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Register a new userSensitivity of the DARWIN observatory to the neutrinoless double beta decay of $^{136}$Xe
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Added by
Patricia Sanchez-Lucas
Publication date
2020
fields
Physics
and research's language is
English
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The DARWIN observatory is a proposed next-generation experiment to search for particle dark matter and for the neutrinoless double beta decay of $^{136}$Xe. Out of its 50$,$t total natural xenon inventory, 40$,$t will be the active target of a time projection chamber which thus contains about 3.6 t of $^{136}$Xe. Here, we show that its projected half-life sensitivity is $2.4times10^{27},$yr, using a fiducial volume of 5t of natural xenon and 10$,$yr of operation with a background rate of less than 0.2$~$events/(t$cdot$yr) in the energy region of interest. This sensitivity is based on a detailed Monte Carlo simulation study of the background and event topologies in the large, homogeneous target. DARWIN will be comparable in its science reach to dedicated double beta decay experiments using xenon enriched in $^{136}$Xe.
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NEXT-100 is an electroluminescent high-pressure xenon gas time projection chamber that will search for the neutrinoless double beta ($beta beta 0 u$) decay of Xe-136. The detector possesses two features of great value for $beta beta 0 u$ searches: energy resolution better than 1% FWHM at the $Q$ value of Xe-136 and track reconstruction for the discrimination of signal and background events. This combination results in excellent sensitivity, as discussed in this paper. Material-screening measurements and a detailed Monte Carlo detector simulation predict a background rate for NEXT-100 of at most $4times10^{-4}$ counts keV$^{-1}$ kg$^{-1}$ yr$^{-1}$. Accordingly, the detector will reach a sensitivity to the bbonu-decay half-life of $2.8times10^{25}$ years (90% CL) for an exposure of 100 $mathrm{kg}cdotmathrm{year}$, or $6.0times10^{25}$ years after a run of 3 effective years.
A high pressure xenon gas time projection chamber with electroluminescent amplification (EL HPGXe TPC) searching for the neutrinoless double beta ($0 ubetabeta$) decay offers: excellent energy resolution ($0.5-0.7%$ FWHM at the $Q_{betabeta}$), by amplifying the ionization signal with electroluminescent light, and tracking capabilities, as demonstrated by the NEXT collaboration using two kg-scale prototypes. The NEXT collaboration is building an EL HPGXe TPC capable of holding 100 kg (NEXT-100) of xenon isotopically enriched in ${{}^{136}rm Xe}$. The installation and commissioning of the NEXT-100 detector at the Laboratorio Subterraneo de Canfranc (LSC) is planned for 2018. The current estimated background level for the NEXT-100 detector is of $4times10^{-4}$ counts/keV-kg-yr or less in the energy region of interest. Assuming an energy resolution of 0.75$%$ FWHM at the $Q_{betabeta}$ and a $0 ubetabeta$ signal efficiency of about 28$%$, this gives an expected sensitivity (at 90$%$ CL) to the $0 ubetabeta$ decay half life of $T^{0 u}_{1/2}>6.0times10^{25}$ yr for an exposure of 275 kg yr. A first phase of the NEXT experiment, called NEW, is currently being commissioned at the LSC. The NEW detector is a scale 1:2 in size (1:10 in mass) of the NEXT-100 detector using the same materials and photosensors and will be used to perform a characterization of the $0 ubetabeta$ backgrounds and a measurement of the standard double beta decay with neutrinos (${2 ubetabeta}$). An 8 sigma significance for the ${2 ubetabeta}$ signal in the NEW detector has been estimated for a 100-day run.
Searching for the Neutrinoless Double Beta Decay (NLDBD) is now regarded as the topmost promising technique to explore the nature of neutrinos after the discovery of neutrino masses in oscillation experiments. PandaX-III (Particle And Astrophysical Xenon Experiment III) will search for the NLDBD of $^{136}$Xe at the China Jin Ping underground Laboratory (CJPL). In the first phase of the experiment, a high pressure gas Time Projection Chamber (TPC) will contain 200 kg, 90% $^{136}$Xe enriched gas operated at 10 bar. Fine pitch micro-pattern gas detector (Microbulk Micromegas) will be used at both ends of the TPC for the charge readout with a cathode in the middle. Charge signals can be used to reconstruct tracks of NLDBD events and provide good energy and spatial resolution. The detector will be immersed in a large water tank to ensure $sim$5 m of water shielding in all directions. The second phase, a ton-scale experiment, will consist of five TPCs in the same water tank, with improved energy resolution and better control over backgrounds.
In the search for the neutrinoless double beta decay of $^{136}$Xe, a high pressure xenon time projection chamber (HPXe-TPC) has two advantages over liquid xenon TPCs: a better energy resolution and the access to topological features, which may provide extra discrimination from background events. The PandaX-III experiment has recently proposed a 200 kg HPXe-TPC based on Micromegas readout planes, to be located at the Jinping Underground Laboratory in China. Its detection concept is based on two results obtained within the T-REX project: Micromegas readouts can be built with extremely low levels of radioactivity; and the operation in xenon-trimethylamine at 10 bar in realistic experimental conditions has proven an energy resolution of 3% FWHM at the region of interest. In this work, two discrimination methods are applied to simulated signal and background data in a generic 200 kg HPXe-TPC, based on two well-known algorithms of graph theory: the identification of connections and the search for the longest path. Rejection factors greater than 100 are obtained for small pixel sizes and a signal efficiency of 40%. Moreover, a new observable (the blob charge density) rejects better surface contaminations, which makes the use of a trigger signal ($T_0$) not imperative in this experiment.
A new method to tag the barium daughter in the double beta decay of $^{136}$Xe is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (Ba$^{++}$) resolution at a transparent scanning surface has been demonstrated. A single-step photo-bleach confirms the single ion interpretation. Individual ions are localized with super-resolution ($sim$2~nm), and detected with a statistical significance of 12.9~$sigma$ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.
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