β-ray spectra continuous not like α and γ ray spectra

Measurement of the total energy of the β decay spectrum

The total energy was less than the theoretical value and energy loss couldn't be explained by neutral gamma energy

Conclusion: missing energy only could be explained by Niels Bohr's statistical energy conservation or by the existence of a new particle.

A proton-electron of nitrogen nucleus with Z = 7 and atomic number = 14 required half integral total spin of 21 fermions. BUT, experimentally band spectra of nitrogen was spin = 1 (Obeyed Bose statistics).

Solution: Wolfgang Pauli's suggestion of a neutral fermion called "Neutron" with mass order of an electron mass and penetrating power stronger than a photon.

NEUTRINO - After Chadwick's discovery of heavy neutron in 1932, Fermi changed Pauli's neutron into NEUTRINO to distinguish from Chadwick's neutron.

In 1934, Fermi developed a β-decay theory based on QED (Quantum Electrodynamics) of Dirac, Heisenburg, and Pauli.

Dirac's Theory: (= quantum mechanics + theory of special relativity) Prediction of positron, the first antiparticle, also the intrinsic spin of the electron.

Gamow & Teller's suggestion of the axial vector current

ψ^*γ_μγ₅ψ (γ₅ = -iγ₀γ₁γ₂γ₃)

Yukawa's π meson decayed into μ meson.

The π meson & the μ meson had unique energy which implies the emission of a very low-mass neutral particle (muon decayed into an electron with a continuous β spectrum of its own.).

A strength of a weak interaction would occur a neutrino to pass through 50 billion miles of water without interaction.

Fred Reines and C. Cowan searched a way to measure inverse β-decay:
(ν_e)^* + p → n + e⁺ (* as anti)

Requirement: low-cross-section, large target, and enormous flux.

Experiment: Nuclear Fission to provide a large amount of antineutrino flux and resulting two 0.5 MeV γ-rays.

Result: Reines and Cowan sent a telegram in June 1956 informing Pauli of their discovery.

By Goldhaber, Grodzins and Sunyar: Neutrino Left handed

νν^* oscillations analogously to K⁰K^*0 (Later to be oscillations into sterile states.)

By Ziro Maki, Masami Nakagawa, and Sakata in 1962.

Ray Davis and his colleagues got first radiochemical solar neutrino results at Homestake Mine in North Dakoda in 1968.

Discovery of ν_τ at SLAC (the Stanford Linear Accelerator Center) by Martin Perl and his colleagues (1976? or 1978?)

Martin Perl and Frederick Reines won the Nobel Prize in 1995 for the discovery of the tau lepton and the detection of the neutrino, respectively.

The IMB Experiment (Irvine Michigan Brookhaven Experiment), using H₂O (Water) detector for the search of proton decay (also detects neutrinos) → Observation of fewer ν_μ-interactions than expected. → The anomaly is, at first, believed to be a detector error.

→ Atmospheric Neutrino Anomaly (Observed by IMB and Kamiokande)

Measurement of Non-zero ν_mass approx. 10^-4 times the mass of the electron by Russian Team → BUT subsequent attempts of independent reproduce measurement FAIL.

In 1986, confirms deficit by first directional counting observation of solar neutrinos.

In 1987, IMB experiment and Kamiokande detect the burst of the neutrinos from Supernova 1987A.

In 1988, Kamiokande's search for proton decay with improved results: distinguishable between ν_μ-interactions and ν_e-interactions.
Measurement of approx. 60% of the ν_μ-interactions

Frejus and NUSEX (The Nucleon Stability EXperiment) used Iron target instead of water (H₂O)
→ Reporting NO DEFICIT of ν_μ-interactions

LEP (Large Electron-Positron) Accelerator Experiment in Switzerland and the SLC (The Stanford Linear Collider) at SLAC (Stanford Linear Accelerator Center: founded in 1962.): Determined only 3 existing light neutrino species (ν_μ, ν_τ, ν_e)

Kamiokande: the Second Experiment to detect solar Neutrinos → Confirmation of the Long-Standing anomaly by finding approx. 1/3 of the expected interactions.

IMB experiment: upgrade → better ability to figure ν_μ-interaction

SAGE (The Russian-American Gallium Experiment, in Russia) and GALLEX (GALLium EXperiment, in Italy)

Kamiokande: Finds Deficit of high-energy ν_μ-interactions. → ν_μ traveling the longest distances from production point to the detector makes the largest depletion.

Kamiokande and IMB groups: Test of the ability of water detectors to distiguish between ν_μ-interactions and ν_e-interactions by using a test beam at KEK.

SNO (The Sudbury Neutrino Observatory): Observation of Neutral Current, Charged Current, and Elastic Scattering from Solar neutrinos → Neutrino oscillations cause the Solar Neutrino Problem.

KamLAND (Kamioka Liquid scintillator Anti Neutrino Detector) started in January. After 10 months, announcement of a detection of a deficit of ν^*_e