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:


Question 1 What is the energy in joules of the following electron energies?   

(a) 100 eV

(b) 100 MeV

(c) 10 GeV              



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 Einsteinís 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 itís the same as rest mass.


Question 2 What is the speed of an electron at:

100 eV;

10 GeV?


Question 3 The mass of a proton is 1.66 ◊ 10-27 kg. What is this in MeV/c2?




Particles and antiparticles

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


We should note the following:



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.

PowerPoint Animation


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.


Question 4 State and explain the sequence of events in an annihilation.