Slowing down high energy neutrinos? Which atoms can slow down high energy neutrinos?
I mean in which medium high energy neutrinos will tend to slow?
 A: I don't think such materials exist (at least we are not aware of any so far). In order to slow down neutrinos, we have to find something which interacts and scatters them. Neutrinos only interact weakly with normal matter, so most of them pass through everything without interacting with it. In fact, the interaction is so weak, that scientists have built a neutrino detector in the Antarctica (called IceCube) to detect neutrinos coming from North; because neutrinos pass easily through Earth(!) while nothing else does.
There are famous examples to illustrate how weak the interactions are, e.g. each second, (the unimaginable number of) 100 billion ($10^{11}$) neutrinos pass through your thumb nail! However their interactions are so weak that, an average person will interact with them once, every 70 years. It's apparently that neutrino which kills us!
A: For the most part nothing affects neutrinos in flight. There are only a few known interaction and behaviors of neutrinos


*

*There are energetic weak interactions with electrons or nuclei (and their parts). These either result in the disappearance of the neutrino (charged current; you get a charged lepton) or in a change in it's momentum (neutral current) but they don't "slow" it down without also changing it's direction. Nor do they affect the neutrinos that do not interact (which is by far the majority).

*There are coherent forward interactions as in the matter effect. These affect the flavor content of the beam without affecting speed1 or direction.

*Flavor oscillation. Again this does not affect speed1  or direction.



1 If you think this through you will notice the difficult little puzzle that comes from flavor changes in flight.
