Why is anti-neutrino called so?

[DrDu]

From what I know of the history of the neutrino, it was first postulated by Pauli to explain momentum conservation in the beta decay. However, nowadays we call the particle emitted in that process anti-neutrino and not neutrino. What is the reasoning behind this change of naming?

 

[humanino]

The reasoning is the conservation of leptonic charge.

Start with a free neutron : there is no lepton. It decays into a proton, plus an electron (which conserve electric charge; the electron is a lepton), plus an anti-neutrino (which carries negative leptonic number).

In the Feynman diagram
[URL]http://upload.wikimedia.org/wikipedia/commons/8/89/Beta_Negative_Decay.svg[/URL]
you can also see the lepton number "carried in" along the arrow by the anti-neutrino, and "carried away" by the electron. By the same token, the hadronic number is conserved along the d->u line. I hope the diagram is not confusing. The anti-neutrino is really outgoing with positive energy, it is represented as a neutrino in-going (backwards in time) with negative energy.

 

[DrDu]

Thank you humanino!
I understand more or less the reasoning, although I cannot see (but imagine) the diagramm.
How is leptonic charge being defined? Is it a conserved charge of the weak interaction?

 

[humanino]

The lepton number is an additive quantum number which is +1 for leptons (electron, muon, tau and their neutrinos) and -1 for antileptons. You may just count them in the initial and final state, and expect this number to be conserved, included in weak interactions.

First note however that this number is already known not to be conserved per family or flavor, as massive neutrinos can oscillate from one flavor to another.

Even worse, although the total number of leptons however has (AFAIK) never been observed to be violated, in principle it could be by very tiny effects called anomalies (the breaking of classical symmetries by quantum effects or loops), even within the standard model. Since it has never been observed, keep in mind that taking lepton number as conserved is an excellent working hypothesis. Effects in which it could be violated also occur beyond the standard model. For instance, there are searches for proton decay into neutral pion plus positron, which violates both baryon and lepton number. Note however that this reaction which has never been observed despite intense searches respects the difference B-L, which in fact is protected against anomalies within the standard model and also respected in many models beyond the standard one.

See also :
http://en.wikipedia.org/wiki/Lepton_number

 

[DrDu]

Is there a (possibly only approximate) symmetry behind the conservation of leptonic and baryonic charge in the standard model?

 

[humanino]

Yes, this is a global symmetry consisting in multiplying all leptons (or hadrons) by a pure phase, so those are two U(1). This is described for instance in
http://arxiv.org/abs/hep-ph/0410370v2