A Brief History of Particle Physics

Geoff Milward

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Particle Physics Chronology

390 bc Atomic Hypothesis Democritus 1890’s Discovery of the Electron 1910’s Nuclear Hypothesis 1930’s The Neutron, Meson and Neutrino 1950’s Particles Proliferate 1960’s Quarks are Proposed 1970’s W, Z and Charm 1990’s The Quarks are Complete 2000 What Next, The Higgs Boson ?

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The Discovery of the Electron

In 1897 Cathode rays were investigated by JJ Thompson. He showed that these particles had sub-atomic masses and were negatively charged. As atoms were neutral he proposed the “plum pudding” model of electrons in a positive surround.

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Rutherford and the Nucleus

Rutherford fired positively charged alpha particles at a thin sheet of gold foil.

The alpha particles were emitted from a sample of Uranium.

He expected the alpha particles to pass through the foil thus verifying Thompson's 'plum pudding' atomic structure.

Alpha particle gun Zinc Sulfide screen Gold foil target 4

Rutherford and the Nucleus (2)

Rutherford found that A few of the alpha particles were deflected at various angles to their initial direction.

An even smaller number of alpha particles rebounded off the foil back towards the sample of Uranium.

Alpha particle gun Zinc Sulfide screen Gold foil target 5

Rutherford and the Nucleus (3)

Rutherford concluded that An atom's mass must be concentrated in a tiny positively charged nucleus as only a very small number of alpha particles either deflected or rebounded off the foil.

Most of the atom must be empty space. This space must contain the electrons.

It turned out Rutherford was rather lucky. Only this type of force gave the same result without using quantum mechanics, a theory which had yet to be developed.

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The Neutron, Meson and Neutrino

In 1932 Chadwick, working with Rutherford, solves the missing atomic mass problem by demonstrating the existence of the neutron.

Bombarding beryllium with alpha-particles produced uncharged particles that are detected when they knock electrons from an electroscope.

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The Neutron, Meson and Neutrino (2)

It looked like theory of atoms was complete but there were outstanding problems. New particles were proposed to explain them.

Why does the nucleus stay together?

In 1934 Yukawa recognised that the nucleon force could be the result of the exchange of a particle between them. He named it the Meson and used QM to predict its mass.

Cosmic ray tracks on photographic plates were used to 'hunt' for the meson, and one was found, but it was the wrong one!

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The Neutron, Meson and Neutrino (3)

Why is energy not conserved in beta decay?

In beta decay an electron is emitted from the nucleus, and should have a fixed energy, but it doesn’t. Energy appears not to be conserved.

In 1930 Wolfgang Pauli proposed that as well as the electron a “little neutral particle”, neutrino, was emitted.

Neutrinos are notoriously difficult to detect. The first direct experiment to detect them was by Clyde Cowan and Fred Reines in 1956.

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Proliferating Particles

There were now getting to be rather a lot of particles! There was a meson that was supposed to hold the nucleus together, but did not interact with nucleons.

There were neutrinos you could not detect, and to make it worse Dirac’s theory said that all particles should have an antiparticle. And then things got even worse … New machines were being developed, the cyclotron, to produce higher energy (& hence heavier) particles. Discoveries of new particles were proliferating so quickly that scientists spoke of a "particle zoo.” It had all got far too complicated, physicists felt there had to be an underlying order.

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Quarks Are Proposed

In 1962 Gell-Mann had glimmerings of that order. The newly found particles seemed to fall into neat patterns, dictated mainly by their mass and electric charge. He proposed that the heavier particles (baryons), such as protons and neutrons, were made up of three fundamental particles that he called quarks 11

Quarks Are Proposed (2)

But this meant the quarks had fractional electric charges, something never observed. The theory was not popular!

One of the patterns had something missing, Gell Mann’s theory predicted an new particle, the Omega minus.

The particle was discovered in 1964 by Samios et al at Brookhaven in the US.

The Quark theory became quickly established.

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Charm, the W and the Z particles

As higher energies were explored by detectors and in 1976 a new particle was discovered which needed a new quark flavour, charm. It can be a sad life being a physicist and opposite is one of the few jokes. The first charmed particle was called the Psi because of the tracks it left in the detector. It is said the Psi is the only particle that signs its own name. 13

Charm, the W and the Z particles (2)

But it wasn’t just new quarks that were needed...

In 1970’s Glashow, Weinberg and Salam developed a theory that unified the weak force and electro magnetism. It needed three new particles to carry the force, the Z, W and W + .

The Z boson was discovered in early 1980’s at CERN by colliding an electron an and anti-electron, 14

Charm, the W and the Z particles (3)

A real picture of Z particle detection.

It is a computer generated image of an experiment at CERN.

It shows the decay of two Z particles, one in yellow, one in cyan.

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The Quarks are Complete

In early 1980’s evidence of a new quark, bottom, was discovered.

But it was not until 1995 that the final quark, top, was found, although predicted by theory.

It’s mass was deduced from the energy of its decay products at 170 proton masses.

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The Quarks are Complete (2)

The six quarks form 3 families of matter.

Each family is heavier than the one before.

Experiments show there appears to be only three families.

The quarks u,c & t all have charge +2/3 but d, s & b are -1/3.

Include electrons and neutrinos and a pattern in charge appears.

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What Next ...

One of the current topics of research in particle physics is the search for the Higgs boson.

The Higgs boson is akin to a rumour spreading through the room.

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