Pair creation and annihilation

touqra

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?

ahrkron

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

smallphi

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.

touqra

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?

smallphi

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.

Barmecides

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.

Nariad

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

Demystifier

They don't. At least not yet.

strangerep

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.

Demystifier

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.

strangerep

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.

meopemuk

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.

MaWM

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.)

Barmecides

The initial question was "How did they experimentally verify that there is pair creation and annihilation in the vacuum?"

ZapperZ

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.

Barmecides

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 !

strangerep

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.