The problem with beta decay
The prehistory of the neutrino starts in 1896 when Henri Becquerel discovered some strange radiation emitted by uranium salts [Bec96]. Isolating the radium, Pierre and Marie Curie named this new phenomenon “radioactivity”. Ernest Rutherford showed that there were two types of radioactivity, alpha and beta, and the Curies identified the beta radiation as electrons.
The beta decay, i.e. the decay of a nucleus AZX into a nucleus AZ+1X with the emission of an electron, was experimentally studied from 1910. In 1914, James Chadwick noted for the first time that the energy spectrum of the electron was continuous, and not the mass difference between the two nuclei [Cha14]. Later, in 1927, Charles Drummond Ellis and William Alfred Wooster [Ell27] made a decisive experiment on radium E (bismuth-210), using a calorimetric technique giving direct proof that the electron spectrum was continuous. To face this contradiction, several important physicists, among them the famous Niels Bohr, suggested iconoclastically that energy was not conserved.
The birth of neutrino
In December 1930, Wolfgang Pauli tried a desperate rescue of “the energy conservation principle”. He suggested that the electron is accompanied by a light, neutral, weakly interacting particle which takes away part of the energy [Pau30]. He called it “neutron” but after the discovery of the neutron by Chadwick in 1932 [Cha32], Enrico Fermi used the term “neutrino” which was immediately accepted by the community. At the Solvay conference in Brussels, in October 1933, Pauli presented “officially” the neutrino [Pau33], with some properties yet to be confirmed: a very small mass (possibly null), a probable spin 1/2. Incorporating the neutrino hypothesis, Fermi built immediately the theory of beta disintegration, which describes the decay of a neutron into a proton, emitting an electron and a “neutrino” [Fer33] (later this “neutrino” will be identified as a “antineutrino”).
Neutrino is a ghostly particle
In 1935, Maria Goeppert-Mayer calculated the (very small) probability of the simultaneous emission of two electrons and two neutrinos (the distinction between neutrinos and antineutrinos was not yet made) [Goe35], the double beta disintegration, observed experimentally much later in the 60’s.
In 1937, Ettore Majorana published his symmetric theory of electron and positron [Maj37], extended immediately to the neutrino by Giulio Racah [Rac37]. Today, we know that the neutrino is the only particle which could be identical to its antiparticle (Majorana type) and several experiments are actively looking for double beta decay without neutrino emission.
In 1934, Hans Bethe and Rudolf Peierls had showed that the cross section (probability of interaction) between a neutrino and a proton should be extremely small and that there was no practicable way of observing the neutrino [Bet34]. In spite of this, many physicists tried, unsuccessfully, to observe the neutrino, until the 50’s. A brilliant idea was proposed in 1946 by Bruno Pontecorvo [Pon46]: the use the inverse beta-process (νe + Z → e⁻ + (Z+1)) to detect the neutrinos, mentioning the famous chlorine-argon reaction (νe + 37Cl →e⁻ + 37Ar followed by the observed 37Ar decay), and quoting the Sun and nuclear reactors as significant sources of neutrinos.
The first detection
It is only in the 50’s that the combination of high intense neutrino sources and large detectors could open the door towards the observation of the neutrino. After a first attempt close to the Hanford reactor in 1953 [Rei53], with a limited statistics, Frederick Reines and Clyde Cowan succeeded to observe the neutrino in 1956 at the Savannah River power plant [Cow56,Rei56]. The high neutrino flux (in fact antineutrinos) was provided by the fission reactions in the core of the plant, and the detector measured the reaction (νe + p → e⁺ + n).
Neutrino properties under study
From then on, neutrinos were fully part of the game of particle physics. In 1957, Tsung-Dao Lee and Chen Ning Yang suggested that parity was not conserved in beta-decay and proposed a two-component theory of the neutrino [Lee57]. A few weeks later, Chien-Shiung Wu and her collaborators observed experimentally that parity was not conserved in beta decay [Wu57]. At the end of 1957, Maurice Goldhaber and his collaborators found in a beautiful experiment that the neutrino was left-handed (had a negative helicity) [Gol58].
The idea that the neutrino coming from the muon decay would be different from the neutrino associated to the electron emerged in the late 50’s in two papers by Pontecorvo [Pon59b] and Schwartz [Sch60]. Leon Lederman, Melvin Schwartz and Jack Steinberger built in 1962 an experiment at the Brookhaven accelerator and discovered the muon-neutrino [Dan62].
The first atmospheric neutrinos coming from the interaction of cosmic rays in the upper atmosphere were observed in 1965 in India and South Africa [Ach65b,Rei65b].
