GKN's 1st lecture meeting on March 1, 2003

  『 Neutrino
        - What a mysterious object - 』
 ◆Speaker : Tetsuro Kobayashi (Former Professor of Toritsu University)
Mitsuko Kazuno (Former Professor of Toho University )
 ◆Place : Ikebukuro Geijutu Gekijo
 ◆Summary : The neutrino was postulated by W.Pauli in 1930 to save the crisis of energy conservation in the βdecay of nuclei, the decay accompanying an electron. In 1914 it was discovered that the electron spectrum of theβdecay showed the continuous character. If theβdecay proceeds to A(nucleus) → B(other nucleus) + @(electron), the energy of the emitted electron is uniquely determined from the energy momentum conservation. The continuous electron spectrum suggests that the βdecay process goes to A → B + @+ X, X being an unseen electrically neutral object. Everybody knows that the electron spectrum from the three body decay should kinematically be continuous. Pauli then called the neutral object X “neutron” which characterizes the electrical neutrality. Later in 1932 the neutron was discovered by J.Chadwick. The present name “neutrinos” for X is after E.Fermi.
 Pauli opened this idea only by his letter to colleagues assembled in Tubingen Conference to discuss the radioactivity from Zurich, and he did not publish it in the form of a paper. He himself called his neutron “Dieses narrische Kind meinen Lebenskrise”.
 In 1933 E.Fermi formulated his theory of theβdecay by taking into account Pauli’s “neutrino” with spin 1/2 and beautifully elucidated the continuous character of the electron spectrum. Albeit the Fermi’s theory of βdecay has widely been accepted since then, nobody has experimentally observed the neutrino due to its electric neutrality, very small mass and weak interactions with others.
However, a plenty of neutrinos exists around us. They come from the sun, stars and high energy cosmic ray particles. Also various kinds of neutrino have been producing in high energy particle and nuclear collision experiments at large accelerator laboratories. Nuclear reactors are another sources of neutrinos.
 At last in 1956, F.Reines and C.L.Cowan confirmed the existence of the neutrino by their experiment at Savannah River nuclear reactor in South Carolina,U.S.A.
 Pauli responded to the telegram informing their discovery from Reines and Cowan as follows : Thanks for message. Everything comes to him who knows how to wait.
 Unfortunately he passed away in 1958 just before the dawn of the neutrino age.
 In the same year as the experimental discovery of the neutrino C.N.Yang and T.D.Lee at the Advanced Study, Princeton,U.S.A. proposed a revolutional law of parity violation in the weak interaction. All experiments in particle physics then carried out have beautifully proved their theory of parity violation. Here it should be emphasized that the neutrino plays the decisive role in the theory of parity violation.
 The 2002 Nobel Prize in Physics has been awarded to Masatoshi Koshiba, a Professor Emeritus of the University of Tokyo, Raymond Davis, a Professor of Physics and Astronomy at the University of Pennsylvania, and Riccardo Gracconi of Associated Universities, Inc. The former two were honored “for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos. Those from the supernova are due to Koshiba and from the sun by Davis.
 Though in the beginning I gave a brief introduction to neutrinos to you, they do not seem to be familiar with you. On this happy occasion I would like to invite you to the wonderful world of neutrinos full of mysteries through the KAMIOKANDE experiment and guide you to its further development culminated in the discovery of the neutrino oscillation--- decisive evidence for the nonzero mass of neutrinos at the Super KAMIOKANDE.
 Since the neutrino, one species of ultimate building blocks of matter is multi-disciplinary, being of vital concern to laymen as well as physicists, it would be appropriate to briefly overview fundamental particles and forces or interactions among them.
 At the most fundamental level matter is composed of two classes of constituent fundamental particles with spin 1/2 : quarks and leptons. They are interacting via four kinds of forces :
 (1)strong , (2)electromagnetic , (3)weak and (4)gravitational.
These forces are mediated by gauge bosons with integer spin (see Table 1 and Table 2). As seen from Table 1 antiparticles are denoted by the bar on their top.
 Quarks feel the strong interactions, and tightly bound inside hadrons, strongly interacting particles. In other words all hadrons are composed of quarks or quark and antiquarks. For example,
Since quarks are confined inside hadrons, one can never take out a free single quark or antiquark from hadrons.
 On the other hand, leptons are structureless point particles and do not feel the strong interaction.
