Antiprotons

[Proteus]

Since there is no reaction between antiprotons and electrons, could it be said that antiprotons would remain stable as long as they were kept away from cosmic rays and hydrogen?

 

Antiprotons and electrons have the same charge, so they'd repell one another, and a reaction between antiprotons and protons would be about as unlikely as interaction between electrons and protons, right?

Edited December 22, 2009 by Proteus

[Mr Skeptic]

I'm pretty sure that any atom of matter will annihilate with an antiproton. The antiproton would be attracted to the nucleus and there's always a proton or more at the nucleus.

[ajb]

You seem to be contradicting yourself here.

 

Antiprotons are charged (-e) just as electrons are. They do interact electromagnetically as you have stated "they repel one another".

 

To what extent antimatter is different to matter is still quite open. The proton is stable acording to the standard model as baryon number is conserved. The CPT theorem then states that the life-time of the antiproton is equal to that of the proton. In the context of the standard model this is infinite.

 

Any method of proton/antiproton decay requires beyond the standard model physics. This is why it is so important to study experimentally the possibility of such decays.

 

Any difference between the proton and antiproton would signify a violation of the CPT theoem. Again, this is partly why the properties of antimatter must be studed carfeully.

[Proteus]

Typo. For interaction read reaction. With "stable," I meant they wouldn't react. Do electrons form a reliable barrier between the protons or antiprotons?

[ajb]

You can overcome any barrier by supplying the particles enough energy. This does not have to be enough to classically overcome the barrier, it could be significantly less due to quantum tunneling.

[Bob_for_short]

...Do electrons form a reliable barrier between the protons or antiprotons?

No, they do not. Take, for example, the Hydrogen atom H0. It can still attract an extra electron and form a negative ion H-. One electron in it is nearly in the same orbit as in H0, another one is much farther from the proton. If you replace one electron with antiproton, I am sure it will take the lowest orbit. Due to its huge mass, the antiproton in such a system will be much closer to proton and will annihilate quickly.

 

It seems to me that the presence of electron in such a system only helps annihilation (a third body to carry away some energy-momentum while p-anti-p recombination). This is my feeling. I do not know anything more concrete about it.

[Proteus]

What if the antiprotons have low momentum? Or is any level of momentum that is obtainably low still be too high?

 

H0? Is that hydroxyl radical you're talking about?

 

I realized hydrogen would be vulnerable, as would be the protons from cosmic rays, but what if the antiprotons were kept away from them? More specifically, could antiprotons be stored, in another way than through electrostatic repulsion in vacuum?

[swansont]

Since there is no reaction between antiprotons and electrons, could it be said that antiprotons would remain stable as long as they were kept away from cosmic rays and hydrogen?

 

Antiprotons and electrons have the same charge, so they'd repell one another, and a reaction between antiprotons and protons would be about as unlikely as interaction between electrons and protons, right?

 

Electrons and antiprotons interact electromagnetically. In fact scattering reactions between the two are one way antiprotons can be slowed down.

[Proteus]

Yes, but as I said, I did mean react, not interact. With react, I mean "interact and undergo a physical change."

 

Or did you mean that while the electrons would slow the protons down, they would not cast them back but on the contrary ensure that they wouldn't miss the much smaller nucleus?

[Bob_for_short]

What if the antiprotons have low momentum? Or is any level of momentum that is obtainably low still be too high?

As any fast charged particle, the antiproton is decelerated in matter to low momenta, mostly by giving away its energy to electrons due to electrostatic interaction.

H0? Is that hydroxyl radical you're talking about?

H0 is a hydrogen neutral atom (p+e). H- is a negative ion (p+2e).

I realized hydrogen would be vulnerable, as would be the protons from cosmic rays, but what if the antiprotons were kept away from them? More specifically, could antiprotons be stored, in another way than through electrostatic repulsion in vacuum?

As any long-range interacting charge the antiproton is isolated from matter with help of long-range electromagnetic forces. I do not know much about charged particle traps though to tell you more.

Edited December 23, 2009 by Bob_for_short

[Proteus]

As any fast charged particle, the antiproton is decelerated in matter to low momenta, mostly by giving away its energy to electrons due to electostatic interaction.

 

You mean, if the antiproton were part of an antimatter atom? I meant, is it possible for free antiprotons to have sufficiently low momentum that they will not react with surrounding atomic protons, except for those in hydrogen? In particular, could they be of sufficiently low momentum that they will not react with protons in metals?

 

H0 is a hydrogen neutral atom (p+e). H- is a negative ion (p+2e).

 

Thanks for clarifying.

 

As any long-ragne interacting charge the antiproton is isolated from matter with help of long-range electromagnetic forces. I do not know much about charged particle traps though to tell you more.

 

So the range of the charge of an electron in noncharged matter is too close to stop the antiproton?

 

Thanks to you all for all the information, by the way!

[Bob_for_short]

You mean, if the antiproton were part of an antimatter atom? I meant, is it possible for free antiprotons to have sufficiently low momentum that they will not react with surrounding atomic protons, except for those in hydrogen? In particular, could they be of sufficiently low momentum that they will not react with protons in metals?

No. The lower momentum, the higher the probability "to get captured" by any atom. Even a neutral atom polarizes and generally attracts a negative charge (electron or antiproton, whatever). Another thing is creating a bound state which is called a negative ion. But the antiproton, being heavy, gets to the lowest orbit and interacts with protons in atomic nucleus - annihilates. In this respect, it is quite different from an electron in atom - there is no Pauli excluding principle for it (no additional "repulsion", for short) and it replaces the electrons easily in atoms if its momentum is low.

 

So the range of the charge of an electron in noncharged matter is too close to stop the antiproton?

The negative and positive charge fields in atoms compensate each other outside of atoms. In this sense, an atom has a short-range interaction potential. In general, we see atoms as neutral.

When a charged projectile moves through matter, it mostly "pushes aside" the light electrons. As a result, atoms get excited and thus the projectile energy is lost progressively. At the end of the trajectory a charged projectile stops and gets bound (sticks) to atoms with forming some ion.

 

The antiproton will penetrate closer to the atomic nucleus due to mutual attraction and this is how it annihilates with a proton in the nucleus. Any atom is "dangerous" for a slow antiproton.

Edited December 23, 2009 by Bob_for_short

[Proteus]

That's very enlightening. Thank you very much.