1. The problem statement, all variables and given/known data Neutrinos with energy of about ##1 GeV## are measured in an underground detector and compared with simulations of neutrinos produced in the atmosphere. Measured flux of upward going muon neutrinos ##(\nu_\mu + \bar \nu_\mu)## is half compared to simulations while measured flux of downward going muon neutrinos matches simulations. For electron type neutrinos, both upward and downward fluxes matches simulations. Why is this so? 2. Relevant equations 3. The attempt at a solution I know that neutrinos are left-handed while anti-neutrinos are right-handed. Also, a muon is much heavier than an electron so any relativistic electrons produced are left-handed, while a muon can be both.