Is the Moon in a Freefall?
Search for Neutrinoless Double Beta Decay
Recent observations of tiny neutrino masses in solar, atmospheric, and reactor neutrino data raised several intriguing questions. Why are neutrinos so much lighter than the other particles? What is the absolute scale of the neutrino mass spectrum? And is the neutrino its own antiparticle, i.e. a Majorana particle? These questions are best addressed by searching for neutrinoless double beta decay (0νββ), an exotic nuclear process which can shed light on both the absolute scale of the neutrino mass spectrum, and the underlying mechanism responsible for the tiny masses that we observe in nature.
The Enriched Xenon Observatory (EXO) is an experimental program designed to search for 0νββ of 136Xe. The current phase of the experiment, EXO-200, uses 200 kg of liquid xenon with 80% enrichment. Double beta decay events are detected in an ultra-low background time projection chamber (TPC) by collecting both the scintillation light and the ionization charge. The detector began taking low background data since April, 2011. The collaboration produced several high-impact physics results, including the first observation and the most precise measurement of two-neutrino double beta (2νββ) decay in 136Xe, as well as one of most sensitive searches for 0νββ decay. The UIUC group is responsible for the electronics upgrade for the second phase of EXO-200 operation, and plays a leading role in EXO-200 data analysis.
The next generation 0νββ experiments aim to increase the experimental sensitivity to the 0νββ search by an order of magnitude to explore the neutrino inverted mass hierarchy region. This aim requires not only significant increase in isotope mass, but also a substantial improvement in detector performance. nEXO is a proposed tonne-scale experiment based on LXe technology demonstrated by EXO-200. Preliminary study shows that significant gain in detector performance can be achieved by optimizing the charge and light detection and associated readout systems. The UIUC group is leading an R&D program to develop low-noise, low-background cold readout electronics for nEXO.
136Xe offers the unique opportunity that its 0νββ decay daughter nucleus 136Ba can be tagged in situ by laser spectroscopy. Such a technique can completely eliminate radioactivity-induced background and make it possible to build a background-free experiment, and is a future upgrade path for nEXO detector to probe the neutrino normal mass hierarchy region. At UIUC, we are using radioactive ion beams at Argonne National Lab to study the barium atom surface desorption, ionization and transport as part of the barium tagging R&D.
Direct Detection of Dark Matter
Astrophysical observations have provided convincing evidence for the existence of dark matter. DAMA experiment claims to have observed the annual modulation signal of dark matter using NaI detectors at the Gran Sasso Lab in Italy. The DM-Ice project aims to conclusively test this claim with a new detector array at the South Pole. The collaboration has successfully deployed and operated a 17 kg detector in the polar ice cap, and is working towards a full scale detector. At UIUC, we are developing ultra-low background NaI powder and crystals as well as trace contaminant measurement techniques.
Measurement of Neutron Electric Dipole Moment
Measuring the neutron electric dipole moment (EDM) is one of the most sensitive ways to search for Charge Parity (CP) violating processes beyond Standard Model. The current null limit has ruled out or greatly constrained many theoretical models. The nEDM project at the Spallation Neutron Source in Oak Ridge, TN, seeks to improve the experimental limit by two orders of magnitude. In my group, we are developing simulation software for scintillation light detection and novel photon detector system that can improve the measurement sensitivity for the experiment.
465 Loomis Laboratory
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