Pair creation and annihilation

In summary, the Casimir effect is a phenomena that occurs when the energy of an electric field is greater than the combined mass of two particles. This effect causes the two particles to repel each other.
  • #1
touqra
287
0
How did they experimentally verify that there is pair creation and annihilation in the vacuum? What kind of particles usually pops in and out of the vacuum?
 
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  • #3
A strong electric field has to create actual observable pairs of charged particles out of otherwise empty space but that is not experimentally verified yet. The lasers we have are still not powerful enough.
 
  • #4
smallphi said:
A strong electric field has to create actual observable pairs of charged particles out of otherwise empty space but that is not experimentally verified yet. The lasers we have are still not powerful enough.

How does a sufficient electric field be able to turn virtual particles from vacuum into real observable particles ?
So, this means that up till today, no one have directly see or verify that there is pair creation and annihilation ?

ahrkron said:
The wikipedia article is a nice starting point for this:
http://en.wikipedia.org/wiki/Casimir_effect

There is also a beautiful picture related to it in
http://antwrp.gsfc.nasa.gov/apod/ap061217.html

From the wiki page, my understanding is that the Casimir effect, through quantum field theory is due to virtual particles being created from vacuum, leading to repulsion or attraction of objects in the submicron scale.
But these virtual particles is just one interpretation of quantum field theory, much like Schroedinger wavefunction mechanics and Heisenberg matrix mechanics.
Have we observe these particles for real yet?
 
  • #5
Quote from V. F. Mukhanov and S. Winitzki "Introduction to Quantum Fields
in Classical Backgrounds", available for free (still) at

http://www.theorie.physik.uni-muenchen.de/~serge/T6/book.pdf [Broken]



A static electric field in empty space can create electron-positron (e+e−) pairs. This
effect, called the Schwinger effect, is currently on the verge of being experimentally
verified.

To understand the Schwinger effect qualitatively, we may imagine a virtual e+e−
pair in a constant electric field of strength E. If the particles move apart from each
other to a distance l, they will receive the energy leE from the electric field. If this
energy exceeds the rest mass of the two particles, leE ≥ 2m_e, the pair will become
real and the particles will continue to move apart. The typical separation of the virtual
pair is of order of the Compton wavelength 2π/m_e. More precisely, the probability of
separation by a distance l turns out to be P ~ exp (−π m_e l). Therefore the probability
of creating an e+e− pair is

P ~ exp(−m_e^2 /eE)

The exact formula for the probability P can be obtained from a full (but rather lengthy)
consideration using quantum electrodynamics.
 
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  • #6
touqra said:
How did they experimentally verify that there is pair creation and annihilation in the vacuum? What kind of particles usually pops in and out of the vacuum?

Particles which can be directly detected are usually called "real". These particles are on-shell. These are the ones you see the effect in your bubble chamber.
On the other hand, quantum field theory allows the creation/annihilation of virtual particles from vacuum. But, as they are not on-shell they do not live long enough to be observed directly. So you observe their effect only indirectly (like forces).

Now, to my point of view, the transition between "virtual" and "real" particles is not really clear in the case of very small energy fluctuations. Is a particle we observe really on-shell I think we cannot proove ? So I would say the only thing we can tell is that we have particles in our models.
 
  • #7
Hawking radiation emited by black holes is also a manifestation of pair-annihilation creation just near the horizon, one particle being trapped by the black hole and the other remaining in the Universe. It is seen then a particle created by the black hole.

This and other outreach articles about particle physics (in Romanian) can be found at our portal: http://fizicaparticulelor.ro [Broken].

Clubul Fizica Particulelor-portal de popularizare a fizicii particulelor: http://fizicaparticulelor.ro [Broken]
 
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  • #8
touqra said:
How did they experimentally verify that there is pair creation and annihilation in the vacuum?
They don't. At least not yet.
 
