Atmospheric Neutrinos

What is Atmospheric Neutrino?

Atmospheric neutrinos are produced by the decay of particles resulting from interactions of cosmic rays (mainly protons) with Earth’s atmosphere. There are two kinds of neutrinos created, muon type and electron type.

Figure 1: Cosmic rays interact with the atmosphere and two kinds of neutrinos are generated.

Figure 2 shows cosmic rays pouring into the earth. Atmospheric neutrinos generated in the atmosphere on all of the earth pass easily through the earth and arrive at the Super-Kamiokande detector. Therefore Super-Kamiokande observes the muon neutrinos and electron neutrinos which have the different path lengths for the zenith angles.

Figure 2:Atmospheric neutrinos are generated uniformly on the earth.

 

Discovery of Neutrino Oscillations

The number of observed atmospheric neutrinos is predicted to be uniform regardless of the zenith angle. However, in 1998 Super-Kamiokande found that the number of upward going muon neutrinos (generated on the other side of the Earth) is half of the number of downward going muon neutrinos. This result can be explained that some of the muon neutrinos change or oscillate into another type of neutrinos which is not observed. This phenomenon is called neutrino oscillation. The discovery of the muon neutrino oscillation indicates the finite mass of neutrinos and suggests to extend the Standard Model.

Figure 3: Zenith angle distribution of the muon neutrinos. The observed number of upward going neutrinos was half of the predictions.

 

Furthermore, in 2004 detailed analysis of atmospheric neutrinos shows a sinusoidal dependence of the event rate as a function of “Length/Energy”, which confirmed the neutrino oscillations.

Figure 4: Survival probability of muon neutrinos as a function of L (neutrino path length)/E (neutrino energy)

Black dots show the observation and blue line shows the prediction based on neutrino oscillation. Data shows a dip around L/E=500km/GeV. The prediction of neutrino oscillation agree well with the dip, but other theories (red and green lines) cannot explain that.