Pair creation and annihilation

  1. 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?
     
  2. jcsd
  3. ahrkron

    ahrkron 734
    Staff Emeritus
    Gold Member

  4. 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.
     
  5. 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 ?

    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?
     
  6. 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



    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.
     
    Last edited: Jul 6, 2007
  7. 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.
     
  8. 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.

    Clubul Fizica Particulelor-portal de popularizare a fizicii particulelor: http://fizicaparticulelor.ro
     
  9. Demystifier

    Demystifier 5,187
    Science Advisor

    They don't. At least not yet.
     
  10. strangerep

    strangerep 2,185
    Science Advisor

    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.
     
  11. Demystifier

    Demystifier 5,187
    Science Advisor

    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.
     
  12. strangerep

    strangerep 2,185
    Science Advisor

    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.
     

  13. 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.
     
  14. 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.)
     
  15. The initial question was "How did they experimentally verify that there is pair creation and annihilation in the vacuum?"
     
  16. ZapperZ

    ZapperZ 30,169
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
     
  17. 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 !
     
  18. strangerep

    strangerep 2,185
    Science Advisor

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

    Eugene.
     
  20. strangerep

    strangerep 2,185
    Science Advisor

    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.
     
  21. Demystifier

    Demystifier 5,187
    Science Advisor

    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
     
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