The University of Alabama

UA Physicists Play Key Role in Resolving Long-Standing Neutrino Question

Dr. Ion Stancu

Dr. Ion Stancu

By Chris Bryant

One of the world’s most closely monitored experiments recently yielded its initial findings. The results were made possible, in part, by three UA scientists who developed one of the primary sets of code used in key portions of the internationally-known effort’s computer analysis.

The experiment’s set-up, constructed at the Fermi National Accelerator Laboratory, is, to the non-scientist, bizarre. It involves a 40-foot spherical tank buried underground and filled with 250,000 gallons of baby oil, 1,520 specialized light bulbs, and the search for subatomic particles that, although invisible, whiz through the human body, and most everything else, by the trillions every second.

These particles, known as neutrinos, have consumed the professional life of Dr. Ion Stancu, associate professor of physics and astronomy, and UA’s lead researcher in the experiment known as MiniBooNE. Stancu, who has been awarded approximately $1 million in Department of Energy funding for UA’s role in the project over the last five years, understands why the nonphysicist would be puzzled by the scientific commotion created by these electrically neutral partners of electrons.

“Neutrinos go through us, through the entire Earth, and don’t really do much,” Stancu said. “So, why the hell should we care? We do care because we want to understand the global picture of all the elementary particles, but also because there are a lot of them in the universe.”

BUNCHES AND BUNCHES
When Stancu says “a lot,” he means it. The sun, alone, produces neutrinos that bombard each square centimeter of the earth’s surface at the rate of 80 million per second. A recent determination that neutrinos, or “little neutral ones,” have mass has ratcheted up their importance.

Stancu, and his UA colleagues, Dr. Yong Liu, an assistant research physicist, and Denis Perevalov, a graduate student, are among the 77 physicists from 17 institutions collaborating on the project.

It was developed to resolve questions surrounding results from a 1995 experiment at the Los Alamos Liquid Scintillator Neutrino Detector, or LSND. That experiment produced surprising evidence related to neutrino “oscillation” or how neutrinos can change from one type, which physicists refer to as a “flavor,” to another. “The only possible explanation for those results was the existence of an unknown neutrino, a fourth neutrino,” Stancu said.

Previously, scientists were only aware of three neutrino flavors and the existence of a fourth would have required the rewriting of the current model scientists use in their attempts to understand the universe.

The MiniBooNE experiment relies on a 250,000-gallon tank filled with mineral oil, which is clearer than water from a faucet. Light-sensitive devices mounted inside the tank are capable of detecting collisions between neutrinos and carbon nuclei of oil molecules.

The MiniBooNE experiment relies on a 250,000-gallon tank filled with mineral oil, which is clearer than water from a faucet. Light-sensitive devices mounted inside the tank are capable of detecting collisions between neutrinos and carbon nuclei of oil molecules.

HOLD OFF ON THAT RE-WRITE

However, the recent announcement, eagerly anticipated by scientists around the world, publicly refutes the earlier experiment. “Consequently, this will also have a serious impact,” Stancu said. “We are confirming that there are only three active neutrinos, and that the presence of a sterile (the proposed fourth) neutrino has been ruled out. This enhances our understanding of the most fundamental properties from the most fundamental particles that make up this universe as we know it.”

The UA group was responsible for one of the key parts of the analysis, the event reconstruction and particle identification component.

A Fermilab accelerator, several miles in diameter and buried underground some 500 meters from the detector, produces beams which, after conversion, primarily consist of one flavor of neutrinos, called muon neutrinos. Knowing the beam is primarily producing muons, scientists can then calculate how many neutrinos would be expected to change flavors.

The neutrinos travel into the detector, filled with ultra pure mineral oil. There, they bump into the nuclei of atoms in the oil, creating charged particles. Devices that look like overgrown light bulbs line the detector’s walls. These bulblike tubes, some eight inches in diameter, are submerged in the oil and serve as the detector’s eyes. The charged particles generate light, which is intentionally slowed by the oil and detectable by these bulbs, known as photomultiplier tubes.

KNOWN BY ITS SIGNATURE
Different reactions generated by different types of neutrinos cause the bulbs to light up in different patterns, or what the scientists call signatures. These signatures enable the researchers to gauge neutrino oscillation. The UA College of Arts and Sciences’ researchers developed one of the two sets of custom-made computer code used in analyzing the data.

While such fundamental research might appear to be detached from every day applications, Stancu said that’s not the case.

“The methods for detecting these particles could have possible applications for nonproliferation studies,” he said. “Whenever people operate a nuclear reactor, it will always produce neutrinos. Nothing can stop those neutrinos. We can detect them and, moreover, from these neutrinos, we can learn, in principle, what kind of nuclear fuel they are burning.”