The nature of dark matter is one of the great mysteries in physics. Direct signals for dark matter have remained elusive. As future searches push to greater sensitivities and smaller interaction cross sections, backgrounds, especially from neutrons, will necessarily arise. Reducing these backgrounds to a negligible level through greater underground depths and more shielding will become impractical for very large detectors. Thus, as we move toward the time when dark matter searches will be performed at the Deep Underground Science and Engineering Laboratory, it is likely that we may have to depart from the zero-background paradigm, instead searching for signals above a well understood neutron background. This problem calls for a new approach to dark matter searches.
The research objective of this award is a search for a dark matter signal above a well understood neutron background, using a liquid Argon (LAr) dark matter detector (MiniCLEAN) underground in SNOLab. The goals of this research are: (i) to develop methods for measuring the neutron background in-situ in LAr dark matter detectors using multiple scattering, inelastic scattering, and neutron capture on 40-Ar; (ii) to develop an active neutron veto surrounding the dark matter detector; and (iii) to apply statistical tools incorporating these neutron background measurements in a dark matter search for signal above background. The MiniCLEAN design is a new direction for dark matter searches. It draws on highly successful, proven approaches of solar neutrino physics to building low-background detectors that scale simply to multi-tonne targets. Successful demonstration of this approach for dark matter by MiniCLEAN will break new ground for future, very large detectors. Success depends critically on background suppression; addressing the difficult neutron component is the focus of this proposal.
Broader impacts: Measurements of the neutron flux using these new methods will provide timely input to designing future large dark matter detectors, as well as shielding for other low background experiments, including neutrino-less double beta decay and low-energy solar neutrino experiments.
The identification of dark matter constitutes one of the major scientific challenges of our time. One compelling explanation for dark matter is the existence of Weakly Interacting Massive Particles (WIMPs). A number of experimental techniques have been developed to directly detect WIMPs by measuring combinations of scintillation light, ionization, or heat, using multiple handles to discriminate a WIMP signal from radioactive backgrounds. The ability to distinguish a potential WIMP signal against the myriad of backgrounds often drives the technological approaches used in dark matter experiments: hence why such experiments are often deep underground, require extreme material screening, and exploit unique signatures that help separate signals from background. Such has also been the focus of the work supported by this NSF grant; to optimize background reduction techniques so as to promptly identify a potential WIMP signal. The approach supported by this grant has moved along two distinct fronts. One focus has been to determine the ability to directly tag potential background events prevalent in dark matter detectors. Nuclear recoils from high energy neutrons often constitute an irreducible background in all WIMP detectors. These high energy neutrons are the product of cosmic ray interactions with the materials surrounding the detector and, as such, are often difficult to model and predict to high accuracy. By tagging such neutrons directly, one can directly measure such backgrounds and the effect they impose on WIMP detection. The MIT group has designed, constructed, and deployed several detectors using liquid scintillator so as to tag high energy neutrons underground. These measurements were conducted at the Waste Isolation Pilot Plant in New Mexico. Such detector technology could be used as a blueprint for a veto system for future large underground dark matter experiments. The second focus undertaken over the past three years has been in the design and construction of the MiniCLEAN dark matter experiment. MiniCLEAN is what is known as a single-phase detector which employs liquid argon as its WIMP target. The single-phase approach detects recoiling nuclei from WIMP interactions in liquid argon using only scintillation light. Backgrounds are rejected by way of highly discriminating pulse-shape analysis and fiducialization. The simplicity of the single-phase technique will allow the construction of very large mass (tens of tonnes or more) detectors at very low cost, thus pushing the major sources of backgrounds well outside the region of interest. Larger detectors, in this approach, are not only better from the standpoint of signal rate, but will have fewer sources of backgrounds and are far easier to model, allowing precise background predictions. The efficacy of this approach has already been employed by the very successful neutrino experiments that discovered neutrino oscillations, demonstrating the benefits of self-shielding and the precision possible in large, monolithic detectors. The MiniCLEAN detector should serve as the first technical demonstration of this approach using liquid argon single phase technology. MIT has helped in the design of the detector, with specific focus on the active veto, the calibration system, and the overall detector operations. The group has had the fortune of attracting faculty a number of undergraduate students to the project. In collaboration with MIT's UROP/IROP program, undergraduate students also had the chance to travel to Sudbury, ON to assist with detector construction. The experiment is nearing the end of construction and should begin first data taking by the end of the year. We thank the NSF for their support of this work.
