A neutrino (/nuːˈtriːnoʊ/ or /njuːˈtriːnoʊ/, in Italian [nɛuˈtrino]) is an electrically neutral elementary particle with half-integer spin. The neutrino (meaning ""little neutral one"" in Italian) is denoted by the Greek letter ν (nu). All evidence suggests that neutrinos have mass but that their masses are tiny, even compared to other subatomic particles. They are the only identified candidate for dark matter, specifically hot dark matter.Neutrinos are leptons, along with the charged electrons, muons, and taus, and come in three flavors: electron neutrinos (νe), muon neutrinos (νμ), and tau neutrinos (ντ). Each flavor is also associated with an antiparticle, called an ""antineutrino"", which also has no electric charge and half-integer spin. Neutrinos are produced in a way that conserves lepton number; i.e., for every electron neutrino produced, a positron (anti-electron) is produced, and for every electron antineutrino produced, an electron is produced as well.Neutrinos do not carry any electric charge, which means that they are not affected by the electromagnetic force that acts on charged particles, and are leptons, so they are not affected by the strong force that acts on particles inside atomic nuclei. Neutrinos are therefore affected only by the weak subatomic force and by gravity. The weak force is a very short-range interaction, and gravity is extremely weak on the subatomic scale. Thus, neutrinos typically pass through normal matter unimpeded and undetected.Neutrinos can be created in several ways, including in certain types of radioactive decay, in nuclear reactions such as those that take place in the Sun, in nuclear reactors, when cosmic rays hit atoms and in supernovas. The majority of neutrinos in the vicinity of the earth are from nuclear reactions in the Sun. In fact, about 65 billion (7010650000000000000♠6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.Neutrinos are now understood to oscillate between different flavors in flight. That is, an electron neutrino produced in a beta decay reaction may arrive in a detector as a muon or tau neutrino. This oscillation requires that the different neutrino flavors have different masses, although these masses have been shown to be tiny. From cosmological measurements, we know that the sum of the three neutrino masses must be less than one millionth that of the electron.