Until recently, there were strong motivations that dark matter (DM) is made out of particles with masses in the scale of 100 times the mass of the proton, the so-called WIMPs (Weakly Interacting Massive Particles). For many years, WIMP searches have dominated dark-matter research but without any clear evidence or hint appearing so far in one of our detectors. This is beginning to trigger a paradigm shift to another dark-matter candidate: a dainty particle called an axion, which could be more than a billion times lighter than an electron and much lighter than the WIMP. The physics of these particles has been considerably developed in recent years, and there are now useful guidelines and powerful motivations to attempt experimental detection.
In a recent seminar entitled “New ideas for the axion dark matter program” Nick Rodd (Miller fellow, UC Berkeley) outlined two key ideas that underpin and inform the future experimental programme in axion searches. “The field of dark matter searches is undergoing a transition to new ideas about the substance of dark matter based on axion or axion-like particles as good candidates”. Axions could also resolve another long-standing puzzle in physics: why do the quarks in a neutron arrange themselves to produce no observable electric dipole moment? In fact axions were first postulated through the Peccei-Quinn solution that addresses this so-called strong-CP problem. For Rodd, “it is encouraging that, as was noted over 40 years ago, a strong independent motivation for the existence of at least one axion comes from the theory of strong interactions. This “QCD axion” would explain why the laws of strong interactions are invariant under changing the direction of time.”
Given the strong theoretical motivation for axions, the experimental landscape is rapidly evolving, with many novel detection techniques and new experimental proposals. According to Rodd, “in the next decade we could have an answer as to whether the axion is DM or if we have been wrong in our guess. There are good reasons to be optimistic about the planned experimental programme” and adds “however this is also the time to reflect on how to maximize the research potential of the instruments that we are going to build. Questions about the possible signatures of DM axions or the amount of information contained in the data that we plan to collect are pertinent and should be addressed in parallel to experimental progress”.
Unlike the WIMP, which we conventionally think of as a particle that we could detect when it bumps into our detectors, the much lighter axion can be described as a wave. This means that our instruments could detect them as oscillating waves and in fact apply interferometry techniques that would give us a wealth of information about their source. “Axion interferometry proposes combining axion search results from different geolocations thus performing interferometry directly on the dark-matter wave”. The second idea Nick Rodd explored in his seminar is the search for relativistic axions that would form a residual Cosmic axion Background (CaB). “Existing searches at, for instance ADMX, are not yet sensitive to a cosmic relic, although if relativistic axions are produced in the late Universe, by for instance dark-matter decaying to axions, then this is a signal the experiments are already sensitive to, but would miss with existing analyses. Qualitatively the signal from CaB would be similar with that of Cosmic Microwave Background (CMB) and we should ensure that we don’t accidentally miss the axion discovery by not carefully analysing the data.”
Fundamentally, a wave is described by its amplitude and phase. Interferometry could give us access to the information contained in the phase of the wave that is related to its momentum. This can be seen by writing the equation for the wave-like Dark Matter as shown below:
We asked our interviewee whether having this information is also relevant/paves new ways in searches of streaming dark matter. “Our best guess at the moment is that the bulk of dark matter is like a gas moving all over the place in the rest frame of the Milky Way. Earth is streaming through this gas and although we don’t know for sure which direction it hits the Earth, our expectation is that it points to the constellation of Cygnus - though this could be wrong if the bulk of local dark matter is in streams. By performing axion DM interferometry and verifying that the signal points to the Cygnus constellation you will have another extremely important and non-trivial confirmation that your signal is related to DM and not an unexpected background”. This is particularly important when searching for such feeble signals. Rodd reminds me that “every new signal, even when it eventually turns out to be a discovery like the Higgs boson, starts with a small significance and our scientific attitude is to question it and perform tests to exclude that it could be a systematic error in our instruments or our analysis”. As new tools will offer much higher sensitivity - allowing us to explore a new regime in the parameter space where axions could live - we could see all sorts of things including unexpected backgrounds. This is why it is important to get more information from interferometry allowing us to verify that our signal is a DM axion. Furthermore, interferometry would prove to be particularly useful in the event that the local DM distribution is more complicated than we imagine. “Part of the DM near the Earth could be in an ultra coherent stream. In this case, we have no expectation as to where this stream might be coming from, and would only be able to test this using interferometry.”
Rodd also emphasizes an interesting experimental fact as interferometry would give a very dramatic fluctuation in the signal even with a single day of data. “We don’t live in the reference frame of DM but in the reference frame of the Earth. So as the Earth rotates the separation between the two instruments is changing with respect to the incident direction of the DM. So the relative phase seen by the two instruments changes throughout the day so when you add the two instruments together the interference pattern changes hugely over the 24h period. This would be very strong evidence that what you see is DM”. This daily modulation, one of the hallmark features of DM interferometry, is shown in figure 2 below.
Fig.3: A Mollweide projection of the Asimov test statistic for the location on the sphere of the boost velocity of the Sagitarrius stream. Brighter locations indicate those locations which are preferred by a likelihood analysis of the interference effect which arises in the analysis of axion direct detection data simultaneously collected at two spatially separated locations. By leveraging the daily modulation of the separation of the two detectors which are confined to the Earth's surface, an increasingly precise determination of the boost orientation can be made. Each frame shows the improvement to the localization based on an additional hour of data collection over a 24 hour collection period. This animation additionally compares the localization for configurations where the detectors are oriented along the East-West (North-South) direction with respect to one another in the left (right) panel. The North-South configuration leads to a better localization since there is a unique global maximum in the Asimov test statistic, whereas the East-West configuration produces an exact degeneracy between two global minima. For more details, see the main work. Copyright: 2020 Joshua Foster (MIT Press).
There are a number of ways we could imagine producing the light axons that make up the CaB. Possible axion sources include thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these sources would give a characteristic frequency spectrum that can be searched for in axion direct detection experiments, and if found would allow us to answer basic questions about the early history of our Universe, as shown in figure 4:
The key insight in the work of Rodd and his collaborators is that the axions that make up the CaB would also create a signal in the same instruments searching for dark-matter axions. Yet critically, the signals would look quite different: whilst dark-matter is predicted to produce all its signal at almost a single frequency (determined by its mass), the CaB would deposit its power over a broad frequency range. Existing dark-matter searches would discard such broad distributions as a background, thereby potentially missing a discovery.
The discovery of one axion would boost our thinking that there are different types of axions, one of which could even constitute dark matter. Furthermore, the search for axions reminds us of the close ties between particle physics and cosmology.
DM axion interferometry and searches for relic axions are both well motivated and present new opportunities for the foreseen instruments currently being developed in different laboratories around the world. In a closing remark Rodd tells me “We want to make sure that we are not biased towards one signal and miss a discovery. The ongoing cross talk is needed as every year new instruments come online and each of them could bring a new discovery.”