Particles and Radiation

Particle physics explores the fundamental building blocks of matter and the forces that govern their interactions. At A-Level, this topic introduces the Standard Model, the classification of particles, radioactive decay processes, the photoelectric effect, and wave-particle duality. This is one of the most conceptually rich areas of A-Level Physics.

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The Standard Model

The Standard Model is the most successful theory describing the fundamental particles and three of the four fundamental forces (electromagnetic, weak nuclear, and strong nuclear). Gravity is not yet incorporated into the Standard Model.

Key Definition: The Standard Model is a theoretical framework that classifies all known fundamental particles and describes the electromagnetic, weak nuclear, and strong nuclear interactions between them.

Fundamental Particles

All known matter is composed of two families of particles:

• Quarks — which experience the strong nuclear force
• Leptons — which do not experience the strong nuclear force

Quarks

There are six flavours of quark, arranged in three generations:

Generation	Quark	Symbol	Charge (e)	Baryon Number
1st	Up	u	+2/3	+1/3
1st	Down	d	−1/3	+1/3
2nd	Charm	c	+2/3	+1/3
2nd	Strange	s	−1/3	+1/3
3rd	Top	t	+2/3	+1/3
3rd	Bottom	b	−1/3	+1/3

Quarks are never found in isolation due to colour confinement. They combine to form hadrons:

• Baryons — made of three quarks (e.g. proton = uud, neutron = udd)
• Mesons — made of a quark-antiquark pair (e.g. pion π⁺ = u anti-d, kaon K⁺ = u anti-s)

graph TD
    A["All Matter Particles"] --> B["Quarks<br/>(experience strong force)"]
    A --> C["Leptons<br/>(do not experience strong force)"]
    B --> D["Hadrons<br/>(composite particles)"]
    D --> E["Baryons<br/>(3 quarks)<br/>e.g. proton uud, neutron udd"]
    D --> F["Mesons<br/>(quark + antiquark)<br/>e.g. pion, kaon"]
    C --> G["Charged Leptons<br/>electron, muon, tau"]
    C --> H["Neutrinos<br/>electron neutrino, muon neutrino, tau neutrino"]

Every quark has a corresponding antiquark with opposite charge, opposite baryon number, and opposite strangeness.

Key Definition: Strangeness is a quantum number assigned to particles containing strange quarks. A strange quark (s) has strangeness −1; an anti-strange quark has strangeness +1. Strangeness is conserved in strong and electromagnetic interactions but can change by ±1 in weak interactions.

Leptons

There are six leptons:

Generation	Lepton	Symbol	Charge (e)	Lepton Number
1st	Electron	e⁻	−1	+1 (Lₑ)
1st	Electron neutrino	νₑ	0	+1 (Lₑ)
2nd	Muon	μ⁻	−1	+1 (L_μ)
2nd	Muon neutrino	ν_μ	0	+1 (L_μ)
3rd	Tau	τ⁻	−1	+1 (L_τ)
3rd	Tau neutrino	ν_τ	0	+1 (L_τ)

Each lepton has a corresponding antiparticle with opposite charge and opposite lepton number.

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Exchange Particles and Feynman Diagrams

Forces between particles are mediated by exchange bosons (also called gauge bosons):

Force	Exchange Particle	Range	Acts On
Electromagnetic	Photon (γ)	Infinite	Charged particles
Weak nuclear	W⁺, W⁻, Z⁰ bosons	Very short (~10⁻¹⁸ m)	All quarks and leptons
Strong nuclear	Gluon (g)	~10⁻¹⁵ m	Quarks and gluons
Gravity	Graviton (hypothetical)	Infinite	All particles with mass

Feynman Diagrams

Feynman diagrams are pictorial representations of particle interactions. Time flows upward (or left to right, depending on convention — in UK A-Level, time typically runs from left to right or bottom to top). Straight lines represent fermions (quarks and leptons), and wavy or curly lines represent exchange bosons.

Described diagram — β⁻ decay Feynman diagram: A neutron (composed of quarks udd) enters from the left. One of the down quarks emits a W⁻ boson and transforms into an up quark (making the neutron into a proton, uud). The W⁻ boson decays into an electron (e⁻) and an electron antineutrino (anti-νₑ). The proton exits to the right along with the e⁻ and anti-νₑ.

Described diagram — β⁺ decay Feynman diagram: A proton enters from the left. An up quark emits a W⁺ boson and becomes a down quark (making the proton into a neutron). The W⁺ decays into a positron (e⁺) and an electron neutrino (νₑ).

Described diagram — Electron-proton collision via weak interaction: An electron enters from the left and a proton from the right. The electron emits a W⁻ boson and becomes an electron neutrino. The W⁻ is absorbed by the proton, which becomes a neutron.

Exam Tip: In Feynman diagrams, always label every particle and exchange boson. Ensure charge, baryon number, and lepton number are conserved at every vertex. The direction of arrows on antiparticle lines is reversed.

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Radioactive Decay at the Particle Level

Beta-Minus (β⁻) Decay

The weak interaction is responsible for beta decay. In β⁻ decay, a neutron transforms into a proton, emitting an electron and an electron antineutrino:

n → p + e⁻ + anti-νₑ

At the quark level: d → u + e⁻ + anti-νₑ, mediated by a W⁻ boson.

Beta-Plus (β⁺) Decay

In β⁺ decay, a proton transforms into a neutron, emitting a positron and an electron neutrino:

p → n + e⁺ + νₑ

At the quark level: u → d + e⁺ + νₑ, mediated by a W⁺ boson.

flowchart LR
    subgraph BetaMinus["β⁻ Decay"]
        A1["Neutron (udd)"] -->|"d quark emits W⁻"| A2["Proton (uud)"]
        A1 -->|"W⁻ decays into"| A3["e⁻ + anti-neutrino"]
    end
    subgraph BetaPlus["β⁺ Decay"]
        B1["Proton (uud)"] -->|"u quark emits W⁺"| B2["Neutron (udd)"]
        B1 -->|"W⁺ decays into"| B3["e⁺ + neutrino"]
    end

β⁺ decay can only occur inside a nucleus (a free proton cannot undergo β⁺ decay because the neutron is heavier than the proton). This process is important in PET (positron emission tomography) medical scanning.

Key Definition: A positron (e⁺) is the antiparticle of the electron. It has the same mass as an electron but carries a charge of +1e.

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Annihilation and Pair Production

Annihilation

When a particle meets its corresponding antiparticle, they annihilate, converting all their mass into energy in the form of photons. To conserve momentum, at least two photons must be produced, emitted in opposite directions.

e⁻ + e⁺ → 2γ (minimum)

The minimum energy of each photon equals the rest energy of one of the particles:

E_photon = m₀c² = 0.511 MeV (for electron-positron annihilation)

Pair Production

The reverse process can also occur: a high-energy photon can spontaneously create a particle-antiparticle pair, provided the photon has sufficient energy. This must occur near a nucleus (to conserve momentum).

γ → e⁻ + e⁺

The minimum photon energy required is:

E_min = 2m₀c² = 2 × 0.511 = 1.022 MeV

Any excess energy becomes kinetic energy of the created particles.