Neutrino

History

Neutrino mass

Interactions with matter

Resources

Neutrinos are elusive subatomic particles that result from certain nuclear reactions. Neutrinos have no electrical charge and only a tiny mass, usually travel at nearly the speed of light, come in three types—electron neutrinos, muon neutrinos, and tau neutrinos—and barely interact with normal matter. Because their interaction rate is so low, neutrinos produced in the core of the sun fly directly out through the outer layers of the sun and flood surrounding space, providing direct (though hard-to-intercept) information about nuclear reactions in the sun’s core. In 1968, however, when scientists first tried to detect electron-type neutrinos emitted by the sun, they found less than half those expected from the then-current theory of nuclear reactions in the sun. This shortage was known as the solar neutrino problem, and was only resolved in 2001 by elaborate experiments that proved that the sun is in fact, producing the number of neutrinos predicted by theory, but that some of these neutrinos change type (electron to muon or tau) en route from the core of the sun to detectors on Earth. The total number of neutrinos detected on Earth is in accord, as it turns out, with the standard model of the solar core.

History

History In the 1920s, physicists noticed some discrepancies in beta decay experiments. In beta decay, a neutron decays into a proton by emitting an electron, also termed a beta particle. It was observed that the total momentum and energy of the electron and proton after the decay was sometimes less than the initial momentum and energy of the neutron. Where did the missing momentum and energy go? According to fundamental laws of physics, the total amounts of both momentum and energy must remain constant. Neither can just disappear.

In 1930, the Austrian physicist Wolfgang Pauli (1900–1958) wrote a letter to a gathering of physicists in TÜbingen, Germany, in which he suggested the idea of neutrinos as the particles that carry away the missing energy. However, he did not publish the idea for another three years. Pauli originally called his suggested particle the neutron, as neutrons had not been discovered in 1930. When neutrons were discovered, the term “neutron” was taken, so Pauli’s particle became the neutrino: literally, the little neutral one.

Italian physicist Enrico Fermi’s (1901–1954) 1934 theory of beta decay used the neutrino hypothesis. (This theory, still used for approximate calculations, was only surpassed for more accurate calculations by theories developed in the 1970s.) But did neutrinos really exist? In the 1930s, no experiments to detect them were possible.

In 1956, after a four-year search, United States physicists F. Reines (1918–1998) and C. L. Cowan (1919–1974) finally succeeded in detecting neutrinos produced by the Savannah River Reactor in South Carolina. By 1962, a particle accelerator at Brookhaven National Laboratory was generating enough neutrinos to conduct detection experiments. Over several months, physicists observed a few dozen neutrino events and found that there were at least two types of neutrinos. The first one discovered was dubbed the electron neutrino, and the second the muon neutrino. Proof of a long-suspected third type of neutrino, the tau neutrino, were first found in late 1998.

Neutrino mass

It was long thought that neutrinos have no mass, because experimental searches had not detected mass. This changed in 1998, with the discovery that at least one of the three types of neutrinos must have mass. The giant Super-Kamiokande detector, a tank buried deep underground in Japan containing 50,000 tons of purified water, detected a difference in the expected numbers of electron neutrinos and muon neutrinos created by cosmic rays. Experimenters found that while the number of electron neutrinos was as expected from theory, the number of muon neutrinos was significantly lower. Scientists concluded that muon neutrinos must be changing, or oscillating, into the other types of neutrinos, which is only possible if some of the neutrinos have mass. In 2001, comparisons of results from Super-Kamiokande and the Sudbury Neutrino Observatory in Canada—a spherical tank containing 1,000 tons of heavy water, located at the bottom of a deep mine, surrounded by thousands of detectors to catch the byproducts of any neutrino interactions— determined the mass of the neutrino more accurately: less than about .000001 times the mass of an electron.