  • #9
smallphi said:
A strong electric field has to create actual observable pairs of charged particles out of otherwise empty space but that is not experimentally verified yet. The lasers we have are still not powerful enough.
Just to clarify... Pair creation by an intense EM field has indeed been
understood and experimentally observed for decades in relativistic heavy-ion
collisions. The setup is that 2 heavy nuclei collide, temporarily creating a state
whose Coulomb field is so immense that it can produce (on-shell) pairs. I.e: the
energy density of the field is greater than the combined masses of the
electron & positron. This is explained in more detailed in some of Greiner's
textbooks, e.g: "QED of Strong Fields".

Regarding lasers, etc, I vaguely recall an announcement claiming such
(on-shell) pair-production from free photons had been done recently
(at SLAC?) but I don't have a reference, sorry.
 
  • #10
strangerep said:
Just to clarify... Pair creation by an intense EM field has indeed been
understood and experimentally observed for decades in relativistic heavy-ion
collisions. The setup is that 2 heavy nuclei collide, temporarily creating a state
whose Coulomb field is so immense that it can produce (on-shell) pairs. I.e: the
energy density of the field is greater than the combined masses of the
electron & positron. This is explained in more detailed in some of Greiner's
textbooks, e.g: "QED of Strong Fields".
I don't think that this can be taken as an experimental proof of the Schwinger effect, in which a static electric field should produce pairs.
 
  • #11
Demystifier said:
I don't think [pair-production from intense
Coulomb field of large-Z nuclei in heavy-ion collisions] can be taken as an experimental proof of the Schwinger effect, in which a static electric field should produce pairs.
I'm surprised to hear such a view, since the results of these experiments
are quite extensive and detailed. They've been in the mainstream for quite
a while.

In the Schwinger effect, an implied consequence is that a sufficiently
strong field could not persist very long - because pair production would
carry away energy-momentum. So creating a nucleus with Z >= 139
and observing the results seems like a reasonable way to investigate
the phenomenon.
 
  • #12
strangerep said:
In the Schwinger effect, an implied consequence is that a sufficiently
strong field could not persist very long - because pair production would
carry away energy-momentum. So creating a nucleus with Z >= 139
and observing the results seems like a reasonable way to investigate
the phenomenon.


Hi strangerep,

I am not familiar with this experiment and its analysis. However, it seems logical to me that when we collide two heavy nuclei with suffient energy, there could be a number of different channels for producing electron-positron pairs. (For example, such pairs can be produced simply in collisions of two electrons if the center-of-mass energy is high enough.) Was it possible to separate all these channels and say exactly which portion of electron-positron pairs was produced by the strong field, as opposed to any other reason?

Thanks.
Eugene.
 
  • #13
Lemme ask this: Isn't there a more common example of this phenomenom?

When a gamma ray photon creates a positron-electron pair, isn't this equivalent to a virtual e+ - e- pair being torn apart and made real by the energy in the photon's electromagnetic field?

(I understand that some posters have specified a *static* field, but others mentioned lasers. This is addressed to the latter.)
 
  • #14
MaWM said:
When a gamma ray photon creates a positron-electron pair, isn't this equivalent to a virtual e+ - e- pair being torn apart and made real by the energy in the photon's electromagnetic field?

The initial question was "How did they experimentally verify that there is pair creation and annihilation in the vacuum?"
 
  • #15
Barmecides said:
The initial question was "How did they experimentally verify that there is pair creation and annihilation in the vacuum?"

But that question, technically, has been answered in MaWM response.

If you shoot a gamma photon into a crystal, and the gamma photon disappears, an electron-positron pair comes out, and the crystal remains the same, have we then shown an experimental verification of the question? I would say it has, and the physics that describes this process would also confirm that.

And in case anyone doubts that this has been done, it has. This process is what will used to generate the positrons for the proposed International Linear Collider (ILC). When you are intending to spend $8 billion, you can't use untested methods.

Zz.
 
  • #16
ZapperZ said:
But that question, technically, has been answered in MaWM response.

If you shoot a gamma photon into a crystal, and the gamma photon disappears, an electron-positron pair comes out, and the crystal remains the same, have we then shown an experimental verification of the question? I would say it has, and the physics that describes this process would also confirm that.

