03/30/06

Physicists say multi-million dollar experiment advancing smoothly

Click image for larger version Fermilab completed the construction and testing of the Neutrino at the Main Injector (NuMI) beam line in early 2005. Protons from Fermilab's Main Injector accelerator (left) travel 1,000 feet down the beam line, smash into a graphite target and create muon neutrinos. The neutrinos traverse the MINOS near detector, located at the far end of the NuMI complex, and travel straight through the earth to a former iron mine in Soudan, Minnesota, where they cross the MINOS far detector. Some of the neutrinos will arrive as electron neutrinos or tau neutrinos.

The ambitious experiment, which involves experts at the University of Wisconsin-Madison, was launched last year to tackle mysteries associated with the origins and mass of neutrinos. Scientists are beaming the particles from Fermilab's facility in Illinois to a massive particle detector located deep in an iron mine in northern Minnesota. During that 450-mile journey, the harmless particles traverse Wisconsin, flowing from the southeast to the northwest reaches of the state.

One year into the five-year experiment-known as the Main Injector Neutrino Oscillation Search (MINOS)-project participants say everything is progressing as planned.

"This early analysis shows that everything is working well, which is very satisfying," says participant Albert Erwin, an emeritus professor of physics at UW-Madison. "The data is already looking better than any other existing data out there." Erwin and his team built instrumentation that helps ensure that the traveling neutrinos hit their target in Soudan, Minn. The UW-Madison scientists also designed a computer system that monitors important ambient features such as temperature.

In nature, neutrinos are born from nuclear reactions that take place within the sun and other celestial objects. Often called "ghost particles," neutrinos have almost no mass, no charge and possess an extremely weak interacting force, which enables them to move through any type of matter, almost as if it wasn't there.

Click image for larger version When operating at highest intensity, the NuMI beam line will transport a package of 20,000 billion protons every two seconds to a graphite target. The target converts the protons into bursts of particles with exotic names such as kaons and pions. Like a beam of light emerging from a flashlight, the particles form a wide cone when leaving the target. A set of two special lenses, called horns (photo), is the key instrument to focus the beam and send it in the right direction. The beam particles decay and produce muon neutrinos, which travel in the same direction. Photo: Peter Ginter.

Neutrinos come in three "flavors," known as electron neutrinos, muon neutrinos and tau neutrinos. As the particles stream through the atmosphere, planets and stars, they switch back and forth between these flavors in a process known as "neutrino oscillation."

The Fermilab experiment emits the neutrinos in their muon form. Scientists have been hoping the long distance to Minnesota will provide enough time for the muon neutrino to transform into its other two flavors, providing new clues about the particle. So far the MINOS experiment has borne that out, scientists said today, with researchers finding that a fraction of the muon neutrinos did indeed disappear, in consistency with theories about neutrino oscillation.

The 6,000-ton steel detector in Minnesota - the Soudan Underground Laboratory - is placed in the path of the neutrinos from Fermilab, and that is where scientists are sampling the neutrino flavors.

But out of trillions of neutrinos produced by the MINOS experiment, only a few thousand will create detectable events at the Soudan Mine. So even though muon neutrinos will be released for ten millionths of a second every two minutes, 24 hours a day, seven days a week, for the next five years, only one hour of data will be analyzed.

"Using a man-made beam of neutrinos, MINOS is a great tool to study the properties of neutrinos in a laboratory-controlled environment," says Stanford University professor Stan Wojcicki, a MINOS spokesperson. "Our first result corroborates earlier observations of muon neutrino disappearance...Over the next few years, we will collect about fifteen times more data, yielding more results with higher precision, and paving the way to better understanding this phenomenon."

The MINOS experiment is primarily funded by the U.S. Department of Energy, with additional support from the National Science Foundation and the United Kingdom's Particle Physics and Astronomy Research Council. The massive initiative has involved around 150 scientists, engineers, technical specialists and students at 32 institutions in six countries, including Brazil, France, Greece, Russia, the United Kingdom and the United States.

Source / Credit: University of Wisconsin / Fermilab