A physical photon cannot convert to a electron-positron pair (no way to satisfy energy-momentum conservation), so there are no experimental observations.

A similar process, which is possible, is obtained by replacing the photon with the neutral vector boson Z. Then both possibilities are theoretically possible (with some relations following from the fact that the parent particle has spin 1), but I doubt that the final spins have ever been measured.

Thanks for your answer, Kåre. I'm also in doubt that the final spins have ever been measured and therefore my question.

The spin preservation would expect the case (2) γ → e+ ↑ + e− ↑ because the spin of the photon is 1.

You answered: A physical photon cannot convert to a electron-positron pair ...

I asked on Nov 23, 2015: Is there information on photon-photon interactions?

Answer André Michaud: "Yes there is.

Electron-positron pair creation during close flyby of two photons, one of which exceeded the 1.022 MeV minimum energy threshold without any atomic nuclei being close by was first experimentally confirmed by Kirk McDonald et al. at the Stanford Linear Accelerator in 1997 with experiment #144"

https://www.researchgate.net/post/Is_there_information_on_photon-photon_interactions?_tpcectx=profile_questions

The downvoter of my question has obviously no idea about physics.

Hans

Hans> would expect the case (2) γ* → e+ ↑ + e− ↑

Only if the spin of the virtual γ* "was pointing in the z-direction".  A massive spin-1 particle can have its z-component quantized to Sz = 1, 0, -1 (in units of the Planck constant), or some superposition of these possibilities. 

Hans> obviously no idea about physics

You did not describe the situation in a clear way.

Note that André Michaud mentions the presence of two photons. I.e. the complete process is not exactly the one you present. Actually, I think the experiment by Kirk McDonald et.al. consisted of colliding an electron beam with a laser beam. I.e., the process is really

e- +  γ →  e-  +  e+ + e-

which is kinematically possible. There are several contributions to the quantum amplitude of this process. One of them can be interpreted as coming from a two-stage process

(1)  e- + γ → e- + γ*

where γ* is a virtual photon, because it does not obey the correct energy-momentum relation (like a real photon does). In this interpretation γ* subsequently decays roughly like the massive spin-1 Z-particle I mentioned,

(2) γ*  →   e+ + e-

Pair production from cosmic gamma-rays in the atmosphere occurs in the presence of atoms and molecules.

Thank you for clarification, Kåre. You are undoubted an expert in theory.

I asked for experimental observations. But there are no spin measurings at pair creation. The electron-positron pair production is obviously a fundamental process in microcosm but it is shabby treated in physics. View for example the decay of the muon:

(1)  μ− → e− + γ

(2)  μ− → e− + e+ + e− 

In the second case emerges an electron positron pair caused by decay energy. You can label it as increase numbered decay because more particle arise as structural might be expected. Many particle conversations are showing this behavior. You can use the theoretical requisite know-how to describe the process but every theory is only an approximately depiction of reality.

I have every reason to ask "stupid" questions. My professional experience tells me that there are no stupid questions but stupid answers.

Hans-G> But there are no spin measurings at pair creation.

I don't know much about the experimental situation, but I do know that high-energy accelerator beams with polarized electrons exists. I now read that they are also been used to produce polarized positrons, essentially through the process you asked (via a virtual photon γ*).  There are further developments in this area, see f.i. the article https://physics.aps.org/articles/v9/58.

So, (i) I was wrong in my doubts that this process had been measured, and (ii) you are wrong that such processes are shabby treated in physics.

The two processes of muon-decay you mention have never been observed. The Particle Data Group lists the branching ratio to be below 5.7 x 10-13 and 1.0 x 10-12 respectively, http://pdg.lbl.gov/2015/tables/rpp2015-sum-leptons.pdf. Due to neutrino masses and mixings they should in principle exist, but with too low branching ratios to make detection possible.

Hans-G> there are no stupid questions

That is true for 2-year olds, but not necessarily for members of ResearchGate (who are easily able to find some basic information on the web first). But I did not find your question stupid (only a bit vague in its premises), and did not downvote it. And I learned something by answering it :-). 

Many thanks for the link, Kåre!

Under the terms of the described experiment we have to assume that possibility (2) γ → e+↑ + e−↑ occurs at pair production.

To the transformations of the muon: The transformation: (1) μ− → e− + γ happens nearly 100%. You are missing the "neutrinos".

It was never observed a neutrino immediately and at the point where it should occur. All neutrino proofs are indirect proofs. All neutrino events are caused by energy amounts which are inexplicable by current theories. We observe a very different internal energy of atoms and molecules always in thermodynamics. Particles and atomic nuclei have not nearly the same internal energy like tennis balls or bullets in mechanics. They are decaying at very different energetic levels. This caused the β-spectrum and leads occasionally to unespected decays of atomic nuclei against the usual direction of decay.

Neutrions are a knowledge destructive invention (in German: erkenntnisdestruktive Erfindung) which make it impossible to recognize the real causes of all the processes which are currently declared by neutrinos. These particles are a circular argument in physics.

