Other candidates for dark matter – the neutralino and the axion
Other possible candidates for dark matter include the neutralino, the lightest of a putative new class of particles that are heavier counterparts of the known particles, and the axion, a super-lightweight particle about one-millionth the mass of the electron. The neutralino is thought to have a mass between 100 and 1,000 times that of the proton which puts it just within reach of the experiments now under way at the Large Hadron Collider at CERN near Geneva. The existence of the axion is hinted at by subtleties that the Standard Model predicts in the behaviour of quarks. Efforts to detect it exploit the fact that in a very strong magnetic field, an axion can transform into a photon[1].
But what exactly is an axion? [2]
Well the story behind its theoretical genesis goes something like this.
- Quantum chromodynamics (QCD), which governs the strong force holding together atomic nuclei, has a problem - a CP (charge parity) problem, under which the flipping of a particle’s charge parity (flipping a particle’s electric charge and viewing it in a mirror) means it will no longer follow the same rules of physics - except that researchers have been unable to find any evidence for this. There is therefore a crack in our best model of particle physics.
- But this problem can be resolved theoretically by utilising the principles of broken symmetries: the concept that sometimes nature is not fully symmetrical when it should be, for example a pencil standing on its end falling in one direction repeatedly when it should fall randomly in different directions. When this happens in the context of particle physics, "a new particle arises" to maintain the underlying symmetry even though it appears to be broken.
- Applying this approach to the strong force, it is speculated that perhaps a hidden type of symmetry relating to this force is also broken, nullifying the anticipated CP difference that should be there but which experiments had failed to detect and that perhaps another new particle, the axion, had caused this.
- By the mid-1980s, theorists concluded that the big bang could have produced enough axions to account for dark matter. But this idea presents significant mass problems .....
Mass problems
- If they are proved to exist and shown to account for dark matter, we know nothing about how heavy axions would be or how likely they would be to interact with normal matter, but, assuming the underlying theory is correct, they would appear to be extremely inert and very lightweight.
- In 1987, Supernova 1987A in the outskirts of the Tarantula Nebula in the Large Megellanic Cloud exploded releasing almost the entire gravitational binding energy of the star in the form of neutrinos, finding their way to underground detectors. If axions had a mass of even a few milli electron volts divided by the speed of light squared (somewhat more than one billionth the mass of the electron) they would have distorted the escape time of the neutrinos on the way to Earth, but there were no such distortions, so axions must have an even smaller mass, meaning they would have extraordinarily feeble interactions with normal matter and radiation; they would be the least interactive particle known.
- Because of the intricacies of the process by which axions were created near the beginning of the universe, the lower the axion mass, the greater the mass density of axions that results, and should the axion mass be too small, the big bang would have produced way more axions necessary to account for dark matter.
- The conundrum is that if axions were too heavy, we would have seen them already through particle colliders or via their effects on the evolution of other stars, and they cannot be too light or there would be too much dark matter.
The search for axions is currently underway at the University of Washington via the Axion Dark Matter eXperiment (ADMX) experiment under the auspices of Leslie Rosenberg[3]. It was originally being conducted in a makeshift home hot-water heater tank, capped with wires inside a supercooled, magnetised vacuum chamber, but now it is much more sophisticated[4] Dark matter cannot consist of ordinary atoms and axions are a prime candidate since they nicely match the inferred properties of dark matter. Their relevance has come to the fore, the more so now that all the detectors searching for WIMPs have come up empty, including the LUX detector beneath the hills of South Dakota.
The search for axions began by searching for them in the explosions produced in particle collider. When this technique produced nothing, the search for cosmic axions turned to the vast pervasive sea of dark matter around us. When an axion interacted with a magnetic field, its total energy would be almost completely converted into a photon, and because the axions’ mass is very small, the resultant photon would appear somewhere in the microwave frequency range, but exactly where is a mystery until we know the precise axion mass[5].
The reasoning goes that if dark matter does consist of particles, they must be continually travelling through earth, and very occasionally, they would decay into microwaves which would produce a weak but detectable signal, and that is what ADMX is listening for. If axions are discovered they will force a revision of the Standard Model of particle physics, do away with the need to modify some of Einstein’s gravity predictions, and provide another link to the much sought after theory of everything. If ADMX does not come up with anything by 2018, it is conjectured their existence can be safely discarded, but perhaps this viewpoint, espoused by Corey Powell in 2015, underestimates the tenacity of Leslie Rosenberg. Now (2017), it is anticipated that eventually:
ADMX and other projects will be able to fully explore the theoretical window of possible dark matter axion masses. The fact that the full plausible mass range is totally accessible to experiments makes axions an attractive candidate for dark matter, compared with some alternatives we may never be able to test completely. ADMX is now sensitive enough to either detect the most plausible versions of axions or rule out the most plausible versions of them within five to 10 years.
Sophisticated cosmological models running on supercomputers are also working more reliable predictions of the axion mass, and it is possible that axions would clump together throughout the universe in a different pattern than WIMPs would, in ways both subtle and dramatic[6]. |
[1] Michael S Turner, “Origin of the Universe”, Scientific American: Special Collector’s Edition: Extreme Physics, Probing the Mysteries of the Cosmos, August 2013, 37 at 41.
[2] This resume is drawn from the article by Leslie Rosenberg, "Searching for the dark", Scientific American, January 2018, 47, 49-50.
[3] Corey S. Powell, “Cleaning up after Einstein”, Scientific American, Special Issue – 100 years of General Relativity, September 2015, 50-55.
[4] See Leslie Rosenberg's article, above.
[5] Ibid.
[6] Ibid, 53