= Yukawa meson theory of nuclear forces
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In 1935, <Hideki Yukawa> proposed a new massive exchange quantum to explain <nucleon> binding. His paper began with the inadequacy of an <Electron>–<neutrino> exchange description of the binding strength. It then required a <potential energy> falling faster with distance than a <Coulomb potential>, introduced an exponentially decaying field, and related its inverse range to the exchange particle's <rest mass>. This was a theoretical proposal, not a detection with an apparatus of his own.<On the Interaction of Elementary Particles. I by Hideki Yukawa>[On the Interaction of Elementary Particles. I, opening motivation, equations (1)–(3) and (12); text inspected in a retypeset mirror]{ref}
The crucial idea is that a finite <rest mass> supplies a length scale even in empty space. In a scalar model, the static limit of a <Klein-Gordon equation> produces the <modified Helmholtz equation>. Its decaying <Green's function> gives the <Yukawa potential>. The derivation below isolates this range mechanism; <one-pion exchange> also carries <spin> and <isospin> structure.
The <nuclear emulsion track experiment> used developed <photographic emulsions> exposed to <cosmic rays> and inspected microscopically. Stopping <pions> gave daughter <muon> tracks with closely grouped ranges. Comparing penetrating <muons> with <pion>-induced nuclear breakup distinguished their interactions; this was not a direct static <force> measurement. The <muon> is a <lepton>, not Yukawa's strongly interacting <meson>.<The Cosmic Radiation by Cecil Frank Powell>[The Cosmic Radiation, printed pp. 144–145 and 151–152, especially Fig. 5; inspected in the Columbia-hosted Nobel volume]{ref}
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