Neutrinos from the Sun
From the 60’s, neutrinos were connecting two domains: particle physics when producing neutrino beams to explore the deep structure of the matter; astrophysics when studying the many sources, as cosmic rays, Sun, supernovae, …
In 1938, Hans Bethe and others developed the theory of solar fusion and energy production in stars, starting with the primary proton-proton fusion in the core of the Sun [Bet38,Bet39]. This reaction, which produces electron-neutrinos, was followed by a complicated cycle of nuclear reactions producing also electron-neutrinos. After the first solar models developed by John Bahcall in 1964 [Bah64], Ray Davis revisited the idea to use the famous radiochemical chlorine-argon reaction (νe + 37Cl →e⁻ + 37Ar followed by the observed 37Ar decay) to observe solar neutrinos in the Homestake mine [Dav64]. The first results, in 1968, showed a deficit of observed solar neutrinos compared to the predictions [Dav68]. This was the start of the solar neutrino problem.
Neutrino families and neutrino oscillation
In 1957, Bruno Pontecorvo proposed that neutrinos could oscillate in to antineutrinos when propagating [Pon57]. In 1962, after the discovery of the second neutrino family, Maki, Nakagawa and Sakata raised the idea of flavor mixing for the neutrinos [Mak62]. Pontecorvo revived his initial idea in 1967 and discussed the oscillation between electron-neutrino and muon-neutrino [Pon67]. Oscillation would imply that neutrinos are massive.
The new accelerators built at CERN and Brookhaven from the early 60’s allowed to build intense neutrino beams. These beams were used to study the properties of the neutrino interactions. The first great success came in 1973 with the discovery of neutral currents (interactions of neutrinos with matter (quarks or electrons) by Z exchange) by the Gargamelle bubble chamber at CERN [Has73a,Has73b]. This discovery was crucial for establishing the electroweak theory (unification of weak and electromagnetic interaction). The neutrino beams were also used to study deep inelastic scattering and test quantum chromodynamics (QCD).
In 1975, at SLAC, the team led by Martin Perl discovered the third charged lepton, the tau, announcing the third neutrino, the tau-neutrino ντ [Per75] and the third particle family. The tau-neutrino ντ was first directly observed in 2001 [Kod01]. In 1989, the LEP at CERN showed that there were only three neutrino families (to be precise three families of active neutrinos), establishing the standard model of particle physics [Aar89,Akr89,Dec89].
The question of the mass of the neutrino (0 or very small) is still at the center of many theoretical and experimental questions. The minimal standard model of particle physics requires a null mass. So a mass for the neutrino gives indications towards extensions of the model. From the experimental side, there are experiments trying to measure directly the neutrino mass using the beta decay of tritium, experiments looking for neutrino oscillation, experiments looking for double beta decay without neutrinos. From the theoretical side, the most popular model is the see-saw mechanism [Min77,Gel79,Yan79]. This mechanism explains why the neutrino masses are so much smaller than the masses of the other leptons and quarks: it generates three very massive right-handed neutrinos which are sterile (do not interact). Still alive, this mechanism has still to be proven.
Neutrinos from supernovae… and from atmosphere
In February 1987, the detectors Kamiokande in Japan and IMB in USA observed approximately 20 interactions due to neutrinos coming from the explosion of the supernova SN 1987A in the Large Magellanic Cloud which occurred 150000 years ago [Hir87,Bio87]. After the paper by Baade and Zwicky [Baa34], such a great event was predicted by George Gamow in 1941 [Gam41], and founded neutrino astronomy.
At the end of the 80’s, the same detectors Kamiokande and IMB showed a possible anomaly in the behavior of atmospheric neutrinos (produced by interactions of cosmic rays in the upper atmosphere): observing less muon-neutrinos than predicted [Hir88,Cas91]. This result, not confirmed by other experiments like the Fréjus one, lasted for almost 10 years. In 1998, the SuperKamiokande detector, 10 times bigger than Kamiokande, showed without ambiguity that muon-neutrinos coming from the antipodes had been partially transformed into tau-neutrinos via the oscillation mechanism [Fuk98b]. This great result solved the atmospheric neutrino anomaly and proved the existence of the neutrino oscillation and consequently that neutrinos were massive.
