AcademyParticles and Cosmology
Academy
History of Fundamental Particles
Level 1 - Physics topic page in Particles and Cosmology.
Principle
The particle model grew from experimental evidence: tracks, scattering, conservation laws, and energy thresholds revealed smaller constituents than atoms.
Notation
\(e\)
elementary charge magnitude
\(\mathrm{C}\)
\(q\)
particle charge
\(\mathrm{C}\)
\(m\)
rest mass
kg, MeV/c^{2}
\(E_0\)
rest energy
J, MeV
\(p\)
momentum
kg m s^{-1}, MeV/c
\(c\)
speed of light
\(\mathrm{m\,s^{-1}}\)
Method
Derivation 1: Charge comes in units
Millikan-type evidence and particle tracks show that observed free charges are integer multiples of \(e\).
Charge unit
\[q=ne\]
Electron charge
\[q_e=-e\]
Derivation 2: Rest energy sets production thresholds
Creating a new particle requires at least its rest energy, plus any momentum requirement imposed by conservation laws.
Rest energy
\[E_0=mc^2\]
Pair threshold
\[E_{\min}=2mc^2\]
Derivation 3: Missing energy and momentum suggest unseen particles
If measured products do not account for the initial energy and momentum, a neutral weakly interacting particle can be inferred.
Energy conservation
\[E_i=E_f\]
Momentum conservation
\[\vec p_i=\vec p_f\]
Rules
Charge quantization
\[q=ne\]
Rest energy
\[E_0=mc^2\]
Pair creation threshold
\[E_{\min}=2mc^2\]
Examples
Question
What minimum photon energy is needed to create an electron-positron pair?
Answer
\[E_{\min}=2(0.511)=1.02\,\mathrm{MeV}\]
Checks
- Use conservation laws before naming a new particle.
- Pair production must conserve charge and lepton number.
- A track in a magnetic field gives charge sign and momentum information.
- Discovery history is evidence-led, not a list of isolated names.