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Poc ar buile
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#1
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I think this is quite an easy question now (having been given the obvious answer), but it stumped me for years...

If a neutron can undergo beta-minus decay and form a proton and an electron (not to forget our friend the anti-electron neutrino), then it seems obvious that a neutron should have more mass than a proton.

If a proton can undergo beta-positive decay to form a neutron and a positron (and an electron neutrino), however. does this imply that the positron has negative mass?
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Spleenus`
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#2
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edit: Ignore - I was thinking the wrong thing. Got my mind going now, unless there's some energy converted to mass but afaik that wouldn't happen as it's radioactive decay... lol what a conundrum
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RobbieC
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#3
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Aren't positrons and neutrinos the same thing??
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Nylex
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#4
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(Original post by RobbieC)
Aren't positrons and neutrinos the same thing??
No, positrons and electrons are pretty much the same thing (positrons just have a +ve charge). Neutrinos are uncharged, have no mass (actually, I think they found that they have a v. small mass) and are produced in the reactions in the Sun.. a lot are produced that way, anyway.
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Hipocrit
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God I hate particle physics, we did it as part of our module 3 physics (AS). The shcool got a PHD student from the university of bath to come and talk to us about it and he told us it isn't taught at university till the third year! :eek:
That was less than reassuring, I can tell you. :rolleyes:

Try a feynmann diagram, just about the only part I understood and it usually helps, antiparticles go in the opposite direction so that might change things, on the other hand I may be talking utter crap.
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Willla2
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neutrons are made of udd quarks. (up down down), and protons are uud. Thus B- decay is a d->u, and B+ decay is u->d.

Now, imagine that in the neutron, the quarks have a sort of instability, and this gives rise to a binding energy. When the quark changes to an up quark, the baryon becomes more stable, and thus emits energy (in the form of kinetic energy of electron and anti-neutrino or something like that).

Now, in the case of a proton becoming a neutron, the baryon has to become LESS stable, it has to ABSORB energy from somewhere to undergo B+. This energy actually comes from the electrostatic repulsion of the proton rich nucleus. There is lots of potential energy available from all the proton repulsion, so when a proton flips to a neutron, that potential energy has decreased. That decrease in PE has been attributed partly to the instability of neutrons being greater than protons.

I may have screwed up a little detail in my explanation because I'm not an expert, but it's all about bearing in mind that energy can manifestate itself as mass. The addition of negative mass can therefore just be the loss of energy.
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elpaw
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(Original post by Willla2)
This energy actually comes from the electrostatic repulsion of the proton rich nucleus.
what if it is just a single proton in vacuum?
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Willla2
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single protons do not decay into neutrons - you can leave it however long you want and it will not decay into a neutron without energy input.

Protons are actually predicted to decay into a neutral pion and a positron by current theory, because these two objects are "more stable" than a proton. The problem we have is that we've never seen a proton do this, even though theory says it CAN do this. But of course, just because it can, doesn't mean it will, but the theory predicts this reaction to proceed at a half life of 10^33 years AT MOST, if you really try to fudge the results. The age of the universe is actually 10^32 years, so this is indeed a very slow reaction. The problem we have is that even when we take enough individual protons and watch them for long enough, theory predicted that at least one should decay within a year, and yet nothing happened. So physicists are a bit puzzled by why a proton refuses to decay when our theory says it can.
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Poc ar buile
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(Original post by Willla2)
Protons are actually predicted to decay into a neutral pion and a positron by current theory, because these two objects are "more stable" than a proton.

Can it? Is a pion not a meson? I suppose that the expulsion of an electron neutrino can conserve lepton number but what about baryon number?

Protons in the confines of a nucleus, as you said earlier, are very different to free protons.
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Willla2
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#10
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well baryon number is not actually an all abiding natural law. Theory says that it can be broken. Please don't ask me how they arrived at that result, but the effective idea is that one up and a down quark sort of merge (i think the technical term is resonate) together producing an anti-up and a positron. It conserves the definite law of conservation of charge, so it's all fine.

This obviously applies to free protons only
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Jonatan
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#11
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(Original post by polthegael)
I think this is quite an easy question now (having been given the obvious answer), but it stumped me for years...

If a neutron can undergo beta-minus decay and form a proton and an electron (not to forget our friend the anti-electron neutrino), then it seems obvious that a neutron should have more mass than a proton.

If a proton can undergo beta-positive decay to form a neutron and a positron (and an electron neutrino), however. does this imply that the positron has negative mass?
Neutrons and protons are not like lego blocks. Particles in particle physics can tranform from one to another provided a few conservation laws are obeyed ( Conservation of total electric charge, energy, momentum and lepton number are the most important ones).

Note that energy and mass are equivalent, thus as long as the overall energy is constant it doesnt matter if it is present in form of kinetic energy, nuclear binding energy, particle masses or any other form of energy. A much more perplexing reaction is the following:

e- + e- -> e- + e- + e- + e+

In this case two electrons have collided and been transformed into three electrons and a positron!!! This reaction can only occur if the original two electrons have great energy which can be transformed into mass and create the new electron and positron. Note that charge is conserved as the total charge is -2 on both sides. This reaction has been observed in particle acceleratiors.
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Chubb
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#12
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Don't bring negative mass into things please - we have enough trouble on our hands trying to find the missing mass though rumor has it that it may be in the foam that you find in parcels...
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Jonatan
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#13
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(Original post by Willla2)
single protons do not decay into neutrons - you can leave it however long you want and it will not decay into a neutron without energy input.

Protons are actually predicted to decay into a neutral pion and a positron by current theory, because these two objects are "more stable" than a proton. The problem we have is that we've never seen a proton do this, even though theory says it CAN do this. But of course, just because it can, doesn't mean it will, but the theory predicts this reaction to proceed at a half life of 10^33 years AT MOST, if you really try to fudge the results. The age of the universe is actually 10^32 years, so this is indeed a very slow reaction. The problem we have is that even when we take enough individual protons and watch them for long enough, theory predicted that at least one should decay within a year, and yet nothing happened. So physicists are a bit puzzled by why a proton refuses to decay when our theory says it can.
I wonder what Rutherford would have said had he seen the stamp collecting we are doing in particle physics...
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Poc ar buile
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#14
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(Original post by Jonatan)
A much more perplexing reaction is the following:

e- + e- -> e- + e- + e- + e+

In this case two electrons have collided and been transformed into three electrons and a positron!!! This reaction can only occur if the original two electrons have great energy which can be transformed into mass and create the new electron and positron. Note that charge is conserved as the total charge is -2 on both sides. This reaction has been observed in particle acceleratiors.
Would this e-/e+ pair not disappear in a flash of gamma radiation pretty quickly? Sounds a bit too much like virtual pair producation to me...
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Jonatan
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(Original post by polthegael)
Would this e-/e+ pair not disappear in a flash of gamma radiation pretty quickly? Sounds a bit too much like virtual pair producation to me...
Virtual particle pairs are created due to the uncertainty in energy during a short time interval. This particle pair is created due to the large ammount of energy in the collision (something completely different). The positron is indeed likely to collide with an electron sooner or later and be anihilated, but unlike virtual particle pairs it is not necessarily the same electron as was created pairwise with it. Thus you could detect this positron directly, unlike positrons created due to teh uncertainty principle.
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