3.2.1 - Particles

The ratio of the charge of a particle to its mass measured in C/kg.

Specific Charge = Charge / Mass

The strong nuclear force holds protons and neutrons together to overcome electrostatic forces of repulsion between protons to keep the nucleus together (stable).

0 - 0.5 fm = repulsive

0.5 - 3 fm = attractive

3 fm = no effect

1 fm = 1 x 10^-15 m

When the nucleus of an atom emits an alpha particle (2 protons and 2 neutrons)

When a neutron in the atom converts into a proton - during which a beta particle (an electron) and an electron anti-neutrino are emitted from the nucleus.

The conservation of energy in Beta decay.

Beta particles emitted had a range of energies up to a maximum so another particle had to be emitted also to conserve the amount of energy lost by the nucleus.

For every particle, there is an antiparticle with equal mass and rest energy but all other properties are opposite - e.g charge.

electron - positron

proton - antiproton

neutron - antineutron

neutrino - antineutrino

A small packet of EM radiation. The energy of a photon is given by the eqn:

E = hf

h - planck’s constant

f - frequency

When a particle and its corresponding antiparticle meet they annihilate and all their mass gets converted into energy in the form of two photons (to account for the conservation of momentum).

2hf = 2E

the combined energy of the photons = the rest energy of the particle - antiparticle pair.

therefore -

hf = E

If a photon with sufficient energy passes near an electron or nucleus then it can be converted into a particle - antiparticle pair; these then move away from each other.

hf = 2E

one photon produces a particle and antiparticle.

Gravity

Electromagnetic

Strong Nuclear Force

Weak Nuclear Force

Gravity affects any particle with mass over an infinite range.

Its exchange particle is the graviton.

The EM force affects any charged particle over an infinite range.

Its exchange particle is the (virtual) photon.

The SNF affects hadrons only over a range of 0 - 3fm.

Its exchange particle is the gluon.

The WNF affects all particles over a range of up to 10^-18 m.

Its exchange particle is the W+ or W- Boson.

It is responsible for Beta decay, electron capture and electron - proton collisions.

They carry energy and momentum between the particles experiencing the force.

e.g repulsion can be imagined as two people throwing a ball between them causing them both to be pushed apart due to momentum; the ball is the exchange particle.

A nucleus converts into a proton and emits a W- boson which then decays into an e- and electron anti neutrino.

A proton converts into a neutron and emits a W+ boson which decays into a positron and an electron neutrino.

When an atomic e- is absorbed by a proton, a neutron and electron neutrino are emitted.

The W+ boson is the exchange particle.

When an electron and proton collide, a neutron and electron neutrino are emitted.

The W- boson is the exchange particle.

All particles are either hadrons or leptons.

A hadron is a sub-atomic particle that interacts through the SNF and is made up of quarks. There are two types of hadron:

Baryons & Antibaryons

Mesons

Particles made up of three quarks.

They have a baryon number of +1. Protons (uud) and neutrons (dud) are both examples of baryons

Particles made up of three antiquarks.

They have a baryon number of -1.

All baryons eventually decay into protons as this is the most stable baryon.

Mesons are quark - antiquark pairs such as kaons or pions. They’re the exchange particle for the Strong Nuclear Interaction.

π+ = up and anti-down quark.

π^0 = up , anti-up or down, anti-down, or strange, anti-strange.

π- = down and anti-up quark.

As neutral pions are made by quarks and antiquarks of the same ‘flavour’ they annihilate and form two photons.

Decay into a muon and an antimuon neutrino (negative pion) or a antimuon and a muon neutrino (positive pion).

Kaons are another type of meson that last longer before decaying than pions; they’re produced by the SNF and decay via the WNF as they’re ‘strange’ particles.

K+ = up and anti-strange quark.

K^0 = down and anti-strange quark.

(anti) K^0 = strange and anti-down quark.

K- = strange and anti-up quark.

Kaons decay into pions or directly into (anti)muons and the equivalent (anti)muon neutrinos.

Quarks are the fundamental particles that all hadrom=ns are composed of. There are three main quarks (up, down and strange).

Leptons are fundamental particles - they cannot be broken down further. Examples include the muon or the electron.

They do not experience the SNF.

Muons are negatively charged and have a mass 200x bigger than the electron so is often referred to as a ‘heavy electron’.

It has a corresponding antimuon (µ+), muon neutrino and antimuon neutrino.

Muons decay into electrons (or positrons) and the corresponding (anti)electron neutrino.

Leptons have a lepton number = 1

Antileptons have a lepton number = -1

Non-leptons have a lepton number = 0

Baryons have a baryon number = 1

Antibaryons have a baryon number = -1

Non-baryons have a baryon number = 0

Anti-strange particles have a strangeness number = 1

Strange particles have a strangeness number = -1

Non-strange particles have a strangeness number = 0

During particle interactions, the following must always be conserved:

• Energy and Momentum

• Charge

• Baryon Number

• Lepton (electron and muon) Number

Strangness is conserved during the SNF but during the WNF it may change by 0, -1 or +1.