# A Pair production without nucleus

1. Nov 26, 2017

As far as I'm aware, pair production always involves the initial energetic photon interacting with another body (e.g. nucleus) to conserve momentum when creating the matter/antimatter products (e.g. electron and positron). Although "secondary" high-energy photons (e.g. $e^+ + e^- \rightarrow 2\gamma$) produced by annihilation of the matter/antimatter products does not require any additional body. I was thus simply wondering about the likelihood of the reverse process of two photons (possibly of different energies) interacting to produce matter/antimatter products without a third body. Does this readily occur and/or has it been observed? To my knowledge, two photons only interact beyond the Schwinger limit, and the above annihilation process is essentially irreversible below this threshold.

Any further thoughts or resources you may have on the above and considerations I may be overlooking would be greatly appreciated.

2. Nov 26, 2017

### Staff: Mentor

In principle the process should occur given sufficiently high energy photons, but such photons would be very hard to produce experimentally. AFAIK this process has not been observed.

3. Nov 26, 2017

Do you know if there's a particular cross-section that's been calculated theoretically for this process (e.g. something analogous to the Klein-Nishina formula but for photon-photon scattering)?

4. Nov 27, 2017

### Staff: Mentor

5. Nov 27, 2017

### vanhees71

This is $\gamma + \gamma \rightarrow \gamma+\gamma$ (Delbrück scattering) but not pair creation, which is $\gamma+\gamma \rightarrow \mathrm{e}^+ + \mathrm{e}^{-}$ (Breit-Wheeler process). I think, it has not yet been observed, as already mentioned in #2. The problem is to produce high-energy high-intensity photon beams. There are attempts in both the accelerator and the laser community to finally observe the process. Theoretically the cross section has been calculated already in 1934 by Breit and Wheeler.

6. Nov 27, 2017

Staff Emeritus
See Burke, D. L. et al. Positron production in multi-photon light by light scattering. Phys. Rev. Lett. 79, 1626–1629 (1997).

Strictly speaking, they don't restrict themselves to exactly two incoming photons (and indeed, I am not sure how you could do so experimentally)

7. Nov 27, 2017

### vanhees71

It seems as if they have indeed used a Compton photon from a laser colliding with an electron which then reacts with a photon or several photons in the laser beam (which of course is a coherent state and thus you can have any multi-photon process in principle) to create the electron-positron pair. I've not been aware about this paper. They however claim only evidence not discovery. Are you aware, whether there is some future work following this setup?

Of course another "holy grail" of this kind of physics is to find empirical evidence or even discover Schwinger pair creation, i.e., spontaneous creation of electron-positron pairs in very strong electrostatic fields (or strong laser beams).

8. Nov 27, 2017

If the electromagnetic field is characterized by photons themselves, is the primary difference now just that two $\gamma$ can create the electron-positron pair as opposed to a photon and a massive body necessary for momentum conservation?

9. Nov 27, 2017

### vanhees71

Yes, it's simply possible because you can fulfill all relevant conservation laws in both cases (2 photons or photon+other object).

10. Nov 27, 2017

Staff Emeritus
I am reasonably sure that there is not, as the facility they used doesn't really exist any more, at least not in that form. It's also more of a curiosity, as the two-photon process in e+e-, i.e. e+e- --> e+e-e+e-, e+e- mu+mu-, and even e+e- + hadrons (like the f2) are well-studied. Yes, the photons are virtual, but the cross-section is dominated by the region of low virtuality. Nobody seriously thinks (or thought) that the process would suddenly shut off when the photons become real.

That said, there have been theoretical advances. One issue with the Burke paper is that they couldn't reliably estimate the trident background; that was calculated in 2010 by Hu et al.

As far as "vacuum sparking", the problem is not with getting the electric fields high enough (most or all atomic nuclei do that) but to get the energy density high enough: you need 1 MeV of energy to make the pair, which means the out state needs to be 1 MeV lighter than the in state. Anything that can get the in state's energy high enough can be recast as producing e+e- pairs by that process, e.g. if you try and do it with a gamma ray, it looks like γ+A→e+e- + A.

11. Nov 27, 2017

### mathman

Immediately after the big bang, the usual description is photon-photon interaction to produce matter-antimatter pairs. The photon density and the photon energies are extremely high.

12. Dec 1, 2017

### Staff: Mentor

$\gamma \gamma \to l^+ l^-$ is easier to observe than photon-photon scattering. Here is an ATLAS note, for example. With “nearly real” photons, similar to the referenced paper.

13. Dec 1, 2017

Staff Emeritus
True, but that's also a lower order process: alpha^2 instead of alpha^4. Those 1/137's really add up.

14. Dec 2, 2017

### Staff: Mentor

That is the main reason it is easier to observe (and muons are cleaner than photons).