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Garbo-Esque Neutrinos May Challenge Standard Model

Date:

March 1, 2000

Source:

Stanford University

Summary:

Shakespeare said that all the world's a stage and all the men and women merely players. But to Stanford physicist Stanley Wojcicki, all the world's a laboratory, and its star player is the neutrino, a subatomic particle that could rightly be called the Greta Garbo of the physics world, for it hardly ever interacts.

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Shakespeare said that all the world's a stage and all the men and women merely players. But to Stanford physicist Stanley Wojcicki, all the world's a laboratory, and its star player is the neutrino, a subatomic particle that could rightly be called the Greta Garbo of the physics world, for it hardly ever interacts.

In 2003, Wojcicki and about 250 scientists from 30 institutions representing five countries will shoot neutrinos from an accelerator at Fermilab in Batavia, Ill., to an underground detector in a former mine 730 kilometers away in Soudan, Minn. The path of the commuter neutrinos is entirely underground, where they will interact with nothing.

On Feb. 20 at the annual meeting of the American Association for the Advancement of Science in Washington, D.C., Wojcicki spoke about long-baseline neutrino experiments, which are providing the first crack in the Standard Model, the prevailing theory that describes elementary particles and forces. Right now physicists think everything is made of 12 fundamental constituents ­ six called quarks, six called leptons. Three of the leptons are neutrinos. (Of the three particles taught in high-school physics ­ electrons, protons, and neutrons ­ only the electron is fundamental. It is a lepton, whereas protons and neutrons are each made of three quarks.)

The long-baseline experiments will explore whether neutrinos have zero mass, as the Standard Model says, and whether lepton number ­ a property thought to be immutable like positive or negative charge ­ is absolutely conserved. Recent Japanese experiments using neutrinos produced by high-energy cosmic rays suggest that neutrinos do have mass.

"If these results are verified, neutrino physics would be the first area to indicate violation of the Standard Model because this theory incorporates both lepton conservation and zero neutrino mass," Wojcicki said."So you have to modify the Standard Model to accommodate these kinds of things." If the experiments reveal, for instance, that neutrinos have mass, however tiny, that discovery will force physicists to paint a new picture of the universe.

"It's very exciting from that point of view that you really may be probing very, very new physics," Wojcicki said. "You really may have your hands on a smoking gun, and if you pursue it, you can really learn more about the murder, if you like."

Wolfgang Pauli first proposed the existence of the neutrino in 1930 to explain an apparent nonconservation of energy when radioactive particles decayed. "I have committed the cardinal sin of a theorist," he is reputed to have said. "I made a prediction which can never be tested, ever, because this particle is so weakly interacting that it may never be seen."

But 26 years later American scientists Frederick Reines and Clyde Cowan detected the first neutrinos. The next year, Italian physicist Bruno Pontecorvo theorized that if different species of neutrinos exist, they might be able to oscillate back and forth between the different species. In 1962, scientists from Columbia University and Brookhaven National Laboratory demonstrated the existence of two species of neutrinos, and a third was found at Stanford Linear Accelerator Center (SLAC) in 1975.

Experiments at the European physics laboratory CERN and at Stanford in 1989 showed that only three species of light (or massless) neutrinos can exist: electron, muon and tau.

Neutrino oscillations transmute one kind of neutrino, such as muon, into another kind, such as electron or tau. But for oscillations to occur, two phenomena must be true: neutrinos must have mass, and lepton number cannot be absolutely conserved.

To solve the mystery of whether neutrinos have mass, physicists performed sophisticated measurements, but all they got was an upper limit. "So we know that neutrinos have a very low mass, if they do have a mass," said Wojcicki.

Regarding whether lepton number (charge) is conserved, neutrinos have no charge that physicists can measure. Here's where the story resembles a detective finding a smoking gun at a crime scene and using deductive logic to reconstruct the murder. When electron neutrinos interact, they make electrons, which do have a charge. Similarly, muon neutrinos interact to make muons, and tau neutrinos interact to make taus. "Knowing what is the lepton in the final state tells you about what kind of neutrino was there initially," explained Wojcicki.

This is all well and good but does not explain why physicists need to shoot neutrinos through the curvature of the Earth from one state to another, as in the Illinois-Minnesota experiment, or from one country to another, as in a similar Switzerland-Italy experiment that CERN will conduct in 2005. (Japan is currently conducting a modest accelerator experiment with a more limited goal of determining whether scientists can observe any oscillations at all.)

