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Some quick particle/nuclear physics questions

Probably lazy, but I figured this might be faster than googling them:

Why are protons stable and neutrons not? What factors broadly affect stability of hadrons?

How does pion exchange work between nucleons? What causes the nucleon to give off a pion (interpret the word 'cause' loosely).

Edit: to clarify this question the specific things I'm having trouble finding an explanation for are: how does the strong force (which acts on colour) cause an attraction between colour-neutral nucleons, by the exchange of colour neutral pions? Also, it would be really nice if someone could present a step by step version of what happens, although if that's impossible then it would be good to know that as well.

Why do pretty much all nuclides have more neutrons than protons?

Edit: I can't find a good answer for this online. My guess is that more neutrons bring an extra bit of stability to a nuclide by increasing the separation between positively charged protons. But I can't find anything to back that up, or suggest otherwise. Simple confirmation/negation would be nice.

Thanks, would be interesting to know a bit more about this topic.
(edited 9 years ago)
Original post by lerjj
Probably lazy, but I figured this might be faster than googling them:

Why are protons stable and neutrons not? What factors broadly affect stability of hadrons?

How does pion exchange work between nucleons? What causes the nucleon to give off a pion (interpret the word 'cause' loosely).

Why do pretty much all nuclides have more neutrons than protons?

Thanks, would be interesting to know a bit more about this topic.


Don't be lazy lerjj, the truth is out there. Put some effort in man! :tongue:
Reply 2
Avatar for Phichi
Phichi
11
Original post by lerjj
..


Question 1

Question 2

Question 3

I got your back man.


Original post by uberteknik
Don't be lazy lerjj, the truth is out there. Put some effort in man! :tongue:


The effort is now minimal, lerjj will make it through this.
Original post by Phichi




The effort is now minimal, lerjj will make it through this.


I love LMGTFY !
Reply 4
Avatar for lerjj
lerjj
OP
13
Original post by Phichi
Question 1

Question 2

Question 3

I got your back man.




The effort is now minimal, lerjj will make it through this.


Wow. I deserved that...

Is there any chance you actually checked what results those searches gave? Because they're not very helpful BTW. For instance, the first search result comes up primarily with details about why nuclides are stable, not why nucleons are/ are not. And their explanation of why they are stable seems to rest mainly on "it's magic!" (not really, but close enough).
Original post by lerjj
Why are protons stable and neutrons not? What factors broadly affect stability of hadrons?

Put simply: you know how the universe, as a whole, favours the lowest-energy possible states? Hence why electrons fill from the centre up, why things fall down to Earth etc. This is entropy encapsulated. Essentially, low energy = stable. This is logical because, to go from low to high energy, work must be done. Inversely, then, systems will spontaneously tunnel from high to low energy until it becomes stable.

Now, protons are stable because baryon number is a conserved quantity, and protons are the baryons with the least mass-energy. Neutrons, however, have slightly greater rest mass than protons, so, after an indeterminate amount of time, neutrons will decay into protons. Protons can't decay because they are already the lowest-energy baryon which exists.

What factor broadly affects stability of hadrons? Generally, in terms of stability and decay, it's all about the energy of a system. Systems at high energy decay into lower-energy states. Equivalently, particles of high mass will decay into particles of lower mass. This is partly why W bosons, for instance, have such incredibly short lifespans: they're incredibly massive. To understand more about this particular scenario, the uncertainty principle actually comes into play by considering energy and time as conjugate variables.
(edited 9 years ago)
Reply 6
Avatar for lerjj
lerjj
OP
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Original post by sjgriffiths
Put simply: you know how the universe, as a whole, favours the lowest-energy possible states? Hence why electrons fill from the centre up, why things fall down to Earth etc. This is entropy encapsulated. Essentially, low energy = stable. This is logical because, to go from low to high energy, work must be done. Inversely, then, systems will spontaneously tunnel from high to low energy until it becomes stable.

Now, protons are stable because baryon number is a conserved quantity, and protons are the baryons with the least mass-energy. Neutrons, however, have slightly greater rest mass than protons, so, after an indeterminate amount of time, neutrons will decay into protons. Protons can't decay because they are already the lowest-energy baryon which exists.

What factor broadly affects stability of hadrons? Generally, in terms of stability and decay, it's all about the energy of a system. Systems at high energy decay into lower-energy states. Equivalently, particles of high mass will decay into particles of lower mass. This is partly why W bosons, for instance, have such incredibly short lifespans: they're incredibly massive. To understand more about this particular scenario, the uncertainty principle actually comes into play by considering energy and time as conjugate variables.


Is there a reason for the mass difference, or is this simply because down quarks are more massive than up quarks and so neutrons just ARE heavier? (I.e. is this the only/most significant factor at play? Or is uud somehow more stable because it forms stronger bonds than udd, or some other explanation.)

Thanks for the help!
Original post by lerjj
Is there a reason for the mass difference, or is this simply because down quarks are more massive than up quarks and so neutrons just ARE heavier? (I.e. is this the only/most significant factor at play? Or is uud somehow more stable because it forms stronger bonds than udd, or some other explanation.)

Thanks for the help!

Yes, down quarks are the lightest of all quarks, whilst up quarks come a close second. So just by their composition, neutrons are slightly more massive than protons.

I'm not aware of the strong interaction itself causing any differences in the stability of either hadrons or nuclei. It pretty much all comes down to the mass-energy of the constituents.

No problem! :smile:

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