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A Brief History of Particle Physics

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He showed that these particles had sub-atomic masses and were negatively charged. ... particles that are detected when they knock electrons from an electroscope. ... – PowerPoint PPT presentation

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Title: A Brief History of Particle Physics


1
A Brief History of Particle Physics
  • Geoff Milward

2
Particle Physics Chronology
  • 390 bc Atomic Hypothesis Democritus
  • 1890s Discovery of the Electron
  • 1910s Nuclear Hypothesis
  • 1930s The Neutron, Meson and Neutrino
  • 1950s Particles Proliferate
  • 1960s Quarks are Proposed
  • 1970s W, Z and Charm
  • 1990s The Quarks are Complete
  • 2000 What Next, The Higgs Boson ?

3
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.
4
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
5
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
6
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.
7
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.
8
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!

9
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
    doesnt. 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.
10
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 Diracs 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.

11
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
12
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-Manns 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.
13
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.
14
Charm, the W and the Z particles (2)
  • But it wasnt just new quarks that were needed...
  • In 1970s 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 1980s at
    CERN by colliding an electron an and
    anti-electron,

15
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.

16
The Quarks are Complete
  • In early 1980s 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.
  • Its mass was deduced from the energy of its
    decay products at 170 proton masses.

17
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.

18
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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