Uncharged neutrinos νe , νμ and ντ are, respectively associated with charged leptons e, μ and τ. Sometimes we call (νe , e) etc. the first generation(family)lepton etc. Similarly (t,b) is called the third generation (family) quark.
 Among these fundamental particles neutrinos are very special.
They are electrically neutral and interact with others only through the weak interaction whose strength is 10-5 compared with the strong interaction. Therefore neutrinos can almost freely pass through everywhere. So far the theory of weak interaction has been successful by assuming zero mass neutrinos. Without a definite principle, however, zero mass neutral particles cannot exist in nature. The problem of the origin of mass would be the most serious and exciting issue. Later I shall come back to this point with reference to Koshiba’s work.
 On 23 February 1997, 7:35:35 UT(Greenwich Standard Time) (±1min) the KAMIOKANDE(Kamioka Neutrino Detector) led by Masatoshi Koshiba observed a neutrino burst coming from the Large Magellanic Cloud 160 light years(ly) far from the earth, where 1 ly is approximately 0.946×1013 km. A huge number of neutrinos(?10-13/ cu) was produced by the explosion of the supernova SN-97A appeared in the large Magellanic Cloud, and they were scattered to the outer space. The obtained signal is consisted of 11 electron events of energy 7.5 to 36 MeV. They were recorded during a time of 13 sec. This is the plain evidence for cosmic neutrinos from the SN-97A.
 KAMIOKANDE started by the brillian idea of Koshiba. It was constructed in the Kamioka mine, Gifu Prefecture. The detector consists of a big tank filled by pure water of 3,000 tons and 948 photomultiplier tubes with its diameter of 50 cm which are embedded in the inner wall of the water tank. The whole apparatus has been put underground of 1,000m in depth.
 Incident neutrinos from SN-97A are elastically scattered by the electrons in water :
The recoiled electrons emit Cerenkov light which is effectively detected by the photomultiplier tubes. Consequently, the Cerenkov light tells us the energy and direction of the electron, and also the direction of incident neutrinos.
 When antineutrinos come in, they react with the proton, the nucleus of Hydrogen atom in water and the positron (antielectron) is emitted by accompanying with the neutron :
Isotropically scattered positrons emit also the Cerenkov light. Since this is the inelastic scattering we cannot know the direction of incident antineutrinos .
We could appropriately call the marvelous KAMIOKANDE experiment the “ Birth of Neutrino Astronomy”.
 Having been encouraged by the success of KAMIOKANDE experiment, Koshiba and his collaborators started in 1995 to construct Super KAMIOKANDE with a bigger water tank of 50,000tons and 10,000 photomultiplier tubes. Its location is very close to KAMIOKANDE.
 In 1998 Super KAMIOKANDE group presented their first exciting data at the International Conference on Neutrino Physics which was held in Takayama, Japan, not far from Kamioka. The world first evidence for the neutrino oscillation by Super KAMIOKANDE surprised all participants. The data from atmosphere neutrinos exhibit a zenith angle dependent deficit of νμ, the muon neutrino. They are inconsistent with expectation based on the calculations of the atmospheric neutrino flux. New Super KAMIOKANDE data are consistent with two neutrino species oscillationνμ ⇔ ντ , ντ being the tau neutrino. Later Canadian Collaboration group at SNO confirmed the Super KAMIOKANDE data.
 The neutrino oscillation is realized only for neutrino having finite mass. The origin of the mass of fundamental particles is an exciting but very subtle problem. In 1962 Z.Maki, S.Sakata and M.Nakagawa proposed a theory of massive neutrinos on the basis of neutrino oscillations. Anyway we could say that the Super KAMIOKANDE has experimentally proved the finite mass of neutrinos from their atmospheric neutrino data on the neutrino oscillation νμ ⇔ ντ . Further precise data are highly expected.
 Still neutrinos are veiled by unsolved mysteries. For example,
(1) Does the right handed massless neutrino exist ?
(2) How many generations of the neutrino are there ?
(3) Is the neutrino Majorana type or Dirac type ?
(4) Why the mass of neutrino is so small ?
(5) Is the dark matter the neutrino ? And so on.
 Recently, K2K and KAMLAND have presented interesting data. We would expect that Japanese neutrino experiments open the next door to mysteries of the neutrino by closely collaborating with theoretical physicists.