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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 section 6.4.4 of the OCR A-Level Physics A specification (H556) — the details of quarks and leptons come in Lesson 7.
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:
Atom: ~10⁻¹⁰ m (0.1 nm)
Nucleus: ~10⁻¹⁵ m (1 fm)
Nucleon: ~10⁻¹⁵ m (1 fm)
Quark: < 10⁻¹⁸ m (point-like as far as anyone can tell)
Electron: < 10⁻¹⁸ m (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).Subscribe to continue reading
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