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Neutrino research has Minnesota roots
by Jeremy Paschke
"The immeasurable of today,” J. J. Thomson once described subatomic particles, “may be the measurable of tomorrow." Then Thompson proved himself correct with his own discovery of the electron, an event that brought dramatic closure to the nineteenth century.
Today, in the final months of the twentieth century, physicists are busy trying to measure the neutrino, another particle once considered immeasurable. Teams of physicists from Japan, Italy, and Minnesota -- including University of Minnesota professors Earl Peterson, Keith Ruddick, and Ken Heller, from the School of Physics and Astronomy -- are vying with each other for first honors in unlocking the mysteries of what Ruddick calls "the sexy particle of the nineties."
The concept of a neutrino dates back to the early 1930s and the subject of beta decay, a nuclear process that occurs when a neutron decays into a proton and an electron. Confusion arose when experimenters observed that momentum was not conserved between the proton and the electron during beta decay. According to the laws of conservation of momentum, the net momentum before a collision must be equal to that after the collision. In terms of decay, a particle cannot decay into a single particle lighter than itself.
Wolfgang Pauli firmly believed that this law could not be violated under any circumstances, so he postulated that -- in the process of beta decay -- an imperceptibly light particle was emitted along with the electron and the proton. This particle carried away some momentum and, thus, explained the momentum imbalance.
In 1934, Enrico Fermi constructed a new theory of beta decay that integrated Pauli's light-weight particle. Fermi affectionately called the particle “neutrino,” Italian for "little neutral one."
Theoreticians used neutrinos to save their concepts, but experimental physicists remained skeptic and demanded evidence that neutrinos really existed. Pauli himself regretted his postulation because he believed the neutrino would never be detected experimentally. More than two decades passed before Fred Reines and Clyde Cowan, two physicists working at Los Alamos Laboratory, experimentally detected the neutrino. Reines' and Cowan's research began in 1951, and was initially titled Project Poltergeist, owing to the neutrino's elusive nature. In 1956, with their research relocated near the Savannah River Nuclear Reactor, Reines and Cowan finally obtained conclusive evidence of neutrinos. Asked to comment on the particle he spent six years chasing, Reines described it as "the most tiny quantity of reality ever imagined by a human being."
After Reines and Cowan's research, the scale of neutrino experiments grew with gigantic strides. Today, many university scientists take their research plans to government operated laboratories, such as Fermilab, a particle collider located 80 kilometers west of Chicago, or CERN in Switzerland.
The University of Minnesota's neutrino experiments take place at the Soudan Underground Research Site, a laboratory leased by the University from the state of Minnesota. Built in 1986, the primary goal of the Soudan laboratory was to detect signs of proton decay. Experiments are conducted 690 meters underground to shield out cosmic rays. Currently, the detector Soudan 2, is being revamped to capture neutrinos and a second detector is being installed for similar neutrino research. In 2004, Fermilab will send a high-energy beam of neutrinos northward through the Earth to the detectors at the Soudan laboratory.
The project is enormous, and representatives from Stanford, the California Institute of Technology, Oxford University, the University of Sussex, and the Institute of Theoretical and Experimental Physics in Moscow will contribute. Along with Peterson, Ruddick, and Heller, Hans Courant, and Marvin Marshack from the University of Minnesota are also leading the research.
The theory that lies at the core of elementary particle physics is called the Standard Model, under which neutrinos can assume three different varieties, or "flavor states" - the electron neutrino, the muon neutrino, and the tau neutrino. The Standard Model seems to favor threes because quarks - the particles that make up protons and neutrons - are also found in threes. However, Peterson points out, the Standard Model does not address mass or particle oscillation.
One source of neutrinos is cosmic rays and cosmic-ray interactions with the atmosphere. Cosmic rays are swiftly moving particles - mainly protons - that relentlessly bombard the upper atmosphere, but do so with erratic energies. When a cosmic ray interacts with an atmospheric atom, a pion particle is produced. The pion then decays into a muon and a muon neutrino. The muon then decays into another muon neutrino and an electron neutrino, with the net result being two muon neutrinos for every one electron neutrino. "We expect to find a ratio of two to one," says Peterson. "But what we do find is wildly different."
