Other problems with this explanation include the rates of nucleosynthesis in the early universe, particularly the neutron to proton ratio, which is fixed by the abundance of light leptons and the number of neutrino flavors. Limits place this maximum mass for the electron neutrino species as ≤ 10 eV. Should the neutrino have a large enough mass, about 30 eV, the predicted abundance of the three known species is sufficient to close the universe and possibly account for the gross features of the missing mass. That these are weak particles means that they decouple from the expansion sooner than the photons and can freely stream on scales larger than those of the photons, also remaining hotter than the photon gas and therefore more extended in their distribution if they accrete onto galaxies or in clusters of galaxies. It is well known that neutrino processes, being moderated by the weak force, permit the particles to escape detection by many of the classical tests. It cannot decay into lower mass species if it is only available in one helicity, and thus would survive from the epoch at which it decoupled from matter and radiation during the radiation-dominated era of the expansion. The electron neutrino (ν e ) is the lightest lepton, and in fact the lightest fermion, making it an interesting particle for explaining DM. Shore, Virginia Trimble, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VIII.D.2.b Neutrinos These studies using the sun as a primary source of electron neutrinos have led to tests of neutrino flavor conservation, neutrino masses, and other fundamental neutrino properties. Later, we discuss means of determining mass differences between neutrino types, or flavors, from observations of neutrinos from the sun. A cosmological argument suggests that the sum of the three known stable neutrino masses should be less than a constant times the square of the Hubble constant, or roughly 100 eV. It is generally presumed that the masses of all neutrinos are very much smaller than those upper limits, probably too small to be measured directly. Only crude upper limits have been set directly on the masses of the muon and tauon neutrinos from the kinematics of their decay modes: 0.2 and 35 MeV, respectively. New experiments to improve this limit continue to be performed. The most sensitive experimental measurement published to date reports an upper limit on the electron neutrino mass of a few electron volts, or less than 1/50,000th the mass of the electron. Many experiments have been performed in an effort to measure the masses of the three neutrinos. The electron, muon, and tauon have masses of 0.511, 105.7, and 1784 MeV, respectively, and the muon and tauon have mean lives of 2.2 × 10 −6 sec and 3.0 × 10 −13 sec, respectively. The masses of neutrinos may be zero and, in any event, are much smaller than the masses of their charged partners. Recently, it has been demonstrated experimentally that only these three relatively low mass, stable lepton families exist in nature. The lepton families are listed in Table I. These are the neutrinos and antineutrinos: ν e, ν ― e, ν μ, ν ― μ, ν τ, ν ― τ. ![]() Associated with each set of charged members is a corresponding set of neutral particles. Each charged set has a negative and a positive member, referred to as particle and antiparticle, respectively. The three lepton families are designated by their charged members: the electron, e − (and e +) the muon, μ − (and μ +) and tauon, τ − (and τ +). With zero or very small mass, all neutrinos spin in a left-handed direction, and all antineutrinos spin in a right-handed direction. As elementary particles, leptons carry an angular momentum of 1 2 unit of spin and are therefore regarded as fermions (particles with 1 2 unit of spin). Charged leptons, of course, may interact with other charged particles and fields through the much stronger electromagnetic force. Leptons, of which there are three known subfamilies, are distinguished from other elementary particles by a unique property: They are coupled to all other particles by the weak force, which results in extremely feeble interactions of leptons with the nuclei of atoms, with other elementary particles, and with themselves. ![]() ![]() Neutrinos belong to the family of elementary particles called leptons (originally, “smaller mass” particles).
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