The experimental finding that the strong interaction observed in atomic nuclei makes no difference between proton and neutron led Heisenberg to the concept of considering the two types of particles formally as two states of one and the same type of particle nucleon ; The only difference between the two states is a quantum number, which appears formally like the spin quantum number for spin 1/2 and is called isospin . “Nucleon” subsequently became the collective name for the two types of particles. The number of nucleons in an atomic nucleus, the mass number , is used to precisely describe the type of atom ( nuclide ) such as B. "Oxygen-16" or "Iron-56".
In today's (2018) standard model , nucleons are defined as those baryons that are composed exclusively of the light up and down quarks and have the isospin 1/2. Besides protons and neutrons in their basic states, this also includes excited states with such a quark composition, the very short-lived nucleons resonances . If you count these as separate particle types - as is usual in this case - there are at least 16 and possibly up to 29 nucleons (as of 2015), according to the Particle Data Group . The charge states are not distinguished here; With this counting method, proton and neutron are one and the same nucleon.
The three quarks mentioned above, the valence quarks , only make up about 5 percent of the mass of a nucleon. The remaining mass is explained with the virtual sea quarks and gluons . The exact breakdown is found to be by 2018 published QCD calculations: quarks (constituent quarks and sea quarks) around 9% to the mass, the remaining shares are taken from the kinetic energy of the quarks with around a third, due to the kinetic energy of the uncertainty being on are “trapped” in a narrow space, and contributions from the gluons (a field strength contribution of around 37 percent and an anomalous gluon amount of around 23 percent).
In 2008, a group at Forschungszentrum Jülich (Budapest-Marseille-Wuppertal collaboration) succeeded in theoretically calculating the masses of protons and neutrons within the framework of the QCD lattice theory with an accuracy of 1 to 2 percent. The simulations were the result of more than two decades of research on supercomputers, QCD in grid area formulation and corresponding algorithms and were error-controlled.
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