3.2.1 - Particles

Specific Charge

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

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Specific Charge = Charge / Mass

What is the Strong Nuclear Force?

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

Strong Nuclear Force - range

0 - 0.5 fm = repulsive

0\.5 - 3 fm = attractive

>3 fm = no effect

What is a femtometer?

1 fm = 1 x 10^-15 m

Alpha decay

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

Beta (minus) decay

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.

Why were neutrinos theorised?

The conservation of energy in Beta decay.

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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.

What is an antiparticle?

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

Antiparticle examples

electron - positron

proton - antiproton

neutron - antineutron

neutrino - antineutrino

What is a Photon?

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

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E = hf

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h - planck’s constant

f - frequency

What is annihilation?

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).

Annihilation Equation

2hf = 2E

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

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therefore -

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hf = E

Pair Production

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.

Pair Production - Equation

hf = 2E

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one photon produces a particle and antiparticle.

The Four Fundamental Forces

Gravity

Electromagnetic

Strong Nuclear Force

Weak Nuclear Force

Gravitational Force

Gravity affects any particle with mass over an infinite range.

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Its exchange particle is the graviton.

Electromagnetic Force

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

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Its exchange particle is the (virtual) photon.

Strong Nuclear Force/ Interaction

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

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Its exchange particle is the gluon.

Weak Nuclear Force/ Interaction

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

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

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

What are Exchange Particles?

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

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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.

WNF - Beta minus Decay

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

WNF - Beta plus Decay

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

WNF - Electron Capture

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

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The W+ boson is the exchange particle.

WNF - electron proton collisions

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

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The W- boson is the exchange particle.

Classification of particles

All particles are either hadrons or leptons.

What is a hadron?

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

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Baryons & Antibaryons

Mesons

Baryons

Particles made up of three quarks.

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

Antibaryons

Particles made up of three antiquarks.

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They have a baryon number of -1.

Baryon decay

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

Mesons

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

Pions

**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.

Neutral Pion Decay

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

Charged Pion Decay

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

Kaons

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.

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**K+** = up and anti-strange quark.

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**K^0** = down and anti-strange quark.

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

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**K-** = strange and anti-up quark.

Kaon decay

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

Quarks

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

Leptons

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

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They **do not** experience the SNF.

Muons

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

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

Muon decay

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

Lepton Number

Leptons have a lepton number = 1

Antileptons have a lepton number = -1

Non-leptons have a lepton number = 0

Baryon Number

Baryons have a baryon number = 1

Antibaryons have a baryon number = -1

Non-baryons have a baryon number = 0

Strangeness Number

Anti-strange particles have a strangeness number = 1

Strange particles have a strangeness number = -1

Non-strange particles have a strangeness number = 0

Conservation Rules

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.