Liquid Argon Detector Calibration R&D for Dark Matter and Neutrino Physics

This proposal is for R&D to improve the performance of experiments testing the fundamental symmetries of nature using neutrinos and searching for interactions of the mysterious dark matter that makes up most of the matter in the universe.
Particle physics seeks to understand the nature of matter at the smallest scales and relate that knowledge to the behaviour of the universe at the largest scales. One of the main questions being addressed by experiments today is why the universe is made of matter and not antimatter. The standard model of particle physics links conservation laws with fundamental symmetries: the missing antimatter is connected to the violation of charge-parity (CP) symmetry---which says that the laws of nature should be the same for matter and for antimatter seen through a parity inversion, equivalent to looking at something upside down in a mirror. All known instances of CP violation are too small to account for the lack of antimatter, so experimenters are now searching for the origin of this using the elusive neutrino.
Astronomical observations have shown us definitively that the matter we see makes up only a small fraction of the universe---just 5%! A significant fraction, 25%, is made up of enigmatic dark matter: massive particles that do not interact with light but do influence the world around us. The standard models of particle physics and cosmology predict that these dark matter particles obey the same laws of nature that normal matter does and so we can devise laboratory experiments to detect evidence of their presence. The leading theory is that dark matter is made up of heavy particles that interact via the weak nuclear force, just like neutrinos do. If so, then it should leave characteristic traces in terrestrial matter: atomic nuclei struck by dark matter particles will recoil with tiny amounts of energy. To test this hypothesis we build experiments designed to observe these tiny recoil energies. Because the rate of these interactions is small, and the recoil energy is tiny, we must build experiments at the cutting edge of particle detection technology to search for this weak signal.
Many dark matter and neutrino experiments use cryogenic (very cold) noble liquids, such as liquid neon, argon or xenon, because of their stable atoms and excellent optical properties. In particular, these liquids emit large amounts of light when the dark matter particles cause nuclear recoils, and that light can be collected and interpreted readily to identify the type of particle that created it: nuclear recoil signal or an interaction of background radiation. Cryogenic noble liquid detector development is pushing the boundaries of particle detection, in a world-wide technology race to study the fundamental questions of the nature of neutrinos and dark matter. However the properties of light in these liquids is not very well understood, and therefore experiments must develop systems to initiate controlled "signals" inside the detectors that test the ability of the experiment to observe the signal, distinguish it from backgrounds and measure its energy correctly. This process is called detector calibration and it is where experimenters spend much of their effort.
This proposal is to develop new technology for detector calibration and to make measurements of the optical properties of cold liquid argon, neon and xenon. Specifically, we will measure the attenuation lengths of light in these liquids; we will measure the angles that light is scattered into from the wavelength shifting films that must be used in cold liquids; and we will test the mechanical utility of certain designs for calibration devices. Once these measurements are made, the calibration of large, liquid noble detectors will be significantly improved, and we will be one step closer to identifying the nature of the puzzling dark matter that accounts for 25% of the universe, and understanding the mystery of the missing antimatter.

The beneficiaries of the proposed work include (i) researchers in the fields of particle physics, nuclear engineering, condensed matter, and astrophysics; (ii) governmental and industrial research entities with interest in neutron and gamma detection; (iii) students who will receive training from the proposed activities; and, (iv) society at large.
Researches in the fields of particle physics, nuclear engineering, condensed matter, and astrophysics will benefit directly from the proposed measurements and technology development. The proposed R&D will improve understanding of particle detection in large, liquid noble detectors, which are of interest for all of these fields for areas of research including neutrino and dark matter detection for fundamental physics, neutron and gamma detection for reactor monitoring, excimer formation and decay relevant for laser development and atomic physics, and optical propagation for studying the chemical composition of the interstellar medium, stars, and galaxy formation. The proposed R&D is aimed at improving particle detection uncertainties, and in general a cost-effective way to maximise physics return on the investment in a large detector is to minimise systematic uncertainties.
Governmental and industrial entities with interest in neutron and gamma detection will benefit from the proposed research because it advances relevant particle detection technology. Liquid noble gas detector offer very good energy resolution for both neutrons and gamma rays, fast response time (and therefore high count-rate capabilities), excellent discrimination between neutrons and gamma rays, and scalability to large volumes. There are efforts underway to develop liquid noble gas detectors for fissionable material detection in cargo based on the extensive and ongoing R&D effort in high-energy physics for dark matter and neutrino-less double beta decay detection. The unique properties of these detectors are promising for Non-Intrusive Inspection for Special Nuclear Materials in cargo, as well as for non-proliferation efforts on reactor monitoring.
Students and post-doctoral researchers will benefit directly from the proposed work because they will receive training in detector design, development, construction, commissioning, data acquisition, as well as detector simulation and analysis. Three PhD students and two post-doctoral researchers will be directly involved in this work, as well as 3-4 MSci. project students over the course of the project (supervised by the three academics plus one RAL staff member on the proposal). Indirectly, high school and undergraduate students will benefit from outreach activities associated with this proposal, such as the RAL Masterclass and Sussex Schools Lab. These activities promote participation in science and technology.
Society at large will benefit from the proposed activities because the research will contribute to answering the big questions in science today, as well as develop detector technology with applications relevant to national security, and train and inspire young people to pursue careers in the areas of science and technology.