Questions on particle interactions and exchange particles

Question 1

Name the four fundamental interactions (forces).

Accepted Answer

The strong (nuclear) interaction and the weak (nuclear) interaction (1); the electromagnetic interaction and gravity (1).

Question 2

Name the exchange particle responsible for the electromagnetic interaction.

Accepted Answer

The (virtual) photon (1).

Question 3

State the exchange particles responsible for the weak interaction.

Accepted Answer

The W⁺ and W⁻ bosons (1). The Z boson may be given as well, but it is not required.

Question 4

State the neutrino or antineutrino emitted in (a) beta-minus decay and (b) electron capture.

Accepted Answer

(a) An electron antineutrino (1). (b) An electron neutrino (1).

Question 5

A neutron becomes a proton during an interaction, so a down quark becomes an up quark. State which fundamental interaction must be responsible.

Accepted Answer

The weak interaction, the only one that changes quark flavour (1).

Question 6

In a simple interaction diagram, state what the wavy line represents, and give the rule that must be satisfied at every junction.

Accepted Answer

The wavy line is the exchange particle (for example a virtual photon or a W boson) (1). Charge must balance at each junction: the total charge entering equals the total charge leaving (1).

Question 7

State the exchange particle associated with each of the four fundamental interactions, and identify the one that is not an established part of physics.

Accepted Answer

Strong: gluons (pions between nucleons) (1). Electromagnetic: virtual photons; weak: W and Z bosons (1). Gravity: the graviton, and this is the one that is not established (1). The first three belong to the Standard Model and are tested to high precision; there is no accepted quantum theory of gravity and no graviton has ever been detected.

Question 8

Explain what is meant by an exchange particle, and how it accounts for a force between two particles.

Accepted Answer

An exchange particle (a virtual particle) is emitted by one particle and absorbed by another (1). In transferring energy and momentum between them (1) it produces the effect we observe as a force; the type of exchange particle determines the interaction (1).

Question 9

Describe the beta-minus decay of a neutron in terms of an exchange particle, naming the exchange particle and all the products.

Accepted Answer

A neutron changes into a proton (1), emitting a W⁻ boson (1). The W⁻ then decays into an electron and an electron antineutrino (1). So n → p + W⁻, then W⁻ → e⁻ + anti-ν_e (1).

Question 10

Describe the beta-plus decay of a proton in terms of an exchange particle and its products.

Accepted Answer

A proton changes into a neutron (1), emitting a W⁺ boson (1), which decays into a positron and an electron neutrino: p → n + W⁺, then W⁺ → e⁺ + ν_e (1).

Question 11

A fast-moving electron collides with a proton and the interaction e⁻ + p → n + ν_e occurs. Name the exchange particle, and use the charges at the two junctions to justify your choice.

Accepted Answer

The W⁻ boson (1). The electron (−1) becomes a neutrino (0), so one unit of negative charge must leave that junction with the exchange particle (1). The proton (+1) absorbs the W⁻ to become the neutral neutron, +1 − 1 = 0, so charge balances at both junctions (1).

Question 12

Two electrons approach each other and are pushed apart without touching. Name the interaction responsible and its exchange particle, and explain how you can tell that the weak interaction is not responsible.

Accepted Answer

The electromagnetic interaction (1), carried by a virtual photon (1). No quark or lepton changes flavour in this event, and flavour change is what marks out the weak interaction (1).

Question 13

Electron capture and an electron–proton collision can both be represented by p + e⁻ → n + ν_e. State one similarity and one difference between the two processes, and name the exchange particle in each.

Accepted Answer

Similarity: the same overall change, a proton and an electron becoming a neutron and an electron neutrino (1). Difference: in electron capture the proton absorbs one of the atom's own inner (orbital) electrons, whereas in the collision a free, fast-moving electron strikes the proton (1). Electron capture is mediated by a W⁺, emitted by the proton (1); the electron–proton collision is mediated by a W⁻ (1).

Question 14

Explain why the photon exchanged in an electromagnetic interaction is described as virtual.

Accepted Answer

It exists only for the brief duration of the exchange (1) and it can never be observed or detected directly (1).

Question 15

Electron capture can be represented by a proton and an electron interacting to give a neutron and a neutrino. State the exchange particle involved and write the overall interaction.

Accepted Answer

The interaction is mediated by a W⁺ boson (1) (a proton emits a W⁺ and becomes a neutron (1); the W⁺ is absorbed by the electron (1)). Overall: p + e⁻ → n + ν_e (1).

Question 16

Explain why the weak interaction has a very short range compared with the electromagnetic interaction.

Accepted Answer

The range of an interaction is linked to the mass of its exchange particle (1). The W and Z bosons are very massive, so they can only exist as virtual particles for an extremely short time (1) and travel a tiny distance, giving the weak force its very short range (1). The photon is massless, so the electromagnetic force has, in principle, infinite range (1).

Question 17

State one similarity and one difference between the electromagnetic and weak interactions in terms of their exchange particles.

Accepted Answer

Similarity: both are mediated by exchange particles (bosons) (1). Difference: the electromagnetic force uses the massless photon (long range) (1), whereas the weak force uses the massive W and Z bosons (very short range) (1).

Question 18

A particle detector records two events. In event 1, a muon turns into an electron, with two neutrinos also produced. In event 2, a moving electron is deflected as it passes another charged particle, with both particles unchanged in type. Deduce which fundamental interaction is responsible for each event, naming the exchange particle involved in each.

Accepted Answer

Event 1: a lepton changes flavour (a muon becomes an electron), which only the weak interaction can do (1); the exchange particle is a W boson (1). Event 2: no flavour changes and two charged particles deflect one another, so the interaction is electromagnetic (1); the exchange particle is a virtual photon (1).

Question 19

In beta-plus decay a proton becomes a neutron in two stages: p → n + W⁺, then W⁺ → e⁺ + ν_e. Show that charge is conserved at each stage.

Accepted Answer

First stage: +1 → 0 + (+1) = +1 ✓ (1). Second stage: +1 → +1 + 0 = +1 ✓ (1). Overall, p → n + e⁺ + ν_e conserves charge: +1 → 0 + 1 + 0 = +1 (1).