Nearly 15 years' worth of data from an experiment initially established by physicists at Indiana University has recently been used to make the first detection of a neutrino with an exactly known amount of energy.
Neutrinos are subatomic particles with nearly zero mass and no electrical charge that can pass through physical matter. Knowing the exact energy of a neutrino is important because many types of subatomic particles can only be studied by examining their interaction with neutrinos. But it's nearly impossible to study these particles with any great accuracy if the energy of the neutrinos used to take the measurement is unknown.
The results of the new study were reported April 6 in the journal Physical Review Letters. The data for the experiment comes from MiniBooNE, a neutrino detector at the Department of Energy's FermiLab. Rex Tayloe, a professor in the IU College of Arts and Sciences' Department of Physics, serves as a co-spokesperson on MiniBooNE. He also helped establish the experiment as its first project manager in 2000.
The new discovery was made possible in part due to measurements previously reported by IU scientists in 2008 and 2010. The lead author on those experiments was IU Ph.D. student Teppei Katori, who is now a lecturer at the Queen Mary University of London. The lead investigators on the new study are based at the University of Michigan and Argonne National Laboratory. Over 40 scientists contribute to the MiniBooNE experiment.
"The only way to understand the oscillation of a neutrino, which is a key aspect of their nature, is by observing how it interacts with matter," Tayloe said. "But these measurements aren't perfect since it's nearly impossible to know a neutrino's energy when it hits an atom. This experiment shows for the first time that it is possible to filter out everything but neutrinos with an exactly known energy."
Specifically, Tayloe said the new study shows that it's possible to pinpoint neutrinos with an energy of 236 million electronvolts, or MeV, out of the many other neutrinos of different amounts of energy created in the same burst from a particle accelerator. Moreover, these "monoenergetic" neutrinos come from an unusual source. Rather than the neutrinos used in most experiments -- which are created from the decay of a subatomic particle called a pion -- the neutrinos used in the new study result from the decay of another, rarer type of subatomic particle call a kaon.
Both particles are examples of a type of particle called a meson. Scientists create neutrinos by smashing protons into a target to create mesons -- mostly pions and kaons -- both of which decay into neutrinos.
"The nuclear physics group in our department here at IU really played an important role in this new study by performing measurements that set the groundwork for the new discovery," Tayloe said. "These earlier results ultimately helped our colleagues separate out everything but the neutrinos with a specific energy."
Surprisingly, Tayloe added, the source for these "kaon neutrinos" is a neutrino beam created for another experiment with connections to IU, the NoVA project. Designed to detect neutrinos with a higher energy than MiniBooNE, NoVA is a different neutrino detector whose creation was led in part by Mark Messier, also professor of physics at IU. Tayloe said that after the activation of the MiniBooNE in 2002, the project's leaders realized they could also detect neutrinos created for this other experiment with a few minor modifications.
"None of these discoveries were our main goal," Tayloe said. "The fact we could detect these other neutrinos -- and now the discovery of these monoenergetic neutrinos from the particles created for NoVA -- were both clever ideas we had after the fact. Now, it looks like they could really lead to some bigger things."