# What distinguishes the neutrinos?

1. Nov 21, 2007

### quantumfireball

What distinguishes the neutrinos???

Assuming Neutrinos are massless
What then distinguishes the 3 neutrinos experimentally.
WHy 3 neutrinos were proposed theoretically in the fisrt place????
Whats the justification behind introducing 3 neutrinos???

Plz dont say that because there are 6 quarks there was a need to have 6 leptons so introducing extra nuetrinos was an abvious choice

2. Nov 21, 2007

### Timo

EDIT2: Ok, I seemed to misunderstand your question slightly. I was just happy that someone was seemingly asking the same stupid questions as I do :P.
Experimental distinguishing: I faintly remember something about "inverse beta decay". More theoretically: A vertex with a charged lepton, a neutrino and a W boson must have the neutrino and the charged lepton to be of the same generation.
Why 3: Similarly as you get spin-up and spin-down as possible spin-adjustments, the SU(2) gauge group gives you +1/2 and -1/2 as "weak isospin" - for each generation. After electroweak symmetry breaking you end up with one of the two having a electromagnetic charge of 1 and the other having a charge of 0.

No, it's actually because we have three charged leptons . Seriously, I have asked for an experimental evidence that the three neutrinos are really distinct particles some time ago (in a slightly different context, though). It turns out that the answer is given in the Wikipedia article about the Neutrino (even with a source):
EDIT: Looking at it, it turns out the source is the PDG: http://pdg.lbl.gov/2005/listings/s007.ps

Last edited: Nov 21, 2007
3. Nov 22, 2007

### drpsycho

They have different interactions with the charged leptons.
Lederman and Steinberger won the nobel prize for having demonstrated experimentally that the neutrinos created together with a muon (in pion and kaon decays) are not able to interact with electrons (while it was already well know that neutrinos created together with an electron, as in usual beta decays, are able to interact with other electrons).
This was the first clear proof of the existence of at least two neutrino types, each associated with a charged lepton.
The experimental evidence cited by Timo arrived some 40 years later, and it was not anymore interesting from the point of view of your question (i.e. "how to know that there are more than one neutrino?") but from the opposite point of view: it demonstrated that there are not more than 3 (light) neutrinos.

The original motivation was explaining why you never observe violations of the electron number and of the muon number.
For example, if there were only one type of neutrino, there would be Feynman diagrams with an intermediate neutrino permitting the existence of the decay $\mu\to e\gamma$.

The conservation of three different leptonic numbers.

4. Nov 22, 2007

### cesiumfrog

Wasn't it because experiments only detected one third as many as originally expected?

5. Nov 22, 2007

### mathman

The fact is that neutrinos have mass nad the three types have different masses.

6. Nov 23, 2007

### drpsycho

no, that's a different business: this observation that you mention, only hinted at the possibility of oscillation (and since oscillation is possible only with different masses, this hinted at the possibility that at least one of the neutrino types is not massless). But I think that at the time there was already the knowledge that at least two neutrinos existed.

7. Nov 23, 2007

### malawi_glenn

$What then distinguishes the 3 neutrinos experimentally.$
What particles they have W boson interaction with differs. For example the $$\nu _e$$ couples more to electrons than muons, and $$\nu _{\mu }$$ couples more to muons than electrons. And so on.

$WHy 3 neutrinos were proposed theoretically in the fisrt place????$
What do you mean by "first place" ? If i remember my neutrino physics correct, the neutrino was introduced before any firm evidence of the existance of the muon, and not the tau-lepton either. But if you mean in the "foundation" of the standard model, you have got some answers to this.

$Whats the justification behind introducing 3 neutrinos???$
besides from the other answers; you also have limits from the width of the Z-boson decay and from the primordial He-abundance (He produced in BB).

Last edited: Nov 23, 2007