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NEUTRINOS: ORIGIN AND CHARACTERISTICS.

I

INTRODUCTION

Neutrino, very small particles with no electric charge and little or no mass. Neutrinos are elementary particles—that is, they cannot be broken into smaller particles. Neutrinos are so small that they pass right through most material. One important kind of neutrino is created in the nuclear reactions that give the Sun its energy. The Sun produces so many neutrinos that 70 billion neutrinos pass through every square centimeter (0.15 sq in) of the surface of Earth every second. Scientists study neutrinos to learn more about the reactions that give the Sun its energy. Similar reactions occur in radioactive substances, or materials made up of atoms that spontaneously change into other particles (Radioactivity). Neutrinos also help scientists understand these radioactive reactions. Neutrinos play an important part in the theory scientists have developed to explain the elementary particles that make up all matter and energy.

II


CHARACTERISTICS AND OCCURRENCE

Neutrinos have so little mass that scientists are not sure that neutrinos have any mass at all. Because they have little or no mass, neutrinos move at speeds near the speed of light (300,000 km/sec, or 186,000 mi/sec). Neutrinos are probably true pointlike particles, meaning they have a radius of zero, or no size.

Neutrinos are affected by at least one of the four fundamental forces that exist in nature. These four forces are the strong force, the electromagnetic force, the weak force, and the gravitational force. The strong force is the force that holds together particles in the nucleus of an atom. It does not affect neutrinos. The electromagnetic force causes particles with electric charges to attract or repel each other. Neutrinos have no electric charge, so the electromagnetic force has no effect on them. The weak force allows particles, even elementary particles, to change form. The weak force does affect neutrinos. The gravitational force causes attraction between particles with mass. If neutrinos do indeed have any mass, the gravitational force affects them, but the mass of neutrinos is so tiny that scientists have not been able to measure gravity’s effect on neutrinos.

A


Place in the Particle Family

Neutrinos are members of a group of elementary particles called leptons. Leptons differ from other elementary particles in a property called spin. Spin is analogous to a measurement of a particle’s angular momentum. Scientists measure the spin of particles in units of a constant number. This constant is equal to a number called Planck’s constant (h) divided by two times the constant pi (). Leptons have spins of + (times the unit h/2). All neutrinos have a spin of +.

Leptons are part of a larger group of particles called fermions. Fermions are defined as particles that obey a rule called the Pauli exclusion principle (named after its developer, Austrian-born Swiss physicist Wolfgang Pauli). The Pauli exclusion principle states that two identical particles cannot occupy the same point in space. The two main types of leptons are those with electric charge and those without electric charge. Neutrinos are leptons without electric charge.

Physicists know of three kinds of neutrinos and three kinds of leptons that are not neutrinos. The three nonneutrino leptons are electrons, muons, and taus. Each nonneutrino lepton has a neutrino partner. The three types of neutrinos are the electron neutrino (νe), muon neutrino (νµ), and tau neutrino (ν). All three of the neutrinos have no electric charge and very small masses (or maybe no mass at all). Despite their similarity, physicists have ways of telling the three types of neutrinos apart. When neutrinos interact with matter, the interactions produce new particles. Any reaction involving a neutrino will produce the neutrino’s charged lepton partner. Physicists can therefore deduce which neutrino was involved in a reaction by detecting the charged lepton that has been produced. If a tau lepton is present in the interaction result, physicists know that a tau neutrino interacted with matter. If a muon or electron is present, physicists know that a muon neutrino or an electron neutrino, respectively, was present before the interaction.

All three types of neutrinos have antiparticles. Antiparticles are opposites of the particles that make up ordinary matter. Particles with electric charge have antiparticles whose electric charges are opposite. The distinction between neutrinos (which have no electric charge) and antineutrinos is more complicated. The direction of a neutrino’s spin is always opposite to the direction of its velocity. The direction of the spin of an antineutrino is always the same as its velocity’s direction. This rule may not work if neutrinos do actually have mass, but physicists have not found a violation of the rule yet.

