Re: If a Neutrino is the smallest subatomic particle, what happens when they co

Good question.

To begin with, terms like "smallest" don't really mean the same thing in the world of subatomic particles. As far as we can tell with current experimental data, neutrinos have no size at all! Neither do electrons, or quarks, or any other "elementary particles". A nice picture of the different elementary particles which have been observed to date can be seen at URL:

http://www.fnal.gov/pub/standardmodel.html.

Neutrinos are, however, much lighter than the quarks, or the charged leptons (such as the electron). In fact, it is not yet known whether their mass is exactly zero, or whether it is merely lower than we have been able to measure up to now.

The second part of your question refers to collisions between neutrinos. In the world we know, things which collide at high energies often break into smaller pieces, and I believe this is the heart of what you are asking...can the neutrino break into anything smaller?

In fact, subatomic particles don't actually hit each other and break apart like objects in the "ordinary" world do. Instead, they interact with each other by exchanging force-carrying particles called "bosons". These include the photon which carries the electromagnetic force, the gluon which carries the strong nuclear force which binds quarks together into composite particles like the proton, and the W and Z bosons which carry the weak nuclear force responsible for processes like nuclear decay.

The special thing about neutrinos, however, is that they are neutral particles which are not affected by the electromagnetic force or by the strong nuclear force. The only way that they can interact with each other or other types of matter is through the weak nuclear force. This means that a neutrino must exchange a neutral Z boson or a charged W boson with a passing particle, a process that occurs very rarely. This is why neutrinos can pass through enormous amounts of matter without ever interacting with it. Billions of neutrinos are passing through us all the time, without any affect.

So, if two neutrinos pass by each other and manage to collide, they will do so by either exchanging a neutral Z boson or a charged W boson. I will "draw" diagrams of what happens in either case:

We begin with the exchange of a Z boson. All this does is transfer some momentum between the two neutrinos, so they will fly away from each other in different directions than they came in:

 -----------+---------
  nu(1)     |    nu'(1) 
            | Z(0)
            |
 -----------+---------
  nu(2)          nu'(2)

time ---->

The other possibility is that they exchange a charged W boson. If this happens, the neutrinos are transformed by the weak nuclear force into charged leptons. To make this example more interesting, I will have two different types of neutrinos collide...an electron neutrino, and a muon antineutrino:

 -----------+---------
  nu(e)     |      e-
            | W+
            |
 -----------+---------
 __
 nu(mu)          mu+

time ---->

Out of the collision between two neutral particles comes a negatively charged electron, and a positively charged muon. As you can see, the total charge coming out (+1 + -1) is the same as the charge going in (0 + 0). Since the W boson also carries momentum, the outgoing particles have different directions and momenta than the initial incoming neutrinos.

I am sure that this answer probably raises more questions than it answers. A good overview of modern particle physics for the layman can be found at http://www.fnal.gov/pub/hep_descript.html, and if there is anything in particular I can help clarify, don't hesitate to contact me at my email address:

silver@physto.se

I hope this helps.