Why don't ups and downs annihilate in nucleons?

  • Thread starter El Hombre Invisible
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In summary: They are particles-antiparticles, and the principle says that no two particles with the same mass and charge can occupy the same space. However, the principle does allow one particle to "occupy" the same space as its antiparticle, because the energies of the two are equivalent. So the principle does allow the up and down quarks to exist in the same space.
  • #1
El Hombre Invisible
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Answer me that! Because they should, shouldn't they? Strong attractive forces, attractive EM forces... you'd think nothing would separate them.
 
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  • #2
Up and down quarks are not each others' anti-particles. Up and anti-up, and down and anti-down, are anti-particles.

- Warren
 
  • #3
I think your question would have been much more interesting,if u had asked it referring to the [itex] \pi^{0} [/itex] meson...

Daniel.
 
  • #4
Hi Warren! Quark-antiquark pairs do annihilate, but what keeps an up from a down (annihilate was a bad word) in a nucleon. They have not one (strong) but TWO (EM) attractive forces between them and yet... never the twain shall meet. What's going on guys?
 
  • #5
Dexter's suggestion of studying the pi-zero is a very good one. Perhaps I'm wrong, but isn't one of the decay modes of the pi-zero annhiliation?

- Warren
 
  • #6
According to [1],there are a lotta decay modes for [itex]\pi^{0}[/itex],but the most probable by far is

[tex] \pi^{0}\rightarrow 2\gamma [/tex] [itex] (98.798\pm 0.032) [/itex]%

,but other decay modes include

[tex] \pi^{0}\rightarrow e^{+}+e^{-}+\gamma [/tex]

[tex] \pi^{0}\rightarrow \gamma+\mbox{positronium} [/tex]

[tex] \pi^{0}\rightarrow e^{+}+e^{+}+e^{-}+e^{-} [/tex]

and so on...So it's not an annihilation,but a decay...:wink:

Daniel.

----------------------------------------------------------
[1]PDG,"Particle Physics Booklet",July 2004,page 28.
 
  • #7
Jesus. Physicists have the attention span of a- Oh, there's a girl joining the forum, look! I jest... tell me about the meson pie, but if that doesn't answer it and anyone does have any nucleon-based thoughts, chuck them my way too. Does it have anything to do with quantum states? I hate those...
 
  • #8
dextercioby said:
[tex] \pi^{0}\rightarrow 2\gamma [/tex] [itex] (98.798\pm 0.032) [/itex]%
Okay, so the pi-zero does indeed annhiliate, and nearly all the time.

So, on to El Hombre's real question: what keeps quarks apart in the nucleon? The Pauli exclusion principle, I presume.

- Warren
 
  • #9
El Hombre Invisible said:
Hi Warren! Quark-antiquark pairs do annihilate, but what keeps an up from a down (annihilate was a bad word) in a nucleon. They have not one (strong) but TWO (EM) attractive forces between them and yet... never the twain shall meet. What's going on guys?

The strong force becomes very weak at short distances.

Charged pions can decay weakly, annihilating to a W which becomes an electron or muon + corresponding anti-neutrino.

No mechanism for a u and anti-d pair (or charge conjugate) to decay via EM.

Keep in the mind that baryon number needs to be conserved.
 
  • #10
So the Pauli (wasn't he in Rocky?) exclusion principal allows a particle-antiparticle pair to occupy the same space, but not two particles? I mean... is nature really going: "Right, I need to get this up and this down in the same place, so- argh no, I can't. It's the Pauli exclusion principal!" Is there some physical reason backing this up or is just a convenient fudge?
 
  • #11
Burt Young (from Rocky) indeed fomulated the Pauli Exclusion Principle. :smile:

What goes on inside a hadron is pretty complex, and I don't think there is any physicist who has a real good understanding of what's going on.

But anyhow - just because you have attractive charges does not mean that things collapse into each other. Look at the hydrogen atom (proton + electron). You can have bound states due to quantum mechanics.
 
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  • #13
I'll... come back when I've learned a thing or two about Schroedinger. About 6 years I think.
 
