# Physics Principles and Problems 2009

## Chapter 30: Nuclear Physics

### Problem of the Week

 (0.0K) Neutrinos Everywhere Every day millions of neutrinos zip through Earth with hardly a scratch to them or to Earth. Neutrinos are particles with no charge that are produced in the sun, stars, and by the cosmic rays that rain down on Earth's atmosphere. They can travel immense distances at the speed of light without being stopped by galaxies, stars, or planets. Neutrinos come in three flavors: electron, muon, and tau. (0.0K) Super-Kam (25.0K) Super-K detects the electron and muon neutrinos that were created at the top of the atmosphere by cosmic rays. Surprisingly, the muon flavor of neutrinos have a greater chance of disappearing when going through Earth than do the electron flavor. The physicists assume that they changed into undetectable tau neutrinos. Such a change in flavor is called neutrino oscillation. The very successful "Standard Model" states that oscillations can occur only if the two neutrinos have different masses. But, if one flavor has a mass, it is very likely that all three do. (0.0K) At the Super-K neutrino detector, the number of neutrinos entering the tank of water from the top is compared with the number coming in from the bottom. Why would the physicists use these two measurements to see if neutrino oscillations have occurred? (0.0K) (5.0K) Preliminary results suggest that the mass difference, which is likely to be close to the mass of a neutrino, is about 20 million times smaller than that of the electron. To get a feel for how small that is, let the electron have the mass of the USS Normandy. What would be a typical object with the mass of the neutrino? (1.0K) Additional news on the measuring the neutrino's mass can be found at http://hep.bu.edu/~superk/news.html. Clicking will launch a new window. (0.0K) (0.0K) (0.0K) (0.0K) (0.0K)