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

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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.