Why don't ups and downs annihilate in nucleons?

1. May 4, 2005

El Hombre Invisible

Answer me that! Because they should, shouldn't they? Strong attractive forces, attractive EM forces... you'd think nothing would separate them.

2. May 4, 2005

chroot

Staff Emeritus
Up and down quarks are not each others' anti-particles. Up and anti-up, and down and anti-down, are anti-particles.

- Warren

3. May 4, 2005

dextercioby

I think your question would have been much more interesting,if u had asked it refering to the $\pi^{0}$ meson...

Daniel.

4. May 4, 2005

El Hombre Invisible

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. May 4, 2005

chroot

Staff Emeritus
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. May 4, 2005

dextercioby

According to [1],there are a lotta decay modes for $\pi^{0}$,but the most probable by far is

$$\pi^{0}\rightarrow 2\gamma$$ $(98.798\pm 0.032)$%

,but other decay modes include

$$\pi^{0}\rightarrow e^{+}+e^{-}+\gamma$$

$$\pi^{0}\rightarrow \gamma+\mbox{positronium}$$

$$\pi^{0}\rightarrow e^{+}+e^{+}+e^{-}+e^{-}$$

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

Daniel.

----------------------------------------------------------
[1]PDG,"Particle Physics Booklet",July 2004,page 28.

7. May 4, 2005

El Hombre Invisible

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. May 4, 2005

chroot

Staff Emeritus
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. May 4, 2005

juvenal

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. May 4, 2005

El Hombre Invisible

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. May 4, 2005

juvenal

Burt Young (from Rocky) indeed fomulated the Pauli Exclusion Principle.

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.

Last edited: May 4, 2005
12. May 4, 2005

dextercioby

13. May 4, 2005

El Hombre Invisible

I'll... come back when I've learned a thing or two about Schroedinger. About 6 years I think.

14. May 4, 2005

dextercioby

A really key subject is group theory...Essential for particle physics.

Daniel.

15. May 4, 2005

wangyi

$\pi^{0}$,but the most probable by far is
$$\pi^{0}\rightarrow 2\gamma$$ $(98.798\pm 0.032)$%
$$\pi^{0}\rightarrow e^{+}+e^{-}+\gamma$$
$$\pi^{0}\rightarrow \gamma+\mbox{positronium}$$
$$\pi^{0}\rightarrow e^{+}+e^{+}+e^{-}+e^{-}$$

these can all be taken as annihilation, because imagine u and $$\bar{u}$$ 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. May 5, 2005

El Hombre Invisible

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. May 5, 2005

masudr

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. May 5, 2005

dextercioby

Actually the Pauli exclusion principle (more general,the symmetrization principle) cannot apply,because the particles involved in the nucleon are not identical...

Daniel.

19. May 5, 2005

El Hombre Invisible

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. May 5, 2005

dextercioby

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

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.

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)