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Spec mapping: OCR H556 Module 6.4 — Particle physics (Standard Model survey). Rutherford alpha-scattering experiment and the nuclear atom; the modern division of matter into 12 elementary fermions (6 quarks plus 6 leptons, arranged in 3 generations); the gauge bosons (γ, W±, Z0, 8 gluons) that mediate the electromagnetic, weak and strong interactions; the Higgs boson (mass-generation); the broad classification of composite hadrons into baryons (qqq) and mesons (qqˉ). Refer to the official OCR H556 specification document for the exact wording of the assessable content.
We began this course with nuclei made of protons and neutrons. In the last few lessons we've treated those nucleons as elementary building blocks of matter. But since the late 1960s, physicists have known that protons and neutrons are themselves composite — made of smaller, truly fundamental constituents called quarks. The modern picture of matter, built up from quarks, leptons and force-carrying bosons, is known as the Standard Model of particle physics. It is one of the most successful theories in the history of science.
This lesson introduces the experimental basis for the modern view of the atom (Rutherford's alpha-scattering experiment), sketches the historical path from nucleons to quarks, and lays out the top-level structure of the Standard Model. It covers the introductory parts of Module 6.4 of the OCR A-Level Physics A specification (H556) — the details of quarks and leptons come in the next lesson, and the conservation laws that govern their interactions in the lesson after that.
In 1909, Ernest Rutherford, along with Hans Geiger and Ernest Marsden, carried out the experiment that established the modern picture of the atom. They directed a beam of alpha particles at a very thin gold foil and measured how they were scattered. Their apparatus was simple: a radioactive alpha source, a thin (0.4 μm) gold foil, and a moveable zinc sulfide scintillation screen that flashed when hit by an alpha particle.
The expected result, on the prevailing plum-pudding model of J. J. Thomson (in which positive charge was spread diffusely through the atom with negative electrons embedded in it), was that alphas would pass through the foil almost undeflected, because the diffuse positive charge would produce only a weak electric field. The observed result was very different:
Rutherford later described the large-angle scattering as "almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you". Such extreme deflections could only be explained by a highly concentrated positive charge at the centre of the atom — a nucleus — containing essentially all the atom's mass in a volume thousands of times smaller than the atom itself.
From the statistics of the scattering, Rutherford deduced that the nucleus had to be at most ~10⁻¹⁴ m across (compared to ~10⁻¹⁰ m for the atom). Most of the atom was empty space.
Rutherford's picture of the atom — refined through work by Bohr, Moseley, Chadwick and others — was:
Typical length scales:
| Object | Size |
|---|---|
| Atom | ∼10−10m (0.1 nm) |
| Nucleus | ∼10−15m (1 fm) |
| Nucleon | ∼10−15m (1 fm) |
| Quark | <10−18m (point-like as far as anyone can tell) |
| Electron | <10−18m (point-like as far as anyone can tell) |
Nuclei come in various sizes, roughly following the empirical relation R \approx R_0 A^{1/3} where R_0 \approx 1.2 fm. This is consistent with nucleons packing together at constant density, so nuclear volume scales with the number of nucleons.
By the 1960s, experiments at new high-energy accelerators had revealed an embarrassing problem: there were too many "elementary" particles. Beyond the familiar proton, neutron and electron, physicists had discovered pions, kaons, muons, various "hyperons" (heavier cousins of the nucleon), dozens of short-lived "resonances"… well over a hundred distinct particles, all apparently with no simple structure.
This so-called "particle zoo" looked like a bad throwback to the pre-periodic-table era of chemistry. Surely all these particles couldn't really be elementary?
The breakthrough came in 1964 when Murray Gell-Mann and George Zweig independently proposed that many of the known particles were in fact composite, built up from a small number of more fundamental constituents that Gell-Mann whimsically called quarks (after a phrase in James Joyce's Finnegans Wake). The quark model classified all the observed hadrons (strongly interacting particles) into families built from just three types of quark: up (u), down (d) and strange (s).
The direct experimental confirmation came from the late 1960s deep inelastic scattering experiments at SLAC, in which electrons were fired at protons. The scattering pattern showed that the proton was not a uniform ball of positive charge: the electrons were being deflected by point-like objects inside the proton. These point-like objects are the quarks.
Since then, three more quarks have been discovered: charm (c, 1974), bottom (b, 1977) and top (t, 1995). Together with six leptons (the electron, muon, tau and their three neutrinos), the twelve fermions make up all ordinary matter in the Standard Model.