Physicists long thought that discovery of a nonzero mass for the neutrino would have important astrophysical implications because of the effects of gravity. Studies of the motions of galaxies show that there is a significant amount of dark matter (i.e., matter invisible through telescopes) in the universe; perhaps as much as 90% of all matter. What comprises this dark matter? Nonzero neutrino mass is now known to account for between 0.1% and 18% of the critical mass density of the universe—the amount of mass required (if gravitation alone is considered) to eventually stop the expansion of the universe. The more likely value is 0.1%, which, though still one-fourth of the mass of all observable stars, is probably not enough to affect the overall geometry of the universe. Thus, the neutrino is probably not a significant contributor to dark matter after all.

Interactions with matter

The size of a subatomic particle is expressed in terms of a cross-sectional area; the larger the cross section, the more likely the particle is to interact with ordinary, solid matter, which consists largely of empty space occupied by matrices of widely separated atoms. The cross section of a neutrino is very small. For a beam of neutrinos passing through the center of Earth, only one in a trillion neutrinos would be blocked by Earth. A slab of lead 100 light years thick will only absorb roughly one third of the neutrinos striking it. This property makes neutrinos difficult to find; they zip right through almost any detector. About 60 billion solar neutrinos pass through every square centimeter of Earth’s surface every second.

The neutrino’s small cross section makes it a useful probe of the sun’s interior, for the energy-producing nuclear reactions in the sun’s core yield very great quantities of neutrinos. A unit of energy produced at the center of the sun takes several million years to migrate to the sun’s surface; the neutrinos, however, zipping through the sun’s substance at nearly the

KEY TERMS

Beta decay— The splitting of a neutron into a proton and an electron.

Beta particle— One type of radioactive decay particle emitted from radioactive atomic nuclei. A beta particle is the same thing as an electron.

Cross section— A measure of the probability that a subatomic particle will interact with matter.

Dark matter— The unseen matter in the Universe, detected by its gravitational effect on the motions and structures of galaxies.

speed of light, make it to Earth in about eight minutes. Neutrinos, therefore, can reveal what is going on in the sun’s core now rather than a few million years ago.

See also Particle detectors; Quantum mechanics.

Resources

BOOKS

Bernstein, Jeremy. The Elusive Neutrino. Honolulu, Hawaii: University Press of the Pacific, 2004.

Fukugita, M., T. Yanagida, and Lutz D. Schmadel. Physics of Neutrinos. New York: Springer-Verlag, 2003.

Shapiro, Maurice, et al. Neutrinos and Explosive Events in the Universe. New York: Springer, 2005.

PERIODICALS

Bahcall, John. “Neutrinos Reveal Split Personalities.” Nature 412 (July 5, 2001): 29–30.

Kirshner, Robert P. “The Full Story on Neutrino Detection.” Science. 303 (2004): 1976-1977.

Normile, Dennis. “Japanese Detector Brings Neutrinos Into Sharper Focus.” Science. 305 (2004): 325.

Siegfried, Tom. “Neutrino Physics Probes Mysteries of ‘Flavor’ and Origins.” Science. 313 (2006): 1880.

Paul A. Heckert

NEUTRINO

In spite of its history of more than seven decades, Wolfgang Pauli's mystery particle, the neutrino, still remains the least known particle. The mystery is attributed to its feeble interaction with others, and recently, Leon Lederman called it a barely existing particle.

In an open letter to a 1930 Tübingen conference, which starts with "Dear Radioactive Ladies and Gentlemen," Pauli introduced the concept of a neutral fermion (a spin-½ particle) and called it a neutron. This was his desperate attempt to rescue the laws of energy and momentum conservation that seemed to be violated by beta-decay processes. However, after James Chadwick's discovery of a heavy particle with no charge in the nucleus, Pauli's neutral fermion, which was actually created in beta decay, was renamed neutrino by Enrico Fermi in 1933. Chadwick's neutral particle kept the name neutron. In the same year, at the Solvay conference, Pauli also speculated that neutrinos may be massless. Fermi recognized the importance of the neutrino and developed his famous beta-decay theory in 1934. Pauli's neutrino is now represented by e since it is accompanied by an electron in most interactions.