To explain the divergence between theory and experiment, physicists hypothesize that neutrinos can oscillate. Oscillation occurs when a particle changes from one "flavor" to another. In the case of cosmic ray interactions with the atmosphere, perhaps some muon neutrinos oscillated into electron neutrinos as the neutrinos traveled from the upper atmosphere to the Earth. While this hypothesis might be true, Peterson explains, studying cosmic rays will never provide the answer because cosmic rays come from unknown sources and have unpredictable energies.
"Watching cosmic rays is kind of like bird watching," Peterson says. "What you see is what you see." But, using cosmic rays is not a systematic and reproducible way to learn about neutrino oscillation. "We want a controlled source," Peterson continues, "where we know the energy of the incoming beam. This is precisely the reason for the Fermilab to Soudan connection."
The concept of neutrino oscillation is strange indeed.
"A single neutrino is actually a quantum mechanical mixture of two other types,"
Ruddick says. In other words, two flavor states can simultaneously exist in the same neutrino as it moves through space unimpeded by detectors. As a result, the neutrino has some probability of existing in one flavor state and some probability of existing in another flavor state. When a neutrino is found in one flavor state, it most likely had a greater probability of being that flavor state than the other.
The probabilities of a moving neutrino being found in one of two flavor states change because neutrinos are so small they can act like waves. The wave / particle duality of matter is an alien concept in modern physics. Light, for example, demonstrates both wave and particle characteristics, as do electrons.
According to Ruddick, both flavors of neutrinos exist side-by-side as two waves. Each neutrino flavor has a distinct frequency to its wave, and the two waves will overlap as they travel through space. Sometimes the waves will be in phase (constructive interference), and sometimes they will be out of phase (destructive interference). Oscillation occurs, Ruddick explains, as the waves move in and out of phase. To prove this hypothesis, physicists need to count a significant number of neutrinos at two distinct points along the neutrinos' path, a task much easier said than done because the neutrino is the smallest particle known to physicists. "Neutrino experiments are about the hardest sorts of experiments you can possibly think of doing," Ruddick says. Rising to confront the challenge are over a hundred scientists, collaborating on what is known as the Main Injector Neutrino Oscillation Search (MINOS) project. In scientific circles, MINOS is momentous because the physics it will unveil move beyond the Standard Model.
The overall plan is simple. Create a beam of neutrinos at Fermilab, then send it 730 kilometers through the Earth to the underground detector at the Soudan laboratory. Neutrinos do not need an excavated tunnel; they bore straight through the Earth, 7 kilometers deep at points below Madison, Wisconsin. Experimenters will measure the ratio of muon to electron neutrinos at Fermilab, and will measure the same ratio at Soudan laboratory. If the ratio changes, then the experimenters will have proof that neutrinos oscillate from one flavor to another. In addition, since only massive particles can oscillate, evidence of oscillation will also confirm that neutrinos have a mass.
To observe a neutrino, the neutrino must interact with other particles. Just as we cannot see the wind, but are aware of it from the rustle of leaves and swaying of flowers, so too must scientists rely on extraneous clues to detect neutrinos. When a neutrino collides with an atom, it may chance to strike the nucleus. If scientists are truly lucky, a reverse beta decay will occur where the neutrino strikes a proton and yields a neutron and a positron. Both the neutron and positron can be detected with scintillation fibers -- long, flexible plastic strands with a unique chemical composition. When a high-energy particle traverses the fiber, a fraction of the particle's energy is pilfered away by the fiber's chemicals, resulting in a blue flash of light. Trapped within the fiber because of total internal reflection, the blue light bounces back and forth until it reaches one end, where a photo-detector reads the light's timing and intensity. Neutrinos can be caught, but only by their coatta ils.