See also: https://www.researchgate.net/publication/279963956_Theory_and_reality_on_the_experiment_of_ReinesCowan_1956

Hans

Research Theory and reality on the experiment of Reines/Cowan 1956

Hand-G> Under the terms of the described experiment we have to assume that possibility (2) γ → e+↑ + e−↑ occurs at pair production.

To nitpick, we cannot assume that this is exactly what they (quite successfully) try to achieve. But it is either this, or the case with both spin directions reversed. I looked up an article about generation of spin-polarized electrons, which is a initiating stage of the process, http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-8035.pdf. In their setup (a majority of) electrons with negative spin orientation, relative to the direction of motion, were produced. This has to do with technical details.

Hans-G> You are missing the "neutrinos".

No, I am not missing (or forgetting) the neutrinos. I took the processes you described at face value, forgetting that this would be a dubious thing to do. But no, if one adds an electron antineutrino and a muon neutrino to the right hand sides of your reactions, one does not get anything near to 100%: One gets processes with branching ratios of respectively of order 1% and 3 x 10-5. With your preferred notation, the process μ− → e− is near to 100%.

I have no comments on your curious dislike for neutrinos. 

You are right, Kåre. I have to correct myself. On closer inspection of the linked article it becomes visible that there are no clear statement about the spin properties of the electron positro pair. Only the illustration purports it.

I have good reasons for my "curious dislike for neutrinos" always since my education. Neutrinos and electromagnetic radiation are similar insofar that they are represent first at all energy. In reality do not exist duplicate positions (in German: Doppelbesetzungen). The energy differences of the beta-spectrum would be compensated where necessary by radiation an not by a no tangibly particle.

A steady stream of energy resp. matter would disappear with the neutrinos into a theoretical adjoining room of physics without return. Reality has no back doors to escape like theory.

Hans-G:

a) I do not understand what you mean to imply by invoking the term "Doppelbesetzungen" (and neither the back doors). [Btw. I do know the word, it's not a matter of (need for) translation]

b) with the risk of exposing a lack of education of mine: I thought that the neutrino came in not just to cover the energetics of beta decay but (and primarily so ?) the lack of angular momentum conservation.

Kai> not just to

and the property of fermionic parity, (-1)F, which should be the hardest one to violate, even in the absence of time and space translation invariance, or rotation invariance (and the resulting violation of their associated Noether conservation laws)

It's straightforward to calculate-not so to produce. An external field is required and it's big, in the scheme of things. But it's possible to measure the reverse process, since it is possible to scatter polarized electron and/or positron beams, that do couple to photons (and to Zs); these processes have been measured in deep inelastic scattering, Cf., also, https://arxiv.org/pdf/hep-ex/9610007.pdf

Dear Kai!

We all have lacks of education somewhere. ;-)

Pauli created the idea of neutrinos original to declare the energie spectre of the β-electrons. (It would be also need for an alpha-trino because those particles show also an energy spectre.) The neutrino get more properties later on par example the receipt of helicity. But in case of the β-conversation of atomic nuclei you can observe great changes of spin inexplicable with neutrinos.

Doppelbesetzung: I mean the fact that Paulis problem to declare the β-spectre by using or define particles as mechanical objects was leading to the invention of a further mechanical object. In reality the different amounts of energy (from a few keV till some GeV) will be released by radiation. The neutrino has taken over somehow the function of the electromagnetic radiation thanks to Pauli.

It is clearly "erkenntnisdestruktiv!"

Hans

Neutrinos are different from photons-and Pauli didn't declare, he explained, the spectrum of β-decay. And measurements since then have led to better quantitative and qualitatitive understanding. However β-decay involves the coupling W-electron-neutrino, not the coupling photon-electron-electron, so it doesn't have anything to do with the question discussed. 

@Stam: What is in your opinion the different between "declare" and "explain"?

A further issue in connection with the invention of more and more particles with only theortical need in physics is the material similarity between all the very different particles. It has taken hundreds of years in chemistry to recognise that atoms in their diversity are the carriers of characteristics and their structural similarity the only reason of their mutual reactivity.

You can declare the diversity of particles and atomic nuclei with similar structures build only by electrons and positrons. Sometimes I think about the way of recognition in physics if electron and positron were for a long time the only one known particles.

It seems that the "neutrino" in physics is the same as the "phlogiston" in chemistry was.

Thesis The Reason of a realistic View to Particles and Atomic Nuclei

The known particles have been  classified: quarks, that have strong, electromagnetic and weak interactions; leptons that have electromagnetic and weak interactions, and  the Higgs boson, that has weak interactions and mediates its own, make up known matter. The interactions are described by the exchange of other particles: gluons (strong interactions), W and Z bosons (weak interactions), photon (electromagnetic interactions). And there's a, new, interaction, mediated by the Higgs.  And this description is quantitative-it's possible to calculate, with controlled errors and measure with controlled errors. And it's possible to predict what would be a new effect, quantitatively. That such new effects are hard to find is a sociological, not a technical issue. It does require learning the technical aspects and the meaning of words. 