Explosion of neutrino studies
In 1989, the Kamiokande detector gave a new important result: the direct detection of the high energy solar neutrinos (so-called boron-8 neutrinos), but with a flux reduction compared to solar models which confirmed the solar neutrino deficit [Hir89]. In 1992, the radiochemical GALLEX experiment (30 tons of gallium at the Gran Sasso laboratory) detected for the first time the solar neutrinos produced in the primary proton-proton fusion in the core of the Sun; but the measured flux was still less than predicted by solar models [Ans92]. The solar neutrino problem was finally solved in 2001 by the Sudbury Neutrino Observatory experiment (SNO). Using 1000 tons of heavy water which allowed to observe all the neutrino flavors, they showed that part of the solar electron-neutrinos had been transformed into muon-neutrinos or tau-neutrinos [Ahm01,Ahm02]. Until SNO, detectors were sensitive essentially to electron-neutrinos. The two messages of SNO were that a) solar models are right; and b) solar electron-neutrinos oscillate between the core of the Sun and the Earth; this was the second proof of the existence of the oscillation mechanism after that observed in atmospheric neutrinos. To be precise, the solar electron-neutrinos are modified by a mechanism involving not only oscillation, but also an adiabatic transformation in the matter of the Sun predicted in 1985 by Lincoln Wolfenstein, Stanislas Mikheyev and Alexei Smirnov [Wol78,Mik85]. The corresponding oscillation parameters have been confirmed by the KamLAND experiment studying electron-antineutrinos produced by nuclear reactors [Egu03].
A new source of natural neutrinos was observed by the KamLAND experiment in 2005: the geoneutrinos coming from the radioactivity in the crust and mantle of the Earth [Ara05]. Statistically marginal, the first observation was confirmed in 2010 by KamLAND and Borexino at Gran Sasso [Bel10].
In 2010, the OPERA experiment at Gran Sasso observed directly the first tau-neutrino candidate produced in a muon-neutrino beam produced at CERN, 732 km away, confirming the oscillation [Aga10].
Having discovered the oscillation mechanism, we come back to the mixing matrix between the flavor eigenstates of two neutrinos and the mass eigenstates proposed by Maki, Nakagawa, Sakata and Pontecorvo (extended later to three neutrinos and parametrized in the so-called PMNS matrix). After the two mixing angles obtained in atmospheric neutrinos (θ23) and solar neutrinos (θ12), physicists began looking for the third angle (θ13). This angle was measured from 2011: a) in an accelerator experiment in Japan, T2K, observing the oscillation between muon-neutrino and electron-neutrino on a distance of 280 km [Abe11a]; b) in reactor experiments observing the oscillation of antineutrinos at a distance of ~1 km; Double Chooz [Abe11b], in France, Daya Bay [An12], in China, followed by RENO [Ahn12], in Korea.
The cosmological neutrinos, produced during the Big Bang, 13.6 billion years ago, constitute with the photons the most numerous particles in the Universe. Studying the cosmic microwave background (the mass of neutrinos has an effect on the CMB power spectrum), the Planck satellite gave in 2013 an upper limit on the total mass of neutrinos of about 1 eV [Ade13].
Already in 1960, Markov had anticipated that high energy neutrinos coming from galactic or extragalactic astrophysical sources could be detected using very large volumes of water, using the Cerenkov technique [Mar60]. In the 70’s, the first proposals were made, like the Dumand project [Rob76]. The first observation of high-energy astrophysical neutrinos has been done in 2013 by the IceCube experiment: a volume of about 1 km3 of ice at the South Pole instrumented with photomultipliers [Aar13a,Aar13b]. In 2018, high-energy neutrinos were observed by IceCube simultaneously with high-energy gammas from a flaring blazar, opening multi-messenger observation from astrophysical objects [Ice18].
In 2014, the Borexino experiment at Gran Sasso observed directly the solar neutrinos produced in the primary proton-proton fusion in the core of the Sun (GALLEX had observed an integral flux) and completed the spectroscopy of solar neutrinos [Bel14]. In 2020, Borexino observed for the first time the solar neutrinos from the so-called CNO cycle [Ago20], which is responsible of only 2% of the energy of the Sun, but which is dominant in more massive stars.
In 2017, the first observation of coherent elastic neutrino-nucleus scattering has been performed by the COHERENT collaboration [Aki17], providing new opportunities to study neutrino properties.
There have been many attempts to measure directly the electron-neutrino mass in beta-decay experiments. The best limit is now that of the Katrin experiment which gave recently an upper value of 0.8 eV [Ake22].
The (hi)story is still continuing: we do not know yet all the properties of the neutrinos, in particular their mass; we do not know if the neutrino is its own antiparticle (Majorana type); we do not know their precise role in supernova explosions; we do not know how much are produced in violent phenomena in the Universe; we have not yet observed cosmological neutrinos, produced 13.6 billion years ago during the big bang…
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