science&news: NEUTRINO HISTORY

This history is like a story. It begins at the beginning and it ends at the end. But, the true history of neutrinos is a story of ideas and, like the story of physics, its chronology does not have to be the one of acts and experiments. Moreover, many experiments and actors of the neutrino history have been forget or deliberately omitted, because this story would have become longer than the trajectory of a neutrino in the universe.

The History before the history (1896-1930)

Most of the scientific discoveries have had their origin in problems that scientific community was facing often without knowing it. Before the neutrino comes, the beta decay problem had to appear. And in order for that problem to appear, radioactivity had to be discovered.

Henri Becquerel in the year 1896, then Pierre and Marie Curie were the first actors of this time. While Henri Becquerel discovered some strange radiation coming from uranium salts, Pierre and Marie Curie isolated radium, a material much more radioactive than uranium.

In 1899, Rutherford shows that two types of radiation exist, that he calls alpha and beta. In 1900, Villard gives evidence for a third type of radiation coming from radium, that he calls gamma radiation. In 1902, Pierre and Marie Curie show that beta radiation was nothing else than electrons, while F. Soddy and E. Rutherford estimate that alpha, beta and gamma radiation are different types of radioactivity.

A crazy race begins to study in details those radiations coming from radioactive materials. Around 1904, Rutherford shows that alpha radiation is made of something like helium atoms.
Finally, three types of radioactivity are definitely asserted:

alpha radioactivity: an Helium 4 nucleus (two protons and two neutrons) comes out of the radioactive nucleus.
gamma radioactivity: a photon of great energy (few MeV) comes out of the radioactive nucleus.
beta radioactivity: an electron comes out of the radioactive nucleus.

The beta radiation (electron), the presumed only particle emitted, should have had a well fixed energy. But, after different studies of this radiation made by Lise Meitner, Otto Hahn, Wilson and von Baeyer, James Chadwick shows in 1914 that this is not the case: the electron energy spectrum is continuous.

Do we have to throw away the energy conservation principle, the sacred principle of scientists always verified by experiment ?... Niels Bohr, among others, dares to believe it. We must wait the year 1930 and Wolfgang Pauli in order the see an other solution.

The birth (1930-1934)

From what we know today, misters Neutrinos were born around 15 billions years ago, soon after the the birth of the universe. Since this time, the universe has continuously expanded, cooled and neutrinos have made their own way. Theoretically, they are now many and constitute a cosmic background radiation whose temperature is 1.9 degree Kelvin (-271.2 degree Celsius). The other neutrinos of the universe are produced during the life of stars and the explosion of supernovae.

But the idea of the neutrino came to life only in 1930, when Wolfgang PAULI tried a desperate saving operation of "the energy conservation principle". The 4th of December 1930, invited at a physicists workshop in Tubingen, he sends to his colleagues a strange letter...

In February 1932, J. Chadwick discovers the neutron, but neutrons are heavy and do not correspond to the particle imagined by Pauli.

At Solvay conference in Bruxelles, in October 1933, Pauli says, speaking about his particles:

"... their mass can not be very much more than the electron mass. In order to distinguish them from heavy neutrons, mister Fermi has proposed to name them "neutrinos". It is possible that the proper mass of neutrinos be zero... It seems to me plausible that neutrinos have a spin 1/2... We know nothing about the interaction of neutrinos with the other particles of matter and with photons: the hypothesis that they have a magnetic moment seems to me not funded at all."

In 1933, F. Perrin shows that the neutrino mass has to be very much lower than the electron mass. The same year, Anderson discovers the positron, the first seen particle of anti-matter, verifying thus the theory of P.A.M. Dirac and confirming the idea of neutrino in the minds of Pauli and Fermi. End of 1933, while Frederic Joliot-Curie discovers the beta plus radioactivity (a positron is emitted instead of an electron), Enrico Fermi takes the neutrino hypothesis and builds his theory of beta decay (weak interaction).
[ Since this time, physicists have made a lot of progress in the understanding of weak interaction and we now speak about protons and neutrons, composed of three quarks each. One of the quarks of the neutron transforms into an one, producing the emission of a W boson, which decays into an electron and an anti-neutrino ]

The neutrino quest begins, but people had to be quite reckless and persevering in those years because, as soon as 1934, Hans Bethe and Rudolf Peierls showed that the cross section (probability of interaction) between neutrinos and matter should be extremely small: billions of time smaller than the one of an electron. This particle interacts so weakly with matter that it can go through the whole earth without deviation.

First quest of the inaccessible star (1935-1956)

Until the end of the forties, physicists try to measure the recoil of a nucleus during its beta decay. All the measurements are compatible with the hypothesis of only one neutrino emitted with the electron. But no direct observation of neutrino seems feasible, since its predicted probability of interaction is too weak for any feasible experiment: a very abundant source of neutrinos and a very sensitive and huge heavy detector are needed. In 1939, Luis Alvarez shows that tritium is radioactive. Until today, beta decay of tritium has been used to give the best limit on the neutrino mass.

