MINOS, or the Main Injector Neutrino Oscillation Search, is now on its third phase of data collection. According to Professor Marvin Marshak, the Fermilab Main Injector accelerator has now aimed more than 5 x 1020 protons at the target in the neutrino beam line. The protons create pions and kaons, which then decay to create the neutrino beam. MINOS has now observed more than 1,000 neutrino interactions in the Far Detector at Soudan MN. “MINOS is clearly in its precision measuring phase."
Marshak says that precision measurements are important for understanding the physics of neutrinos. While the unexpected phenomenon of neutrino oscillation was first discovered using neutrinos naturally created by the Sun and by cosmic rays, accelerator-made neutrinos have fewer uncertainties and yield more precise results. Physics has often moved from observation to experiment, Marshak says. When we observe interactions at Soudan by Fermilab-created neutrinos, we know precisely how long the neutrinos have been traveling and what direction they are going. We can compare observations in a Near Detector at Fermilab, which observes the neutrinos before an oscillation can occur, with what we see at Soudan, after the possibility of oscillation.
Marshak says that MINOS is refining two parameters. The first is the difference in mass difference between the muon neutrino and the tau neutrino and the second is the “coupling constant” or the probability that a neutrino will oscillate. “When we started, we were unsure how frequently neutrinos would oscillate,” Marshak says. “We knew the probability was large, but now, with more precise data, it appears to be 100% of the time.” He continues, “We don’t know why the probability should be so large.”
Marshak also says that by pinning down the muon to tau neutrino oscillation MINOS has helped to shift attention in the neutrino physics world toward the final oscillation that has yet to be observed, the muon to electron neutrino oscillation. This final observation will be sought by the NOνA experiment, which will share the NuMi beam with MINOS when it is complete.