This morning's announcement of the Nobel Prize in Physics for the discovery of neutrino oscillations has a deep Notre Dame connection. Back in the mid-1980s, a group of mostly U.S. physicists, including Notre Dame's Prof. John LoSecco, were observing a gigantic tank containing 2.5 millions gallons of water, deep in a salt mine outside Cleveland, Ohio.
The group’s goal was to catch a proton, one of the fundamental building blocks of atoms, in the process of decaying. Though the IMB experiment never saw protons decay (nor has anyone since), LoSecco and colleagues did notice an anomaly in the number of muons entering their detector. Muons are subatomic particles created by the interactions of one kind of neutrino with the Earth, while the neutrinos are themselves created as a by-product of cosmic rays passing through the Earth's upper atmosphere. The experiment was recording only half of the expected number of muons, and by 1986, LoSecco and colleagues announced the discrepancy had reached 3.5 standard deviations, even though no competing experiment had seen any similar effect.
This so-called "atmospheric neutrino anomaly" was confirmed two years later, in a 1988 paper by Japanese competitors working on the Kamiokande experiment. Eventually that group, led by Takaaki Kajita, built an even larger detector, containing over 12 million gallons of water, dubbed Super-Kamiokande, or SuperK for short. With the added size, SuperK was able to conclusively demonstrate in 1998 that the atmospheric neutrino anomaly was due to neutrinos changing form, or oscillating, which can only happen if neutrinos have non-zero mass. For this work, Kajita shared in the 2015 Nobel Prize in Physics today.
But it was LoSecco and the small team on IMB that first noticed the disappearing muons and began the decade-long odyssey that helped uncover the secrets of the one of nature's most mysterious particles, the elusive neutrino.