In 1934 Hans Bethe and Rudolf Peierls expressed a pessimistic view on the direct detection of neutrinos because their interaction strength was so weak. In the early 1950s Frederick Reines and Clyde Cowan looked for a way to detect neutrinos by observing the inverse beta decay (a process by which ν_e turns into a positron). Since this required a large target and an enormous flux source of neutrinos, Reines and Cowan even considered a nuclear explosion; however, they chose a nuclear reactor instead. In June 1956 they sent a telegram to Pauli informing him that neutrinos had been detected.

Second and Third Neutrinos

A large discrepancy between the predicted rate of muon decay μ →e + ϓ and the assumption of only one kind of neutrino resulted in the speculation about the existence of a second neutrino. A new neutrino with a different lepton number or flavor, the muon neutrino ν_μ, was necessary. This would not permit the process ν_μ + P → e⁺ + n . Indeed, the absence of this process was confirmed in 1962 at the Brookhaven National Laboratory, establishing the existence of ν_μ. After the dramatic discovery of charm quarks in 1974, the presence of two families of quarks and leptons was established.

Soon after, the third charged lepton τ was discovered by a collaboration led by Martin Perl in 1975 at the Stanford linear electron-positron collider. This immediately suggested the possibility of a third family of quarks and leptons. A doublet of bottom (b ) and top (t ) quarks was subsequently discovered, and the existence of the third neutrino, the tau neutrino ν_τ, was generally accepted. This is because, in each family, quarks and leptons must appear as a doublet so that no anomalies (unwanted infinities) appear in the Standard Model. However, a direct observation of the process ν_τ → τ has remained elusive because the accelerator-produced ν_τ did not have enough energy to produce a clear signal of the production of τ by ν_τ. Finally in 2000 the signal was observed at Fermilab, confirming the existence of the tau neutrino and completing the three families of quarks and leptons.

Neutrino Mass

The mass of neutrinos is complicated. Originally, Wolfgang Pauli proposed that the neutrino mass was approximately that of an electron. Later, at the 1933 Solvay conference, he speculated that it could be massless. For some period, the only way to measure the neutrino mass was to see possible deviations of the Kurie plot of the electron energy spectra in beta decays. At the highest electron energy, the neutrino carries the least energy so that the shape of the spectrum is sensitive to the mass of the neutrino. A large number of such experiments have been carried out. The most popular process used is ³H → ³He + e⁺ + ν_e, which has the maximal kinetic energy of the electron. So far, all the results based on various beta decays have been negative, yielding the following limits: m (ν_e) < 3 eV, m (ν_μ) < 0.19 eV, and m (ν_τ) < 1.92 MeV. A substantial improvement in these results is not feasible in the near future.

Neutrino Oscillations

An entirely new approach to probe neutrino mass began with Bruno Pontecorvo's proposal of neutrino oscillations in a series of papers in the late 1950s. In 1957 he proposed the conversion of a neutrino into antineutrino, which would be possible for neutrinos with mass. Recall that only one neutrino was known at that time. When the second neutrino ν_μ was discovered in 1962, Bruno Pontecorvo, and Ziro Maki, Masami Nakagawa, and Shoichi Sakata independently entertained the possibility of ν_μ ↔ ν_e oscillations. The physics of neutrino oscillations can be understood as follows.

When those particles produced and detected in an experiment differ from those that govern the propagation, oscillation phenomena can occur. For example, when ν_μ is produced from the π^- decay, this process produces a neutrino with a definite flavor (in this case, ν_μ); however, when ν_μ travels, its motion is governed by an equation of motion for a particle with a definite mass. If neutrinos have mass, the ν_μ state produced is not a state with a definite mass (a pure mass eigenstate). Instead, it is a linear combination of three mass eigenstate neutrinos. That is, ν_μ does not have a definite mass! Moreover, when it propagates, it cannot be described by a single equation of motion because all three components have different values of mass.