In 1945, the first atomic bomb explodes. Despite of the horror it inspires, it is for the physicists a remarkable powerful source of neutrinos. Frederick Reines, who is working at Los Alamos, speaks to Fermi in 1951 about his project to place a neutrino detector near an atomic explosion. In 1952, he meets Clyde Cowan and they finally agree to use a more "pacific" source of neutrinos: the nuclear plant of Hanford, Washington.
The detector is quickly built. Their experiment is proposed in February 1953, realized in spring and their results come out during the summer 1953. But the signal is not convincing. They do again their experiment in 1956, more carefully and this time near the nuclear plant of Savannah River, South Carolina. The improvements made, especially do decrease the background signal, give them the jack pot! The neutrino is here. Its tag is clearly visible in the detector, well above the backgrounds like cosmic radiation signals.
[ Near the same nuclear plant, other physicists, like Luis Alvarez or Ray Davis, had tried to detect neutrinos using carbonate chloride solutions, where chlore should have transform into radioative argon under the interaction with a neutrino. Unfortunately for them, a nuclear plant delivers only anti-neutrinos! ]

Reines and Cowan experiment principle consisted in using a target made of around 400 liters of a mixture of water and cadmium chloride. The anti-neutrino coming from the nuclear reactor interacts with a proton of the target matter, giving a positron and a neutron. The positron annihilates with an electron of the surrounding material, giving two simultaneous photons and the neutron slows down until it is eventually captured by a cadmium nucleus, implying the emission of photons some 15 microseconds after those of the positron annihilation. All those photons are detected and the 15 microseconds identify the neutrino interaction.

Detector of the 1953 experiment

Scheme of the 1956 experiment

The second quest (1957-1962)

The neutrino (or more precisely the anti-neutrino) coming out of a nuclear reactor is an electron type neutrino ( nue

), because, in beta decay, he is emitted together with an electron. Is it different from the muon type neutrino ( numu

), that could be associated to the muon, an other particle observed in cosmic rays?... Or is this difference only a theoretical arbitrary convenient assertion? Reines was not permitted to continue his researches. He had to come back at Los Alamos.

Others made the continuation and, in 1959, in the cafeteria of Columbia university, New-York, began the quest of the numu neutrino. After a discussion between T.D. Lee and M. Schwartz, this last one realized the possibility to produce a quite intense neutrino beam from decay of pions, that are particles produced in great quantity when a proton beam of GeV energy encounters matter. T.D. Lee and C.N. Yang are enthusiastic about the idea and begin to calculate the expected cross sections, while Schwartz associates to Leon Lederman, Jack Steinberger and, later, a young physicist from Orsay Jean-Marc Gaillard. They find the ideal detector for their experiment while looking at the spark chamber built by J. Cronin and his team, at Princeton.

In 1960, Lee and Yang are more and more convinced that if a reaction like is not observed, this is because two types of neutrinos exist. Meantime, the construction of spark chambers (a 10 tons set full of neon gas) runs rapidly. At the beginning of 1962, all is ready to run. The accelerator of Brookhaven delivers some hundreds of millions of neutrinos per hour, among which about 40 interact clearly with the detector. In 6 cases over 40, the particle coming out of the interaction is identified as an electron, which is approximately the expected background. In 34 cases over 40 , the output particle is identified as a muon. Conclusion: the numu is a different particle.

If the numu , and the nue have been a unique and same neutrino, our neutrino hunters would have obtain the same numbers of muons and electrons.

The neutrino is CERNed (1963-1983)

The discovery of nue

, and

implied a natural fever among the physicists. Numerous experiments and breakthroughs, almost in the same time, about quarks and leptons, came one after an other.

During the sixties and seventies, electrons and neutrinos of high energy are used to probe the composition of nucleons (that is neutrons and protons). The evidence for quarks, then the study of it, is quite grateful to the neutrino. One sees at CERN, during the seventies, especially in 1975 and 1976, the experiments CDHS, CHARM and CHARM II, then BEBC, which all give remarkable results about quark structure of nucleons and allow to better encompass this strange force: the weak interaction.

Some photographs of detectors

BEBC
CHARM
CHARM II
CDHS

In 1970, Glashow, Illiopoulos and Maiani make the hypothesis of the existence of a second quark family. At the end of 1974, their hypothesis is confirmed by two american experiments. Second family of neutrinos, second family of quarks: a nice bridge begins to be drawn between leptons families and quark families.

In 1973, after an incredible race between the Fermilab team and the CERN team of the bubble chamber "Gargamelle", neutral currents (neutrino interaction with matter where neutrino is not transformed into an other particle like muon or electron) are discovered. In 1977, the team lead by Leon Lederman discovers at the Stanford accelerator the b quark, that is a third quarks family. Approximately at the same time, Martin Perl discovers the particle tau, that is the third leptons family. The nutau neutrino is there: physicists can smell it but have not seen it! In 1998, it has still not been observed experimentally!

In 1983, the W boson shows its existence to the experiment UA1 by decaying in electron and anti-neutrino. Then comes the Z boson. The weak interaction and the neutrino print definitively their mark in physics history. A long way has been covered since the first beta decay recognized by Curie and Rutherford in 1898.

