Although dark energy is all the rage these days, dark matter still has enough mystery to keep physicists entertained. We don't actually know what dark matter is. Yes, we know that it hangs around in galaxies, modifying their rates of rotation. We know that without dark matter in the early Universe, there would have been insufficient gravitational attraction to generate the Universe we observe today. We have even seen dark matter that's physically independent of normal matter out there in the Universe.

But we really want to get a hold of it and find out how it fits with the pantheon of particles we do know about. Doing this will require some experimental ingenuity and possibly all sorts of new approaches.

Currently, our experiments are silent on what dark matter actually is. Even worse, dark matter doesn't fit into the Standard Model—the theory that tells us about everything from quarks and neutrinos to where all the missing socks are. Extensions to the Standard Model, like supersymmetry, produce a large menagerie of particles. Each of these possible particles might look like dark matter, but theory doesn't really pin their properties down very precisely.

This leaves the field rather open as far as potential experiments go, so there's a lot to try—and none of it is easy. Think of it as painting a landscape you have never seen, in a dark room, without knowing where the paint is.

Many hands make light work?

The key to success here is to explore as many different avenues as possible. In the spirit of these discussions, allow me to present a new salty, shiny dark matter detector (yet to be built). The target of this detector is the potential dark matter particle called the axion, which is a very light supersymmetric particle. The nice thing about axions is that their mass is low, so if they are around, we should be able to detect them in laboratory experiments. Indeed, we have reported in the past on experiments that have placed limits on the mass of the axion.

Likewise, because the axion is light, it will have a direct impact on the evolution of stars. Stars begin by burning hydrogen; when they run out of hydrogen, they begin to burn helium. For a given stellar mass, the timing of this switch is fairly predictable. However, axions will change that time by a small amount. By carefully examining the statistics of stars, it is possible to place limits on the way that axions affect normal matter.

Even more noticeably, during a supernova, a dying star will emit a burst of neutrinos. If they interact too strongly, axions will quench that burst. So supernova SN1987a provides some limits on their interactions as well. Finally, white dwarfs, which are the slowly cooling corpses of stars, will cool slightly faster if the axion is around.

Guess what? White dwarfs do cool just a little faster than predicted by theory.

The picture that emerges is that we have a couple of tantalizing hints that the axion might be real. Even better, if it does exist, it still influences ordinary matter, even if weakly. So with the right experiment, we can detect it.

Making axions flip magnets

Pierre Sikivie from the University of Florida has proposed an interesting approach to detecting axions. The basic idea is that every now and again, an axion will bang into an atom or molecule and place it in an excited state. Given a hypothetical axion density and a sample of atoms, we can predict how many atoms should be in the excited state at any one time. Working backward, if we continually count the number of atoms in the excited state, we can work out how many axions there are, as well as how massive they are.

Unfortunately, this is easier said than done. To excite an atom into an electronic excited state usually requires energies on the order of an electron volt, or 0.2aJ (1aJ = 10-18J). Axions must have energies between 100,000 and a million times smaller, so electronic excited states are out. Instead, an axion can excite the spin states of electrons.

Picture it like this: an electron is like a small bar magnet, and this bar magnet is running around inside a magnetic field that is composed of the magnetic field from neighboring electrons and the nucleus. This field may give the electron a preferred or lower energy orientation of its bar magnet. We can also control this energy difference by applying a magnetic field. With the field applied, the excitement of the electron involves sufficient energy to flip its magnet from aligned with the applied field to aligned against the applied magnetic field.

If we set the energy difference between the two spin states to the energy of the axion, then the axion will generate atoms in the excited spin state. Then we simply count the number of atoms in the excited state and await the adoring crowds that will descend on our lab.

Sikivie proposes using salt crystals like iron floride because they have been well-studied and have a relatively clean set of excited spin states that are dominated by the magnetic properties of iron.

So we can set up a crystal of material that will respond optimally to dark matter, but we still need to count excited atoms. To make this work, we need to have as few atoms in the excited state as possible. Unfortunately, the excited state is very close to the ground state (remember, we're talking less than 10-23 Joules here), so thermal energy is enough to excite atoms. Thus, the sample material needs to be held at well under 1K (say ~100mK). Even there, if you have a few milligrams of sample, you would still expect a large background of excited atoms. To get around that, you have to sweep the magnetic field strength so that a change in the number of excited atoms is detected, rather than the absolute number.

Finally, Sikivie proposes to count the number of excited atoms through laser absorption or fluorescence. Shine a laser that is tuned to push atoms from the excited state to a much higher energy state, which I will call the signal state. After that, detection is easy: either look for glowing atoms or measure a dip in the laser beam intensity after it has passed through the sample. Either way, we can measure that at the single-atom level.

Hmm, will that really work?

Regular readers know that I love lasers. If anything can be done with a laser, you can expect me to be cheering the project on. But in this case, I think there are better ways. In the paper, Sikivie doesn't seem to pay much attention to the practicalities of this sort of detector. It's true that a laser with the right wavelength to excite atoms from the excited state to the signal state will not have the right wavelength to excite atoms from the ground state to the signal state. But the transition we don't want to see is just very improbable, not impossible, so the laser will generate a small background. Worse, that background will increase as we try to detect lighter and lighter axions.

I think Sikivie is also relying on a nice rule of physics: essentially, if exciting the atom from the excited state to the signal state conserves angular momentum, then exciting the atom from the ground state to the same signal state will not conserve angular momentum. These are called forbidden transitions. Unfortunately, not only are they not entirely forbidden (just very weak), but there is usually a transition from the ground state to a different signal state that requires a very similar light frequency.

All in all, we are looking at a rather large background.

I think I would go for something much less direct. For instance, the population in the excited state will change the dielectric constant of the material. If you placed the material inside a very high quality cavity for, say, microwaves (~2GHz), then the resonant frequency of the cavity would change as the population of the excited state increased or decreased. Since we can measure frequency to 1 part in 1015, this would make for an extremely sensitive detector. Furthermore, the radiation in the cavity has much lower energy than the axion interaction, so it will only contribute a tiny amount to the background.

To sum up: I think using electronic and nuclear spin states to detect axions is a very exciting idea. By using spin states, you can sweep the strength of a magnetic field through different values until you hit on one that matches the energy of the axions. If the axions have a distribution of energies, that is also detectable. I do have problems with the proposed detection scheme, but there are many different possibilities of counting the atoms in the excited state, so I don't see that preventing further work.

Physical Review Letters, 2014, DOI: 10.1103/PhysRevLett.113.201301