I'm not sure that the initial question was talking about photon conversion into an e+e- pair when interacting with an electromagnetic field.
He wanted to know if we already have been observing directly pair creation from vacuum which is quite different !
 
  • #17
meopemuk said:
I am not familiar with this experiment and its analysis. However, it seems logical to me that when we collide two heavy nuclei with suffient energy, there could be a number of different channels for producing electron-positron pairs. (For example, such pairs can be produced simply in collisions of two electrons if the center-of-mass energy is high enough.) Was it possible to separate all these channels and say exactly which portion of electron-positron pairs was produced by the strong field, as opposed to any other reason?
I'm not an expert on the details, so I'll mention the few things I'm aware of...

Both the theoretical and experimental analysis are very difficult. The
experiments involve "gentle" colliding of (say) a stripped Uranium nucleus
with a target, such as Uranium or Curium or some other very heavy
element. One needs Z > 173 (my previous recollection of 139 was wrong).

At certain energies, this can produce "quasi-molecules" where
the binding energy become supercritical, allowing spontaneous pair
creation to occur, manifesting as positron emission. Detailed theoretical
investigation involves multipole analysis of a 2-centre Dirac eqn, which
is remains difficult, even numerically. I'm not familiar with the gory
details. Slightly higher energies overcome the Coulomb repulsion further
to allow formation of a superheavy nucleus, and of course different
experimental effects.

So I think the short, inadequate, answer to your question is that while
some unwanted effects can be reduced by careful choice of the
collision energy, there are still multiple effects occurring, which
need to be teased apart.

The textbooks from which I originally read about this subject were:

Greiner & Reinhardt: "Quantum Electrodynamics" 1994

Greiner, Muller, Rafelski: "QED of Strong Fields" 1985

Both of these books are bit old now, so I'm sure the state of
the art has advanced since then. Sorry I can't be more definitive.
 
  • #18
strangerep said:
Sorry I can't be more definitive.

Thank you, strangerep, your answer is more than sufficient.

Eugene.
 
  • #19
Barmecides said:
I'm not sure that the initial question was talking about photon conversion into an e+e- pair when interacting with an electromagnetic field.
He wanted to know if we already have been observing directly pair creation from vacuum which is quite different !
Yes, I was just about to say the same thing. So let me try to answer the
original question...

It depends on what one means by "vacuum". If one has a naive picture of
the vacuum as empty space with no particles of matter in it, but allows
the possibility of static fields, then the previous answers are relevant.
However, in QFT the notions of particle and field become hard to
separate. The "vacuum" is merely the state of lowest energy and this
state is common to all types of particles and fields. So a region of
otherwise empty space upon which an intense Coulomb field has been
imposed certainly does not qualify as "vacuum" in the QFT sense.
So my answer to the original question is that spontaneous creation of
real (on-shell) particles from the physical QFT vacuum does not occur
(neither theoretically nor experimentally). If it did, the vacuum would
be unstable, and none of us would exist.
 
  • #20
strangerep said:
I'm surprised to hear such a view, since the results of these experiments
are quite extensive and detailed. They've been in the mainstream for quite
a while.

In the Schwinger effect, an implied consequence is that a sufficiently
strong field could not persist very long - because pair production would
carry away energy-momentum. So creating a nucleus with Z >= 139
and observing the results seems like a reasonable way to investigate
the phenomenon.
My point is, if we deal with collisions, then the field is far from being static. On the other hand, the Schwinger effect talks specifically about static fields. I am not surprised that a collision may lead to a pair creation, but I am very skeptical about the claim that a STATIC field may produce a pair. See, e.g.,
http://xxx.lanl.gov/abs/hep-th/0103251
http://xxx.lanl.gov/abs/hep-ph/0105176
 
  • #21
I have more elementary questions about pair production. Why does it have to be near a large nucleus. I've read that it's because of relativity and some observer may see a reduced frequency of the photon because of the relativistic doppler effect. Why does a large nulceus fix that problem? Is it because the mass of the nucleus is dilated and provides the extra energy/mass for the pair production? Also, I understand that if one of the pair is charged, the other particle has to have the opposite charge, but why do the particles have to have any charge at all? Is it because the only particles that we know of with small enough mass are charged?
 