@Stam: Plase name me any evidence for "quarks".

As the LHC-ecperiment was starting in 2009 it was a big euphoria to prove them. But there was nothing - apart from Higgs-boson. Somehow the involved parties had to save the honour. They are looking for quarks this very day. Physics should be more critical towards its own theories.

The proton consists really of three subparticles but not "quarks". The reaction between proton and antiproton leads to the conclusion that the well known light mesons (→ μ−; μ+; π+; π−; π+; K−; K+) are the substructural building blocks of protons and all heavier particle including atomic nuclei. Any nuclear fragmentation shows a high amount of light mesons and other heavier fragments. The inability of knowledge in physics would be funny if it would be not too expensive.

I refer to the linked paper above.

Hans

Deep inelastic scattering experiments probe properties of quarks. It's not necessary to have particles as asymptotic states, in order to establish their properties. The quantitative description of the properties of quarks and their bound states is in terms of perturbative QCD at high energies and lattice QCD at low energies. 

It's wrong to claim that nothing has been discovered at the LHC, apart from the Higgs boson. What has, also, been discovered is that the self-coupling of the Higgs remains in the domain, where perturbation theory about free fields is consistent. This measurement implies the existence of new particles, that remain to be discovered. 

The statements about the mesons are, simply, wrong. That mesons are produced in proton-antiproton collisions doesn't mean that protons or antiprotons are made up of mesons (leptons are, also, produced in such collisions, as well). 

There's no such notion as ``realistic'' in physics. The only notion is that of calculation, under controlled approximations, consistent with what's known, that is the basis for imagining experimental protocols-which is what's going on. That the time to generate and analyze data in high energy physics is incompatible with a 24/7 news cycle is a sociological fact.

Hans-G> As the LHC-ecperiment was starting in 2009 it was a big euphoria to prove them [quarks].

Huh??? No serious particle physicist has doubted the existence of quarks since the "November revolution" in 1974. The LHC-experiments have used the collisions of quarks (of known existence, used to design the experiments) to produce Higgs particles. They also hoped to find the supersymmetric partners of known particles; this has not been successful (at least not so far).

What is your criteria for a particle to "realistically exist"? Where do you draw the line in the listings of the Particle Data Group, http://pdg.lbl.gov/2014/reviews/contents_sports.html? Have you ever seen a pion? Why do you consider it to have a more realistic existence than a quark? The pi0-meson has a lifetime of about 10-16 s; or as long as light needs to move 26 nm. I would say that the quarks created in f.i. electron-positron collisions exists as genuine particles much longer than that (eventually they convert to other particles, but so do most of the particles you promote to have a realistic existence).

How do you explain particle masses, lifetimes, and other properties with your 'realistic View' of particle physics?

Quote: "... da alle zuverlässigen (Experimente) davon negative Resultate geliefert haben. Wenn Quarks gefunden werden, dann wid die Aufregung so enorm sein, dass die relevanten Experimente sehr schnell bekannt werden." [H.Fraunfelder/E.M.Henley: Teilchen und Kerne, R. Oldenbourg Verlag München Wien 1999, page 477]

You should stay with the truth, Kåre!

Without knowledge of its history and context, I cannot comment on that quote.

The November 1974 discoveries certainly lead to "enorm Aufregung", and Nobel Prizes in Physics two years later. Quark-Antiquark collisions were used to discover the W and Z bosons in 1983.

The tendency of self-evidence is inherent in every theory. All observations and experimental results are evaluated with reference to the predictions of the theory which has to be proven. An outstanding example of self-evidence is the Savannah-River-Experiment in 1956. Stray fission neutrons are causing positrons and also neutrons. But those both particles are considered as proof of neutrinos. Every further comment becomes superfluous with it.

There's no absolute stability in the world of nuclei and particles even in the depths of mines, lakes or glaciers. Some of the unimaginable amount of atomic nuclei show a constant conversion diametrically opposed to the normal decay. This is caused by the spectre of internal energy and also by energy amounts and particles realased by natural radioactivity. Especially nuclei with small difference of bond energy (for example: 31-Ga-71 ↔ 32-Ge-71 + e−) exhibit this "abnormally behaviour". The current physics stays in the belive, that this is caused by "neutrinos" or better by its energy.

The detectors for the OPERA-experiment were indicating a continuous background noise of instability regardless of the proton pulses and assumed "neutrino beams" at CERN. ...

Hans-G> The tendency of self-evidence

Have you tried to publish your alternative explanations? With quantitative evidence (vague dreams tend to vaporize in the presence of equations and numbers)? What were the objections of the referees?

In summary: There are no informations about the spin characteristics of electron and positron generated by pair production. I would espect anti-parallel spin.

@Kåre: You can read in every textbook of physics vague assumptions about reality as following: "One did believe..." (HUH!), "even if one accepts that...", "in more reasonable line..." but 51% deviation between real measuring and theoretical calculation, "We believe currently..." and so on.This vague assumptions are the foundations of physics, especially particle physics.

We believe what we wish to believe. I do not belive!