Particles & Antiparticles Particle Physics Lesson 4

Today’s Objectives Compare rest mass energies of particles and anti-particles Describe the processes of pair production and annihilation. Calculate energies involved in pair production and annihilation.

Video The Matter with Antimatter.wmv

Particle Physics Particle physics is concerned with fundamental particles, which means that the particles can't be broken down any further..  It used to be thought that protons, neutrons and electrons were the fundamental particles of matter.  However it has been found that nucleons (proton and neutron) are made up of smaller particles, so nucleons are now not fundamental. 

Paul Dirac (1902-1984) Wrote an equation obeyed by an electrons which had a kind of mirror image solution Predicted the existence of anti-particles in 1928.

Discovery of the Positron Cosmic ray experiment. Charged particles leave an ion trail as they pass through a super-cooled gas. Causes vapour to condense to leave vapour trails – like jet plane. Magnetic field causes charged particles to bend. The particle bent the wrong way in a magnetic field.

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.

Positron Emission Also called β+ decay. Remember is isn’t a ‘natural’ form of decay – only occurs in experiments. Happens when there are too many protons.

Notes 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) other neutral particles and antiparticles are not identical.

Complete this table Particle Mass compared with proton Charge Electron 1/2000 -1e Positron Proton Antiproton Neutron Antineutron

Albert Einstein From Einstein’s special theory of relativity:- Equivalence of energy and mass: Energy can turn into mass Mass can turn into energy Have a go! Calculate the rest mass energy of a proton in eV.

Annihilation If a particle meets with its corresponding anti-particle they annihilate. All the mass of the particle and anti-particle mass is converted into energy in the form of photons (γ-rays).

Pair Production

Pair Production In pair production a photon creates a particle and a corresponding anti-particle (usually e+ and e-). Only occurs if one photon has enough energy to produce the total rest mass of the two particles. Where fmin is the minimum photon frequency required, E0 is the rest mass energy of the particles.

Cloud Chamber Pictures Can you see where pair production happens? Photons enter from the top of the image.

Cloud Chamber Pictures The top interaction includes a “knock on” electron.

Example Questions What is the rest mass energy of an electron? What is the minimum energy required for pair production of an electron and a positron to take place? What is the minimum frequency required for pair production of an electron and a positron to take place? … and the wavelength? To which part of the electromagnetic spectrum does this correspond?

Application - PET Scanners Positron Emission Tomography Positron emitting isotope is given to patient – some reaches the brain. Positrons only travel a few mm before annihilating with electron and photons produced are detected.

Albert Einstein Showed that the mass of a particle increases as its speed increases with the energy supplied to the particle to its increase mass by E=mc2. The mass of a particle when it is stationary its rest mass (m0) corresponds to rest energy m0c2