All subjects · Physics · WPH14 (Unit 4)

Particle interactions, tracks and probing the nucleon

Particle physicists work backwards from tracks to the particles that made them, using conservation laws. To see inside protons and neutrons, they need very high energies.

Conservation in interactions

In every particle interaction, charge, energy (including rest mass energy) and momentum are conserved. Kinetic energy alone need not be conserved, because mass and energy can convert into each other.

Reading tracks

Why high energies?

To probe a structure, the wavelength of the probe must be similar to or smaller than the structure. A particle's de Broglie wavelength is λ = h / p. Nucleons are about 10-15 m across, so the probing particles need very large momentum and therefore very high energy (GeV). High energies are also needed to create new, massive particles, since E = c2Δm.

Worked example

What momentum must an electron have to have a de Broglie wavelength of 1.0 x 10-15 m?

  1. p = h / λ = 6.63 x 10-34 / 1.0 x 10-15

Answer: p = 6.6 x 10-19 kg m/s, which needs an energy of about 1 GeV

Key idea: Charge, energy and momentum are always conserved. Only charged particles leave tracks. Probing nucleons needs λ = h/p of about 10-15 m, so very high energies.

Check yourself

  1. An electron has a momentum of 6.63 x 10-19 kg m/s. Find its de Broglie wavelength, in units of 10-15 m.
    Show answer

    1 x 10-15 m. λ = h / p = 6.63 x 10-34 / 6.63 x 10-19 = 1.0 x 10-15 m.

  2. Find the momentum needed for a de Broglie wavelength of 0.50 x 10-15 m, in units of 10-18 kg m/s, to 3 s.f.
    Show answer

    1.33 x 10-18 kg m/s. p = h / λ = 6.63 x 10-34 / 0.50 x 10-15 = 1.33 x 10-18 kg m/s.

  3. A particle at rest decays into two particles. One moves east with momentum 2.0 x 10-20 kg m/s. Find the size of the other's momentum, in units of 10-20 kg m/s.
    Show answer

    2 x 10-20 kg m/s. Total momentum is zero, so the other has 2.0 x 10-20 kg m/s, moving west.

  4. How does a neutral particle show up in a bubble chamber photograph?
    • It leaves a straight, thick track
    • It leaves no track
    • It leaves a tight spiral
    • It leaves a track that curves to the left
    Show answer

    It leaves no track. Neutral particles do not ionise the liquid, so they are inferred from gaps and momentum.

  5. A track spirals inwards in a magnetic field. Why?
    • The particle gains energy
    • The magnetic field is getting weaker
    • The particle's charge is increasing
    • The particle loses energy and momentum as it ionises the material
    Show answer

    The particle loses energy and momentum as it ionises the material. As p falls, r = p / BQ falls, so the path tightens.

  6. Two tracks curving in opposite directions start at the same point, with no track leading to it. What is the best explanation?
    • A charged particle stopped
    • A neutral particle produced a pair of oppositely charged particles
    • Two charged particles collided
    • The detector failed
    Show answer

    A neutral particle produced a pair of oppositely charged particles. No incoming track means a neutral particle; opposite curvature means opposite charges.

  7. Why are very high energies needed to investigate the structure of nucleons?
    • Nucleons only exist at high temperatures
    • High energy particles are easier to detect
    • Low energy particles have no charge
    • The probing particles need a de Broglie wavelength as small as a nucleon
    Show answer

    The probing particles need a de Broglie wavelength as small as a nucleon. λ = h / p must be about 10-15 m or less, so p and E must be large.

  8. Which quantity is NOT necessarily conserved in a particle interaction?
    • Charge
    • Momentum
    • Kinetic energy
    • Total energy
    Show answer

    Kinetic energy. Kinetic energy can turn into rest mass energy and back.

Practise with instant feedback

Sources

Last updated 7 November 2026. Spotted a mistake? Tell us.