Physics Colloquium

For many years, neutrinos were thought to be massless. However, in the last decade or so, measurements of neutrino mass squared differences in neutrino oscillation xperiments gave a small but nonzero result. To find the actual neutrino masses, a complementary but very sensitive experiment is required. Large scale single and double beta decay experiments are currently being pursued to address this fundamental question. The EXO (Enriched Xenon Observatory) experiment, a liquid xenon double beta decay experiment, will be described. The goal of EXO is to search for neutrinoless double beta decay in liquid Xe-136 with ZERO background, leading to a mass sensitivity as low as 0.007 eV. This is made possible by precision measurement the total energy of the two decay electrons AND, for the first time, detection of the daughter Ba-136 ion at the determined decay site.
The speaker will focus especially on work in his group and at Stanford/SLAC on the challenge of detecting a single Ba+ ion in liquid xenon, preparatory to detecting the one Ba+ daughter ion that may be created in 1-10 tons of xenon in 5-10 years of EXO operation. Much of the current work involves new questions in condensed matter physics, such as, “How do you grab a single ion in liquid Xe and then release it efficiently in a linear trap in vacuum”, or for direct Ba+ tagging in the liquid Xe, “What is the configuration of the ion’s neighbors – is it a bubble or is it a Xe snowball?” and “How spectrally broad and efficient are the absorption and emission of a Ba+ ion in liquid Xe?” Mobility measurements and laser spectroscopy are beginning to answer these questions.