Focus

Leptons, Quarks and Electroweak Theory

Electrons, muons and tau particles, along with their corresponding neutrinos, are collectively known as leptons, the Greek word for "slender", as they are all significantly less massive than most other elementary particles, "tte leptons are thought to be fundamental particles that do not consist of anything else; they are therefore amongst the basic building blocks out of which the Universe is constructed.

We also now know that protons and neutrons are not themselves fundamental, but consist of smaller particles, the quarks, buried deep within them, "tte whimsical name quark was taken from James Joyce's (1882-1941) Finnegan's Wake, Book 2, Episode 4, including the phrase '"ttree quarks for Mustar Mark!"

Two kinds of quarks, called up and down, respectively reside within the proton and neutron, and the transmutation of a proton into a neutron involves changing an up quark into a down quark. Every time one quark is changed into another, it produces a neutrino.

Neutrinos occasionally interact with other sub-atomic particles through a force that is at least 1,000 times weaker than the electromagnetic force and 100,000 times feebler than the strong force, "tte electromagnetic force binds electrons to protons, and the strong force holds protons and neutrons together in the nucleus, "tte electromagnetic force and the weak force are unified in a single electroweak theory developed by Sheldon Glashow (1932-), Abdus Salam (1926-1996) and Steven Weinberg (1933- ); in 1979 they were awarded the Nobel Prize in Physics for this feat, even before the discovery of the electroweak forces that were predicted by their theory. In the standard electroweak theory, the neutrinos are assumed to be completely without mass.

tide with which it is most likely to interact (Focus 3.1). All of the neutrinos generated inside the Sun are electron neutrinos, designated v ; this is the kind that interacts with electrons, denoted e~. tte other two flavors, the muon neutrino, v , and the tau neutriii'

no, v , interact with muons, and tau particles, t, respectively, tte muon and tau particles are unfamiliar to most of us because they die shortly after birth, tte muon decays into an electron, a muon neutrino and an electron anti-neutrino in just two millionths, or 2X10~6, of a second, and the tau particle disappears just three-tenths of a million-millionth, or 3 X 10-13, of a second after it is made, tte muon was discovered in 1936 as part of the particle fall out generated when a high-energy cosmic-ray particle slams into the Earth's atmosphere. Indirect evidence for the tau neutrino was obtained from the decay of the tau particle, discovered by Martin L. Perl (1927- ) and his colleagues in 1975 when using the linear particle accelerator at Stanford University; he shared the 1995 Nobel Prize in Physics for this discovery, with Frederick Reines (1918-1998) for his discovery of the neutrino. More direct evidence for the tau neutrino was detected in 2000, using another particle accelerator at Fermilab, near Chicago.

Neutrinos apparently have an identity crisis! Each type of neutrino is not completely distinct, and the different types can be transformed into each other. In the language of quantum mechanics, neutrinos do not occupy a well-defined state; they instead consist of a combination or mixture of states, each with a specific mass. As neutrinos move through space, the mass states come in and out of phase with each other, so the mixture they form changes with time.

tte effect is called neutrino oscillation since the probability of metamorphosis between neutrino types has a sinusoidal, in and out, oscillating dependence on path length, tte chameleon-like change in identity is not one way, for a neutrino of one type can change into another kind of neutrino and back again as it moves along. Like the Cheshire cat, the elusive neutrino can appear and disappear.

In 1967 the Italian atomic physicist Bruno Pontecorvo (1913-1993) proposed the idea that one type of neutrino might transform, or oscillate, into another type in the vacuum of space; he was also the first person to propose using a chlorine detector to study neutrinos. And in 1969, Pontecovo and Vladimir Gribov (1930-1997), working in Russia, proposed that the discrepancy between the observed solar neutrinos and theoretical expectations for the Sun could be explained if solar neutrinos switch from electron neutrinos to another type as they travel in the near vacuum of space from Sun to Earth, thereby escaping detection. Almost a decade later, the American physicist Lincoln Wolfenstein (1923- ) showed that the neutrinos could oscillate, or change type, more vigorously by interacting with matter, rather than in a vacuum, and in 1985 the Russian physicists, Stanislav P. Mikheyev (1940- ) and Alexei Y. Smirnov (1951- ) explained how the matter oscillations might explain the Solar Neutrino Problem.

tte theory, named the MSW effect after the first letters of the last names of the scientists who developed it, proposed that the electron neutrinos generated in the solar core could change type on their way out of the Sun. ttis metamorphosis would happen extremely rarely in the vacuum of space, but might be amplified in the dense interior of the Sun. Interactions between the electron neutrinos and the densely packed solar electrons can, when the density is just right, alter the mass state of a neutrino traveling out through the Sun, thereby changing it into a muon neutrino. Once formed, the muon neutrino does not change back into an electron neutrino; it travels out into space and remains invisible to the first solar neutrino detectors.

In order to change from one form to another, neutrinos must have some substance in the first place, ttey can only pull off their vanishing act if the neutrino, long thought to have no mass, possesses a very small one. Such a tiny neutrino mass has been inferred from measurements of non-solar neutrinos using the high-tech S100 million Super-Kamiokande detector (Fig. 3.5), which replaced the older, nearby Kami-okande instrument in 1996.

