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Neutrino Oscillations

Experiments of the past forty years have revealed three families of the ghostly subatomic particles called neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino. The continuing study of neutrino physics suggests that a neutrino of one family might sometimes change into a neutrino of a different family, by a mechanism known as neutrino oscillation. The MINOS experiment will use the NuMI neutrino beam at Fermilab to search for this phenomenon as a way to answer the question whether neutrinos have mass.


What are oscillations? We all know what happens when we push a child in a swing. Like the pendulum of a clock, the swing goes as far as it can in one direction, then returns to the other side and goes back again, continuing this motion, back and forth, for a long time. Such oscillations, or periodic changes from one state into another and back again, are common in nature. Over the course of the year we go from cold winter to warm summer and back again. Over the course of a day we go from dark to light to dark again. More fancifully, we might say that Clark Kent oscillates into Superman and back again to Clark Kent.


Do neutrinos oscillate?

Theoretical inclinations support the notion that the subatomic particles called neutrinos should oscillate as well, changing from one flavor of neutrino to another and back again.


Two conditions are necessary for neutrinos to oscillate—to change, for example, from to :

  • Neutrinos must have mass, and the mass cannot be identical for all three neutrino flavors.
  • There must be no rigorous law forbidding a transition between neutrino species.

For the fundamental particles called quarks, neither of these constraints applies. All the quarks have masses, and transitions can and do take place between quark families—between bottom and charm quarks, for example. There appears no a priori reason why the situation should be different for the leptons, the family that includes neutrinos. There are no theoretical arguments forbidding massive neutrinos or transitions between different kinds of leptons.


If the physics of the quarks carries over to the leptons, neutrinos will oscillate from one kind to another. Imagine a beam of pure electron neutrinos, produced, for example, in the sun. If oscillation occurs in the beam, by the time it strikes the earth, muon neutrinos and tau neutrinos will also exist in the beam. We say that the electron neutrinos have oscillated into muon neutrinos or tau neutrinos.

Why do neutrino oscillations matter?

Because the issue of the mass of the elementary building blocks of matter is one of the most fundamental problems in particle physics today, the question of neutrino mass acquires great significance. Direct measurements have shown that neutrino masses, if they exist, are much smaller than the masses of their brother charged leptons—at most, one-thousandth to one-hundred-thousandth as great. For example, we know that if the electron neutrino has a mass, it is less than one hundred-thousandth of the mass of the electron.


Theoretical models provide possible explanations for this great mass difference. Experimentally, however, it will be very difficult to push direct neutrino mass measurements much beyond what we know today. We must turn to indirect experiments such as neutrino oscillation experiments to find the tools to probe the neutrino mass scale far beyond what we can learn from direct measurements.

THE PHYSICS OF NEUTRINO OSCILLATIONS

Of all nature's building blocks, the subatomic particles called neutrinos are the most elusive. Neutrinos are difficult particles to pin down because they have no electric charge and almost never interact with matter. Fermilab's MINOS (Main Injector Neutrino Oscillation Search) experiment will attempt to discover if neutrinos have mass by trying to observe neutrino oscillations, or changes in "flavors," among the three types of neutrino.

I  Neutrinos

Some 40 years after the first direct observation of the electron neutrino, we know very little about neutrinos. As early as the late 1950s, physicists suggested that neutrinos could have mass, and, if they do, would be able to transform from one flavor to another. Theory does not forbid such a transformation. Nevertheless, no one has ever made a direct experimental observation of this phenomenon, which is known as neutrino oscillation. The three flavors of neutrino are the electron neutrino , the muon neutrino , and the tau neutrino .

II  Oscillation

Oscillation, broadly defined, means the change from one state to another and then back again. Theory supports the notion that neutrinos should oscillate from one flavor to another. More specifically, neutrinos of one flavor have the "potential" to transform into another.

III The Fundamental Questions

Physicists will measure two basic parameters with the MINOS experiment:

1. What is the maximum fraction of the neutrino beam that can change from one flavor to another? For example, scientists will investigate how much of the beam will transform from muon neutrinos to tau neutrinos. We can call this maximum fraction the oscillation potential.

2. If neutrinos oscillate, how great or small is the oscillation length? The oscillation length is the distance a beam of neutrinos, travelling at near the speed of light, would take to transform from one neutrino flavor to another—up to its full potential—and back again.

IV  Neutrino Oscillations

The circles directly below represent the population of a beam of muon neutrinos traveling at almost the speed of light. They show the oscillation of muon neutrinos into tau neutrinos and (at a much lower rate) into electron neutrinos.

V  The Experiment

To determine whether neutrinos oscillate from one flavor to another, and, hence, have mass, a collaboration of 200 scientists from 26 institutions worldwide are planning the MINOS experiment. (See diagram.)


VI  The Physics of Minos

View Diagram.


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