AS AQA Phys Unit 1- Particles and Antiparticles

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Paul Dirac

Dirac, an English physicist, predicted antimatter before the first antiparticle, the positron, was discovered. When he wrote down the equation obeyed by electrons, Dirac found a mirror image solution.

- It predicted the existence of a particle like the electron but with opposite electric charge.

- The cosmic ray experiment carried out later identified the positron. Positrons have identical mass (and identical energy in MeV) to electrons but carry a positive charge.

Dirac's theory of antiparticles predicted that, for every type of particle there is a corresponding antiparticle which;

  • Annihilates the particle and itself if they meet, converting their total mass into photons.
  • Has exactly the same rest mass as the particle.
  • Has exactly opposite charge to the particle if the particle has a charge.

Antiparticles (antimatter)

Matter and antimatter particles make up everything in the universe.

Each matter particle has a corresponding antiparticle with the same mass but with opposite charge. For instance, an anti-proton is a negatively-charged particle with the same mass as the proton;

Particle: proton, p, +1 Antiparticle: antiproton, p (with line above), -1

neutron, n, 0. antineutron, n (with line above), 0

electron, e^-, -1. positron, e^+, +1

electron-neutrino, Ve, 0. electron-antineutrino, Ve (with line above), 0

Matter and Antimatter from energy

There is an equivalence of energy and mass. This is due to Einstein's Theory of Relativity. Energy can change to mass and mass to energy; E=mc^2. When energy is converted into mass it results in equal amounts of matter and antimatter being produced.

Einstein stated that the mass of a particle when it's stationary i.e. rest mass (m0), corresponds to rest energy (m0c^2) locked up as mass. He showed that rest energy must be included in the conservation of energy. Dirac (metioned earlier) predicted the existence of antimatter particles (antiparticles) that would unlock rest energy, whenever a particle and corresponding antiparticle meet and annihilate each other.

In the Large Hadron Collider the kinetic energy of charged particles is accelerated to create collisions. When these particles collide;

  • Total energy of particles and antiparticles before collison = their rest energy + kinetic energy
  • Total energy of particles and antiparticles after collision = their rest energy + kinetic energy

Using conservation of energy,

rest energy of


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