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Start The Standard Model of Particle Physics
03 February 2014

The Standard Model of Particle Physics

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The Greeks called an “atom” the smallest particle that could be obtained by splitting matter. It was a purely theoretical concept, but it was all they knew about it and this remained just as abstract until very recently. The model broke down in the mid-nineteenth century, when they began to think that the atom had “parts”.  In 1897 J.J. Thomson (as distinct from W.Thomson, Baron Kelvin) discovered the electron as an elementary particle with a negative electric charge, which was part of the atom. Since atoms were electrically neutral, it was thought that the atom’s matter had a positive electric charge that was neutralized by electrons. The atom was, therefore, a “plum pudding” where the electrons nested.

Ernest Rutherford, a physicist from New Zealand based in England, took a giant leap in 1911 when he discovered the proton in the nucleus of the atom. He was an “experimental” physicist (as opposed to a theoretical physicist) and his successful experiment consisted of bombarding a piece of thin gold foil with ionized helium (helium atoms stripped of their electrons). To his surprise he saw that most of the helium ions passed through the gold foil with ease, but some “bounced” as if they had hit something solid. He deduced that the nucleus of the gold atoms had a hard particle, which was later called the proton. The atom, therefore, had a hard nucleus of protons. He received the Nobel Prize in Chemistry for this discovery, which was to his disgust to a certain extent as he considered himself a physicist and a not chemist (“All science is either physics or stamp collecting” is one of his quotes).

Niels Bohr, a Danish physicist, perfected the atomic model and described it as a miniature Solar System, in which negative electrons revolved around the positive nucleus. The space in between was empty. The model was completed by James Chadwick around 1932 when he discovered the neutron as part of the nucleus. The Solar System model, however, was incorrect and did not explain why electrons, upon losing energy, did not fall into the central nucleus, in spite of the electromagnetic attraction between electrons and protons.

It was quantum mechanics that solved the problem by determining that electrons moved in different layers or “orbitals” and could not move from the quantum energy level of an orbital to another level without receiving or yielding a “quantum” of energy. The Schrödinger equation thus determined the structure of the atom, accompanied by Wolfgang Pauli’s “exclusion principle”, in which there can only be one electron in the same orbital (or two, if their “spins” or intrinsic angular momenta are opposite). Wave functions or the Schrödinger equations determine the “probability” that an electron is in a particular location but not its exact position. With his “uncertainty principle”, Werner Heisenberg had indicated that it was impossible to determine simultaneously both a particle’s position and momentum.

The Second World War arrived with this intellectual background and the fission of uranium in the atomic bomb. Thereafter, physics began to explore the “parts” resulting from splitting the atom. More and more particles were discovered (up to a few hundred), giving rise to a chaotic situation that has been described as the “particle zoo”.

It was Murray Gell-Mann, Professor of Physics at Caltech and 1969 Nobel Prize winner, who brought order to the zoo, determining what has been called the “standard model of particle physics”. The essence of the model is not overly complicated, but the details are for specialists and professionals. Particles are split into two groups: those with mass and those transmitting a force of nature. Particles with mass are those which form the protons and neutrons of the atomic nucleus and also form the electrons orbiting the nucleus.

The component particles of protons and neutrons are called “quarks” and are elementary particles, i.e., they are not composed of smaller parts. Electrons are also elementary particles. There are three families or groups of quarks with peculiar names like “up & down”, “charmed & strange” and “top & bottom”. The three families have increasing amounts of mass. The most common particles in nature are “up & down”. In addition, each family has its own electron and corresponding neutrino. The second family’s electron is called the muon and that belonging to the third family is called the tau.

In principle force-transmitting particles do not have mass and the generic name for them is the boson. Each of the forces of nature has its specific transmitting particle: photon (electromagnetic force), gluon (strong nuclear force) and W-weak and Z-zero bosons (weak nuclear force). The graviton, corresponding to the force of gravity, has not been found so far. Gravity is the most “rebellious” of the four classes of existing forces and no-one has managed to include it in any model. What is known as “the theory of everything” (the unification of the four forces) is a pipe-dream as of yet. Einstein himself failed in his quest, even though he spent the last decades of his life in search of this.

A peculiarity of the W and Z bosons is that they do have mass, mass conferred to them, it is believed, by the famous Higgs boson, as recently discovered at CERN, upon crossing with them in the Higgs field. To some extent this is a break up of the symmetry of the standard model, which is otherwise quite symmetrical.

On a purely theoretical level, sometimes a super-symmetry is spoken about, which is similar to the standard model, where each particle has its corresponding equivalent, though with greater mass and therefore heavier. None of these super-heavy particles has been discovered so far. The supersymmetric model could be used, among other things, to explain the extra dimensions of hyperspace or the nature of dark matter. Subject matters to let your imagination run wild …

Ramón Reis

Economist, Madrid (Spain)

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