brief history of neutrino
Brief History of the Neutrino 1) 1896: Henri Becquerel discovers natural radioactivity while studying phosphorescent properties of uranium salts. Éarays: easy to absorb, hard to bend, positive charged, mono-energetic; Ébray: harder to absorb, easy to bend, negative charge, spectrum?; É grays: no charge, very hard to absorb. 2) 1897: J.J. Thompson discovers the electron. 3)1911: Lise Meitner and Otto Hahn first shown that the energies of electrons mitted by beta decay had a continuous rather than discrete spectrum. 4) 1914: Chadwick presented definitive evidence for a continuous beta-ray spectrum. F. A. Scott, Phys. Rev. 48, 391 (1935) spectra were continuous Instead of discrete Origin continuous b-ray spectrum was unknown. Different options include several different energy loss mechanisms. It took 15 years more to decide that the “real” beta- ray spectrum was really continuous. Reason for continuous spectrum was a total mystery: + QM: Spectra are discrete + Energy-momentum conservation: N → N’ + e - electron energy and momentum well defined Nuclear physics before 1930: nucleus = n P P+ n e e- É Example: 4 He = 4P +2e - work well. É However: 14 N = 14P +7e - is expected to be a fermion, but it was experimentally known that is a boson! É There was also a problem with the magnetic moment of nuclei: m N , m P á m e (m=eh/4mc). How can the nuclear magnetic moment be so much smaller than the electron one if the nucleus contains electrons? SOLUTION: bound, nuclear electrons are very weird! + This can also be used to solve the continuous b-ray spectrum: energy need not to be conserved in nuclear processes! (N. Bohr) “…This would mean that the idea of energy and its conservation fails in dealing with processes involving the emission and capture of nuclear electrons. This does not sound improbable if we remember all that it has been said about peculiar properties of electrons in the nucleus.” (G. Gamow, 1931) Enter the neutrino… Weakly interacting massless neutral fermion 1930: Postulated by Pauli: the "neutron", a new spin 1/2 particle with small mass and no electric charge in order that: + resolve the problem of continuous beta-ray spectra, + reconcile nuclear model with spin-statistics theorem. 1932: Chadwick discovered the neutron. Chadwick’s neutron if different from Pauli’s neutron = neutrino (Fermi). Adapted summary of an English translation to Pauli’s letter dated December 4, 1930 observing the unobservable… 1) 1956: “Discovery” of the electron neutrino (Reines and Cowan) in the Savannah River Nuclear Reactor site. 2) 1962, the second neutrino: n m ∫ n e (Lederman, Steinberger, Schwarts at Brookhaven National Laboratory-BNL How to see them ? (Ex) Superkamiokande: 22,500 tons of water observing atmospheric neutrinos 300 events/yr Need huge detector 3) 2001: n t directly observed (DONUT experiment at FERMILAB: . Brief History of the Neutrino 1) 1896: Henri Becquerel discovers natural radioactivity while studying phosphorescent properties of uranium salts. Éarays: easy. neutrino (Fermi). Adapted summary of an English translation to Pauli’s letter dated December 4, 1930 observing the unobservable… 1) 1956: “Discovery” of the electron neutrino (Reines and Cowan). second neutrino: n m ∫ n e (Lederman, Steinberger, Schwarts at Brookhaven National Laboratory-BNL How to see them ? (Ex) Superkamiokande: 22,500 tons of water observing atmospheric neutrinos