Particle Physics Tutorial 6 - Particles and Anti-Particles

Particle physics is concerned with fundamental particles.  It used to be thought that protons, neutrons and electrons were the fundamental particles of matter, which could not be broken down into anything smaller.  However it has been found that nucleons are made up of smaller particles, so nucleons are now not fundamental. 

Units in Particle Physics

You will often see some strange looking units when reading about particles.  Joules and kilograms are far too big and clumsy at the particle level.

The main unit you will see is the electron-volt.  It is not a voltage, but a unit of energy.  It is the amount of energy that a single electron has when it is accelerated by a potential difference of 1 volt.  You have already looked at energies in electron-volts in previous tutorials.

1 eV = 1.6 × 10^-19 J

You will also see multiples like keV, MeV, and GeV:

• 1 keV = 1000 eV = 1.6 × 10^-16 J

• 1 MeV = 1 × 10⁶ eV = 1.6 × 10^-13 J

• 1 GeV = 1 × 10⁹ eV = 1.6 × 10^-10 J

You will also see the atomic mass unit, u (it will not be examined at AS):

1 u = 1.661 × 10^-27 kg

You will also come across an odd expression rest energy.  At the subatomic level, mass and energy are one and the same thing.  Mass can be turned into energy, and energy can be made into mass.  They are linked by Einstein’s famous simple equation:

E = mc²

The rest energy is expressed in MeV.  Rest mass is given in MeV/c² where c² is the square of the speed of light (9 × 10¹⁶ m² s^-2).  It can be shown that J/ m² s^-2 = kg.

The rest energy (mass) of an electron is 0.511 MeV/c² = 9.11 × 10^-31 kg.

The rest energy of a muon = 105.7 × 10⁶ × 1.6 × 10^-19 = 1.88 × 10^-28 kg

                                                            9 × 10¹⁶

 In this unit we will talk about rest energy, but it’s the same as rest mass.

Particles and antiparticles

Each particle has an antiparticle.  However, antiparticles are not found in normal matter, but arise in:

• high-energy collision experiments,

• interactions with cosmic rays,

• radioactive decay.

We should note the following:

• an antiparticle has the same mass as its particle,

• a particle and its antiparticle have equal but opposite charge

• an unstable particle and its antiparticle have the same lifetime.

• some neutral particles and their antiparticles are identical (e.g. photon and p^o meson)

• other neutral particles and antiparticles are not identical.

Annihilation

When particles and antiparticles meet, they annihilate each other, releasing their combined mass as energy in the form of photons. 

Because momentum and energy have to be conserved, two or three photons are created.  If there is sufficient energy, other particles may be created as well.  For example, the collision between an electron and a positron may give rise to two muons:

e-   +   e+ → m+      +     m-

The reverse process can apply as well.  Electrons and positrons can be formed when a gamma ray passes through matter.  A gamma photon can give rise to an electron and a positron, provided the energy of the photon is more than twice the rest mass of an electron, and that it is near a nucleus. This pair production is a good illustration of how mass and energy can be changed from one to another.

Antiparticles can be made in large quantities in accelerators, resulting from high-energy collisions.  They have short lifetimes, about 10^-10 s because when they meet their equivalent particle, they annihilate each other in a burst of energy.  It is even possible to make simple anti-atoms. 

It is thought that there is more matter than antimatter in the Universe.  It is possible that antimatter exists in large quantities somewhere, and that there are antimatter stars and planets.  None have yet been detected.