The Standard Model is the current theoretical framework describing all known elementary particles and three of the four fundamental forces (electromagnetism, weak nuclear force and strong nuclear force; gravity is not included). It has been tested to extraordinary precision and agrees with almost every experimental result of the past fifty years.
The particle content of the Standard Model has three components:
OCR A-Level Physics A focuses on a simplified subset of this picture:
u), down (d) and strange (s).You need not memorise the full Standard Model table, but you should recognise the broad family structure we'll see in Lesson 7.
Every particle has a corresponding antiparticle — a partner with the same mass and spin but with opposite electric charge and opposite values of all other "quantum numbers" (like baryon number, lepton number, strangeness). Some examples:
| Particle | Symbol | Antiparticle | Symbol | Charge | Mass (MeV/c²) |
|---|---|---|---|---|---|
| Electron | e⁻ | Positron | e⁺ | \mp 1 | 0.511 |
| Proton | p | Antiproton | p̄ | \mp 1 | 938.3 |
| Neutron | n | Antineutron | n̄ | 0 | 939.6 |
| Neutrino | ν_e | Antineutrino | ν̄_e | 0 | ≈ 0 |
u quark | u | Anti-up | ū | +2/3 \mp 2/3 | ~2.2 |
d quark | d | Anti-down | d̄ | -1/3 \mp 1/3 | ~4.7 |
Antiparticles were first predicted by Paul Dirac in 1928 as a consequence of combining quantum mechanics with special relativity. The positron was discovered by Carl Anderson in 1932 in cosmic ray cloud chamber photographs. The antiproton was produced at the Bevatron accelerator in 1955. Today, antiparticles are routinely produced and studied in accelerators.
When a particle meets its antiparticle, they annihilate: their combined mass-energy is released as radiation (typically photons). We shall see the details in Lesson 8.
flowchart TB
SM["Standard Model"]
F["Fermions<br/>(matter)"]
B["Bosons<br/>(force carriers)"]
Q["Quarks<br/>u, d, c, s, t, b"]
L["Leptons<br/>e, μ, τ, ν_e, ν_μ, ν_τ"]
G["Gauge bosons<br/>γ, W, Z, gluons"]
H["Higgs boson"]
SM --> F
SM --> B
F --> Q
F --> L
B --> G
B --> H
γ) for electromagnetism, W and Z for the weak force, gluons for the strong force.The 12 fermions are organised into 3 generations or families, each containing 2 quarks and 2 leptons. The first family consists of the particles that make up ordinary matter:
| Generation | Quarks | Leptons |
|---|---|---|
| 1st | u, d | e⁻, ν_e |
| 2nd | c, s | μ⁻, ν_μ |
| 3rd | t, b | τ⁻, ν_τ |
Each generation is a heavier copy of the previous one. The second-generation muon is essentially a heavy electron; the charm and strange quarks are heavier analogues of the up and down. Why nature repeats itself three times is one of the unsolved questions of modern physics.
Ordinary matter is made almost entirely from the first generation: protons and neutrons (made of u and d quarks) plus electrons. The higher generations show up in cosmic rays, radioactive decays and particle accelerators.
The masses of the quarks and leptons span an extraordinary range:
| Particle | Mass (MeV/c²) |
|---|---|
ν_e (neutrino) | < 10⁻⁶ |
e⁻ | 0.511 |
u quark | ~2.2 |
d quark | ~4.7 |
s quark | ~95 |
μ⁻ | 105.7 |
c quark | ~1270 |
τ⁻ | 1777 |
b quark | ~4180 |
t quark | ~173 000 |
The top quark is ~400 000 times heavier than the electron, which itself is ~500 000 times heavier than the neutrinos. No current theory explains this pattern — the masses are input parameters of the Standard Model.
The fermions, gauge bosons and the Higgs can be laid out in a single chart that has become the iconic visual of modern particle physics:
This is the entirety of the visible "Standard Model": twelve fermions, four kinds of gauge boson (γ, W±, Z0, and the gluon — which technically comes in 8 colour-combinations but is usually grouped under the single name "gluon"), and the Higgs. Every other particle ever observed — proton, neutron, pion, kaon, hyperon, J/ψ — is a composite built from these by the strong interaction. The chart contains, in this sense, all of matter.
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