Dark matter annihilation into gamma rays

[VantagePoint72]

This question straddles this forum and the "Beyond the Standard Model" one a bit, so if a mentor thinks it belongs better elsewhere, please feel free to move it.

I've seen references in the "popular science" press about the possibility of indirectly detecting dark matter by looking for gamma rays produced in annihilation. This is a process that seems to be expected specifically with WIMPs and possibly with other models. For example, here. I know pop sci press can be misleading, but the literature seems to agree, e.g. arXiv:1203.5636 and arXiv:1303.3284

What I don't understand is how dark matter particles annihilating each other can produce gamma rays. That would require a vertex in the Feynman rules linking the dark matter field with the electromagnetic potential, which in turn would contradict the basic assumption about dark matter: that it doesn't interact electromagnetically. How do you get gamma rays out of something that, by definition, isn't connected to the EM field?

My first guess would have been that the dark matter particles are not producing gamma rays directly but that their annihilation products scatter off of regular matter and produce high energy photons. This doesn't seem to be the case, though. According to this http://kipac.stanford.edu/kipac/research/FGST: "In some of these models, the dark matter particle may self-annihilate or decay into standard model particles, including photons with energies as large as the dark matter particle rest mass." So it seems like the expectation is that DM particles can produce gamma rays directly. How is this possible?

 

[Orodruin]

It may happen that dark matter interacts, although very weakly, with photons. This is typically induced by the dark matter to SM interactions being mediated by something like a Z' which could mix with the photon, thus effectively producing dark matter with fractional charge.

Other processes might be higher order processes and, as you say, the results of the annihilation products being charged.

 

[mfb]

How do you get gamma rays out of something that, by definition, isn't connected to the EM field?

With loop processes. The type of loop depends on the dark matter model.

 

[VantagePoint72]

With loop processes. The type of loop depends on the dark matter model.

Could you expand on this? Maybe with an example Feynman diagram? I don't understand what loops have to do with it.

 

[Orodruin]

Could you expand on this? Maybe with an example Feynman diagram? I don't understand what loops have to do with it.

If the DM has a coupling to a charged SM particle, you can have a Feynman diagram where this SM particle forms a loop to which you can couple photons.

 

[mfb]

Could you expand on this? Maybe with an example Feynman diagram? I don't understand what loops have to do with it.

First page in google image search for "dark matter annihilation":
diagrams
more diagrams
and so on. It is really not hard to find examples.

 

[VantagePoint72]

Thank you, both. @Orodruin, is this also the kind of process by which you meant, "It may happen that dark matter interacts, although very weakly, with photons"? So, that being the case, dark matter particles would interact electromagnetically with ordinary charged matter, just with extremely low amplitude?

Also, I just want to double check: @mfb, in your first diagram is the propagator labelled "h_i" a Higgs boson? I'm assuming yes, I've just only ever seen it labelled "H" in diagrams.

 

[Orodruin]

Thank you, both. @Orodruin, is this also the kind of process by which you meant, "It may happen that dark matter interacts, although very weakly, with photons"? So, that being the case, dark matter particles would interact electromagnetically with ordinary charged matter, just with extremely low amplitude?

I was thinking of both this kind of process as well as kinetic mixing of two gauge sectors (which, on the other hand would happen through similar loop processes as well).

Also, I just want to double check: @mfb, in your first diagram is the propagator labelled "h_i" a Higgs boson? I'm assuming yes, I've just only ever seen it labelled "H" in diagrams.

In many BSM models there are additional Higgses, even charged ones.

 

[VantagePoint72]

Thanks again. If you wouldn't mind indulging me for one final question to make sure I'm not missing anything. In this guest post on Sean Carroll's blog, Katherine Freese writes about galactic gamma ray signals in the DM search:

This line, originally proposed by Stockholm’s Lars Bergstrom, would have been the expectation if two WIMPs annihilated directly to photons.

So, when someone talks about two WIMPs annihilating directly to photons, it's still mediated by various loops? I guess my confusion here was mostly a semantic one, if that's the case. To me, saying two WIMPs directly annihilate into photons implies a Feynman rule linking two WIMP propagators and two photon propagators at a vertex. I guess, in hindsight, that's not really a reasonable interpretation since that's not even what happens when an electron and positron annihilate.