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 VOL. 23, NO. 10NOVEMBER 21, 1997 


In Search of the 'Little Neutral One'

In Giant Helium Balloon, Janet Conrad Pursues the Theoretical Heavy Neutrino


Physicist Janet Conrad and her 34-foot-long helium balloon, inside which her experiments search for heavy neutrinos. She is conducting the research at the Fermi National Accelerator Laboratory in Batavia, Ill. Photo by Fermilab Visual Media Services.
There's a lighter side to Janet Conrad's research.

  To find an elusive heavy neutrino predicted by physics theory, Conrad, assistant professor of physics, has constructed a detector that uses a series of four long helium bags. Each is 34 feet long and 16 feet in diameter, and is secured by ropes to the floor at the Fermi National Accelerator Laboratory in Batavia, Ill. Each bag, built by the same company that constructs the Macy's Thanksgiving Day Parade balloons, could lift three physicists.

  A beam of particles from Fermilab's Tevatron accelerator is shot through the detector. If the particle beam includes heavy neutrinos, they will decay to two charged particles within the detector, which consists of the helium bags interspersed with instruments that record the path taken by the charged particles.

  The search is not an easy one.

  "Heavy neutrinos are expected to be produced very rarely and to decay very rarely," Conrad said. "In this sense, they are harder to observe than light neutrinos, which are easy to produce and which interact with other particles more often."

  Neutrinos and other particles can interact with molecules of air, so ideally any particle search should be conducted in a vacuum. "That would have to be one enormous vacuum, and it was not practical," Conrad said. Instead, she has filled the balloons with helium, which is less dense than air and will produce fewer background interactions that could provide misleading data.

  Part of the reason neutrinos are so difficult to find is that they are neutral particles. Enrico Fermi coined the term; "neutrino" means "little neutral one" in Italian. Charged particles can be detected by their electrical interactions; neutrinos can't. Instead, physicists hope that passing heavy neutrinos will decay within range of their detectors and produce signature charged particles that can be detected. Heavy neutrinos have a very distinctive signature and should be easy to find if they decay inside the helium bags, Conrad said.

  She led a group of six Columbia and Barnard students, both graduate and undergraduate, in preparing the electronic equipment and building the neutrino detector.

  The project is funded by an $85,000 Outstanding Junior Investigator Award from the U.S. Department of Energy, which has been renewed for 1997-98, and a $49,860 Career Advancement Award from the National Science Foundation.