B


Origins

Most of the neutrinos that reach Earth are electron neutrinos. Electron neutrinos come from the Sun, from collisions in the atmosphere, and from the decay of radioactive elements. Many muon neutrinos also reach Earth. Muon neutrinos come from collisions in the atmosphere. Tau neutrinos are much less well known than electron neutrinos and muon neutrinos are. Scientists are not sure how many tau neutrinos reach Earth or what their origins are.

While many subatomic particles, or particles that make up atoms, can only penetrate into objects a very small distance, neutrinos easily pass through an object as large as Earth. Almost all the neutrinos that reach Earth pass right through the planet. Billions of neutrinos pass through every human body every second.

Most of the neutrinos that pass through Earth are electron neutrinos from the Sun. In the fusion reactions that fuel the Sun, four protons (positively charged particles much larger than neutrinos or electrons) come together to form the nucleus of a helium atom. The nucleus of a helium atom contains two protons and two neutrons (particles about the same size as protons, but with no electric charge). Two of the original protons must change into neutrons to create a helium nucleus. The transformation of each proton releases a neutrino, an antielectron (also called a positron), and energy. The Sun releases 4 × 1038 neutrinos every second. The number 4 × 1038 is very large—written out, it would by the digit 4 followed by 38 zeros. The Sun’s neutrinos leave the Sun in equal numbers in all directions, so only a small fraction of the Sun’s neutrinos hit Earth.

Collisions between particles in the atmosphere create electron neutrinos, electron antineutrinos, muon neutrinos, and muon antineutrinos. Many particles called cosmic rays enter Earth’s atmosphere every second. Cosmic rays are electrons, protons, or atomic nuclei with varying speeds that enter Earth’s atmosphere from space. They come from a variety of sources—the Sun, other bodies in the solar system, distant stars, and faraway galaxies. About 3,000 cosmic rays hit every square meter (10.8 sq ft) of the top of Earth’s atmosphere every second.

Most cosmic rays collide with atoms in the atmosphere before they hit the ground. When cosmic rays collide with atoms, the collision produces up to 30 particles called pions. Pions are unstable particles that spontaneously change into other particles soon after their creation. About 75 percent of the pions created by atmospheric collisions have electric charge. The rest are neutral. Neutral pions do not produce neutrinos. A positively charged pion decays, or changes, into an antimuon and a muon neutrino. A negative pion decays into a muon and a muon antineutrino. Muons and antimuons are also unstable particles. A muon changes into an electron and an electron antineutrino. An antimuon decays into a positron and an electron neutrino. Therefore, each pion ultimately decays into an electron or positron, plus an electron neutrino or electron antineutrino, plus a muon neutrino or muon antineutrino. Each cosmic ray can produce up to 30 pions, so each cosmic ray can release up to 60 neutrinos in a collision. The number of neutrinos from the atmosphere is still far less significant than the number of neutrinos that come from the Sun.

A smaller number of neutrinos on Earth are electron neutrinos and electron antineutrinos created by radioactive decay, or the transformation of an atom of one element into an atom of another element. The reactions that cause radioactive decay are similar to the reactions that occur in the Sun in that both involve the conversion of protons to neutrons. In radioactivity, these reactions are called beta decay. When one or more protons in an atom change into neutrons, the protons release electron antineutrinos, electrons, and energy. The atom’s atomic number, or the number of protons in its nucleus, goes down. An atom’s atomic number determines which element the atom is. See also Elements, Chemical.

C


Interactions and Detection

Neutrinos rarely interact with other particles. Given the high density of neutrinos in the universe, especially near stars, the universe would be very different if neutrinos interacted more often. When an electron neutrino does interact with matter, it collides with an atomic nucleus. The atomic nucleus absorbs the electron neutrino and undergoes a type of reverse beta decay. The electron neutrino combines with a neutron to produce a proton, an electron, and energy. The atom’s atomic number goes up, changing it into a different element. When a muon neutrino interacts with a nucleus, the collision releases a muon. When tau neutrinos interact with nuclei, they produce tau particles.

