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Particles and Nuclear Structure

Testing a reaction against the conservation rules

  • 3 things to know
  • AS Unit 1

Overview

Recall questions on Testing a reaction against the conservation rules.

What you need to know

Answer each one before you reveal it.

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Key term

Test whether the reaction p + p → p + p + π⁰ is allowed by the conservation rules.

Key term

Explain why the reaction n → p + e⁻ + νₑ is forbidden.

Key term

Test whether the reaction π⁻ + p → n + π⁰ is allowed.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Name the three quantum numbers that must be checked when deciding whether a particle reaction is allowed.

    Model answer

    Charge, baryon number and lepton number must all be conserved in the reaction.

  2. 2

    Which two other quantities must be conserved in every particle reaction?

    Model answer

    Total energy and total momentum must be conserved, as in every other physical process.

  3. 3

    How is the baryon number of a particle found from its quark content?

    Model answer

    Each quark contributes +⅓ and each antiquark contributes −⅓, and these are added together. Three quarks therefore give +1 and a quark with an antiquark gives 0.

  4. 4

    Test whether the reaction p + p → p + p + π⁰ is allowed by the conservation rules.

    Model answer

    The charge is +2 on each side, the baryon number is +2 on each side and the lepton number is 0 on each side. The reaction is therefore allowed provided the incoming protons carry enough kinetic energy to create the pion.

  5. 5

    Test whether p → n + e⁺ + νₑ satisfies the conservation rules, and explain why a free proton does not decay this way.

    Model answer

    Charge goes from +1 to 0 + 1 + 0 = +1, baryon number stays at +1 and lepton number goes from 0 to −1 + 1 = 0, so the rules are satisfied. A free proton still cannot decay this way because the neutron is more massive than the proton, so there is not enough energy available. Inside a suitable nucleus the surrounding binding energy can supply the difference.

  6. 6

    Explain why the reaction p → n + e⁺ is forbidden.

    Model answer

    Lepton number is zero before the reaction but the positron on its own gives −1 afterwards. Lepton number is not conserved, so the reaction cannot happen.

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