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Pair creation and annihilation

by touqra
Tags: annihilation, creation, pair
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strangerep
#19
Aug2-07, 08:15 PM
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Quote Quote by Barmecides View Post
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.
Demystifier
#20
Aug6-07, 03:11 AM
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Quote Quote by strangerep View Post
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
passingthru
#21
Jun26-09, 12:19 PM
P: 19
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?
malawi_glenn
#22
Jun26-09, 12:53 PM
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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.
Bob S
#23
Jun26-09, 01:11 PM
P: 4,663
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.
Abbas Sherif
#24
Jul13-09, 11:52 AM
P: 27
Quote Quote by smallphi View Post
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-muench...ge/T6/book.pdf



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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