From the Electron to the Particle Zoo

Particle physics is the study of the elementary constituents of matter and the interactions among them. The modern picture — a dozen matter fermions, a handful of force carriers, and one scalar — is the endpoint of a century of experiment that repeatedly resolved what looked elementary into something composite, and that repeatedly discovered particles nobody had asked for. This lesson traces that history in order, because the logic of the Standard Model is easiest to follow along the path that produced it. The vocabulary introduced here in narrative form is made precise in the two lessons that follow.

Four eras

The subject splits into four eras, each defined by the shortest distance experiment could resolve. Each era found the atom of the previous one to be built from smaller pieces.

• Atomic era (roughly 1897–1932). Matter is atoms; atoms contain electrons and a positive nucleus. The relevant scale is the atomic size, $\sim 10^{-10}$ m.
• Nuclear era (1932–1960s). The nucleus is protons and neutrons bound by a strong short-range force. The scale is the nuclear size, $\sim 10^{-15}$ m = 1 fm.
• Subnuclear era (1960s–1970s). Protons, neutrons, and the flood of new hadrons are bound states of quarks; the leptons remain pointlike. The resolved scale drops below $10^{-16}$ m.
• Standard Model era (1970s–present). Quarks and leptons interact through gauge bosons of $SU(3)\times SU(2)\times U(1)$; the Higgs boson completes the picture in 2012.

The atomic era

The electron. J. J. Thomson established in 1897 that cathode rays are a stream of identical negatively charged particles, far lighter than any atom, by deflecting them in crossed electric and magnetic fields to measure the charge-to-mass ratio $e/m$. The same particle emerged from every cathode material, which identified it as a universal constituent of matter. The electron is the first elementary particle and remains elementary today.¹

The photon. Planck's 1900 fit to the blackbody spectrum introduced quantized energy exchange $E=h\nu$; Einstein's 1905 explanation of the photoelectric effect promoted the quantum to a localized particle of light. Compton scattering in 1923 confirmed that light carries momentum $p=h\nu /c$ in discrete quanta by measuring the wavelength shift of X-rays scattered from electrons. The name photon was attached later, but the particle of the electromagnetic field was established: the first force carrier.

The nucleus. Rutherford's group fired alpha particles at a thin gold foil in 1909–1911 and found a small fraction scattered through large angles, some nearly backward. A diffuse atom could not do this; the deflections required the atom's positive charge and nearly all its mass to sit in a tiny core, $\sim 10^4$ times smaller than the atom. This is the nucleus, and its discovery opened the nuclear era. Rutherford identified the hydrogen nucleus as a constituent of heavier nuclei in 1919, disintegrating nitrogen with alpha particles and detecting ejected protons.

The nuclear era

Two 1932 discoveries organized the nucleus and complicated the count of particles.

• The neutron. Chadwick showed that the penetrating neutral radiation emitted when beryllium is bombarded with alpha particles is a particle of nearly the proton's mass, not a photon: a massless photon could not, by energy and momentum conservation, eject protons of the observed energy from paraffin. The neutron gave the nucleus a two-constituent composition, $Z$ protons and $N$ neutrons, and posed the question of what binds them against Coulomb repulsion.
• The positron. Anderson, photographing cosmic-ray tracks in a cloud chamber in a magnetic field, found a track curving as a positive charge but with the ionization and range of an electron. This is the positron, the electron's antiparticle, whose existence Dirac's relativistic equation had predicted. It was the first antiparticle and the first confirmation that antimatter is real.

Yukawa's meson. The strong nuclear force is short-ranged: it acts across a nucleus but not between nuclei. In 1935 Yukawa proposed that a force carried by exchange of a massive boson has a range set by the carrier's Compton wavelength,

so a range of about 1.4 fm requires a carrier of mass

intermediate between the electron and the proton. Yukawa named it the mesotron, later meson. The prediction is quantitative: fix the range from nuclear data and the carrier mass follows.

The muon–pion confusion

In 1937 Anderson and Neddermeyer found a charged particle in cosmic rays with mass near $100\text{MeV}/c^2$, close to Yukawa's estimate, and it was provisionally identified as the meson. The identification was wrong. Yukawa's particle is the carrier of the strong force and must therefore interact strongly with nuclei. In 1947 Conversi, Pancini, and Piccioni showed that the cosmic-ray particle, brought to rest in matter, does not get captured and absorbed by nuclei as a strongly interacting particle would; negative ones survived long enough to decay. The cosmic-ray particle is a heavy lepton, the muon $\mu$, blind to the strong force.

