２２ Neutrino【What is the Universe？】Dialogue with the Universe[1]

２２　Neutrino【What is the Universe？】Dialogue with the Universe[1]

○A neutrino is an electrically neutral elementary particle with tiny mass. The neutrino (meaning "little neutral one" in Italian) is denoted by the Greek letter ν (nu). Neutrinos typically pass through normal matter unimpeded and undetected.

○Number of particles
・Comparing the number of particles in the universe, photons and neutrinos are overwhelmingly large.
Photons and neutrinos　　10e+3　(300) particles／ｃｍ３
　Protons and electrons 10e-6
Neutron 10e-7
Dark matter 10e-8

○ Neutrinos always have left-handed helicities (spins antiparallel to momenta) and all antineutrinos have right-handed helicities.
※Antineutrinos are very heavy. Particles with this much weight can not exist without going back to just after the universe began.
○Because antineutrinos and neutrinos are neutral particles, it is possible that they are actually the same particle.

○ By the way, neutrino is lighter than other nine fermions (particles that make up the substance).

Neutrino　　　　　　　 1e-3～1e-1 (eV)
other nine fermions　　1e+6～1e+12 (eV)
Higgs boson 1e+11 (126GeV)

○Immediately after the universe was born, Higgs bosons are flying around too much heat. Then, symmetry is maintained, weak force and the electromagnetic force behave in the same way. In such cases, quarks and neutrinos can change places with each other.

○Neutralino that is the lightest and stable particle in supersymmetric particles is one of the leading candidates for dark matter.
・Neutralino, that is the partner of photon and Z boson or the the Higgs particle, and that is electrically neutral. It's spin is 1/2. So neutralino particles is such as relatives of neutrino.

【References】
1.Wikipedia:Neutrino