Search for axions in streaming dark matter
Streaming dark matter (DM) axions may be the better source for their discovery than the widely assumed isotropic DM. Because, a large axion flux enhancement can take place, temporally, due to gravitational lensing when the Sun and/or a planet are aligned with the stream or an axion caustic pointing to the Earth. This concept requires a wide-band axion antenna with fast scanning mode, and can run parasitically at no extra cost. Some new DM axion detectors have this scheme built-in, though unnoticed. In order not to miss a discovery, a coordinated network of axion haloscopes can provide full time coverage and large axion mass acceptance. Of interest are also axion miniclusters (MC) in particular if the solar system has trapped one during its formation. If we were to cross the path of a minicluster during a few days per year, the dark matter density would increase by ρMC~105•ρ<DM> and the temporal sensitivity gain a factor 105 with a duty cycle of about 1% compared to a conventional search using a narrow band and slow scanning detector.
The identification of the constituents of the mysterious dark matter (DM) in the Universe is one of the few fundamental open questions in physics. The well-motivated axion has not been discovered as yet, in spite of the efforts undertaken world-wide. The axion naturally arises from a suggestion by Peccei and Quinn to solve the strong CP-problem, whose striking manifestation is the negligibly small electric dipole moment of the neutron. This remains a serious and challenging issue for the Standard Model. Discovering the axion may thus solve both a problem in particle physics and an enduring mystery in cosmology. The fact that axion DM is at large for such a long time, a re-orientation of the applied detection concepts outside the widely accepted DM picture seems inevitable. To some degree, the same reasoning applies also to other DM candidates.
The search for DM, and in particular for relic axions, has been focused on the assumed isotropic halo of our Galaxy, with an expected broad velocity distribution around 240 km/s and an average density of ρ<DM> ~ 0.3 GeV/cm3. However, this target choice might have been the reason behind the failure in detecting the axion so far. Instead, the proposed new strategy for the detection of the elusive axion is based on streaming DM axions. Even a tiny flux might get temporally strongly enhanced due to gravitational lensing effects, surpassing thus on the long term the isotropic local DM density. This can happen, if the Sun or some planet is found along the direction of a DM stream propagating towards the Earth.
The planetary motion opens additional windows of opportunity for focusing and bending of DM constituents. For example, the “Sagittarius Stream” (Figure 1) and/or the dense tidal streams of axion-miniclusters, along with the actually overlooked planetary gravitational lensing effects for slow speed particles (~10-3c) are fitting in. Interestingly, the expected mass of a trapped axion-minicluster (~10-12 Msun) is about 100 times below a conservative bound on possible DM in the Solar System following many high precision positional observations of planets and spacecraft, but it is still >105 times above the average DM density. Crossings may last a few days per year without assuming gravitational lensing. If this happens, it should imply temporally a 105 times better performance than a conventional search with the same set-up. I.e., after 1 year of data taking the sensitivity on the DM cross section is improved by five orders of magnitude compared to data taking with the same narrow-band and slow scanning detector, which is sensitive only to the isotropic DM axions.
Other DM streams include caustics, eventually also one associated with the annual alignment (within 5.5o) Galactic Center --> Sun --> Earth in December, and probably more unpredictable ones. Ideally, this new concept benefits largely from a joint effort to reach complete annual coverage and the maximum possible axion mass range (see below).
While streaming DM and (planetary) gravitational lensing are not new, however, both combined together may become instrumental in axion research, provided axion haloscopes have built-in:
- a wide-band receiver: the associated decreased detection sensitivity should be well compensated by the temporally increased influx of DM, and
- a continuous fast axion rest mass scanning: the faster the scanning the shorter dense axion bursts can be utilized
Both conditions seem feasible for the ~1.8K CAST magnet and eventually down to the mK range for other haloscopes, where heating-up effects should be the limiting factor. Remarkably, some novel DM detector schemes are wide-band by default, e.g. CASPEr and GNOME in Mainz, soon also ABRA-10 cm in MIT, etc. Interestingly, in such cases, these two requirements make no sense. Though unnoticed, this new relic axion detection proposal is already built-in and in operation! However, possible burst-like events from such detectors may be rejected, since they do not fit the general picture of the isotropic DM. In addition, the unavoidable machine stops require a network of co-ordinated axion antennae, preferentially around the Globe, in order not to miss an axion signal hidden in an unpredictable dense structure.
CAST has been converted to a DM-axion antenna, with the participation of the CAPP institute from Korea, and might well profit from including this new concept, when, as we expect, test measurements in Korea have been successfully completed.
In conclusion, this new concept can be implemented, to some degree, in every axion haloscope, if it has not been the case already (even though not on purpose). We stress that this scheme should actually run parasitically, providing thus an additional window of opportunity in the field.
Figure 1. The Sagittarius Stream is a complex structure made of tidally stripped stars and dark matter from the Sagittarius Dwarf Galaxy due to the ongoing merging with our Galaxy the last ~5 Gyears. The locations of the Sun (●) and the galactic center (GC) are given. The inserts show expanded the inner galactic region. The three most prominent distant branches of the Sgr stream are labeled NW (Northwest), NE (Northeast) and S (South). Courtesy Abraham Loeb / Harvard.
Further Reading: https://arxiv.org/abs/1703.01436