Super-Kamiokande can observe both solar electron neutrinos and atmospheric muon neutrinos, tte electron neutrinos are created by nuclear fusion at the center of the Sun, while the muon neutrinos are created when fast-moving cosmic rays enter the Earth's atmosphere from outer space, tte solar electron neutrinos are distinguished by their relatively low energy, near the 5 MeV threshold of the detector. A high energy of 1,000 MeV is typical of the atmospheric muon neutrino. Neutrinos of higher energy produce a tighter cone of light, so the solar electron neutrino makes a fuzzy, blurred and ragged light pattern, while the atmospheric muon neutrino produces a neat, sharp-edged ring oflight.

After monitoring light patterns for more than 500 days, the Super-Kamiokande scientists reported in 1998 that muon neutrinos produced in the atmosphere change type in mid-flight. Subsequent experiments using neutrinos generated on Earth have

FIG. 3.5 Super-Kamiokande This neutrino detector has been built a kilometer underground in a Japanese zinc mine. The huge stainless-steel vessel, 40 meters tall and 40 meters wide, has been filled with 50,000 metric tons (50 million liters) of highly purified water. About 13,000 light sensors, called photo-multiplier tubes, are uniformly arranged on the inner walls of the vessel. The photo-multiplier tubes are so sensitive that they can detect a single photon of light - a light level approximately the same as the light seen on Earth from a candle on the Moon. The light sensors can monitor the entire volume of the pure transparent water for the blue flash of Cherenkov light generated by an electron recoiling from a direct hit by a neutrino. (Courtesy of Yoji Totsuka, Institute for Cosmic Ray Research, University of Tokyo.)

FIG. 3.5 Super-Kamiokande This neutrino detector has been built a kilometer underground in a Japanese zinc mine. The huge stainless-steel vessel, 40 meters tall and 40 meters wide, has been filled with 50,000 metric tons (50 million liters) of highly purified water. About 13,000 light sensors, called photo-multiplier tubes, are uniformly arranged on the inner walls of the vessel. The photo-multiplier tubes are so sensitive that they can detect a single photon of light - a light level approximately the same as the light seen on Earth from a candle on the Moon. The light sensors can monitor the entire volume of the pure transparent water for the blue flash of Cherenkov light generated by an electron recoiling from a direct hit by a neutrino. (Courtesy of Yoji Totsuka, Institute for Cosmic Ray Research, University of Tokyo.)

FOCUS 3.2 Terrestrial neutrinos

Although our planet is perpetually bathed with electron neutrinos, produced by nuclear fusion reactions in the Sun's core, other kinds of neutrinos are also produced on Earth by cosmic rays entering the atmosphere, by man-made, high-energy particle accelerators, and by man-made nuclear reactors, "ttey have all been used to demonstrate that neutrinos change type, or flavor, oscillating between types as they travel through material on the Earth.

In 1998 the Japanese Super-Kamiokande group reported the discovery of neutrino oscillations when observing muon neutrinos generated by cosmic rays interacting with the atmosphere, "ttere were roughly twice as many muon neutrinos coming from the atmosphere directly over the Super-Kamiokande detector than those coming up from the other side of the Earth, "tte muon neutrinos are produced in the atmosphere above every place on our planet, but some of them apparently disappeared while traveling through the Earth to arrive at the detector from below.

"tte further a neutrino travels, the more time it has to oscillate, and that would account for why there are fewer muon neutrinos arriving from the back side of Earth, some 12,700 kilometers away, than from 40 kilometers up in the atmosphere directly above the detector. Some of the muon neutrinos generated in the atmosphere on the Earth's far side had apparently changed or oscillated into undetected tau neutrinos somewhere along their way through the Earth, "tte atmospheric neutrino data was subsequently used to show how the expected neutrino oscillations depend on the neutrino energy and the travel distance.

Intense muon neutrino beams generated by a Japanese particle accelerator were then directed through the Earth to the Super-Kamiokande underground neutrino detector, located about 250 kilometers away, "tte observed deficit of detected muon neutrinos confirmed the disappearing atmospheric muon neutrino result.

American and Japanese scientists next teamed up to construct the $20 million Kami-oka Liquid scintillator Anti-Neutrino Detector, abbreviated KamLAND, at the site of the older Kamiokande solar neutrino detector. KamLAND detects electron anti-neutrinos that have traveled through the Earth from 51 nuclear reactors in Japan plus 18 reactors in South Korea, "tte measurements, reported in 2003 to 2005, show that some of the electron anti-neutrinos are disappearing when traveling to the detector, "ttis work confirmed the solar electron neutrino deficit that had been detected for more than three decades, and provided compelling evidence for neutrino oscillation and mass that had previously been demonstrated by observations of solar neutrinos from the Sudbury Neutrino Observatory, or SNO for short, "tte combined SNO and KamLAND results have constrained the neutrino mass and oscillation parameters, and indicated that most of the solar neutrino metamorphosis occurs in the Sun's interior.

confirmed the effect (Focus 3.2). ttey suggest that although all solar neutrinos are born electron neutrinos, they do not stay that way. Nevertheless, the terrestrial neutrinos did not come from the Sun, and are not directly related to nuclear fusion reactions there. So the solution to the Solar Neutrino Problem was not definitely known until 2001, when a new underground solar neutrino detector in Canada, the Sudbury Neutrino Observatory, demonstrated that solar neutrinos are changing type when traveling to the Earth.

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