Direct measurement of nuclear polarization for decay correlations testing the Standard Model

点击翻译按钮获取中文摘要

This proposal addresses our largest systematic errors in determining the degree of spin polarization of our decaying nuclei. We need more accuracy to improve most of our TRIUMF neutral atom trap (TRINAT) program, so that our tests of the Standard Model predictions of decay correlations remain competitive with and complementary to other improving particle physics experiments.  We want to measure directly the degree of spin polarization we achieve by optically pumping our potassium atoms. The most direct way to do that is to first split the spin projections of our atomic levels in energy-- using an external magnetic field, the Zeeman effect-- and then scan over them with a resonant laser, exciting an optical transition in the atom. However, we must excite all these sublevels nearly uniformly when we spin-polarize them by optical pumping, so we must keep the energy splitting well below the natural linewidth of our optical pumping transition (technically, the 6 MHz-wide 4S to 4P1/2 770 nm transition). So with this RTI proposal we would resolve the Zeeman-split atom sublevels using the narrower-linewidth 1.1 MHz 4S to 5P resonant transition at 405 nm. The measured relative populations would give us a direct model-independent nuclear polarization measurement, a critical test of our present less direct determination as we seek to improve our accuracy to better than 0.1%.   A further future research application, making our laser-cooled potassium atom cloud substantially colder and smaller, is also described, along with its potential impacts on our precision measurements by improving the angle resolution between our decay products.   I have guided two successive co-op research terms with patient students trying to make this new approach work with an obsolete first-generation commercial laser at 405 nm. While completing much simpler measurements, we've identified several issues that make these tricky measurements unfeasible with present equipment. This RTI would upgrade to a much better model 405 nm laser with specified narrower linewidth, better spatial mode quality, and sufficient power available to uniformly probe the entire atomic ensemble.   It would be used by an existing Canadian Ph.D. thesis student for the spin-polarized experiments, along with help from the P.I. It would hopefully be the basis of one to three undergrad co-op research terms through the TRIUMF student program (all ads invite all to apply, and we are grateful some students thank us for that in their cover), as we consider safety of hands-on tutorials starting summer 2021 and possible pandemic status. These, like roughly half our undergrad research projects, could be done by a year 2A student even without the lab courses (not always available at smaller universities), and all cover letters are of course read by the PI.