  • #14
A really key subject is group theory...Essential for particle physics.

Daniel.
 
  • #15
[itex]\pi^{0}[/itex],but the most probable by far is
[tex] \pi^{0}\rightarrow 2\gamma [/tex] [itex] (98.798\pm 0.032) [/itex]%
[tex] \pi^{0}\rightarrow e^{+}+e^{-}+\gamma [/tex]
[tex] \pi^{0}\rightarrow \gamma+\mbox{positronium} [/tex]
[tex] \pi^{0}\rightarrow e^{+}+e^{+}+e^{-}+e^{-} [/tex]

these can all be taken as annihilation, because imagine u and [tex]\bar{u}[/tex] collide, what will be produced? we can work out the ratio, and find the
IR-safe cross section ratio is nearly the same value.
 
  • #16
Well, my next question was going to be: why should the Pauli exclusion principal apply (as opposed to why do we need it to apply)? Why is it that an electron and positron, or quark and corresponding antiquark, or two photons can meet but not an up and a down, or an electron and a nucleus? In the former, the energies of each particle are equal and their charges add to zero, while in the latter either the masses or charges or both are different. If two particles of equal energy and opposite but unequal charge were combined, charge would not be conserved. Does this make a difference?
 
  • #17
El Hombre,

the main thing is all energy eigenstates (and since *all* states are given by linear combinations of these eigenstates, we can get *all* the possible states) are given by the solution of the time-independent Schrodinger equation. The solution of this equation will show that even the lowest energy state corresponds to both particles existing without them "hitting" each other.
 
  • #18
Actually the Pauli exclusion principle (more general,the symmetrization principle) cannot apply,because the particles involved in the nucleon are not identical...

Daniel.
 
  • #19
Good point, Daniel. The exclusion principal cannot stop an up and a down cohabiting a point in spacetime. But it can prohibit two ups. Why? Physically, I mean. Has anyone come up with a physical theory for the exclusion principal.

Hi masudr - like I said... I need to learn a thing or two about Schroedinger. But again... why? Some guy with crazy hair cannot be the reason why matter does not collapse in on itself. I mean, if you placed a proton near to an electron with both at rest, because they are free particles they would (would they not) form a neutron. What is it about atomic electrons that overcomes the EM attraction? Where does a quantum state come from?
 
  • #20
El Hombre Invisible said:
Good point, Daniel. The exclusion principal cannot stop an up and a down cohabiting a point in spacetime. But it can prohibit two ups. Why? Physically, I mean. Has anyone come up with a physical theory for the exclusion principal.

Nope,for now,there's no theory whose consequences would include the 6 axioms of nonrelativistic QM,symmetrization principle (sic!) included...

El Hombre Invisible said:
Hi masudr - like I said... I need to learn a thing or two about Schroedinger. But again... why? Some guy with crazy hair cannot be the reason why matter does not collapse in on itself. I mean, if you placed a proton near to an electron with both at rest, because they are free particles they would (would they not) form a neutron.

Nope,most certainly not.Depending on the energy of the incoming electron,the scattering of an electron off a proton can be very different;at high energies,the electron would penetrate the proton & would scatter off the 3 quarks.

El Hombre Invisible said:
What is it about atomic electrons that overcomes the EM attraction? Where does a quantum state come from?

The principles of quantum mechanics give the answer to why the hydrogen atom doesn't collapse under coulombian attraction between the electron & the proton.

Daniel.

P.S.Are u a girl?(reference to an earlier post)
 
  • #21
El Hombre Invisible said:
Why is it that an electron and positron, or quark and corresponding antiquark, or two photons can meet but not an up and a down, or an electron and a nucleus?

Let's consider the possibilities. If the process takes place at all, it has to take place via the electromagnetic, strong, or weak interaction.

It can't be the elctromagnetic interaction because the up and down quarks, or the electron and proton, aren't a particle-antiparticle pair.

It can't be the strong interaction, because gluon exchange changes only the color (red/green/blue) of a quark, not its flavor (up/down/charm/strange/top/bottom). So for an "annihilation" into a gluon to take place, you'd have to have (I think) a quark and antiquark of different colors.

