Physics A-Level (AQA) : Particles

Simple model of the atom

Atom contains a positively charged nucleus composed of protons and neutrons and electrons that surrounds the nucleus.

Charge and mass of proton, neutron and electron in SI units

Proton: charge +1.60x10^-19 C, mass 1.67x10^-27 kg
Neutron: charge 0 C, mass 1.67x10^-27 kg
Electron: charge -1.60x10^19 C , mass 9.11x10^-31 kg

Charge and mass of proton, neutron and electron in relative units

Proton: charge +1, mass 1
Neutron: charge 0, mass 1
Electron: charge -1, mass 1/1840

What is specific charge?

Charge divided by mass, unit C kg-1

What is proton number (atomic number)?

Number of protons in the nucleus, symbol Z

What is nucleon number (mass number)?

Number of protons and neutrons in the nucleus, symbol A

Example of nuclide notation

What are isotopes?

Atoms with the same number of protons and different number of neutrons

What is the role of the strong nuclear force?

Overcomes electrostatic repulsion between protons and keeps the nucleus stable

How does the strong nuclear force vary with separation?

Closer than 0.5fm - repulsive
Between 0.5-3.0fm - attractive
Further than 3.0fm - no effect / zero

What is alpha decay?

Unstable nucleus emits alpha particle (helium nucleus)
Equation: X(A,Z) -> Y(A-4,Z-2) + α(4,2)

What is beta (minus) decay?

A neutron in the nucleus changes into a proton and emits fast-moving electron and electron antineutrino
Equation: X(A,Z) -> Y(A,Z+1) + e-(0,-1) + _νe(0,0)

Why was the existence of the neutrino hypothesised?

To account for conservation of energy in beta decay. Observation showed energy of particles after beta decay was less than it was before. Some of the energy must had been carried away by undetected particles (neutrino).

What is antiparticle?

For every type of particle, there is a corresponding antiparticle

Comparison of particle and antiparticle masses, charge and rest energy

Particle and its corresponding particle have equal masses and rest energy, but opposite charge.

Antiparticles of the electron, proton, neutron and neutrino

Positron, antiproton, antineutron, antineutrino

Photon model of electromagnetic radiation

Electromagnetic waves are emitted as discrete wavepackets and each wavepacket is referred to as a photon.
E = hf, E = hc/λ
E = photon energy, J
h = planck constant, 6.63x10^-34 J s
f = frequency, Hz
c = speed of light, 3.00x10^8 m s-1
λ = wavelength, m

What is annihilation?

A particle and a corresponding antiparticle meet and their mass is converted into radiation energy as two photons. Two photons are produced in this process to conserve momentum.

What is pair production?

A photon interacts with a nucleus or an electron and creates a particle-antiparticle pair, its radiation energy is converted into mass.

Energies involved in annihilation and pair production

Rest energy and kinetic energy of the particle-antiparticle pair is equal to the energy of the photon / two photons

What are the four fundamental interactions?

Gravity, electromagnetic, weak nuclear, strong nuclear/interaction

What is the concept of exchange particles?

Exchange particles are transferred between particles when a force acts between them. Exchange particles transfer energy and momentum.

What is the electromagnetic force?

The force that acts between charged particles

What is the exchange particle of the electromagnetic force?

Virtual photons - they have zero mass, infinite range and no charge.

What is the weak interaction?

The force that is responsible for β- decay, β+ decay, electron capture and electron-proton collisions.

What is exchange particle of the weak interaction?

W bosons - they have a non-zero rest mass, a short range of no more than 0.001fm, and are positively charged (W+ boson) or negatively charged (W- boson).

Feynman diagram: β- decay

Feynman diagram: β+ decay

Feynman diagram: electron capture

Feynman diagram: electron-proton collisions

What are hadrons?

Particles that are subject to the strong interaction

The two classes of hadrons

Baryons / antibaryons and mesons

What are (anti)baryons?

Particles that consist of three (anti)quarks

Examples of baryons and antibaryons

Baryons: proton, neutron
Antibaryons: antiproton, antineutron

What are mesons?

Particles that consist of one quark and one antiquark

What is baryon number?

A quantum number that must be conserved in all interactions.
Baryon: 1
Antibaryon: -1
Non-baryon: 0

What is the only stable baryon?

The proton, into which other baryons eventually decay

What is the pion?

The exchange particle of the strong nuclear force

What is the kaon?

A strange particle that can decay into pions

What are leptons?

Leptons are fundamental particles and are not subject to the strong interaction

Example of leptons and anti-leptons

Leptons: Electron (e-), muon (μ-), electron neutrino (νe), muon neutrino (νμ)
Their antiparticles: Positron (e+), anti-muon (μ+), electron antineutrino (_νe), muon antineutrino (_νμ)

What is lepton number?

A quantum number that must be conserved in all interactions; lepton number for electron leptons and muon leptons must be conserved in all interactions.
Lepton: 1
Anti-lepton: -1
Non-lepton: 0

What is the muon?

A particle that decays into an electron

What are strange particles?

Strange particles contain strange quark. They are produced through the strong interaction and decay through the weak interaction.

What is strangeness?

A quantum number to reflect the fact that strange particles are always created in pairs. It is conserved in strong interaction but can change by 0, +1 or -1 in weak interaction.
Strange quark: -1
Anti-strange quark: 1

Properties of quarks and antiquarks

Quark combinations of hadrons

Proton: u u d
Neutron: u d d
Antiproton: _u _u _d
Antineutron: _u _d _d
π+: u _d
π-: _u d
π0: u _u, d _d, s _s
K+: u _s
K-: _u s
K0: d _s
_K0: _d s

Decay of the neutron

n -> p + e- + _ve

Change of quark character in β- decay

d -> u + e- + _ve

Change of quark character in β+ decay

u -> d + e+ + ve

What are conserved in interactions?

Energy, charge, momentum, baryon number, lepton number are conserved in all interactions. Strangeness is not conserved in weak interaction.