Suppose one tries to detect ν_μ with a detector a distance L from ν_μ source. Since all three components (mass eigenstates) have different values of mass, they propagate in different manners, creating phase differences. This means the neutrino that reaches the detector is no longer the original ν_μ because the detected neutrino contains, in addition to the original ν_μ, additional ν_e and ν_τ components. Hence, the detector can detect all three neutrinos of different flavors. The necessary and sufficient conditions for the oscillation are as follows: (1) The neutrinos have mass, and (2) neutrinos with flavor are linear combinations of mass eigenstate neutrinos. Oscillations are characterized by the quantity where m₂ and m₁ are the mass of ν₂ and ν₁, respectively. Thus, although the observation of neutrino oscillations cannot pin down the values of mass, it shows that some neutrinos have mass. This is the reason why the search for neutrino oscillations has gained a great deal of attention. Since the Standard Model of particle physics has been constructed with the assumption of massless neutrinos, the discovery of neutrinos with mass signals new physics beyond the Standard Model.

Numerous attempts to observe neutrino oscillations, using ν_e from nuclear reactors and ν_μ from accelerators, all failed. The problem was the lack of information of the values of Δm². In order to observe oscillations, one has to design the experiment so that Δm²L /4E (the combination appearing in the oscillation formula) approximately equals 1. In reality, the energy of neutrinos E is fixed by an accelerator. One then has to determine the location L of a detector. This cannot be done unless one knows the value of Δm². With past experimental setups with L ranging from about 10 m to 1 km, no oscillations have been seen.

In 1998 oscillations of atmospheric neutrinos were confirmed for the first time by the detector at Kamioka in Japan. The detector was originally designed as a nucleon decay experiment and was thus named Kamiokande. The larger version at Kamioka is called the Super-Kamiokande (SK). The atmospheric neutrinos, produced by cosmic rays, consist of ν_μ and ν_e whose ratio on the surface of the Earth is roughly two to one. Both Kamiokande and SK observed a smaller number of ν_μ than expected whereas the number of ν_e was consistent with theory. The latest experimental results have confirmed that ν_μ's from cosmic rays oscillate into ν_τ's that escape detection by the current experimental setups, whereas ν_e's do not oscillate into other neutrino flavors. The latest quantitative results are expressed by the finding that and that the mixing is almost maximal. This has been supported independently by other groups. It is generally believed that these findings are very convincing evidence for neutrinos with mass.

A more significant experiment in neutrino physics is the measurement of the solar neutrinos (located in Homestake, Idaho), which was originally designed to probe the solar core. The observed rate of solar neutrinos is roughly one-half the value expected with the standard solar model developed by John Bahcall and others. Recently, it has been concluded that solar ν_e's are depleted because their oscillations, most likely into ν_μ's that cannot be detected since the energy of the original ν_e is too low for a converted ν_μ to produce μ. The results of all the solar neutrino experiments suggest that Although two different values of Δm² appear to be determined, thus confirming massive neutrinos, the determination of individual neutrino mass value still remains a major task for future experiments.

The recent K2K long-base-line experiment, so named because ν_μ's from the accelerator at KEK in Tsukuba, Japan, are sent to Kamioka located 250 km away, is exclusively designed to confirm the SK results, without relying on the atmospheric neutrinos. The distance of 250 km is long enough to see the oscillations suggested by the atmospheric neutrino experiment. Since 1999 the SK detector has observed ν_μ-induced events (forty four) that are about one-third less than the expected number (sixty-six) without oscillations, indicating that about one-third of ν_μ's have most likely transformed into ν_τ's, consistent with the atmospheric neutrino results. This may be, when confirmed independently, the first positive result of neutrino oscillations with human-made neutrinos. Similar long baseline experiments, for example, with a distance of 730 km, are under construction including MINOS (Fermilab to Soudan mine in Minnesota) and OPERA (CERN to Gran Sasso National Laboratory in Italy).

