Close-up of the tank of the MiniBooNE neutrino experiment before it was filled with ultraclean mineral oil. Image: Fermilab
Two Fermilab experiments have put new boundaries on a search for a possibly undiscovered type of neutrino, leaving prior measurements unexplained.
So far, scientists have observed three types, or flavors, of neutrino: the electron neutrino, the muon neutrino and the tau neutrino. But physicists have seen hints that this may not be the whole picture.
Expanding the neutrino clan would have far-reaching effects, said Los Alamos physicist Warren Huelsnitz of Fermilab’s MiniBooNE experiment. “If we discovered a new type of neutrino, it would expand our understanding of the Standard Model,” he said. “It would cause cosmologists to recalculate their predictions of the early universe.”
One of scientists’ first clues that neutrinos were more than what they seemed came when they measured fewer electron neutrinos than expected streaming from the sun. A deficit also appeared when proton decay experiments measured the number of muon neutrinos produced in the Earth's atmosphere. Physicists eventually discovered that the missing neutrinos were simply in disguise; they had oscillated into another flavor of neutrino as they traveled.
Flavor oscillations solved the solar and atmospheric neutrino problems, but subsequent experiments have come up with more puzzling results. The Liquid Scintillator Neutrino Detector experiment at Los Alamos National Laboratory found an excess of electron antineutrinos coming from a muon antineutrino beam at short range. Reactor and radioactive-source experiments have found similar-seeming deficits of electron antineutrino and neutrino events, respectively.
If the excess and deficits were due to oscillations, that could point to the existence of at least one other, intermediary type of neutrino, to which muon neutrinos would need to oscillate before becoming electron neutrinos, Huelsnitz said. “There would have to be a new type of neutrino involved as a go-between,” he said.
The new type of neutrino would be sterile – not affected by the weak nuclear force like other types of neutrinos – and therefore would not interact in today’s neutrino detectors.
The MiniBooNE experiment at Fermilab has found an excess of electron antineutrino events that may be related to the LSND result. But when MiniBooNE scientists searched for a corresponding deficit in muon neutrinos or antineutrinos, they found no evidence of one.
This month, they updated their results in partnership with another short-baseline neutrino experiment, SciBooNE. Once again, they found no shortage of muon antineutrinos.
The new result put tighter parameter constraints on the possibility of sterile neutrinos. But the excess of electron antineutrinos at Los Alamos and Fermilab remains a mystery.