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Neutrinos and How to Catch Them 1 Introduction Neutrinos and how to catch them D. Indumathi, M.V.N. Murthy and G. Rajasekaran The Institute of Mathematical Sciences, Chennai 600 113, India Important developments have occurred recently in neutrino physics. These in- clude the discovery of neutrino mass and the direct experimental proof of the thermonuclear hypothesis for the energy production mechanism in the Sun and other stars. These have led to world-wide plans for new neutrino laboratories. The India-based Neutrino Observatory (INO) is a project aimed at building a world-class underground neutrino laboratory. In this article we describe recent discoveries in neutrino physics as well as the INO project at an elementary level. 1 Introduction Neutrinos are ghostly particles that can penetrate the whole Earth without getting stopped. They are everywhere in the Universe. In fact, trillions of neutrinos are passing through our body every second. All the stars including our Sun produce neutrinos copiously in their core. This is because the nuclear fusion processes that are the source of solar and stellar energy also generate neutrinos. Neutrinos are also produced by exploding stars (supernovae). Other sources of neutrinos include cosmic rays, nuclear reactors, particle accelerators and radioactive ore deep inside the Earth. In addition there are cosmic relic neutrinos from the birth of the Universe. The energy spectrum of the naturally produced neutrinos spans nearly thirty orders of magnitude, from 10−5 eV to 1025 eV. Very important discoveries have been made recently in neutrino physics and neutrino astronomy. Neutrinos, which are the charge-neutral spin-1/2 particle emitted along with electrons (so-called beta-rays) in the radio-active decay of nuclei, were thought to be massless. The new discovery is that they are actually massive, although much lighter than any other particle including the electron. While there is now experimental proof, through the detection of solar neutrinos, that the Sun and other stars are powered by thermonuclear fusion reacions, the same experiments also clearly indicated that neutrinos exhibit the quantum-mechanical phenomenon called oscillation. Impelled by these exciting discoveries, world-wide plans are being made now, for further studies of neutrinos and their properties: their masses, the nature of neutrino oscillation and the nature of the neutrino itself. India was a pioneer in neutrino experiments. In fact, cosmic-ray-produced neutrinos were first detected in the deep mines of Kolar Gold Fields (KGF) in 1965. Although the KGF experiments were closed down because of the closure of the mines, it is now planned to revive underground neutrino experiments in India. The India-based Neutrino Observatory (INO) is a project proposal for an underground laboratory that can house neutrino experiments. We shall give an elementary review of the recent discoveries in neutrino physics and also describe the INO project. The article will attempt to paint an elementary picture of neutrinos and their interactions while highlighting the exciting discoveries that have been made and can yet be made in this field, and to enthuse students, scientists and engineers alike about the vast possibilities in neutrino physics in the world, and, in particular, at the future INO laboratory. The plan of the article is as follows. After a brief history of neutrino physics, we take up solar neutrinos. Because of the historical importance of the solar neutrino problem and its solution, solar neutrinos are discussed in detail. This is followed by brief accounts of 1 atmospheric, reactor and geo-neutrinos. Finally in section 9 we describe the INO project. 2 Brief History of Neutrino Physics The chief landmarks in the history of neutrino physics are given below. We shall describe them in more detail in the rest of the article. • 1930: Wolfgang Pauli proposes the existence of a neutral particle. • 1932: Enrico Fermi gives it the name neutrino and constructs the theory of beta decay. • 1956: Clyde Cowan and Frederick Reines detect the neutrino for the first time. • 1965: The KGF experiment in India observes the neutrinos from atmosphere. • 1967: Raymond Davis’ experiment and the genesis of the solar neutrino problem. • 1998: Discovery of neutrino mass by the Super-Kamioka experiment in Japan. • 2002: Solution of the solar neutrino problem by the SNO detector in Canada. • 2002: The KamLAND experiment in Japan confirms oscillations by a terrestrial ex- periment. • 2005: Detection of geo-neutrinos by KamLAND. Pauli proposed the existence of the neutrino in order to save the principle of conservation of energy in the beta decay of nuclei. He hypothesised that a neutrino is always produced along with the electron in nuclear beta decay. Fermi incorporated the notion of neutrinos into his theory of beta decay. Because of the success of Fermi’s theory in explaining quantitatively all the experimental data on nuclear beta decays, there