Neutrinos interact so rarely because of their tiny size, their neutral electric charge, and their very high speed. The pointlike nature of neutrinos means that they can pass through even very dense matter without much chance of hitting an atom. A neutrino has no electric charge, so the electric charges of other particles cannot change a neutrino’s path to draw it close to a nucleus or other particle. The high speeds of neutrinos give them less time and therefore less chance to interact with other particles as they pass by. The probability that a neutrino will interact with matter as it passes through Earth is about 5 × 10-13 to 1. The number 5 × 10-13 is very small; written out, it would be a decimal point followed by 12 zeros and the digit 5. Scientists can only detect neutrinos because a huge number of neutrinos are constantly passing through Earth at any given time.

Physicists and astronomers detect neutrinos by watching for the rare instances in which neutrinos interact with matter. Scientists use two main types of neutrino detectors. Detectors for solar electron neutrinos are placed far underground, on the ocean floor, or under polar ice. These detectors are shielded from interference from other particles by a thick layer of earth or water. They count neutrino interactions either by counting the number of atoms changed by incoming neutrinos or by looking for light produced when neutrinos interact with water. Underwater detectors can also detect muon neutrinos.

The second type of neutrino detector takes in neutrinos from a well-known source, such as a nuclear power plant (see Nuclear Energy) or a particle accelerator, a device that brings particles to high speeds in order to produce collisions that will form new particles. This type of detector has a target composed of some dense material, often lead, and some way to record the path of particles that come out of the target. Scientists look for evidence of muon neutrinos and tau neutrino interactions by looking for their by-products: muons and tau particles.

Underground neutrino detectors are filled with a liquid that is particularly susceptible to changing atomic number in neutrino interactions. The most commonly used liquids are chlorine and gallium. Chlorine becomes argon when it interacts with electron neutrinos. Gallium becomes germanium. The physicists operating the experiment drain the tank on a regular basis and are able to find and count the changed atoms. This allows them to count the number of neutrino interactions that took place since the tank was filled.

Underwater neutrino detectors and neutrino detectors embedded in ice rely on a phenomenon called Cherenkov radiation. Cherenkov radiation is a flash of light produced when charged particles move through water or ice at a speed greater than the speed of light in these substances. Only charged particles produce Cherenkov radiation, so neutrinos must interact with matter to produce an electron (in the case of electron neutrinos) or a muon (in the case of muon neutrinos) before these detectors can count them.

When neutrinos do interact with matter and produce a charged particle, the initial speed of the neutrino gives the new charged particle enough energy to move off nearly as quickly as the neutrino came in. Neutrinos always move at the speed at which light travels in a vacuum, no matter what material they travel through. The speed of light in water or ice is slightly slower than the speed of light in a vacuum, so the charged particle initially moves faster than light does in the water or ice. Muons have larger masses than electrons have, so the flash of light that they produce is different enough to allow the detectors to differentiate between the two kinds of particles. The deep ice at Earth’s North and South poles is especially good for detecting neutrinos. The lower layers are composed of snow that fell millions of years ago, making the ice very pure. The snow has been compressed by the layers above it until no air bubbles exist in the ice, making the ice so clear that detectors can see flashes far away.

III


HISTORY AND CURRENT RESEARCH

Scientists first theorized the existence of neutrinos while studying beta decay in the 1920s. In beta decay, protons change into neutrons and electrons, and energy is released. They discovered that the sum of mass and energy carried by the particles before the decay did not equal the sum of mass and energy carried by the particles after the decay. According to the rules of physics called conservation laws, the sum of mass and energy must stay the same in all reactions. Rather than abandon this fundamental principle of physics, Wolfgang Pauli came up with a radical solution. He proposed that a new particle that had not yet been detected must carry the missing energy. In order to follow the rules of physics, this particle must have no electric charge, little (or no) mass, and the same spin as that of protons and electrons. It was given the name neutrino, which means “little neutral one” in Italian.

