Neutrino

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 can 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. In addition, some high-energy cosmic events, such as exploding stars called supernovas and matter falling into supermassive black holes at the center of galaxies, are thought to generate neutrinos (see Neutrino Astronomy).

II

Characteristics and Occurrence of Neutrinos

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.

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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 (p). Leptons have spins of +y (times the unit h/2p). All neutrinos have a spin of +y.

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 (ν_e), muon neutrino (ν_µ), and tau neutrino (ν_t). 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 of Neutrinos

Most of the neutrinos that reach Earth are thought to be 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 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.

Neutrinos that reach Earth can also come from cosmic events beyond the Sun (see Neutrino Astronomy). In 1987 two neutrino telescopes on Earth detected a burst of neutrinos from a supernova, or exploding star, in the Large Magellanic Cloud, a companion galaxy to the Milky Way. The neutrinos are thought to have come from the collapse of the star’s core into a neutron star, although astronomers have not found a neutron star in the supernova remnant. The origins of other neutrinos from space may be more difficult to pinpoint. Possible sources of high-energy cosmic neutrinos include collapsing galaxies, exploding galactic nuclei, matter-antimatter annihilation, gamma-ray bursts, and black holes. The big bang that started the universe is thought to have released enormous amounts of neutrinos that would now have very low energies, making them hard to detect (see Astrophysics; Cosmology).

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