Results from the MINOS experiment at the Fermi National Accelerator Laboratory (FNAL)

On June 24 scientists from the MINOS experiment at the Fermi National Accelerator Laboratory (FNAL) just outside Chicago, announced the results from a search for a rare phenomenon, the transformation of muon neutrinos into electron neutrinos. A team of physicists at the Cavendish Laboratory, led by Professor Mark Thomson, played a central part in producing this potentially exciting result. The new result is consistent with and significantly constrains a measurement reported 10 days previously by the Japanese T2K experiment. The results of these two experiments could have implications for our understanding of the role that neutrinos may have played in the evolution of the universe. If muon neutrinos transform into electron neutrinos, neutrinos could be the reason that the big bang produced more matter than antimatter, leading to the universe as it exists today.

The MINOS experiment consists of an intense beam of muon neutrinos produced at FNAL which are then detected 735 km away in the large underground MINOS detector, located in a deep mine in Minnesota. It has been known for some time that, during the journey, some of the muon neutrinos transform into tau neutrinos, an effect known as neutrino oscillations.  The first indication that the muon neutrinos also transform into electron neutrinos was announced recently by the T2K collaboration. Searching for rare electron neutrino interactions in the 5400 ton MINOS detector is extremely challenging. When a neutrino interacts in the MINOS detector it leaves a pattern of hits rather like a very poor quality photograph of the interaction. The pattern of hits from the interesting electron neutrino interactions are (sometimes subtly) different from the vast majority of neutrino interactions that are observed. To maximise the chance of correctly identifying the interesting patterns of hits, Professor Thomson and his team developed a new pattern recognition technique. Using a sophisticated simulation of the detector, the expected patterns of hits for different types of neutrino interactions in the MINOS detector was studied.  A library of over 50 million pictures of these patterns of hits was constructed, forming a gallery of neutrino "mugshots"; half are made from simulations of the interesting electron neutrino interactions and the other half from normal neutrino interactions. When a real neutrino interaction is observed in the MINOS detector, its pattern of hits is compared to the library of mugshots. The 50 mugshots which most resemble the observed event are identified; if the majority of these come from simulated electron neutrino interactions the event is declared as an interesting event, i.e. a candidate electron neutrino interaction. Physicists from Cambridge, in collaboration with colleagues from the California Institute of Technology, used this technique to analyse the data recorded in the MINOS detector. A total of 62 electron neutrino-like events were identified. If muon neutrinos do not transform into electron neutrinos, then only 49 events should have been seen. However, the 71 such events would have been expected if neutrinos transform as frequently as suggested by recent results from the Tokai-to-Kamioka (T2K) experiment in Japan which uses a different method and analysis technique to look for this rare transformation. The results combined provide the first indication that the fundamental parameter, theta13, is relatively large. If confirmed this result is not only scientifically extremely interesting, but would be central to determining the future direction of the field of neutrino physics.
Ruth Toner, a final year graduate student at the Cavendish laboratory, who performed much of the detailed data analysis, said "it is immensely satisfying to see what has been several years of hard work culminate in such an interesting result. The combined results of MINOS and T2K suggest that the next few years will be very exciting for neutrino physics.  I personally can't wait to see what happens next. "