In light of the newly developed model (M3 and SM3 ), if further confirmed, most effort of current dark matter search will be destined to failures. Indeed, there is nothing to detect if there is no direct interaction, however weak, between normal particles and dark (mirror) particles. This makes all the Weakly-Interacting-Massive-Particle-like (WIMP-like) or axion search programs to no avail. However, the advancement of the detection technology with the past efforts including those for the detection of neutrinos could be rekindled to a new life for the studies of mirror matter.
The idea is that oscillations between normal and mirror neutrons (i.e., n-n’) can be detected in a large-volume scintillation detector made of absorption-free dense materials in addition to other laboratory tests.
Resonant n-n’ oscillations could occur when neutrons travel in dense enough materials due to the medium effects, similar to the so-called matter effects for the normal neutrino oscillations. Unfortunately, the density condition for such resonant oscillations is about 100-1000 g/cm3 (or hundreds to thousands of times the water density), which is typical or easily found in burning regions of stars. Such a condition, however, is not feasible on Earth, as the densest natural materials like Osmium and Platinum have a density of about 22.6 g/cm3.
Another obstacle to observe n-n’ oscillations on Earth is that neutron capture reaction cross sections are pretty large for many isotopes or nuclei. So when neutrons interact with other nuclei in the terrestrial environment, more often than not, they’ll be captured to form heavier isotopes before the n-n’ oscillation could play a role. Fortunately, there are indeed some materials that have almost zero neutron-capture cross sections, i.e., nearly absorption-free, such as helium-4 and heavy water(D2O).
The last issue is the size of the detector for n-n’ oscillations. On average, a neutron particle will oscillate into a mirror one by a very tiny chance of about 10-5 after each collision with another nucleus in the detector material. It is so rare that we have to make the detector large enough to observe the oscillations before neutrons escape from the detector.
Here we can see how the technology from dark matter search and neutrino studies can be utilized.
For example, we can construct a liquid helium-4 (LHe4) detector, similar to the liquid argon (LAr) or liquid xenon (LXe) detectors used in dark matter search. All these materials are noble gas elements and have similar chemical and scintillation properties. The cryogenic techniques developed for these types of detectors can be readily used for the helium-4 detector. The good thing about helium-4 is that it is basically absorption free. So the only competing process to n-n’ oscillations is the neutron beta decay. It can be calculated that the n-n’ oscillation rate is about six times the beta decay rate in this case, making the signal easy to be detected.
Unfortunately, helium-4, even in liquid form, is not very dense with a density of about 0.125 g/cm3. Therefore, the mean free path of a neutron (even at extremely low temperatures like 4 K) inside the he-4 liquid is fairly large, about 40 cm. To keep the neutrons inside within the oscillation time scale of about 160 seconds, we have to build the detector of about 100 meters in size. Nevertheless, our experiences with ton-size LAr and LXe detectors for dark matter search should help on building such LHe4 scintillation detectors.
Similarly, the techniques used for neutrino detection with a large volume of water could also be used for the observation of n-n’ oscillations. Instead of normal water, heavy water would be a good detection medium like the old SNO which had unfortunately ceased operations. The advantage of this medium is that it can work at room temperatures. But deuterium and oxygen in heavy water still have small yet nonzero neutron capture cross sections. As such, the n-n’ oscillation rate is only about 16% of the absorption rate, which is a much smaller effect compared to LHe4. Another advantageous point of using heavy water is that the neutron mean free path is fairly small, about 1.5 cm such that the detector size does not have to be as large as LHe4. As a matter of fact, 5-meter in size would be enough to contain neutrons within the oscillation time scale of about one second.
I hope that the communities from dark matter search and neutrino detection will recognize the importance of applying their existing detector technology to the tests of n-n’ oscillations of M3 and SM3 . More technical details can be found in the paper arXiv:1906.10262.