was hardly any doubt (at least in theorists’ minds) that neutrinos existed; however, a direct detection of the neutrino came only in 1956. This achievement was due to Cowan and Reines who succeeded in detecting the antineutrinos produced from fission fragments in a nuclear reactor. In 1965, neutrinos produced from cosmic-ray interactions with Earth’s atmosphere were detected at KGF in India and later in many other experiments in the world. Solar neu- trinos were detected by Davis et al. in 1967 in the U.S.A. and later by other detectors. The Kamioka detector in Japan and other detectors also observed neutrinos from the su- pernova explosion SN1987A. The Super-Kamioka detector discovered neutrino mass through the study of cosmic-ray-produced neutrinos in 1998, while the Sudbury Neutrino Observa- tory (SNO) in Canada solved the solar neutrino problem in 2002. The KamLAND detector in Japan confirmed the neutrino oscillation in a purely terrestrial experiment, using an- tineutrinos from nuclear reactors, in 2002 and the same detector detected geo-neutrinos in 2005. 3 Solar Neutrinos In the 19th century, the source of the energy in the Sun and the stars remained a major puzzle in science, which led to many controversies. The answer to this question held the key to several related questions: • What is the age of the Sun? 2 • This in turn is intimately related to the origin of life on Earth, since the ultimate source of energy for everything that happens on Earth is derived from the Sun. Charles Darwin had a crude estimate of the age of the Earth as about 300 million years old, based on the rate of erosion of the Wealds in the South of England. This seemed to Darwin long enough to produce the varied species on Earth through natural selection. This was contradicted by the physicists, most notably William Thomson, Lord Kelvin, at that time. Kelvin gave an estimate of 30 million years as the age of the Sun by calculating its gravitational energy divided by the rate at which the energy is radiated. Obviously the age of the Earth could not have been larger than the age of Sun. Certainly life on Earth could not have existed before the Sun! Finally the problem was solved only after the discovery of the atomic nucleus and the tremendous energy locked up inside the nucleus. It was Arthur Eddington who, in 1920, suggested nuclear energy as the source of solar and stellar energy. It took many more years for the development of nuclear physics to advance to the stage when Hans Bethe, the Master Nuclear Physicist, analysed all the relevant facts and solved the problem completely in 1939. A year earlier, Carl von Weizs¨acker had given a partial solution. Bethe’s paper is a masterpiece. It gave a complete picture of the thermonuclear reactions that power the Sun and the stars. However, a not-so-well-known fact is that Bethe omitted the neutrino that is emitted along with the electron, in the reactions enumerated by him. The neutrino, which became a part of Fermi’s theory for beta decay, was already a well- known entity in nuclear physics. So it is rather inexplicable why Bethe ignored the neutrinos in his famous paper. The authority of Bethe’s paper was so great that astronomers and astrophysicists who followed him in subsequent years failed to note the presence of neutrinos. Even many textbooks in Astronomy and Astrophysics written in the 1940s and 1950s did not mention neutrinos! This was unfortunate, since we must realize that, in spite of the great success of Bethe’s theory, it is nevertheless only a theory. Observation of neutrinos from the Sun is the only direct experimental evidence for Eddington’s thermonuclear hypothesis and Bethe’s theory of energy production. Herein lies the importance of detecting solar neutrinos. The basic process of thermonuclear fusion in the Sun and stars is four protons combining into an alpha particle and releasing two positrons, two neutrinos and 26.7 MeV of energy. However, the probability of four protons meeting at a point is negligibly small even at the large densities existing in the solar core. Hence the actual series of nuclear reactions occurring in the solar and stellar cores are given by the so-called pp chain (see Box 1) and the carbon- cycle (see Box 2). In the carbon cycle the four protons are successively absorbed in a series of nuclei, starting and ending with carbon. In the pp-chain two protons combine to form the deuteron and further protons are added. In the Sun, the dominant process is the pp-chain. In heavier stars, it is the carbon cycle. 3 Box 1: The pp chain. + + p+p → D + e + νe p+p+e → D+ νe Eν ≤ 0.423 MeV Eν = 1.442 MeV 99.77% 0.23% D+p → He3 + γ 3 3 4 3 4 7 3 4 + He + He → He + p + p He + He → Be + γ He +p → He + e + νe 84.92% 15.08% 10−5% Eν ≤ 18.77 MeV 7 − 7 7 8 Be + e → Li + νe Be +p → B + γ 15.07% 0.01% Eν = 0.861 MeV (90%) Eν = 0.383 MeV (10%) 7 4 4 8 4 + Li +p → He + He B → 2He + e + νe Eν ≤ 15 MeV 4 + In all branches of the pp chain, the net process is 4p → He + 2e + 2νe.
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