See also:Case Study: Super-Kamiokande and the Discovery of Neutrino Oscillations; Conservation Laws; Lepton; Particle; Standard Model

Bibliography

Boehm, F., and Vogel, P. Physics of Massive Neutrinos (Cam-bridge University Press, Cambridge, UK, 1987).

Kim, C. W., and Pevsner, A. Neutrinos in Physics and Astrophysics, Vol. 8: Contemporary Concepts in Physics (Harwood, Lang-horne, PA, 1993).

Chung W. Kim

Neutrino

Neutrinos are elusive subatomic particles that result from certain nuclear reactions. Neutrinos have no electrical charge and only a tiny mass , travel at nearly the speed of light , come in three types—electron neutrinos, muon neutrinos, and tau neutrinos—and barely interact with normal matter . Because their interaction rate is so low, neutrinos produced in the core of the Sun fly directly out through the outer layers of the Sun and flood surrounding space, providing direct (though hard-to-intercept) information about nuclear reactions in the Sun's core. In 1968, however, when scientists first tried to detect electron-type neutrinos emitted by the Sun, they found less than half those expected from the then-current theory of nuclear reactions in the Sun. This shortage was known as the solar neutrino problem, and was only resolved in 2001 by elaborate experiments that proved that the Sun is in fact, producing the number of neutrinos predicted by theory, but that some of these neutrinos change type (electron to muon or tau) en route from the core of the Sun to detectors on Earth . The total number of neutrinos detected on Earth is in accord, as it turns out, with the standard model of the solar core.

History

In the 1920s, physicists noticed some discrepancies in beta decay experiments. In beta decay, a neutron decays into a proton by emitting an electron, also termed a beta particle. It was observed that the total momentum and energy of the electron and proton after the decay was sometimes less than the initial momentum and energy of the neutron. Where did the missing momentum and energy go? According to fundamental laws of physics , the total amounts of both momentum and energy must remain constant. Neither can just disappear.

In 1930, the Austrian physicist Wolfgang Pauli (1900–1958) wrote a letter to a gathering of physicists in Tübingen, Germany, in which he suggested the idea of neutrinos as the particles that carry away the missing energy. However, he did not publish the idea for another three years. Pauli originally called his suggested particle the neutron, as neutrons had not been discovered in 1930. When neutrons were discovered, the term "neutron" was taken, so Pauli's particle became the neutrino: literally, the little neutral one.

Italian physicist Enrico Fermi's (1901–1954) 1934 theory of beta decay used the neutrino hypothesis. (This theory, still used for approximate calculations, was only surpassed for more accurate calculations by theories developed in the 1970s.) But did neutrinos really exist? In the 1930s, no experiments to detect them were possible.

In 1956, after a four-year search, U.S. physicists F. Reines (1918–1998) and C. L. Cowan (1919–1974) finally succeeded in detecting neutrinos produced by the Savannah River Reactor in South Carolina. By 1962, a particle accelerator at Brookhaven National Laboratory was generating enough neutrinos to conduct detection experiments. Over several months, physicists observed a few dozen neutrino events and found that there were at least two types of neutrinos. The first one discovered was dubbed the electron neutrino, and the second the muon neutrino. Proof of a long-suspected third type of neutrino, the tau neutrino, were first found in late 1998.

Neutrino mass

It was long thought that neutrinos have no mass, because experimental searches had not detected mass. This changed in 1998 with the discovery that at least one of the three types of neutrinos must have mass. The giant Super-Kamiokande detector, a tank buried deep underground in Japan containing 50,000 tons of purified water , detected a difference in the expected numbers of electron neutrinos and muon neutrinos created by cosmic rays. Experimenters found that while the number of electron neutrinos was as expected from theory, the number of muon neutrinos was significantly lower. Scientists concluded that muon neutrinos must be changing, or oscillating, into the other types of neutrinos, which is only possible if some of the neutrinos have mass. In 2001, comparisons of results from Super-Kamiokande and the Sudbury Neutrino Observatory in Canada—a spherical tank containing 1,000 tons of heavy water, located at the bottom of a deep mine, surrounded by thousands of detectors to catch the byproducts of any neutrino interactions—determined the mass of the neutrino more accurately: less than about .000001 times the mass of an electron.

