# Neutrino collision with a proton,neutron or an electron?

1. Oct 19, 2011

### jayaramas

what happens if a neutrino hits head on with a proton,neutron or an electron?

2. Oct 19, 2011

### grzz

Re: neutrino

A neutrino is highly non interacting. But there are possibilities for interaction so long as no conservation law is broken.

3. Oct 19, 2011

### jetwaterluffy

Re: neutrino

From what I remember, a neutrino does interact with the weak force. So if it hits a proton or neutron head on, then it will be affected. An electron is a point particle, so you can't get another point particle, such as a neutrino, to hit "head on" with it in some sense. The electron doesn't interact with the weak force anyway, from what I know, so there would be no effect, except perhaps gravitational forces.

4. Oct 19, 2011

### Bill_K

Re: neutrino

Several possibilities. Among them:

νe + p → n + e+

νe + e → μ + νμ

5. Oct 19, 2011

### jayaramas

Re: neutrino

so if u calculate number of protons in earth and still considering that all solar neutrinos pass through it is not justifiable is it?

6. Oct 19, 2011

### phinds

Re: neutrino

I assume you mean it is unlikely, yes? Unlikely is not the same as impossible. It is a valid question.

7. Oct 19, 2011

### jayaramas

Re: neutrino

what is the reverse process? any way neutrino came from a nuclear reaction, so a fast moving neutrino can hit and attains its position back. why is it not likely?

8. Oct 19, 2011

### kurros

Re: neutrino

Consider
$$n → p + e^{-} + \bar{\nu}_{e}$$
The chances of the products all colliding back together at the same time are negligibly tiny.

but ok fine you could have a slightly different process:
$$p + \bar{\nu}_{e} → n + e^{+}$$
In this case I think the problem is that neutrinos interact very weakly and the scattering has to exceed a minimum threshold energy to happen (since this reaction requires energy input), and this has a very low chance of occurring. i.e. you would need very fast neutrinos indeed to start with, and even then they will hardly interact anyway.

Last edited: Oct 19, 2011
9. Oct 20, 2011

### DrDu

Re: neutrino

Basically, these inverse decay processes are how neutrinos are detected in neutrino detectors.

10. Oct 20, 2011

### bbbeard

Re: neutrino

Not true. Actually, beta decay is the paradigmatic weak force interaction.

11. Oct 20, 2011

### jetwaterluffy

Re: neutrino

Yeah, I worked that out after I had already posted. However, I still can't see how they can "collide".

12. Oct 20, 2011

Re: neutrino

Think of it as an interaction, rather than a collision.

A particular particle interaction has a measure of reactivity which is termed a cross-section. As the name suggests, this quantity is actually in units of area - it could be expressed in square metres, though the more commonly used unit is the http://en.wikipedia.org/wiki/Barn_(unit)" [Broken]. But you can loosely think of this as the "target area" that the other particle has to "hit" for an interaction to occur.

An electron and an (anti-)neutrino can combine to form a W- boson. In any practical scenario, this W- will be virtual as its mass is far larger than those of the incoming particles, so it will almost immediately decay again, and can usually only decay back into another e and neutrino because any other decay route would produce particles of higher energy. So the end result in this case is simply an elastic collision.

But electrons and (non-anti-)neutrinos can also interact. In this case, the electron could give off a W- and thereby transform itself into a neutrino, while the W- is absorbed by the original neutrino and is thus transformed into an electron. Swap ya! (Alternatively, the neutrono could emit a W+ which the electron then absorbs, with the same end result.) Again, this is an elastic collision.

The second type of interaction can also take place but with a Z0 instead of a W. In this case the electron could interact with either a neutrino or an anti-neutrino. Interactions with Z0 do not change the identities of the incoming particles.

These interactions all have extremely small cross-sections - but they do happen. The W interactions are the basis of the http://en.wikipedia.org/wiki/MSW_effect" [Broken] which has to be taken account of when looking at neutrino oscillations where the neutrinos have travelled through regions containing matter.

Last edited by a moderator: May 5, 2017