Investigate the Standard Model of matter, including quarks, leptons and the fundamental forces, and the role of particle accelerators in confirming the existence of these particles

What this dot point is asking

NESA wants you to outline the Standard Model: the elementary fermions (six quarks and six leptons in three generations), the gauge bosons that mediate three of the four fundamental forces (photon, W and Z, gluons), the Higgs boson, the place of gravity (outside the Standard Model), and the role of particle accelerators in producing and confirming these particles. You should distinguish elementary from composite particles and know a few example reactions.

The answer

Why the Standard Model

After Chadwick (1932), the atom was understood as a nucleus of protons and neutrons surrounded by electrons. By the 1950s, cosmic ray experiments and early accelerators were producing a confusing menagerie of new particles (pions, kaons, lambdas, hyperons). The Standard Model, developed through the 1960s and 1970s and steadily verified, classifies all observed particles as combinations of a small number of truly elementary constituents.

Two categories of matter particles (fermions)

Fermions have spin 1/2 and obey the Pauli exclusion principle. They come in two families.

Quarks. Feel all three Standard Model forces (strong, electromagnetic, weak). Carry fractional electric charge ($+2/3$ or $-1/3$). Always confined inside composite particles (hadrons). Six flavours in three generations:

A proton is $uud$ (charge $+1$); a neutron is $udd$ (charge $0$).

Leptons. Do not feel the strong force. Charged leptons feel electromagnetic and weak; neutrinos feel only the weak force (and gravity). Six in three generations:

The muon and tau are heavier, unstable versions of the electron. Neutrinos are extremely light and electrically neutral.

Each fermion has an antiparticle of opposite charge. Ordinary matter is made of first-generation particles (up, down, electron, electron neutrino).

The four fundamental forces

Gauge bosons have spin 1 (spin 2 for the graviton). They are exchanged between fermions to mediate the forces. The Standard Model is a quantum field theory of these particles, with three forces unified within it; gravity is described separately by general relativity and is not part of the Standard Model.

Strong force. Binds quarks into protons, neutrons and other hadrons. The residual strong force (mediated by pions, themselves quark-antiquark pairs) binds protons and neutrons into nuclei.

Electromagnetic force. Long range, infinitely so for static fields. Holds electrons in atoms, binds atoms into molecules, and underlies all of chemistry, biology and macroscopic phenomena.

Weak force. Responsible for beta decay (transmuting a down quark into an up quark, or vice versa). Mediated by the massive $W$ and $Z$ bosons, which limit the range. Combined with electromagnetism into a single "electroweak" theory at high energies.

Gravity. Predicted to be mediated by the spin-2 graviton, never detected. Described classically by general relativity. Outside the Standard Model.

The Higgs boson

The Higgs field, postulated in 1964 and finally detected as the Higgs boson in 2012 at CERN's LHC, is the mechanism by which the $W$, $Z$ and the fundamental fermions acquire their masses. The Higgs is spin 0 (a scalar boson) and is the only known elementary scalar. Its discovery completed the Standard Model as originally formulated.

Hadrons: composites of quarks

Quarks are never observed in isolation (the property of "confinement"). They appear in two main bound-state types:

• Baryons: three quarks (or three antiquarks). Examples: proton ($uud$), neutron ($udd$), lambda ($uds$). Baryons are fermions.
• Mesons: a quark and an antiquark. Examples: pion $\pi^+$ ($u \bar{d}$), kaon $K^0$ ($d \bar{s}$). Mesons are bosons.

In recent years more exotic hadrons (tetraquarks, pentaquarks) have been detected, all consistent with quark substructure.

How particle accelerators confirmed the Standard Model

Particle accelerators bring beams of electrons, protons or heavier nuclei to high energies and collide them. By $E = m c^2$ (and conservation of energy-momentum), high-energy collisions produce particles that do not exist in ordinary matter. Detectors track the outgoing particles' trajectories and energies to reconstruct what happened.

Landmarks:

• SLAC (Stanford), 1968-1973. Deep inelastic scattering of high-energy electrons off protons revealed point-like substructure: the quarks. Friedman, Kendall and Taylor won the 1990 Nobel Prize.
• Brookhaven and SLAC, 1974. Discovery of the charm quark (the $J/\psi$ meson).
• Fermilab, 1977. Discovery of the bottom quark.
• CERN SPS, 1983. Discovery of the $W$ and $Z$ bosons by the UA1 and UA2 detectors, confirming the electroweak unification.
• Fermilab Tevatron, 1995. Discovery of the top quark, the heaviest elementary particle ($\sim 173$ GeV/$c^2$, about a gold atom's mass in a single quark).
• CERN LHC, 2012. Discovery of the Higgs boson by the ATLAS and CMS detectors. Completed the experimental verification of the Standard Model.

In each case the produced particles decayed essentially immediately, but the kinematics of their decay products in the detector (and statistical analysis of millions of events) reconstructed their mass and properties.

What the Standard Model does not explain

Despite its success, the Standard Model leaves several open questions:

• Dark matter (about 27% of the universe's energy density) is not in the Standard Model.
• Dark energy and the accelerating expansion of the universe are not explained.
• Why gravity is so much weaker than the other forces (the hierarchy problem).
• Why there are three generations of fermions, and why their masses are so different.
• Whether neutrinos are their own antiparticles, and the mechanism of their (very small) masses.
• A quantum theory of gravity unifying with the other forces.

Research at the LHC and elsewhere continues to look for "physics beyond the Standard Model".

Common traps

Confusing the gauge bosons with the forces. The forces and their carriers are different concepts. The strong force is what holds quarks together; the gluon is its mediator. Saying "the gluon force" is incorrect.

Listing gravity as a Standard Model force. It is not. The Standard Model contains three forces (strong, weak, electromagnetic) and the Higgs. Gravity is general relativity.

Calling the proton or neutron elementary. They are composite (three quarks). Quarks are elementary, but always confined.

Saying neutrinos have no mass. Originally postulated as massless in the Standard Model, neutrinos are now known to have very small but non-zero masses, established by neutrino oscillation experiments (Super-Kamiokande, SNO).

Saying particle accelerators only smash known particles. Their key purpose is to create new ones, using $E = m c^2$ to convert kinetic energy into mass. The heavier the particle, the higher the accelerator energy needed.

In one sentence

The Standard Model classifies all known elementary particles as six quarks and six leptons (each in three generations), with three of the four fundamental forces mediated by gauge bosons (gluons for strong, photon for electromagnetic, $W$ and $Z$ for weak), the Higgs giving the others their mass, and all of this experimentally established through particle-accelerator discoveries from quarks (SLAC, 1968) to the Higgs (LHC, 2012); gravity remains outside the model.