"We really want to do very precise, quantitative studies to measure the mass difference, measure the mode of oscillations, what is the final state, and all of this," Wojcicki said.

To do that, scientists look at the probability that one type of neutrino will change into another type. The probability of that transmutation oscillates like a wave, and it increases as the distance grows between where the particle is created and where it is detected. It also depends on the difference in mass between the first type of neutrino and the second. Since the probability can be anywhere from zero to one, if the difference in mass is small (as it would be if particles have little or no mass), the distance between particle birthplace and detector ­ called the baseline ­ must be large.

Though Wojcicki's research base is SLAC in California, he travels to Illinois's Fermilab for his neutrinos. His project, called MINOS, for Main Injector Neutrino

Oscillation Search, requires more neutrinos than SLAC can produce. "You need to have a very intense neutrino beam because first of all neutrinos do not interact very much, and secondly you'll be detecting them very, very far away," Wojcicki said. "It's like a beam of flashlight where the farther you go, the fainter is the light."

Accelerators allow scientists to pulse the beam. "It's like a pulsar that emits bursts of energy on a periodic basis," Wojcicki explained. So while the background noise from natural radiation sources, such as cosmic rays, will be constant, the neutrino signal will be pulsed from the direction of Fermilab. The underground location of the detector shields it from cosmic rays, reducing the background significantly.

"The way it works is the accelerator accelerates protons to higher energy," Wojcicki said. "Then protons are allowed to strike the target. They make short-lived particles called pions. You make a beam of the pions, and they travel through a vacuum pipe about half a mile long. As they travel, some of them will decay, and they will decay into neutrinos."

Another class of long-baseline experiments employs nuclear reactors to study electron neutrinos. (Whereas accelerators produce muon neutrinos with ease, reactors are a good source of electron neutrinos.) With reactor-generated neutrinos, the detector need only be as far as one kilometer away, as these neutrinos have a much lower energy than their accelerator siblings. (Energy is inversely proportional to the odds that one kind of neutrino will transmute into another kind, so the lower the energy, the shorter the baseline.) Stanford Associate Professor Giorgio Gratta is working on an experiment in Japan that will look at neutrinos from all reactors in Japan that, serendipitously, happen to be far enough away from a central detector to be useful (most are more than 100 kilometers away).

The cost of the MINOS program is about $120 to $130 million, according to Wojcicki, with about $70 million to build the Illinois infrastructure and equipment and about $50 million for the Minnesota detector. But unlike a single-purpose experiment, such as a clinical trial designed to find out if a specific drug is effective in treating a disease, long-baseline experiments may satisfy many goals. "The simplest measurement, which is just to look at the muons and compare the rates at the far detector and the near detector, we can do in a few months or a year," Wojcicki said. "More sophisticated measurements ­ the detailed, quantitative measurements, or looking for the different modes ­ may take three, five years."

Although Wojcicki called his research "very, very basic," it has spawned a practical application: a new technique for making inexpensive scintillator, or material that produces light when charged particles pass through it and that is used extensively in medical research today. His experiment will use an unprecedented 250 tons.

New knowledge of neutrinos also may benefit scientists in other fields. For example, it could help cosmologists understand the evolution of supernovae.

In May, neutrino physicists from around the world will meet in Monterey, Calif., to discuss experiments with even longer baselines ­ ones that join one continent to another. Such experiments face considerable technical challenges, such as building a very intense proton source, creating targets that can tolerate intense beams, and concentrating beam particles so that they can be guided around a storage ring.

Wojcicki is official spokesperson for the MINOS project. Of the 250 people involved worldwide, most have doctoral or advanced engineering degrees. While only 15 are graduate students, Wojcicki expects that number to triple in the next two years because the scheduled date to begin data collection makes the involvement of graduate students, who usually stay four to six years, appropriate at that time. Stanford researchers are largely involved in the project's software issues and include senior research associate George Irwin and postdoctoral researchers Robert Hatcher, Larry Wai and Carlos Arroyo. Wojcicki also has worked with several undergraduate students, including Brad Patterson, who is working on an honors thesis. "There's a strong educational component in addition to the research objective," Wojcicki said.

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