It can't be a weak-interaction annihilation into a [itex]Z^0[/itex], because that requires a particle-antiparticle pair like with the electromagnetic interaction.

It can't be a weak "annihilation" into a [itex]W[/itex] because that requires a combination like an up and an anti-down, or an electron and an anti-electron-neutrino (note the particle and antiparticle are different types, and the charge adds up to +1 or -1).

These combinations come from the rules for constructing Feynman diagrams for the various interactions, which in turn come from the fundamental assumptions about how the interactions work, that is, what kind of gauge invariance produces them.
 
  • #22
dextercioby said:
Nope,most certainly not.Depending on the energy of the incoming electron,the scattering of an electron off a proton can be very different;at high energies,the electron would penetrate the proton & would scatter off the 3 quarks.
So neutron decay is not a reversible process? I'm talking a direct hit here... one dimensional kinematics.
dextercioby said:
The principles of quantum mechanics give the answer to why the hydrogen atom doesn't collapse under coulombian attraction between the electron & the proton.
So what you're saying here is that Schroedinger must be correct because Schroedinger said so? I have no problem when somebody says something DOESN'T happen in ACCORDANCE with quantum theory, but I have trouble when somebody says something CAN'T happen BECAUSE of quantum theory.
dextercioby said:
P.S.Are u a girl?(reference to an earlier post)
To which earlier quote are you referring, miss?
 
  • #23
jtbell said:
It can't be the elctromagnetic interaction because the up and down quarks, or the electron and proton, aren't a particle-antiparticle pair.
Eh? Since when are EM interactions restricted to particle-antiparticle pairs?
 
  • #24
El Hombre Invisible said:
So neutron decay is not a reversible process? I'm talking a direct hit here... one dimensional kinematics.

Yes,neutron decay is not reversible..."A direct hit" is called scattering (the quantum term for 'collision').Mott,Coulomb,Rutherford,inelastic,all are scatterings...

El Hombre Invisible said:
So what you're saying here is that Schroedinger must be correct because Schroedinger said so?I have no problem when somebody says something DOESN'T happen in ACCORDANCE with quantum theory, but I have trouble when somebody says something CAN'T happen BECAUSE of quantum theory.

To me the two ideas are synonimous... :tongue2:

El Hombre Invisible said:
To which earlier quote are you referring, miss?

I take it that you're not a girl... :tongue2:

Daniel.
 
  • #25
El Hombre Invisible said:
Eh? Since when are EM interactions restricted to particle-antiparticle pairs?

Of course not.Incidentally,the [itex] \pi^{0} [/itex] 2 photon decay mode is an electromagnetic process...Neutron decay is weak.

Daniel.
 
  • #26
I'm considering first-order Feynman diagrams with a single vertex here. When most people talk about an "annihilation" via the EM interaction, they're talking about a particle and antiparticle producing a (virtual) photon.
 
  • #27
If you're talking about pair annihilation in QED,both the 2 incoming particles (lepton-antilepton) and the 2 outgoing photons are very real...

It may be true that the single vertex in QED can be interpreted as an incoming lepton & an incoming antilepton annihilating into a virtual photon,but i don't recall any process in which this could be really happening...

Daniel.
 
  • #28
El Hombre Invisible said:
So what you're saying here is that Schroedinger must be correct because Schroedinger said so? I have no problem when somebody says something DOESN'T happen in ACCORDANCE with quantum theory, but I have trouble when somebody says something CAN'T happen BECAUSE of quantum theory.

All theories that we learn in college and grad school, such as quantum mechanics, relativity, and quantum electrodynamics, are correct in the sense that they yield more accurate experimental predictions than other theories.

Have you taken ANY physics at all? Have you learned about conservation of momentum, for example? That principle is used to make theoretical predictions of experimental phenomena. Do you have a problem with me saying something is incorrect because it violates conservation of momentum?
 
  • #29
Only free positrons annihilate with real electrons

El Hombre Invisible said:
Eh? Since when are EM interactions restricted to particle-antiparticle pairs?

