Why Does Θ-Dependence Vary with Neutrino Energy in Monte Carlo Simulations?

[cj7529]

Hi I've been asked to give "A physical explanation of why the Θ-dependence of multi-GeV Monte Carlo data is qualitatively different from the sub-GeV data."
i.e why is the monte carlo simulated data peaked around Cos(theta) = 0 for high energy neutrinos but not low energy ones.

I'm not sure if I'm going the right way with this.
I have read that there should be an up down symmetry of neutrino events. But due to the geo magnetic field this is altered for low energy neutrinos. i.e there is an asymetry in the low energy case. This seems to be the right ballpark but I'm not sure where to go next.
I have some questions:

1)Cos(zenith angle)= 0
does this mean horizontal?
2)If so, why would the high energy flux peak horizontally?
Any help would be greatly appreciated, I'm stumpted!

Thanks

 

[Nate810]

!1) Yes, cos(zenith angle) = 0 does mean horizontal. The zenith angle is the angle between the zenith (the point directly above you) and the direction of the particle. So when cos(zenith angle) = 0, the particle is travelling horizontally. 2) The peak around cos(theta) = 0 for high energy neutrinos is due to the fact that the Earth's atmosphere is denser the closer it is to the surface. This means that high energy neutrinos are more likely to interact with the atmosphere near the surface (where the density is higher), which causes them to be deflected more when they travel through the atmosphere. This results in a peak at cos(theta) = 0, because this is the angle at which the neutrinos have travelled the shortest distance through the atmosphere, and thus been deflected the least. In contrast, low energy neutrinos don't interact as strongly with the atmosphere, so they are not affected by the atmospheric density as much, resulting in an absence of a peak at cos(theta) = 0.