Deep inside the atom

More particles than expected

In 1932, matter seemed simple: protons, neutrons and electrons, plus the photon. That year Carl Anderson found the positron in cosmic ray tracks, the antimatter partner Paul Dirac had predicted. Then the surprises kept coming:

• the muon (1936), in cosmic rays: like a heavy electron, about 207 times its mass. "Who ordered that?" asked the physicist I. I. Rabi.
• the pion (1947), predicted by Hideki Yukawa as the carrier of the force between nucleons, found in cosmic rays at high altitude.
• the kaon and other "strange" particles (late 1940s and 1950s), which were produced quickly but decayed surprisingly slowly.
• the neutrino (detected 1956, beside a nuclear reactor), the antiproton (1955, at the Bevatron accelerator), and from the 1950s, dozens of short-lived particles made in accelerators and photographed in bubble chambers.

By the early 1960s over a hundred particles were known, a "particle zoo". A list that long suggested these were not fundamental at all, just as the periodic table's pattern had suggested atoms have an internal structure.

Evidence for quarks

In 1964 Murray Gell-Mann and George Zweig independently noticed that the hadrons (the particles that feel the strong force) fitted patterns that could be explained if they were built from three smaller particles, which Gell-Mann called quarks: up, down and strange, with charges of $+\tfrac23 e$, $-\tfrac13 e$ and $-\tfrac13 e$. The pattern predicted a missing particle, the $\Omega^-$ (three strange quarks), with a particular mass. It was found at Brookhaven later in 1964, with the predicted mass.

Many physicists still thought quarks were just a mathematical device, since nobody had seen a particle with a fractional charge. Direct evidence came from experiments at the Stanford Linear Accelerator Center (SLAC) between 1967 and 1973.

The experiment used the same idea as Geiger and Marsden's, with electrons of up to 20 GeV in place of alpha particles. At these energies, an electron's de Broglie wavelength is about $10^{-16}$ m, a tenth of a proton's diameter, so it can probe inside. If the proton's charge were spread smoothly, electrons would scatter only slightly. Instead, many more than expected scattered through large angles and lost large amounts of energy (the scattering was "deep inelastic"). The results showed point-like charged particles inside the proton, with fractional charges matching quarks. Other experiments also showed that about half the proton's momentum is carried by neutral particles: the gluons.

The Standard Model

The Standard Model, developed in the 1960s and 1970s, describes all known matter with 12 fundamental fermions (6 quarks and 6 leptons, each with an antiparticle), the force-carrier bosons, and the Higgs boson.

Quarks come in six flavours in three generations: up and down; charm and strange; top and bottom. They have fractional charge and are the only matter particles that feel the strong force. They are never found alone (confinement): the strong force between quarks does not weaken as they are pulled apart, and the energy put in creates new quark–antiquark pairs instead.

Leptons don't feel the strong force. There are three charged leptons (electron, muon, tau, charge −1) and three neutrinos (electron, muon and tau neutrinos, neutral and almost massless). Leptons appear to be truly fundamental, with no internal structure.

Hadrons are particles made of quarks, held by the strong force. There are two kinds:

• Baryons: three quarks. The proton is uud (charge $\tfrac23 + \tfrac23 - \tfrac13 = +1$); the neutron is udd (charge $\tfrac23 - \tfrac13 - \tfrac13 = 0$). Each quark has baryon number $+\tfrac13$, so a baryon has baryon number +1.
• Mesons: one quark and one antiquark, such as the $\pi^+$ (u$\bar{\text{d}}$) and $K^-$ (s$\bar{\text{u}}$). Mesons have baryon number 0 and are all unstable.

The quarks inside a proton account for only about 1% of its mass. The rest is the energy of the gluon fields and the quarks' motion, through $E = mc^2$.

Fundamental forces

In the Standard Model, forces are transmitted by the exchange of gauge bosons:

Gravity is not part of the Standard Model: no one has yet combined it with quantum theory. The Higgs boson is not a force carrier; it is the particle of the Higgs field, through which the W and Z bosons, quarks and charged leptons get their masses.

The weak force is the only one that can change one flavour of quark into another. In β⁻ decay, a down quark in a neutron becomes an up quark by emitting a W⁻ boson, which immediately decays into an electron and an electron antineutrino. So udd (neutron) becomes uud (proton).

Conservation laws

Every particle reaction must conserve energy, momentum and charge. Two further numbers are always conserved:

• Baryon number $B$: quarks $+\tfrac13$, antiquarks $-\tfrac13$, so baryons +1, antibaryons −1, mesons and leptons 0.
• Lepton number $L$: +1 for leptons, −1 for antileptons, 0 for everything else, conserved separately for each family (electron, muon, tau).

Checking these tells you whether a proposed reaction can happen. For example, $n \rightarrow p + e^- + \bar{\nu}_e$: charge $0 = +1 - 1 + 0$ ✓; $B$: $1 = 1 + 0 + 0$ ✓; $L_e$: $0 = 0 + 1 - 1$ ✓. Allowed.

Particle accelerators

Accelerators use electric fields to speed up charged particles and magnetic fields to steer and focus them. They serve two roles in testing theories: high energy gives a short de Broglie wavelength to probe small structures (as at SLAC), and high energy can be converted into the mass of new, heavy particles ($E = mc^2$) to test predictions.

• Linear accelerators (linacs) push particles along a straight line through a series of tubes, with alternating voltages timed so the particles are always pulled forward at each gap. SLAC's linac was 3.2 km long. Smaller linacs are used in hospitals to make X-rays for cancer treatment.
• Cyclotrons, invented by Ernest Lawrence in 1932, use a magnetic field to bend particles into a circle so the same accelerating gap is used again and again. As the particles speed up, the radius $r = mv/qB$ grows and they spiral outwards. Cyclotrons are used to make medical isotopes such as fluorine-18.
• Synchrotrons keep particles on a fixed circular path by increasing the magnetic field as their momentum rises. They reach the highest energies. The Australian Synchrotron in Melbourne accelerates electrons to 3 GeV in a 216 m ring and uses the intense X-rays they emit to study materials, proteins and medicines.

The Large Hadron Collider at CERN is a synchrotron 27 km around, buried about 100 m under the French–Swiss border. Two beams of protons, each at up to 6.8 TeV, travel in opposite directions and collide head-on at four points, where huge detectors record the debris. Colliding beams head-on puts all the energy into making new particles, rather than wasting much of it on the recoil of a fixed target. Australian physicists from the universities of Melbourne, Sydney and Adelaide work on the ATLAS detector.

Accelerators have validated the Standard Model step by step: the W and Z bosons were discovered at CERN in 1983 with exactly the predicted masses; the top quark at Fermilab in 1995; the tau neutrino in 2000; and the Higgs boson at the LHC in 2012. They have also shown what the Standard Model does not explain. It includes no dark matter particle, gives neutrinos no mass (but they do have tiny masses), and doesn't include gravity. Experiments such as SABRE South, in a former gold mine at Stawell in Victoria, are searching for dark matter now.