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From the first particle accelerators to the discovery of the Higgs boson: A brief history of CERN
Simulated image of the Higgs boson decaying into four muons.
Image credit: CERN/CC-A.
First published on 30th July 2013. Last updated 1 January 2020 by Dr Helen Klus
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Life before CERN ↑
Before construction began on the European Organization for Nuclear Research (CERN) in 1954[1a], the atom was known to be composed of electrons (an elementary particle, and a type of lepton) and a nucleus containing neutrons and protons (which are hadrons, particles now known to be made of smaller particles called quarks and gluons), and all of these particles were thought to have an antimatter partner.
Fusion and fission reactions had taken place, and new particles such as muons (another elementary particle, and type of lepton) and pions and kaons (which are also hadrons) had been discovered in cosmic rays, using particle detectors like cloud chambers and bubble chambers. 'Cosmic ray' is a general term for different types of high-energy particles that originate from space, including photons, which are 'particles' of light.
1.1 The cloud chamber and bubble chamber ↑
British physicist Charles Thomson Rees Wilson invented the cloud chamber in 1911[2]. He received the 1927 Nobel Prize in Physics for his invention and help in its development.
A cloud chamber is made using a jar of gaseous alcohol, which has been cooled with dry ice. Particles move through the gas, removing electrons from the atoms it passes. This causes the rest of the gas to condense near them, creating a visible track, which could be photographed.
If a magnetic field is placed across the chamber, then it will deflect the path of charged particles. Austrian physicists Marietta Blau and Hertha Wambacher showed how the shape of the track could be used to determine the energy and charge of the particle[3].
American physicist Donald Glaser invented the bubble chamber in 1952[4]. This improved upon the cloud chamber and Glaser was awarded the 1960 Nobel Prize in Physics. In cloud chambers, particles travel through a superheated liquid rather than a cooled gas.
A diagram of a bubble chamber, where particles travel through a liquid, inside of a magnetic field. A camera is placed above the liquid.
A simple bubble chamber. Image credit: Stannered/aarchiba/Public domain.
In order to for a particle to be detected, it must be energetic enough. Cosmic rays are extremely energetic before they enter the atmosphere - reaching energies of up to 100 billion, billion electron volts (1020 eV), which is 16 Joules - but they cannot travel through the atmosphere[5]. This means that they could only enter chambers that were taken to the top of mountains, or above the atmosphere in balloons, and they could not be controlled.
At sea-level, the particles emitted by radioactive material produce tracks, but these only reach energies of up to about 150 million eV (150 MeV) or 40 billionths of a Joule (2.4 × 10-11 J)[6]. Physicists wanted to see what atoms are made of, and what keeps them together, as well as to see short-lived particles that could only be created in high-energy collisions. If this were to happen, then energies of more than 150 MeV would have to be reached. Kinetic energy is proportional to velocity. This means a higher energy is reached at higher speeds.
More systematic and precise experiments could only be conducted if it were possible to produce particles with higher energies on demand, at sea level, and this was achieved with the invention of particle accelerators.
A photograph from a bubble chamber, showing the effects of the decay of a kaon particle.
An image from a bubble chamber, showing the decay of a positive kaon. The decay products move in a spiral due to the magnetic field. Image credit: CERN/CC-A.
1.2 Linear accelerators ↑
The simplest particle accelerator is a battery. A battery has negative and positive ends. This difference creates a voltage, which produces an electric field. This accelerates electrons that travel along a wire. This is where the unit of the electron volt comes from; it is the unit of energy gained by one electron, accelerated by a voltage of 1 volt. This is a useful unit to use when the number would be extremely small if measured in Joules.
The first linear particle accelerator was built by Rolf Widerøe in 1928[7]. This increases the velocity of charged particles by subjecting them to a series of alternating voltages. Like in a single battery, the particle is accelerated across the gap between differing voltages, but here they then meet another gap and travel across this. More and more gaps can be added, making the particle travel faster and faster. The longer the accelerator, the faster the particle can travel. Particles were fired at a fixed target and the aftermath could be recorded in cloud or bubble chambers.