Proton is a positively charged composite particle found in ordinary atomic nuclei. It has electric charge equal in magnitude and opposite in sign to the charge of the electron and has a rest mass of approximately 938 MeV when expressed through mass energy equivalence. Proton is a baryon and therefore belongs to the family of hadrons that experience the strong interaction. Unlike an electron, a proton is not elementary. Its internal structure is described primarily in terms of three valence quarks consisting of two up quarks and one down quark together with a dynamic sea of gluons and quark antiquark pairs.
The proton has spin one half and is therefore a fermion. Its positive electric charge is produced by its internal quark and field structure rather than by a single elementary constituent carrying the entire charge. The proton is held together by quantum chromodynamics in which quarks interact through gluons and colour charge. The proton therefore illustrates an important distinction between elementary particles and composite particles. Its measured properties include not only mass and charge but also magnetic moment, charge distribution and internal momentum distributions that arise from its strongly interacting constituents.
The proton was recognised as a constituent of atomic nuclei during the development of nuclear physics in the early twentieth century. Ernest Rutherford played a central role in establishing the existence of the hydrogen nucleus as a positively charged nuclear constituent. The term proton derives from a Greek word meaning first and was introduced during the early development of nuclear terminology. The later discovery of neutrons and the development of the quark model transformed the understanding of the proton from an apparently elementary nuclear constituent into a composite hadron with rich internal structure.
The proton is stable according to all established observations. Some grand unified theories predict extremely long proton lifetimes through processes that violate the accidental baryon number conservation of the Standard Model, but no confirmed proton decay has been observed. Experimental searches therefore place very large lower limits on proton lifetime for various decay channels. Proton stability is important to the persistence of ordinary matter because ordinary atoms would be profoundly different if protons decayed on accessible timescales.
Inside a proton the two up valence quarks and one down valence quark do not behave as three independent classical particles. Quantum chromodynamics describes a constantly interacting system of quarks and gluons in which gluon fields carry colour charge and can generate additional quark antiquark pairs. The term valence quark refers to the net flavour content that determines the protons quantum numbers while sea quarks and gluons contribute substantially to its mass momentum and other measurable properties. The protons mass is therefore not simply the sum of the rest masses of its three valence quarks.
Protons participate in electromagnetic, weak and strong interactions. Their strong interaction is fundamental to hadronic and nuclear physics while residual strong forces between protons and neutrons help bind atomic nuclei. Two protons electrically repel each other because they carry the same positive charge, but inside nuclei the short range residual strong interaction can overcome electromagnetic repulsion under suitable conditions. Nuclear stability depends on the balance among electromagnetic repulsion, strong nuclear effects, neutron proton composition and quantum mechanical structure.
Proton beams are important in particle physics, medicine and research. Accelerators can produce high energy proton beams for collider experiments and fixed target experiments. Proton therapy uses the energy deposition characteristics of protons to treat selected cancers, with the depth dependence of energy deposition allowing treatment planning that can reduce dose to some surrounding tissues compared with certain alternatives. Protons are also used to investigate nuclear reactions, materials and fundamental particle interactions.
Proton should not be confused with a generic positive ion even though a hydrogen ion without its electron is a proton. A proton is specifically the hydrogen nucleus consisting of its composite quark and gluon structure. In an atom the number of protons determines the chemical element and is called the atomic number. Changing the number of protons changes the element itself, whereas changing the number of neutrons produces an isotope of the same element. Changing the number of electrons produces an ion without changing the elemental identity.