"It's remarkable when you think about it," muses Ruddick. "By chucking a few bits of plastic together you are asking deep questions about the universe." Galileo might have thought along similar lines after he pieced together glass lenses and discovered the moons of Jupiter. However, a look at the machinery behind MINOS reveals that there is much more involved than just a few bits of plastic.
Noting the irony of huge detectors built for minuscule particles, Ruddick says that the MINOS detector "is one of the biggest detectors ever built." Each layer of scintillating fibers will be followed by a layer of inch-thick iron -- resulting in 600 layers all together. The completed detector will loom eight meters high and stretch out for one mile. If Fermilab plays the pitcher in this game of high-energy physics, then the MINOS detector will wear a catcher's glove that measures one mile across and be positioned a half a mile below the Earth's surface. Once the detector is in place, it cannot be retrieved. It must work correctly the first time.
Capturing neutrinos is not cheap. The project's budget, provided mainly by the U.S. Department of Energy, exceeds $135 million dollars. Such a hefty sum, critics argue, could be invested in other projects. High-energy physics faces a growing list of financial concerns. Because no one wants potential knowledge about the universe to go undiscovered because the process costs too much, for the moment, knowledge takes precedence over spending.
"Neutrinos are crucial for solving the big puzzles," argues Ruddick. Understanding neutrinos may answer deeply profound questions that have remained unanswered so far -- How was the universe formed? What is the universe's ultimate fate? Will matter endure forever, or will it decay into pure energy?
Besides, Ruddick adds, "there are so many neutrinos the universe, they have got to be important."
Looking at some numbers may help fathom the abundance of neutrinos. On Earth, roughly 40 billion neutrinos pass through an area the size of a penny every second. In the time it takes to read this paragraph, trillions of neutrinos will penetrate this very page. Most of these neutrinos come from the sun, where intense nuclear reactions are constantly occurring. Other sources of neutrinos include nuclear power reactors, and the Earth's natural radioactive processes. A surprising source of neutrinos are people. The human body contains roughly 20 milligrams of potassium 40, an isotope that undergoes beta decay and emits neutrinos. Therefore, without even knowing it, you emit close to 340 million neutrinos each day.
Theorists estimate that the neutrino density in the universe is 330 neutrinos for every cubic centimeter. The predicted mass of an electron neutrino lies between 8 and 10 electron volts, an incredibly minute quantity. An electron is a hundred thousand times heavier than its neutrino. Mass is measured in terms of electron volts, or energy, because in relativity, where E = mc2, the mass of a particle depends upon its energy. Although such figures for mass are incomprehensibly small, combining a small mass with the abundance of neutrinos in the universe incurs some interesting results. If neutrinos have mass, then they might account for some or all of the dark matter in the universe. One of the most profound riddles that neutrino research might someday solve is the question of mass itself. Modern physics lacks a grand unified theory, mainly because of unanswered questions pertaining to mass. The topic of mass lies outside the Standard Model. And occurrences such as gravity -- why does mass attract other masses? -- are easy to observe, but difficult to explain.
"The mystery is mass," says Heller. "It's always been a mystery in physics why the neutrino doesn't have mass. If we could either know that the neutrino has mass or that it is massless, it would help focus our thinking on the subject of mass."
Philosophically, says Heller, if every elementary particle had an equal mass, then all matter could be viewed as a collection of tiny bricks. Research, however, indicates different masses for the few things we call elementary. Even so, knowing that the neutrino has mass will advance physics. Heller is enthusiastic. "I think most researchers today would bet that the neutrino has mass," he says.
Indeed, neutrinos help choreograph the dance of our active universe. Increasing our knowledge about them may unravel cosmological questions that have long remained unanswered.
A single answer, however, will undoubtedly engender hundreds of more puzzling questions. We ought not be dismayed, though, because opening one door in science always leads to many other doors still closed, and ensures that the human attempt to understand nature is always challenging and vivacious.
