Last modified: Wednesday, March 26, 2008
Updating the "Bible of Particle Physics"
After many trials and tribulations, Indiana University Professor Stuart Mufson and former Indiana University graduate student Brian Rebel made a discovery that was recently reported in a book Mufson calls "The Bible of Particle Physics." The book, published by the Journal of Physics, is better known as the Review of Particle Physics.
"I've been studying cosmic rays for 25 years and have never had a result reported there before," said Mufson. "And that's it. I'll probably never do it again."
The discovery involves two types of elementary particles; muons and neutrinos, which scientists study to learn how the universe works at the most basic levels imaginable. Billions of neutrinos pass through the Earth every single day without ever hitting another particle. But when they do strike one, the interaction often creates muons, which are what scientists use to study neutrinos.
Unlike neutrinos, muons are very easy to detect directly, as long as the right equipment is used. For Mufson and Rebel, the "right equipment" was a 500-ton iron detector in the Soudan Underground Laboratory, which is located a half mile underground in Minnesota's first iron mine that is now a state park.
The detector was built to study the muons created by a stream of neutrinos beamed through 450 miles of solid rock from the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. The experiment is underground to shield the detector from the trillions of naturally occurring muons coming from the atmosphere. Despite the 2,341 feet of rock, millions of natural muons still reach the detector every year. But that's just fine with Mufson and Rebel, because these are the particles they study.
Cosmic rays in the Earth's atmosphere create trillions of muons and neutrinos. Additionally, these neutrinos occasionally create even more muons. In order to study neutrinos from the atmosphere, scientists must distinguish between the trillions of muons created by cosmic rays and the few hundred created by the neutrinos -- a task which is like looking for a needle in a haystack.
In addition to their search for the "needles," Mufson and Rebel decided to analyze the "haystack," or the millions of high-energy muons created by cosmic rays. This may seem like an obvious track to take, but cosmic rays have been studied extensively for nearly a century. And because cosmic rays have been studied for so long, new results usually come about only when studied in an unexplored energy range, as was done in their investigation. By studying cosmic rays at these energies, Mufson and Rebel discovered they create significantly more positively charged muons than negatively charged ones than previously believed.
"It was like looking for a needle in a haystack your entire life and then finding out the haystack is the more interesting of the two," said Mufson. "Other scientists in the field thought it was a pretty cool result."
Although the results obtained were interesting and significant -- which is why they made their way to the Review of Particle Physics -- bringing the paper out of the MINOS collaboration proved quite contentious. In fact, Mufson said, "It was easily the most difficult scientific process I've ever had in my life."
In the end, the MINOS collaboration was convinced. Once submitted, the paper was approved for publication within weeks. And Mufson can now proudly see his study in the "Bible of Particle Physics."
"Finding something new in cosmic rays is pretty rare," said Mufson. "We had to study out the wazoo to do it."