Muon Neutrinos vs Electron neutrinos I read somewhere that for muon neutrinos, why is the upward measured flux half of what simulation of neutrino production in atmosphere predicts while the downward flux is the same? Whereas for electron neutrinos it's the same for both upward/downward. Has this got anything to do with helicity? I checked that the energy of neutrinos from the sun is roughly $1GeV$.
 A: I found a very good explanation from the book "Beam Line: Fall 2001". The sun only produces electron-type neutrinos, so any muon neutrinos present must have been due to interactions.
So basically muon upward-going neutrinos originated from distant parts of the atmosphere, below the horizon and have been around for a long time. In contrast, downward moving neutrinos started much closer at the atmosphere. 
So upward moving muon neutrinos have had time to oscillate, while downward moving muon neutrinos don't. Atmospheric muon neutrinos oscillate into a tau neutrino, as the probability is related to the difference in masses. Thus it oscillates between a muon neutrino and a tau neutrino. Since the oscillation is $\sim sin^2(\Delta M (\frac{L}{E}))$, the average observed upward moving is $\frac{1}{2}$ of the downward moving.
For electron neutrinos, they are produced in the sun, and have had time to oscillate on their way to earth. So measured upward and downward fluxes are equal for electron neutrinos.
