section_path: The neutron in elementary particle physics - the Standard Model
section_name: The neutron in elementary particle physics - the Standard Model
paragraph_text: The neutron in elementary particle physics - the Standard Model
  
  Within the theoretical framework of the Standard Model for particle physics, a neutron comprises two down quarks with charge − 1 / 3 e and one up quark with charge + 2 / 3 e. The neutron is therefore a composite particle classified as a hadron. The neutron is also classified as a baryon, because it is composed of three valence quarks. The finite size of the neutron and its magnetic moment both indicate that the neutron is a composite, rather than elementary, particle.
  
  The quarks of the neutron are held together by the strong force, mediated by gluons. The nuclear force results from secondary effects of the more fundamental strong force.
  
  The only possible decay mode for the neutron that conserves baryon number is for one of the neutron's quarks to change flavour via the weak interaction. The decay of one of the neutron's down quarks into a lighter up quark can be achieved by the emission of a W boson. By this process, the Standard Model description of beta decay, the neutron decays into a proton (which contains one down and two up quarks), an electron, and an electron antineutrino.
  
  The decay of the proton to a neutron occurs similarly through the weak force. The decay of one of the proton's up quarks into a down quark can be achieved by the emission of a W boson. The proton decays into a neutron, a positron, and an electron neutrino. This reaction can only occur within an atomic nucleus which has a quantum state at lower energy available for the created neutron.