onepion

One-pion exchange, often abbreviated as OPE, is a mechanism in nuclear physics in which two nucleons interact by exchanging a single pion. It accounts for the longest-range part of the nuclear force and is rooted in the idea that the nuclear interaction is mediated by mesons predicted by Yukawa.

Historically, the concept came from Hideki Yukawa, who in the 1930s proposed that a light meson could mediate the strong force between nucleons, giving the nuclear force a finite range. Pions come in charged and neutral varieties, and their exchange yields isospin-dependent forces between nucleons. The resulting interaction is described by a pion-nucleon coupling and can be expressed in a potential form that contains central and tensor components, reflecting the spin and orbital angular momentum coupling involved in the exchange.

Physically, the central part of the OPE potential contributes to the overall attraction or repulsion between nucleons, while the tensor part, arising from the spin structure of the pion exchange, mixes different partial waves. The tensor force from OPE is particularly important for describing the deuteron, influencing its binding and the mixing of S and D waves. OPE also shapes nucleon-nucleon scattering phase shifts at long range and forms the foundation of many phenomenological nucleon-nucleon potentials and approaches in quantum chromodynamics-inspired models.

Limitations and extensions: OPE describes only the long-range part of the interaction. Short- and intermediate-range forces require additional mechanisms, such as multi-pion exchanges and heavier mesons, as well as contact interactions. In modern theory, one-pion exchange is embedded within frameworks like chiral effective field theory, where pions are explicit degrees of freedom alongside higher-order contributions.

See also: Yukawa potential, pion, nucleon-nucleon interaction, tensor force, chiral effective field theory.

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