EXO-200 Experiment (Enriched Xenon Observatory)

Waste Isolation Pilot Plant (WIPP), Carlsbad, NM
link to http://www.wipp.energy.gov/%5C/science/UG_Lab/UG_LabNew.html
Liang Yang (Co-Spokesperson)

EXORecent 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 anti-particle, i.e. a Majorana particle? These questions are best addressed by searching for neutrinoless double beta decay, an exotic nuclear process which can shed light on both the absolute scale of the neutrino mass spectrum, and on the underlying mechanism responsible for the tiny masses that we observe in nature.

Neutrinoless Double Beta decay

Double beta decay is the simultaneous beta decay of two nucleons inside a parent nucleus. There are two possible decay modes. The first decay mode involves the emission of two neutrinos along with two electrons and the final state nucleus (2νββ) , while the second decay mode has no emitted neutrino in the final state (0νββ). The 2νββ decay mode is allowed by the Standard Model and has been observed in several isotopes. The 0νββ decay mode, on the other hand, is a lepton-number-violating process that can occur only if neutrinos are massive Majorana particles (i.e. their own antiparticles).

EXO-200 New 0νββ Search Results Announced

The new neutrinoless double beta decay search results from EXO-200 was announced at the TAUP meeting (July 2017) after one year of Phase-II operation. In Phase-II, the detector energy resolution at the decay energy region is improved to 1.23%. External radon background between the cryostat and lead shield has been suppressed with radon free air. In addition, new analysis techniques for discriminating the gamma background has been implemented. Incorporating both hardware and software upgrades, the combined sensitivity for Phase-I and Phase-II data improved 2-fold to 3.7 x 1025 years. No statistically significant signal was observed, leading to a lower limit on the 0? half-life of 1.8 x 1025 yrs at the 90% confidence level. The UIUC group led the front-end electronics upgrade for Phase-II and contributed substantially to the analysis. Graduate student Shaolei Li studied the wire gain calibration and checked the grid correction for Phase-II. Graduate student Matthew Coon presented the new results at the 2017 Meeting of APS Division of Particle & Fields.

Image (right): The EXO 0νββ search result and its comparison with other experiments.

EXO-200 Phase-I Results

EXO-200 is one of the largest running double beta decay experiments. Its central component is a liquid xenon time projection chamber with ~110 kg active volume. EXO-200 began low-background data taking in June 2011. Using the first two years of data, the collaboration published a 0νββ decay search result with total 136Xe exposure of 100.0 kgyr, and set a lower limit for neutrinoless double beta decay half-life at 1.11025 yr for 136Xe. This is one of the most sensitive searches for this type of rare decays. To appreciate the extreme long life time of the decay, we can compare it to the age of the Universe, which is merely 1.4 x 1010 years.

Image (right): One half of the EXO-200 TPC during assembly.

EXO-200 Phase-II Upgrade and Experimental Sensitivity

EXO-200 underwent two major detector upgrades in 2016. First, the frontend readout system was upgraded to reduce coherent noise in the scintillation channels and lower the threshold for the V wire channels. Second, a Rn-suppressed air system was commissioned to purge the air gap between the cryostat and lead shielding.

The Illinois group successfully led the upgrades to the front-end electronics system. The new frontend readouts for the APD channels removed coherent noises and had an immediate impact on the detector energy resolution. The time averaged detector resolution is 1.58% at the 0νββ decay Q value (2.46MeV). After the electronics upgrades and optimizing detector drift field, the energy resolution at the Q value improved to 1.28%. Further improvements are possible with new analysis techniques. Furthermore, elimination of the APD coherent noise has also lowered the scintillation reconstruction threshold, enabling us to probe physics channels at lower energies with the Phase-II data. The EXO-200 electronics upgrade work was supported by DOE award DE-SC0014332.

Image above: EXO-200 Detector Resolution in Phase I and Phase II

EXO-200 Phase-II low background data taking began on April 29, 2016 and will continue until 2018. Sensitivity studies indicate that with the detector upgrades and analysis improvements, EXO-200 can reach a 0νββ decay half-life sensitivity of 5.71025 years, using combined Phase-I and Phase-II data. The EXO-200 result will provide one of the most sensitive searches for 0νββ decay and demonstrate the capabilities of LXe TPC for future tonne-scale detectors.

Student Research Opportunities

We look for talented students to join the team to develop new analysis techniques to enhance the detector energy resolution and improve its background rejection capabilities. These new techniques can extend the physics reach of EXO-200 and future tonne scale detector, nEXO. The work is supported by NSF CAREEER award, PHY-1654495.

EXO-200 Interview