The genuine Yukawa particle appeared the same year. Powell's group, exposing nuclear emulsions at high altitude, found a heavier particle that stops in the emulsion and decays to the lighter one: the pion $\pi$ decaying to a muon (plus an unseen neutrino), the muon in turn decaying to an electron. The pion is strongly interacting, with the predicted mass ($139.6\text{MeV}/c^2$ charged, $135\text{MeV}/c^2$ neutral), and is Yukawa's meson. The two particles differ by a factor of order unity in mass but by everything in behavior.

The confusion clarified the two families that persist to this day. The muon is a lepton, like the electron but heavier, feeling only the electromagnetic and weak forces. The pion is a hadron, feeling the strong force. Rabi's reaction to the muon — who ordered that? — captured the central surprise: nature repeats the electron at a higher mass for no evident reason, the first hint of generations.

Strange particles

Also in 1947, Rochester and Butler photographed cosmic-ray events in a cloud chamber in which a neutral particle, leaving no track, decayed into two charged particles that appeared as a V opening from a point in the gas. The inferred parent masses did not match any known particle. These are the first strange particles, the neutral kaon $K^0$ and the lambda ${\Lambda }^0$.

Strange particles are produced copiously (a strong-interaction rate) but decay slowly (a weak-interaction lifetime, $\sim 10^{-10}$ s). Pais explained the mismatch by associated production: strange particles are made in pairs by the strong force, which conserves a new additive quantum number, but each decays alone through the weak force, which does not conserve it. Gell-Mann and Nishijima named the quantity strangeness and fixed its bookkeeping, the first of the new quantum numbers that would sort the coming flood.

The accelerator era and the zoo

Postwar accelerators replaced cosmic rays as the source of new particles. The Bevatron produced the antiproton in 1955 by colliding protons at an energy above the pair-production threshold, confirming that antimatter is generic. Then the count exploded. Bubble chambers and rising beam energies revealed dozens, then hundreds, of hadrons: pions and kaons of several charges, the $\eta$, the vector mesons $\rho$, $\omega$, $\varphi$, baryon resonances $\Delta$, ${\Sigma }^{\ast }$, ${\Xi }^{\ast }$, and more. Most were resonances — states so short-lived ($\sim 10^{-23}$ s) that they are seen only as peaks in scattering cross sections, not as tracks.

No principle limited the list, and none of the new hadrons behaved like an elementary particle. The situation resembled chemistry before Mendeleev: a catalogue with regularities but no organizing scheme. Fermi's remark that a student able to name all the particles should have been a botanist expressed the discomfort. Two levels of order resolved it.

• Classification. Gell-Mann and Ne'eman's Eightfold Way (1961) arranged the hadrons into geometric multiplets (octets and a decuplet) using the group $SU(3)$. The scheme predicted a missing corner of the baryon decuplet, the ${\Omega }^{-}$ with strangeness $-3$, whose 1964 discovery at the predicted mass confirmed the pattern.
• Composition. Gell-Mann and Zweig proposed in 1964 that the multiplets follow from building hadrons out of three fundamental quarks ($u$, $d$, $s$) and their antiquarks: baryons are three quarks, mesons a quark–antiquark pair. The fractional charges $+\frac{2}{3}$ and $-\frac{1}{3}$ were the price, and free quarks were never seen, so the model was at first treated as a mnemonic.

Deep inelastic scattering at SLAC in 1968 gave the quarks physical reality: electrons scattered off protons at large momentum transfer as though striking hard pointlike constituents inside, exactly the parton picture the quark model required. The subnuclear era had begun.

Completing the Standard Model

The remaining particles arrived as the electroweak and strong theories were built and tested.

The Standard Model that these discoveries assembled has three generations of quarks and leptons, the photon and gluons and weak bosons that carry the gauge forces, and one Higgs boson. It is the subject of this course. The next lesson fixes the classification of particles — leptons, quarks, baryons, and mesons — and the antiparticle and Feynman-diagram vocabulary; the lesson after that sets out the four interactions and their carriers. Every later module makes one piece of the chart quantitative, beginning with the natural units and relativistic kinematics that all the calculations rest on.