Positron Capture

[Jay-qu]

Since I cant find any references about it, it seems that the process of a neutron capturing a positron is impossible. I assume this could precede through a u-channel W boson and the anti-neutrino could carry off the excess energy..

 

Is this possible, if not why not?

[Qfwfq]

it seems that the process of a neutron capturing a positron is impossible.

Could it simply be there isn't much interest in it? Have you looked only through searchable media?

 

As you suggest, it is easily relatable to beta decay as far as diagrams go, at first sight I would expect it to have a cross section to some degree comparable to electron capture, I'm not sure what conservation rule it would violate, except maybe you should try working out the energy-momentum in c. m. coordinates, in which the sole massless anti-neutrino would have to carry the exact opposite of the recoiling proton which of course isn't massless. Perhaps this can be defeated by moe than the single antineutrino oucome but the cross section might end up just too low... Just kinda guessing.

[Jay-qu]

Yeah I have only searched through google and the arXiv. I have now found one paper where the process is mentioned in the context of a plasma of positrons, electrons and nucleons. So suffice to say it is possible, but I was just wondering about it happening in more generic circumstances.

[CraigD]

Since I cant find any references about it, it seems that the process of a neutron capturing a positron is impossible. I assume this could precede through a u-channel W boson and the anti-neutrino could carry off the excess energy..

 

Is this possible, if not why not?

Not possible, I think – though I've a wikipedia self education in particle physics, not a decent academic one. :(

 

The inverse beta decay process (follow the link, reverse time and change particles, as I’m to lazy to draw my own diagram :)) would be [math]V_e + e^+ \to W^+[/math], [math]W^+ + \hat{d} \to \hat{u}[/math], so it’d have to be an antineutron ([math]\hat{u} \, \hat{d} \, \hat{d}[/math]) capturing the positron ([math]e^+[/math]) to turn into a antiproton ([math]\hat{u} \, \hat{d} \, \hat{d}[/math]).

 

I don’t think a positron and a neutron would have any interaction.

[Jay-qu]

Craig, the second interaction you wrote violates charge conservation. It should be a down and up not anti down and anti up. I'm now sure this process happens (at least in a plasma) but I'm not sure about it happening with a nucleus.

[Rade]

First a question.

 

If we discuss the possibility of positron (e+) capture by a neutron (n), we should also at the same time discuss possibility of electron (e-) capture by neutron (n)--correct ?

 

As I understand the situation, we only have knowledge of electron (e-) capture by a proton (p)---this is well known.

 

I am not aware of any information of either (e-) or (e+) capture by a neutron ? Is this correct ?

 

==

 

Next an observation.

 

In the Nov-Dec 2010 journal "American Scientist" is a paper by T.P. Smith (now at Dartmouth) called "The Anatomy of the Neutron". He reviews recent experiments on the neutron. He concludes with this statement..."the surface is dominated by negative down-quarks". Now, a negative down quark has a charge = (d = -1/3).

 

So, based on these data, I would predict that it may be possible for a neutron to capture a positron (e+), if the surface of the neutron has (d-) quarks present in large numbers. Although one is matter (quark) and one antimatter (positron) they are not identical entities and Pauli Exclusion does not apply, thus no annihilation is predicted, only binding or not. The (+) and (-) charges should attract.

 

Conversely, the neutron with (d- quark at surface) would not be predicted to capture an electron (e-)--this is the event that would be impossible, two (-) charges do not attract.

 

OK, where does my logic fail ?

[CraigD]

Craig, the second interaction you wrote violates charge conservation. It should be a down and up not anti down and anti up.

Doh! :doh: Not only am I a poor amateur physicist, I’m not even a good accountant!

 

The 2nd and 3rd steps of the process could be [math]W^+ + \hat{u} \to \hat{d}[/math], and [math](\hat{u} \, \hat{d} \, \hat{u}) \to (\hat{u} \, \hat{d} \, \hat{d})[/math], but that’s just inverse beta decay using all antimatter, no neutrons or antineutrons absorbing anything, not what you’re talking about, Jay.

 

If the 2nd and 3rd steps are [math]W^+ + d \to u[/math], and [math](u \, d \, d) \to (u \, d \, u)[/math], I’ve got what you’re talking about. So, having finally understood the basics of the question, I’ll change my answer from “impossible” to “possible”.

 

If we discuss the possibility of positron (e+) capture by a neutron (n), we should also at the same time discuss possibility of electron (e-) capture by neutron (n)--correct ?

...

OK, where does my logic fail ?

An electron and an electron antineutrino produce an anti W boson ([math]e^- + \hat{V}_e \to W^-[/math]), which with an up quark produces a down quark ([math]W^- + u \to d[/math]). A neutron doesn’t have an extra up quark, so can’t do this.

 

Put in other words (which I find helpful in imagining this stuff), a neutron capturing a positron to become a proton is like a neutron beta decaying into a proton, releasing an electron. Both change the charge of the universe outside the nucleon by -1.

 

It doesn’t seem like it would take terribly hard experiments to observe [math]n + e^+ \to p[/math]. Just get a positron source and some easy-to-get isotope that beta decays into a stable isotope (eg: cesium-137 to barium-137), run it for a while, separate and measure the stuff, and show that it’s “decaying” significantly faster than its know decay rate predicts. It seems so easy, I’d be surprised if it hasn’t been done, perhaps many times, already.

[Rade]

See page 30 of the Google Book:

http://books.google.com/books?id=whIbrWJdEJQC&pg=PA28&lpg=PA28&dq=n+%2B+e%2B&source=bl&ots=6v5nfZQAt4&sig=XqG2XMQYSEKMbftSdTuC7uqP23M&hl=en&ei=L7e3TMbQMoWenwf5k_GVCQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CB8Q6AEwAzgK#v=onepage&q=n%20%2B%20e%2B&f=false

 

It discusses a 1959 experiment by Reines and Cowan of the reverse reaction:

 

antineutrino + p ---> e+ + n

 

Perhaps it also goes in the reverse direction, but I have not yet found any experiments.

[Jay-qu]

Thanks Rade, but as I said earlier - I know it is possible now.