Dark Matter Weakly Interactive Massive Particles (WIMP) in the Galactic halo should have an apparent motion from the Cygnus constellation to the Earth. Most sensitive detectors for dark matter candidates currently in operation cannot measure the correlated direction of the WIMP scattered ion in the target bulk.
A new solution to the problem of dark matter directional detection might come from the use of new detectors constituted by large arrays of aligned carbon nanotubes (CNTs), as WIMP targets. A brush of aligned CNTs oriented in the direction parallel to the WIMP wind might represent an anisotropic medium for the carbon ion scattered off the CNT surface by the WIMP. This idea has been investigated with simulations in three papers (two published and one in preparation) authored by some of the participants. Carbon ions scattered by the WIMPs would induce defects into the CNT honeycomb mesh itself. We expect the density of defects being dependent on the relative orientation of the impinging WIMP and the CNT axes.
We propose to experimentally test this idea by emulating the scattered carbon ion with calibrated ion beams. The induced defect anisotropy can be directly measured by core-level X-ray photoemission (XPS) and Raman spectro-microscopies, both highly sensitive techniques to accurately control the chemical state, variation of bonding and density of defects and lattice strain in carbon-based materials. Preliminary XPS results carried out with low-energy (2-5 keV) Ar+ ions as a function of incidence angle with respect to the CNT axes reveal a slightly yet measurable modification of the C 1s lineshape, very promising for demonstrating directionality, whose angular and energy dependence will be fully determined in the context of this project.
Predictions for WIMP masses span several orders of magnitudes. Current direct WIMPs search experiments have very limited sensitivity (if none) in the low mass range, that is in the region of few GeV/c2. Direct search experiments are in fact using relatively high mass nuclei as target. CNTs - on the contrary - will offer a target with low mass nuclei as carbon, extending the sensitivity reach.
It is therefore very interesting to develop a novel WIMP detecting strategy based on carbon-based materials. A possible strategy could be represented by the quantitative observation of the defect density induced by WIMP induced carbon ion recoils. Moreover, if this project proves that the damage due to ions crossing the CNTs is a function of the relative orientation of the CNT axis, this would represent a powerful tool to discriminated the signal of WIMP-induced ion recoils from the background.
Currently, none of the existing direct search experiments for WIMP is able to discern nuclear recoil direction. If this anisotropic feature of a CNT target will be proved, we might envisage in the future to build an experiment of the size needed to detect dark matter particles.
It must be mentioned that in the future the integrated exposure of the current direct search experiments will be such to make them sensitive to the solar neutrinos scattering. This weakly interacting particles have well known interaction with matter but they leave the same signature in the detectors as WIMPs would do. The directional information for nuclear recoils will then become unavoidable for further searches of WIMPs to discriminate events originated by WIMP in the direction of the Cygnus constellation from neutrinos coming from the Sun.
An accurate determination of the anisotropic behaviour of well-aligned CNTs towards controlled damage via the integrated analysis of the C 1s lines-hape and of the Raman spectroscopy constitutes an important and new approach to obtain a quantitative observation of defect density as induced by Ar+ bombardment. We remark that, while particle-irradiation induced effects on CNTs have been studied [16, 20], no analysis of the angular dependence, thus of the possible anisotropy, nor of the threshold dose on highly-aligned CNTs is available to date.
This in turn might be the basis for a new WIMP detection technique. In fact, the controlled ion bombardment would represent a fundamental step to simulate the effect of defect formation associated to WIMP and to eventually correlate them with the initial direction of the WIMP.
If nano-materials as CNT might effectively be used as anisotropic target, further developments might be envisaged as the functionalization of CNT with other atomic species: this - for instance would pair the anisotropic response of the CNT supporting structure with a CNT dopant made of different nuclei and therefore sensitive to different mass ranges.
This project might represent the seed for investigation of the interaction of low energy charged ions with other future nano-materials. It might represent a powerful tool to test new nano-materials that might be eventually optimized for a future detector of these elusive dark matter particles.