By the 1950s most physicists accepted the existence of the neutrino, but it was not discovered until 1956, about 25 years after it was initially proposed. The first neutrino detected was actually an antineutrino, but the detection of an antineutrino confirmed the existence of neutrinos. American physicists Frederick Reines and Clyde Cowan, Jr., created a neutrino detector inside a nuclear power plant complex in South Carolina. The radioactive decay of the reactor fuel of the power plant provided plenty of antineutrinos. The detector was composed of radiation detectors surrounding a tank filled with cadmium chloride dissolved in water.



Reines and Cowan hoped that antineutrinos would enter their detector’s tank and collide with protons, producing neutrons and positrons. The positron is the antiparticle of the electron. When an antiparticle collides with its equivalent particle, they annihilate each other and the mass of both particles becomes energy. In this case, the energy was in the form of gamma rays, electromagnetic radiation with a very short wavelength. The neutron created in the antineutrino-proton reaction collides with and is absorbed into a cadmium nucleus. This interaction also releases a burst of gamma rays. Reines and Cowan calculated the amount of time that should pass between the gamma-ray burst caused by the positron-electron annihilation and the gamma-ray burst caused by the absorption of the neutron by the cadmium nucleus. Their detector did indeed find consistent spacing between bursts of gamma rays, proving that both reactions were occurring and that antineutrinos and neutrinos do exist.

In 1962 American physicists Leon Lederman, Melvin Schwartz, and Jack Steinberger at Brookhaven National Laboratory in New York examined the particles produced when pions decay with an apparatus similar to that of Reines and Cowan. The Brookhaven team found a second neutrino that was distinguishable from the neutrino that Reines and Cowan had found. The new neutrino was called the muon neutrino because of its association with the muon. The first neutrino became known as the electron neutrino.

In 1975 a group of physicists led by American physicist Martin Perl found the tau lepton in an experiment at the Stanford Linear Accelerator Center (SLAC) in California. Physicists assume that the tau has an associated neutrino, just as the other two charged leptons do. The tau neutrino has not yet been directly detected, but experiments through the end of the 1990s indicated with great precision that three distinct neutrino types exist.

Detection of solar neutrinos began in the 1970s. The first solar neutrino detector was built in an abandoned gold mine called the Homestake Mine in Lead, South Dakota. The Homestake detector used a tank of chlorine atoms, which changed to argon atoms if they interacted with a neutrino. Through the 1970s, 1980s, and 1990s, several more major solar neutrino detectors were built around the world. All of the results from these solar neutrino detectors show only about one-third of the number of neutrino interactions that scientists expected to see. Less neutrinos are detected when the detector is on the side of Earth not facing the Sun. If neutrinos really do pass unimpeded through matter, the number of neutrinos coming at Earth from the Sun should be almost exactly the same as the number of neutrinos coming out the other side of Earth. The problem of the missing neutrinos is called the solar neutrino problem.

In experiments performed in 1998, Japanese physicists found strong evidence that one type of neutrino can change into another type of neutrino. Subsequent research performed at another neutrino detector, near Sudbury, Canada, has indicated that electron neutrinos oscillate into muon or tau neutrinos as they travel from the Sun to Earth. If confirmed, this conclusion would provide a solution to the solar neutrino problem by explaining why the number of electron neutrinos detected is so much lower than scientists expected.

Neutrino oscillation can only occur if neutrinos have mass. The mass of neutrinos must be very small, but if they do indeed have mass, it could greatly affect scientists’ ideas of the universe. Physicists and cosmologists have long suspected that the universe must be filled with matter that they cannot detect. They can see stars and galaxies by the light that they emit. Scientists can even detect gas and dust by seeing how starlight changes as it passes through regions of gas and dust. Models of the universe indicate, however, that much more matter must be present in addition to the matter that scientists can detect. If neutrinos have mass, they could account for some of this missing matter. See also Dark Matter; Cosmology.

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