AcademyParticles and Cosmology
Academy
Fundamental Interactions
Level 1 - Physics topic page in Particles and Cosmology.
Principle
Particle processes are governed by four interactions: strong, electromagnetic, weak, and gravitational. Their different ranges, strengths, and exchange particles explain which processes occur.
Notation
\(\gamma\)
photon, electromagnetic exchange particle
\(g\)
gluon, strong exchange particle
\(W^\pm\)
charged weak exchange bosons
\(Z^0\)
neutral weak exchange boson
\(r\)
interaction range
\(\mathrm{m}\)
\(m_X\)
exchange-particle mass
kg, GeV/c^{2}
Method
Derivation 1: Range depends on exchange mass
Massless exchange particles give long-range interactions. Massive exchange particles give short-range interactions.
Range estimate
\[r\sim\frac{\hbar}{m_Xc}\]
Massless exchange
\[m_X=0\Rightarrow r\to\infty\]
Derivation 2: Identify the interaction from what changes
Electromagnetic processes conserve particle flavour. Weak processes can change flavour and can involve neutrinos.
Electromagnetic
\[q\ \mathrm{unchanged}\]
Weak decay
\[n\to p+e^-+\bar\nu_e\]
Derivation 3: Strong interaction confines quarks
Gluons couple to colour charge, so isolated quarks are not observed at low energies.
Hadron condition
\[\mathrm{colour\ neutral}\]
Confinement
\[q\ \mathrm{not\ isolated}\]
Rules
Range estimate
\[r\sim\frac{\hbar}{m_Xc}\]
Beta decay example
\[n\to p+e^-+\bar\nu_e\]
Charge conservation
\[\sum Q_i=\sum Q_f\]
Examples
Question
Which interaction is responsible for beta decay?
Answer
The weak interaction, because a neutron changes into a proton while emitting an electron and antineutrino.
Checks
- Strong interactions bind quarks inside hadrons.
- Electromagnetic interactions act on electric charge.
- Weak interactions allow flavour change and neutrino processes.
- Gravity is negligible in most particle experiments but important cosmologically.