The neutrino dance (1983-1996...)

In the eighties, "theoretical and practical considerations" push progressively some physicists to agree on a non zero mass neutrino. Quantum mechanics allow then the oscillation of neutrinos A nue

can, along its travel in the universe, becomes a numu

and vice-versa.

In 1979, in an other experiment, F. Reines, still near the nuclear plant of Savannah River, undertakes the measurement of the ratio between neutral current and charged current, with anti-neutrinos on deuterium. The result is not compatible with theoretical predictions and could be explained by the oscillation of neutrinos. But no conclusion yet.

Under the sting of this result (that will later be corrected by other experiments), a team of ILL (Laue Langevin Institute) in Grenoble try to find oscillation of neutrinos near the nuclear reactor of its Institute. Then, many experiments search oscillation of neutrinos near nuclear plants around the world. Especially, the team of ILL gives birth to two teams: Goesgen in Switzerland and Bugey in France, between Chambery and Lyon. This last one implies five french laboratories, including LAPP. The two experiments Goesgen and Bugey find in 1984 two opposite results: Bugey has seen oscillations and Goesgen not. Finally, Bugey corrects the shooting direction and gives also a limit on neutrino oscillation. No neutrinos oscillation yet.
Result: if the mass difference between numu and nue is not very small (more than 0.1 eV), there is no more than 10% of mixing between the neutrinos, that is nue has not more than 10% of chance to transform into a numu .

But the history does not stop there, because the neutrino is facetious. Since 1969, a physicist named Ray Davis tried, in Homestake mine, under 3000 meters of earth and stones, to catch a few solar neutrinos per year, using a very big detector made of 600 tons of industrial solvent based on chlorine. His first results surprised. They are confirmed today, after 20 years of data: above a neutrino energy of 1 MeV, the sun emits three times less neutrinos than predicted by the standard solar model.

Astrophysicists scraped their heads and other experiments were undertaken in order to confirm this unexpected deficit. Especially, three experiments: GALLEX, SAGE and KAMIOKANDE. The experiment of Davis, HOMESTAKE, uses chloride, GALLEX uses gallium and KAMIOKANDE uses water. Theoretically, the different experiment does not see the same neutrinos, according to their origin from thermonuclear reactions in the sun.

Could the observed deficit of solar nue neutrinos come from neutrinos oscillation?... The idea is attractive, but the results of the experiments show that it is difficult to make it practical.

On an other hand, in 1985, S.P. Mikheyev and A.Y. Smirnov developped the work of L. Wolfenstein about neutrino oscillation enhanced by the presence of matter: this is the MSW effect. The solar neutrino deficit could come also from an oscillation during the path inside the sun. But experiment is the only truth.

Neutrinos are re-CERNed (1989-1996...)

In 1989, as early as the first months of data taking at LEP, the new CERN particle accelerator, the study of the Z boson lifetime allows to show that only three light neutrino families exist. This was a major physics result!

The shorter is the lifetime of a particle, the more undetermined is its mass. One says that its mass distribution has a width. And this width increases with the number of possibilities for the particle to decay. The Z boson, whose mean lifetime is around 10^-23 second, can decay in pairs particle, anti-particle like neutrino, anti-neutrino. The more is the number of neutrino families, the larger is the width of the Z mass distribution.

In 1992, two experiments at CERN are built in order to detect neutrino oscillations: NOMAD and CHORUS, which, thanks to neutrino oscillations, hope to see some nutau neutrinos inside a beam of numu produced with the protons of the CERN SPS accelerator. The data taking began in 1994 and first results were given in 1998.

The neutrino still dance (1993-1998...)

In 1995, the GALLEX experiment publishes its final result: a deficit of about 40% over almost the whole spectrum of solar neutrinos. SAGE confirms wisely this number and KAMIOKANDE observes a deficit of 50%, but for the Bore neutrinos only (above 7.5 MeV). The HOMESTAKE experiment, sensitive to Bore and Beryllium neutrinos had seen a deficit of 70%. The mystery is still there. Depending on the solar neutrino energy, the deficit is not the same. The MSW effect could explain this selective deficit.

Finally, KAMIOKANDE observes a deficit of numu in the cosmic rays showers. To try to interpret all those deficits by using only neutrino oscillations interpretation is not an easy work!... And it becomes very difficult if you try to incorporate the results obtained by LSND experiment at Los Alamos: an oscillation between anti- nue and anti- numu .

In 1996, LSND experiment announced new results: 22 interactions of anti- nue when 4 such interactions were expected. The non zero neutrino mass hypothesis seems to be stronger, but the scientific quest is like a mountain climber: it must make sure of its hanging points before continuing to progress. Results of other experiments and confirmation from LSND are thus impatiently waited...

1998, the LSND confirmation arrives and a Japanese experiment, Super-Kamiokande, has seen also an anomaly in the number of atmospheric neutrinos and solar neutrinos. The results are still preliminary but all seems in place for a massive neutrino or for a new gift from nature (that is a new scientific discovery).