Hi Hombre,
The annihilation of an anti-proton impinging on a real hydride target is a multiple step process that first produces three pair of +/- pions each of which loses mass that is converted to kinetic energy and which decays to three pair of +/- muons; these also lose mass that is converted to kinetic energy and these ultimately disintegrate into a total of three positrons all of which annihilate, thus producing 6 photons. Note that the last step is the only one where annihilation takes place.
There is a fourth delayed pair of +/- pions that occur because of the kinetic energy of the impinging anti-proton coupled with the electrostatic attraction between these interacting +/- nucleons. See it all in a modern Physics textbook. Cheers, Jim
 
  • #30
juvenal said:
All theories that we learn in college and grad school, such as quantum mechanics, relativity, and quantum electrodynamics, are correct in the sense that they yield more accurate experimental predictions than other theories.

Have you taken ANY physics at all? Have you learned about conservation of momentum, for example? That principle is used to make theoretical predictions of experimental phenomena. Do you have a problem with me saying something is incorrect because it violates conservation of momentum?
Easy, tiger! What I meant was that when an event occurs, its cause is not page 176 of some textbook. You said it yourself: "That principle is used to make theoretical predictions of experimental phenomena." Damn right. The principal is used to make a predication. When we make the prediction that such and such a particle will have such and such a velocity after an event, we can be so sure because it is in accordance with the conservation of momentum. But us knowing about the conservation of momentum ain't the damn reason why momentum is conserved. Likewise, the Pauli exclusion principal doesn't hold just because Pauli discovered it, and certain quantum states aren't allowable simply because Shroedinger says so.

Your counter-example suggests my point was not well made. No - I do not have a problem with you saying something is incorrect because it violates conservation of momentum. BUT nor do I have a problem with you saying something is incorrect because all of the quantum states have already been filled, or such a one isn't allowable. That falls into something working in accordance with quantum theory. But when I ask WHY momentum is conserved, the answer is not the conservation of momentum.

I am a physics undergrad. Isn't that enough for you? Now I feel so inferior. :cry:
 
  • #31
Also, note I said "but I have trouble when somebody says ...", not "I have a problem with people who say...". As in: I personally can't accept that and process it and be satisfied. Not as in: I'm going to pick a fight with ANYONE who DARES say that to me! Okay? Now have another cup of coffee.
 
  • #32
El Hombre Invisible said:
But when I ask WHY momentum is conserved, the answer is not the conservation of momentum.

Umm - yes it is. You could talk about the laws of physics being translationally invariant in space (if you've studied Noether's theorem), but then you could ask "Why are the laws of physics translationally invariant in space?" The answer - because they are.

Why do the laws of quantum mechanics hold? Because they do. I don't think any real physicist can tell you that the world HAS to work this way. It just does.
 
  • #33
The answer 'because it is' is not one I accept. Progress is made because people DON'T just settle for 'because it is'. Why does metal conduct electricity? Because it does? Or because the outer electrons form a sea of particles moving from one atom to another carrying current? Why does matter exist? Because it does? Or because photons can cause pair creation? Why is the speed of light constant in any frame of reference? Because it is? Or because time progresses differently for observers moving at different speeds? If all scientists lumped for 'because it is' then we'd still be living on a flat earth. If you have a problem with me asking 'why', seriously mate it's just tough. Get the flip over it! This is, after all, a physics forum open to anyone to ask questions. Do you go from thread to thread just saying 'because it is'?
 
  • #34
If you know so many answers to difficult questions,why can't u answer the question:why is free space-time homogenous and isotrope...?

Daniel.
 
  • #35
dextercioby said:
If you know so many answers to difficult questions,why can't u answer the question:why is free space-time homogenous and isotrope...?

Daniel.
Eh? Are you talking to me? Why are you beating up on me? I never said I knew jack. If I knew so much, why would I be here asking questions? My argument is to juvenile's insistence that I should just accept things the way they are and not ask questions about their meaning is that in the history of our race scientific progress has been made by people not accepting things the way they are and asking why they occur. Is this really such a controversial idea that it warrants instant hostility?
 

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