  • #22
The large nucleus makes the calculations simpler, since it's kinetic energy gain in the "collision" will be neglected. There is possibility for pair production "on" electrons as well (but then there will be triple production if I remember my class in radiation physics 3years ago LOL)

The particle has to be charged, the photon couples to electric charge.
 
  • #23
Virtual pair production, and its polarization in a Coulomb field, leads to the renormalization of "bare" charge. The charge that we all use in our calculations is the renormalized charge, corresponding to 1.6 x 10-19 Coulombs per electron (or proton). The present renormalization calculations are based on an article published by Uehling in 1935 (google Uehling integral). When two charges are closer than an electron Compton wavelength, the charges begin to "see" some un-renormalized charge, and this correction (called vacuum polarization) must be included in energy calculations. Perhaps the best example of this are the energy level calculations of muonic atoms (because the muons are very close to nuclei), where the measured atomic transition energies agree very well with calculations using a muon mass based on muonic g/m and g-2 experiments. The comparison of electron and muon g-2 measurements also provide a very accurate estimete of the virtual electron-positron pair production in a Coulomb field.
Pair production and annihilation is not limited to electrons. In muonic g-2, for example, there are corrections for both electronic and muonic pair production. In principle, any charged particle-antiparticle pairs could be included, but the higher mass severely limits the rate.
 
  • #24
smallphi said:
Quote from V. F. Mukhanov and S. Winitzki "Introduction to Quantum Fields
in Classical Backgrounds", available for free (still) at

http://www.theorie.physik.uni-muenchen.de/~serge/T6/book.pdf [Broken]



A static electric field in empty space can create electron-positron (e+e−) pairs. This
effect, called the Schwinger effect, is currently on the verge of being experimentally
verified.

To understand the Schwinger effect qualitatively, we may imagine a virtual e+e−
pair in a constant electric field of strength E. If the particles move apart from each
other to a distance l, they will receive the energy leE from the electric field. If this
energy exceeds the rest mass of the two particles, leE ≥ 2m_e, the pair will become
real and the particles will continue to move apart. The typical separation of the virtual
pair is of order of the Compton wavelength 2π/m_e. More precisely, the probability of
separation by a distance l turns out to be P ~ exp (−π m_e l). Therefore the probability
of creating an e+e− pair is

P ~ exp(−m_e^2 /eE)

The exact formula for the probability P can be obtained from a full (but rather lengthy)
consideration using quantum electrodynamics.
"the energy exceeding the rest mass of the two particles which in turn means exceeding the rest energy of the two particles". Are you implying translation from virtual particles to real particles violates conservation of energy? I find this hard to believe. Please make yourself clearer.
 
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1. What is pair creation and annihilation?

Pair creation and annihilation is a phenomenon in quantum physics where a particle and its antiparticle are created or destroyed simultaneously.

2. How does pair creation and annihilation occur?

Pair creation and annihilation can occur when a high-energy photon interacts with a nucleus or an electric field, resulting in the creation of a particle and an antiparticle or the annihilation of a particle-antiparticle pair.

3. What is the importance of pair creation and annihilation in particle physics?

Pair creation and annihilation are important in particle physics as they help explain the behavior of subatomic particles and their interactions with each other. They also play a crucial role in the study of quantum field theory and the Standard Model.

4. Can pair creation and annihilation be observed experimentally?

Yes, pair creation and annihilation have been observed experimentally in particle accelerators such as the Large Hadron Collider (LHC) and in cosmic ray experiments. These observations provide evidence for the existence of antiparticles and confirm the principles of quantum physics.

5. What is the relationship between pair creation and annihilation?

Pair creation and annihilation are inverse processes of each other. In pair creation, a particle and its antiparticle are created from energy, while in annihilation, a particle and its antiparticle are destroyed, resulting in the release of energy.

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