MadSci Network: Physics

Re: What happens in neutrino antineutrino annihilation ?

Date: Mon Oct 16 11:30:47 2006
Posted By: Samuel Silverstein, Lecturer in physics
Area of science: Physics
ID: 1158908092.Ph


There are a lot of questions here, so I'll try answering them in order.

What happens in neutrino antineutrino annihilation ?

The two particles meet at a single point and annihilate each other, producing a virtual Z boson, which is the neutral (i.e. no electric charge) carrier of the weak nuclear force. This Z boson then immediately decays to produce another particle/antiparticle pair, either a new pair of neutrinos, two charged leptons, or a quark/antiquark pair. What you can produce depends on how much energy there is from the colliding neutrinos.

Wouldn't it be far easier to detect neutrinos by annihilating them ?

Well, I wouldn't say "far easier". In fact, "far harder" would be much more accurate. The neutrino-neutrino collision probability is very small, so to observe such collisions on earth you would need to aim two narrow, intense beams of neutrinos at each other. Plus, the neutrino energies must be high enough that they produce pairs of detectable particles like electrons (otherwise the only possible collision products are more neutrinos, which are very hard to see!)

We can and do produce neutrino beams, but since neutrinos are chargeless they can't be focused electromagnetically (i.e. with magnets). So the beams would be too diffuse to actually produce enough collisions to be useful. So although in principle neutrino-neutrino collisions would make for some very nice physics (clean signals!), we can't do it.

In fact, the most likely place I can think of to observe neutrino-antineutrino annihilation would be in a collapsing supernova. I haven't found any simple websites describing this, but here is an example of a physics paper on the subject.

Shouldn't it be possible to detect the Sun's neutrinos using the antineutrinos of one of our earthbound nuclear reactors ? What would be the particles produced in this annihilation ? Shouldn't they be far easier to detect than the neutrinos and antineutrinos themselves ?

Not really (see above). The intensities are too low, and so are the energies, so you couldn't produce any significant number of collisions, and most of the time the collision products would be neutrinos. Researchers DO detect solar and cosmic neutrinos at several massive experiments around the world. See, for example, this list on Wikipedia of current and future experiments, including links to their respective homepages.

A related question is would you please explain the current controversy over whether there even exists any distinction between neutrinios and antineutrinos (some claim they are the same particle), has this been settled experimentally or not ?

The Standard Model of particle physics only allows neutrinos to have one helicity, so neutrinos are left-handed and antineutrinos are right-handed. This is what has been observed in many particle physics experiments over the years. But now we know that neutrinos have mass, so it is possible to see right-handed neutrinos and left-handed antineutrinos (this hasn't been experimentally seen yet, to my knowledge).

If handedness is the only difference between neutrinos and antineutrinos, then the absolute distinction between the two is lost. Then you would have the possibility of violating conservation of lepton number. If there is yet another property that distinguishes them, then lepton number is conserved. There are interesting arguments on both sides, and we'll have to wait for new theoretical work and experimental data before we know the answer.

I hope this helps.

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