(Image from the MiniBooNE archive. This depicts a neutrino signal in the detector. )
There was a new result announced today. The MiniBoone collaboration has confirmed the more “standard” scenario for neutrino oscillations, and ruled out suggestions by an earlier experiment (LSND3) from a while back that might have implied the existence of sterile neutrinos. Two commenters, Scott H. and Bee, alerted me to this in another thread. Rather than me trying to paraphrase things, I’m going to first reproduce Scott H.’s comment here (he came back from a colloquium by Jocelyn Monroe on the topic earlier today):
Jocelyn Monroe just gave a colloquium on the MiniBoone results today. It was a fantastic talk â€” anyone who wants to get an overview from an expert should invite her! Iâ€™m not even approximately an expert on this subject and am essentially just trying to regurgitate what I learned from her talk, from memory.
One of the major motivations of MiniBoone was trying to understand the LSND result, which indicated an excess of oscillations into electron neutrinos. One possible interpretation of their result was that there exist sterile neutrinos, and that the excess of electron neutrinos in their beam was due to oscillations of sterile neutrinos. This would be one hell of an exciting result if true, so people have been pretty eager to test it.
Hence MiniBoone. MiniBoone makes a beam of muon neutrinos, sends it through half a kilometer of dirt, then dumps the beam in a tank of mineral oil (which is outfitted with a gajillion phototubes). Some of the muon neutrinos oscillate into electron neutrinos en route. The various interactions of the different neutrino flavors with the target produce signatures which they measure in their phototubes; by understanding those things, they can measure the oscillations into electron neutrinos.
Jocelynâ€™s talk was devoted hugely to the details of how the backgrounds are understood and the data are analyzed. It was impressive. Jocelyn was deeply involved in this process and did a really thorough job going over everything.
The punchline: NO excess. I canâ€™t even say no â€œsignificantâ€ excess; there just wasnâ€™t any. They cannot explain the LSND result. (Itâ€™s worth mentioning that a good fraction of the MiniBoone team is also associated with LSND â€” quite an honest approach, it seemed to me.) It remains a mystery what is going on with the LSND data; but, MiniBoone seems to be pointing to a fairly standard result for neutrino oscillations. No sterile neutrinos needed, it would seem.
Take all the details with a grain of salt â€” the punchline was clear, but this is rather far from my expertise, so Iâ€™m likely to be botching things.
…and next I am going to point you to a press release on Interactions (thanks Bee), which is here. Here’s a bit of what they say:
Currently, three types or â€œflavorsâ€ of neutrinos are known to exist: electron neutrinos, muon neutrinos and tau neutrinos. In the last 10 years, several experiments have shown that neutrinos can oscillate from one flavor to another and back. The observations made by the LSND collaboration also suggested the presence of neutrino oscillation, but in a neutrino mass region vastly different from other experiments. Reconciling the LSND observations with the oscillation results of other neutrino experiments would have required the presence of a fourth, or â€œsterileâ€ type of neutrino, with properties different from the three standard neutrinos. The existence of sterile neutrinos would throw serious doubt on the current structure of particle physics, known as the Standard Model of Particles and Forces. Because of the far-reaching consequences of this interpretation, the LSND findings cried out for independent verification.
The MiniBooNE collaboration ruled out the simple LSND oscillation interpretation by looking for signs of muon neutrinos oscillating into electron neutrinos in the region indicated by the LSND observations. The collaboration found no appearance of electron neutrinos as predicted by a simple two-neutrino oscillation scenario.
“It was very important to verify or refute the surprising LSND result,” said Robin Staffin, DOE Associate Director of Science for High Energy Physics. “We never know what nature has in store for us. The MiniBooNE experiment was an important one to do and is to be complimented for a job well done.”
… and third, I’ll point you to more information over on Cosmic Variance on the topic: A guest post by Heather Ray.