quantum theory of beta decay in 1934, rechristened it the “neutrino,” the “little neutral one,” to distinguish it from Chadwick’s heavy neutron. It was twenty-six years before anyone caught one. In 1956 Clyde Cowan and Frederick Reines parked a tank of cadmium-doped water next to a Savannah River reactor, waited for the antineutrinos that should have been streaming out, and on June 14 sent Pauli a telegram: “We are happy to inform you that we have definitely detected neutrinos.” Pauli, in Zurich, drank a case of champagne.

Then came the deluge. From 1947 onward, cosmic-ray plates and the new accelerators at Berkeley, Brookhaven, and CERN started pulling up particles by the handful. The pion arrived in 1947, predicted in 1935 by the Japanese theorist Hideki Yukawa as the carrier of the strong nuclear force; G.D. Rochester and C.C. Butler at Manchester saw the first “V particles” in cloud-chamber tracks the same year (today we call them the kaon K and the lambda Λ). The sigma Σ followed, then the xi Ξ, then the cascade of resonances at Brookhaven’s Cosmotron. By the early 1960s there were over a hundred “elementary” particles in the books and physicists had stopped trying to memorize their names. Enrico Fermi famously told a young Leon Lederman, “Young man, if I could remember the names of all these particles, I would have been a botanist.” Robert Oppenheimer suggested the Nobel Prize should go to the physicist who failed to find a new one that year. The state of affairs was rude and getting ruder. Underneath, something had to be ordering this menagerie. Either fundamental physics had become very ugly, or the particles weren’t fundamental.

The answer, it turned out, was that the particles weren’t fundamental. In 1964 Murray Gell-Mann at Caltech and (independently) George Zweig at CERN proposed that the hundred-strong zoo of hadrons (mesons and baryons) could be assembled from three smaller building blocks. Gell-Mann named them quarks, a Joycean coinage he lifted from Finnegans Wake (“Three quarks for Muster Mark!”). Each came in three “flavors,” up, down, and strange, with fractional electric charges of +2/3 and −1/3, an outrageous proposal at the time because no one had ever measured a fractional charge in the laboratory. The proton was uud, the neutron udd, the pion a quark and an antiquark. The hundred-particle zoo collapsed onto a chart with three rows.

The fractional charges felt absurd because no one had ever seen one drift across a Wilson cloud chamber. But in 1968 a deep-inelastic-scattering experiment at the Stanford Linear Accelerator Center (SLAC) under Jerome Friedman, Henry Kendall, and Richard Taylor fired high-energy electrons at protons and saw them bouncing off small, hard, point-like things inside. The pattern of scattering looked like Rutherford’s gold-foil result transposed to a smaller scale: instead of a smooth proton, electrons were ricocheting off internal scatterers each carrying about 1/3 the proton’s charge. Quarks were real. The chase for the heavier flavors went on for thirty more years: charm was discovered simultaneously at Brookhaven (as the J meson, by Samuel Ting) and SLAC (as the ψ meson, by Burton Richter) on November 11, 1974, an event so dramatic that the entire physics community telephoned each other across the country and the resulting particle is still called the J/ψ in honor of the tie. Bottom arrived at Fermilab in 1977 in the Υ meson found by Leon Lederman. Top, predicted by symmetry but stubbornly heavy, took until 1995 to fall, also at Fermilab, with the CDF and DØ collaborations needing 173 times the mass of a proton in collision energy to make a single top quark. The third generation of leptons rounded out: Martin Perl saw the tau lepton at SLAC in 1975, and the tau neutrino was finally cornered at Fermilab in 2000.

Q(proton)  = Q(u) + Q(u) + Q(d) = (+2/3) + (+2/3) + (−1/3) = +1
Q(neutron) = Q(u) + Q(d) + Q(d) = (+2/3) + (−1/3) + (−1/3) =  0

The bosons (the force carriers) came in alongside. The photon, the carrier of electromagnetism, had been in physicists’ hands since Einstein in 1905. The W⁺, W⁻, and Z⁰, the carriers of the weak force (the force responsible for beta decay), were predicted by the electroweak unification of Sheldon Glashow, Abdus Salam, and Steven Weinberg in the late 1960s and discovered at CERN by Carlo Rubbia’s UA1 and UA2 collaborations in 1983. The gluon, the carrier of the strong force that binds quarks into hadrons, was first observed (as three-jet events in electron-positron collisions) at DESY in Hamburg in 1979. That left one piece. The whole electroweak theory depended on a missing particle, a scalar boson with no spin and no charge, whose interaction with all the others was what gave them mass in the first place. Robert Brout, Francois Englert, and Peter Higgs (and a few others, in three papers within a few weeks in 1964) had laid out the mechanism. The particle it implied came to be called, against Higgs’s polite objections, the Higgs boson. It took 48 years and a 27-kilometer ring buried under the Franco-Swiss border to find it.

The list is, today, seventeen species. Twelve fermions: six quarks (up, down, charm, strange, top, bottom), three charged leptons (electron, muon, tau), and three neutrinos. Four gauge bosons: the photon, the gluon, and the W and Z. One scalar: the Higgs. Add antiparticles and color charges and you get to 61 distinct objects, but the bookkeeping all flows from those seventeen. Compare that to the situation in 1960, when there were more than a hundred “elementary” particles and no chart that fit them. The progress is real. The model is, on its own terms, complete. And yet it is plainly not the end: it has no place for gravity, no explanation for why the masses run over twelve orders of magnitude from the electron neutrino to the top quark, no candidate for dark matter, no reason that any of these particular constants take any particular values. The chapter you have just read is the story of how seventeen entries got onto the chart. The chapters that follow will explain what each entry is, why it exists, how its interactions are organized into the Standard Model’s three forces, and where the chart’s silence is loudest.

The chronology, taken as a whole, has a shape worth pausing on. Three particles arrived in the 35 years from 1897 to 1932 (electron, proton, neutron); then forty years of frantic addition during which the field nearly drowned in its own discoveries; then a long, slow polishing of the chart from 1974 onward, with each new particle requiring a larger machine and a longer wait than the last. The list grows by a constant factor in a sea of exponentially rising effort. That is what completion looks like in physics. You can read off, from the timeline alone, that the field is approaching some kind of closure, and the gap between 1995 (top) and 2012 (Higgs) tells you exactly how hard the last entries had to fight to get in.