The pion as the exchange particle of the strong nuclear force.

The idea

The strong nuclear force binds nucleons inside the nucleus. In AQA A-level Physics, the attractive part of this force is modelled as a short-range interaction mediated by the exchange of pions. Pions are mesons made of a quark and an antiquark. A nucleon emits a virtual pion, which is absorbed by a neighbouring nucleon; this exchange transfers momentum and produces an attractive force. The strong force is attractive at separations around 1 fm and becomes negligible beyond roughly 3 fm. While the strong force is repulsive below 0.5 fm to prevent nuclear collapse, this repulsive core is not explained by pion exchange. Pions are virtual exchange particles, meaning they exist only during the interaction and are not permanent constituents of the nucleus.

What earns the marks

• Pions are mesons composed of a quark and an antiquark, for example π⁺ is u d̄ and π⁻ is ū d.
• The attractive strong nuclear force between nucleons is modelled as the exchange of virtual pions.
• Exchange of a virtual particle transfers momentum and produces an attractive force between the nucleons.
• The strong force has a short range, being attractive around 1 fm and negligible beyond 3 fm, with a repulsive core below 0.5 fm (though repulsion is not due to pion exchange).
• Pions are virtual, so they are not counted as permanent constituents of the nucleus and are not observed directly in the exchange process.
• The strong force acts equally between protons and neutrons, consistent with pion exchange being possible between all nucleon pairs.

Common mistakes and how to put them right

• Treating pions as permanent particles inside the nucleus.

  Put it right by describing them as virtual exchange particles that exist only during the interaction.

• Attributing the repulsive core of the strong force to pion exchange.

  Put it right by stating pion exchange only models the attractive residual force between nucleons.

• Claiming the strong force has infinite range.

  Put it right by stating that pion exchange gives a short range of a few femtometres, unlike the infinite range of the electromagnetic force.

Exam tips

• Link the exchange particle explicitly to the attractive nature of the strong force, but avoid claiming pion exchange causes the repulsive core below 0.5 fm.
• Use the correct symbols π⁺, π⁻ and π⁰ and state the quark composition of at least one pion when describing the exchange.
• When explaining nuclear stability, refer to the balance between the short-range strong force and the longer-range electrostatic repulsion.