Physicists long thought that discovery of a nonzero mass for the neutrino would have important astrophysical implications because of the effects of gravity. Studies of the motions of galaxies show that there is a significant amount of dark matter (i.e., matter invisible through telescopes) in the Universe; perhaps as much as 90% of all matter. What comprises this dark matter? Nonzero neutrino mass is now known to account for between 0.1% and 18% of the critical mass density of the Universe—the amount of mass required (if gravitation alone is considered) to eventually stop the expansion of the Universe. The more likely value is 0.1%, which, though still one-fourth of the mass of all observable stars, is probably not enough to affect the overall geometry of the Universe. Thus, the neutrino is probably not a significant contributor to dark matter after all.

Interactions with matter

The size of a subatomic particle is expressed in terms of a cross-sectional area; the larger the cross section , the more likely the particle is to interact with ordinary, solid matter, which consists largely of empty space occupied by matrices of widely separated atoms . The cross section of a neutrino is very small. For a beam of neutrinos passing through the center of the Earth, only one in a trillion neutrinos would be blocked by the Earth. A slab of lead 100 light years thick will only absorb roughly one third of the neutrinos striking it. This property makes neutrinos difficult to find; they zip right through almost any detector. About 60 billion solar neutrinos pass through every square centimeter of the Earth's surface every second.

The neutrino's small cross section makes it a useful probe of the Sun's interior, for the energy-producing nuclear reactions in the Sun's core yield very great quantities of neutrinos. A unit of energy produced at the center of the Sun takes several million years to migrate to the Sun's surface; the neutrinos, however, zipping through the Sun's substance at nearly the speed of light, make it to Earth in about eight minutes. Neutrinos, therefore, can reveal what is going on in the Sun's core now rather than a few million years ago.

See also Particle detectors; Quantum mechanics.

Resources

books

Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell'sExploration of the Universe. 7th ed. Philadelphia: Saunders College Publishing, 1995.

Fukugita, M., T. Yanagida, and Lutz D. Schmadel. Physics ofNeutrinos New York: Springer-Verlag, 2003.

periodicals

Bahcall, John. "Neutrinos Reveal Split Personalities." Nature 412 (July 5, 2001): 29–30.

Taubes, Gary. "Neutrino Watchers go to Extremes." Science 263 (January 7, 1994): 28–30.

Steinberger, J. "What do we Learn from Neutrinos?" Science 259 (March 26, 1993): 1872–1876.

Paul A. Heckert

KEY TERMS

Beta decay

—The splitting of a neutron into a proton and an electron.

Beta particle

—One type of radioactive decay particle emitted from radioactive atomic nuclei. A beta particle is the same thing as an electron.

Cross section

—A measure of the probability that a subatomic particle will interact with matter.

Dark matter

—The unseen matter in the Universe, detected by its gravitational effect on the motions and structures of galaxies.

neutrino (symbol v) Uncharged elementary particle with little or no mass, spin ¹/₂ and travelling at the speed of light. US physicist Wolfgang Pauli predicted its existence. Classified as a lepton, it has little reaction with matter and is difficult to detect. There are three known species. The electron neutrino is closely associated with the electron, and is produced when protons and electrons react to form neutrons, as in the Sun. The muon neutrino is associated with the muon, and occurs in high-energy reactions. The tau neutrino is associated with the tau particle.

neu·tri·no / n(y)oōˈtrēnō/ • n. (pl. -nos) a neutral subatomic particle with a mass close to zero and half-integral spin, rarely reacting with normal matter. Three kinds of neutrinos are known, associated with the electron, muon, and tau particle.