Axion vs. WIMP / Is Axion a WIMP?

Question

What is the difference between an Axion and a WIMP?

A WIMP is just defined as a class of particles interacting via gravity and potentially via weak interaction (or a new force that is even weaker than that). Why is the Axion not a type of WIMP?

Answer 1

WIMPs (Weakly Interacting Massive Particles) are a class of hypothetical particles which can be dark matter candidates which most notably have masses near the $\bf{weak}$ scale, $m_{\rm{wimp}} \approx \mathcal{O}(100 GeV)$, and interact with the same strength (or less) than the weak interactions.

Axions are also hypothetical dark matter candidates with entirely different origins. They arise from spontaneous symmetry breaking and thus have very small masses, $m_{\rm{axion}} \ll m_{\rm{wimp}}$. They interact with through QCD and QED in very specific ways depending on the specific model of the axion.

Answer 2

It is a strange but true fact in the history of dark matter that the axion started out with its interaction strengths set by an energy scale --- the axion decay constant --- that was near the rest-masses of the charged W and neutral Z vector bosons that carry the weak nuclear force. Meanwhile, the WIMP started out with a rest-mass that was ten times lighter than this scale. On the original weak scale for the axion decay constant, see for example the original papers proposing the existence of a new light boson with the same quantum numbers as the neutral pion, that is, the axion. They were written by Steven Weinberg and by Frank Wilczek and were both published in the prestigious journal Physical Review Letters in 1978 [1,2].

On the WIMP side, four groups of theoretical particle physicists independently proposed in 1977 that, in the Big Bang, you can make the dark matter in the universe in the form of spin one-half electrically neutral, colorless fermions that feel the weak nuclear force [3,4,5,6]. But, these weakly interacting massive particles, or WIMPs, would have to have rest-mass about ten times less than the weak energy scale. Steven Weinberg was part of one of these groups and the paper he co-authored ends with a specific quote for the original mass scale of the WIMP that is ten times lower than the masses of the W boson (84 GeV) and Z boson (91 GeV): “Of course, if a stable heavy neutral lepton [meaning a spin one-half fermion that is electrically neutral, colorless under the strong nuclear force, but able to feel the weak nuclear force] were discovered with a mass of order 1–15 GeV, the gravitational field of these heavy neutrinos would provide a plausible mechanism for closing the universe [that is, the WIMPS would form the dark matter in the universe].”

So what then is the difference between a WIMP and an axion? In practice, the difference boils down to how you look for them. Two experiments in the late 1980s set the course of dark matter history in this regard.

The original WIMP searches grew out of an experiment by David Caldwell and others who were then based at the University of California [7]. They were staring at hockey pucks of the semiconducting material germanium and looking for nuclear decays that are forbidden provided that --- as the absence of signal rising above the low background noise in their experiment strongly suggested --- the electron-type anti-neutrino and neutrino are distinguishable just as positrons and electrons are distinguishable. It was therefore decided that a WIMP should have rest-mass around the mass of a germanium nucleus, which is about the same as a W boson, so that you would get a good signal in the germanium hockey puck when a WIMP from the local dark matter halo of the galaxy collides with the germanium nucleus.

Meanwhile, the original axion searches grew out of a superconducting solenoid magnet from the 1960s that Adrian Melissinos and his team based at Brookhaven National Lab happened to have on hand [8]. The inner diameter of the solenoid set the size of the biggest microwave cavity that could be fit inside the magnet. Therefore, it was decided that the axion must have a rest-mass energy around 10 micro-electron-Volts to coincide with the energy of a microwave photon at the resonance frequency of the cavity.

But, physically what is the difference between the axion and a WIMP? Maybe nothing. The case of the DAMA/Libra dark matter experiment suggest as much.

The long-running DAMA/Libra experiment based at the Gran Sasso National Laboratory was conceived, designed, and has been run as a WIMP search by Rita Bernabei and her team [9]. For the past two decades, the experiment has seen a dark matter signal with significance that long ago passed the conventional 5 standard deviation threshold for the discovery of a new phenomenon in particle physics [10]. Yet, the signal seems unlikely to be coming from WIMPS with the mass of a germanium nucleus or even something ten times less [11].

Perhaps axions are the cause of the DAMA/Libra dark matter signal. For that to be the case, though, the axion would have to have rest-mass energy more like 0.5 electron-Volts. That is the typical energy needed to effect the event rate in the kind of "scintillation" detectors used by DAMA/Libra experiment [12].

On the other hand, if dark matter is really made of axions with rest-mass energy 0.5 eV, then almost all the past, present and planned axion dark matter experiments have no chance of seeing it, at least with their current conception, design, and running conditions [13]. In practice, such a 0.5 eV axion could then have already been seen for the past two decades in an experiment looking for WIMPS --- the DAMA/Libra experiment --- but would never be seen in these axion dark matter experiments. The physical distinction between an axion and a WIMP would then evaporate, since axion dark matter experiments would be blind to the axion while a WIMP search would already have seen the axion.

[1] F. Wilczek, Phys. Rev. Lett. 40, 279 (1978).

[2] S. Weinberg, Phys. Rev. Lett. 40, 223 (1978).

[3] P. Hut, Phys. Lett. B 69, 85 (1977).

[4] B. W. Lee, and S. Weinberg, Phys. Rev. Lett. 39, 165 (1977).

[5] K. Sato and M. Kobayashi, Prog. Theor. Phys. 58, 1775 (1977).

[6] M. Vysotsky, A. Dolgov, and Y. Zeldovich, JETP Lett. 26, 188 (1977).

[7] D. O. Caldwell et al. Phys. Rev. Lett. 54, 281 (1985).

[8] S. DePanfilis et al. Phys. Rev. Lett. 59, 839 (1987).

[9] C. Bacci et al., Phys. Lett. B 293, 460 (1992).

[10] R. Bernabei et al. Nucl. Phys. At. Energy 22, 329 (2021).

[11] R. Maruyama, Nucl. Phys. B 1003, 116457 (2024).

[12] J. B. Birks, Theory and Practice of Scintillation Counting (Pergamon Press, 1967), pg. 453, 455.

[13] Y. K. Semertzidis and S. Youn, Sci. Adv. 8, eabm9928 (2022).