Double Beta Decay: Questions from a Newbie

[seerongo]

I am new to this forum and hoping all you experts here can help explain some things that have been bugging me.

In a double beta decay (any kind, with or without neutrinos), I read that the decay occurs from two separate neutrons, apparently simultaneously. Is this the case, or can one neutron decay into two protons somehow? The Feynman diagram I saw implies two separate neutrons. If the decay is from two separate neutrons, by what mechanism can two separate neutrons "know" to decay simultaneously, even if they are neighbors? Is there some trigger that affects both at once?

Ron

 

[daveb]

If one neutron decayed to two protons, this would violate Conservation of Baryon number

 

[Astronuc]

If one neutron decayed to two protons, this would violate Conservation of Baryon number

Not to mention conservation of mass/energy.

 

[humanino]

In a double beta decay (any kind, with or without neutrinos), I read that the decay occurs from two separate neutrons, apparently simultaneously. [...] If the decay is from two separate neutrons, by what mechanism can two separate neutrons "know" to decay simultaneously, even if they are neighbors? Is there some trigger that affects both at once?

A neutron decays into a proton, an electron and an antineutrino. Alternatively, neutron decay can be triggered by the absorption of a neutrino, leading to a proton and an electron.

So, if inside the nucleus the anti-neutrino somehow transforms into a neutrino, it likely to trigger such a "baby" neutron decay, almost simultaneous (for all purposes). Obviously this violates lepton number conservation (since neutrino and anti-neutrino have opposite lepton number).

This is possible only if neutrino have mass, and are of Majorana type. In that case, they differ only by their handeness (you can take that as the definition of "Majorana type"), and if they have mass they should be able to oscillate between the two handeness state. If neutrino were really masseless, they would conserve their hadeness. So early searches for neutrino mass (and type) were focusing on (neutrinoless) double-beta decay.

This is now rather old stuff however. See NEMO web page

edit
BTW, welcome to PF
Don't be offended by comments, we are actually having a lot of fun here
Such as for instance "Violation of energy conservation !? Stop it, you are killing me ! "

 

[olgranpappy]

and charge

If one neutron decayed to two protons, this would violate Conservation of Baryon number

Not to mention charge.

 

[seerongo]

Thanks for the replies. I guess I was looking for some triggering event. I figured it made no sense for one neutron to decay to two protons (although I didn't really know why...)
Thanks, Humanino, for the Nemo site link. I saw that site and that's what got me thinking about it. The Feynman diagrams explain the processes nicely but did little to explain the simultaneity. Your scenario would explain simultaneity but only if one of the antineutrinos became a neutrino. In the originally predicted (and as I understand, observed) double beta - double antineutrino decay where both antineutrinos excape, the question (for me) remains.

 

[humanino]

In the originally predicted (and as I understand, observed) double beta - double antineutrino decay where both antineutrinos excape, the question (for me) remains.

This one is rather trivial. In some rare cases, there is simply no favorable simple beta decay. For the decay to release energy, you need to turn 2 neutrons into 2 protons. This is illustrated in the following plot from NEMO site :

Simple beta decay would be Z -> Z+1 (which is not possible).
Only double beta is possible Z -> Z+2

 

[Rade]

Perhaps the simultaneity results from the short time formation of an unstable dineutron (see: http://en.wikipedia.org/wiki/Dineutron ) in those few isotopes where double beta decay has been observed, that is, a [NN] unstable nucleon cluster in the outer "halo" area of certain isotopes. It is known that dineutron has a very weak and unstable subthreshold resonance of ~ 0.067 MeV (see: Sofianos et al, 1997, J. Phys G: N Part Phy 23: 1619-1629 ). So, if both neutrons in the dineutron [NN] form such a resonance, and then decay at "the same time", one would observe double beta decay experimental results. Both dineutron and double beta decay are rare events, so this hypothesis that they are coupled physical events seems to make sense to me--but I am no expert, just some thoughts to add to the discussion.