# 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 $1GeV$.

• You are presumably talking about results from Super Kamiokande (often abbreviated as "super-K"). I haven't enough time to write a complete answer now, but it's about neutrino mixing. – dmckee --- ex-moderator kitten Jun 6 '15 at 15:33
• Ok, I went to read about the experiment and neutrino mixing. The basic explanation is that the neutrinos hop from one flavour to another. But doesn't that apply to both electron and muon neutrinos? – user44840 Jun 6 '15 at 15:36

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.