Monday, 23 September 2013

Axion Dark Matter in Tabletop Experiments

A paper from last week offered a very interesting suggestion for a new type of search for axion dark matter, as well as the possibility that it may already have been seen.

I've talked about dark matter a few times on this blog, but I don't think I've yet mentioned axions.  Part of the reason for this is that axions are somewhat outside my area of expertise.  Still, the main points that are needed here are that axions are very light, very weakly interacting particles.  By light, we are talking at least a billion times lighter than the electron.  Like WIMPs, axions are introduced for unrelated reasons yet can serve as natural dark matter candidates.
The extremely small interaction strength is a problem for verifying the existence of axions, and testing if they are dark matter.  All the traditional WIMP searches—direct detection from scattering off nuclei, indirect detection from annihilation products in the galaxy or production in a particle collider—fail for this reason.  Other searches do exist, such as looking for photons converting to axions in the presence of a magnetic field, or for axions decaying to photons in a resonant cavity.  To this stable, Christian Beck has proposed an interesting idea based on tabletop physics.

The idea makes use of a Josephson junction, a junction of two superconductors separated by an ordinary conductor.  Superconductivity is an intrinsically quantum phenomenon, and the Josephson junction turns out to be a simple, clean example of a quantum system.  It's so simple and well understood, in fact, that I did an experiment with one as an undergrad; and remember, I'm neither a condensed matter physicist nor an experimentalist.

One of the characteristic features of the Josephson junction are the Shapiro steps.  When you apply an electric potential across a Josephson junction and expose it to microwaves, the current does not increase  linearly with the current.  Rather, it remains constant unless the potential increases by at least a particular amount, proportional to the frequency of the microwave radiation.

The key observation to Beck's idea is that in the presence of an axion background, a similar effect can happen where the axions stimulate tunnelling of Cooper pairs across the junction.  The background magnetic field associated with the supercurrent allows the axion to convert to a photon of the same energy, essentially just the axion mass-energy.  This photon can then induce current to cross the junction as well as any normal photon.  The signal is then a step-like response at the junction without direct irradiation.

Now, as I said I'm not an expert in either Josephson junctions nor axions.  But the analysis in the paper seems reasonable to my educated lay perspective.  Perhaps most interestingly of all, though, is the citation of an earlier study of these junctions that found a consistent, temperature-independent background peak.  Beck suggests that this can be interpreted as dark matter; the resultant signal would correspond to axions comprising about one-sixth of the total dark matter.  This passes the obvious sanity test of being less than one, and raises the interesting possibility that something we've been looking for for some time has been hidden in an unwanted background!
Figure 2 of Beck's paper.  The small plots to the left and right are from the previous work, with the arrows pointing to the unexplained background peak.  This peak is temperature-independent, consistent with an external source.  Interpreting this peak as a dark matter signal gives axions with a mass of 5 billions times smaller than an electron.

In any case, it should be relatively simple and inexpensive to check this possibility.  Together with the gravity wave ideas that were suggested at SUSY, it is another use of precision condensed matter experiments in fundamental physics.

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