Editing Nuclear physics (section)

===Yukawa's meson postulated to bind nuclei===
In 1935 [[Hideki Yukawa]]<ref>{{cite journal |last1=Yukawa |first1=Hideki |title=On the Interaction of Elementary Particles. I |journal=Proceedings of the Physico-Mathematical Society of Japan |series=3rd Series |date=1935 |volume=17 |pages=48–57 |doi=10.11429/ppmsj1919.17.0_48 |doi-access=free |url=https://www.jstage.jst.go.jp/article/ppmsj1919/17/0/17_0_48/_pdf/-char/en |url-status=live |archive-url= https://web.archive.org/web/20231122190008/http://www.jstage.jst.go.jp/article/ppmsj1919/17/0/17_0_48/_pdf/-char/en |archive-date= Nov 22, 2023 }}</ref> proposed the first significant theory of the [[strong force]] to explain how the nucleus holds together. In the [[Yukawa interaction]] a [[virtual particle]], later called a [[meson]], mediated a force between all nucleons, including protons and neutrons. This force explained why nuclei did not disintegrate under the influence of proton repulsion, and it also gave an explanation of why the attractive [[strong force]] had a more limited range than the electromagnetic repulsion between protons. Later, the discovery of the [[pi meson]] showed it to have the properties of Yukawa's particle.

With Yukawa's papers, the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force, unless it is too large. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron (or [[positron]]). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high-energy photons (gamma decay).

The study of the strong and weak nuclear forces (the latter explained by [[Enrico Fermi]] via [[Fermi's interaction]] in 1934) led physicists to collide nuclei and electrons at ever higher energies. This research became the science of [[particle physics]], the crown jewel of which is the [[Standard Model|standard model of particle physics]], which describes the strong, weak, and [[Electromagnetism|electromagnetic forces]].