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Signal-background discrimination with convolutional neural networks in the PandaX-III experiment using MC simulation

Science China Physics, Mechanics & Astronomy, Vol.61, 101007(2018)

The PandaX-III experiment will search for neutrinoless double beta decay of ${}^{136}Xe$ with high pressure gaseous time projection chambers at the China Jin-Ping underground Laboratory. The tracking feature of gaseous detectors helps suppress the background level, resulting in the improvement of the detection sensitivity. We study a method based on the convolutional neural networks to discriminate double beta decay signals against the background from high energy gammas generated by ${}^{214}Bi$ and ${}^{208}Tl$ decays based on detailed Monte Carlo simulation. Using the 2-dimensional projections of recorded tracks on two planes, the method successfully suppresses the background level by a factor larger than 100 with a high signal efficiency. An improvement of 62% on the efficiency ratio of $\epsilon_{s}/\sqrt{\epsilon_{b}}$ is achieved in comparison with the baseline in the PandaX-III conceptual design report.


Design and commissioning of a 600 L Time Projection Chamber with Microbulk Micromegas

Journal of Instrumentation, Vol.13, P06012(2018)

We report the design, construction, and initial commissioning results of a large high pressure gaseous Time Projection Chamber (TPC) with Micromegas modules for charge readout. The detector vessel has an inner volume of about 600 L and an active volume of 270 L. At 10 bar operating pressure, the active volume contains about 20 kg of xenon gas and can image charged particle tracks. Drift electrons are collected by the charge readout plane, which accommodates a tessellation of seven Micromegas modules. Each of the Micromegas covers a square of 20 cm by 20 cm. A new type of Microbulk Micromegas is chosen for this application due to its good gain uniformity and low radioactive contamination. Initial commissioning results with 1 Micromegas module running with 1 bar argon and isobutane gas mixture and 5 bar xenon and trimethylamine (TMA) gas mixture are reported. We also recorded extended background tracks from cosmic ray events and highlighted the unique tracking feature of this gaseous TPC.


Constraining WIMP-Nucleon Effective Interactions from PandaX-II Experiment


We present constraints on the WIMP-nucleon effective interaction from the PandaX-II experiment. We consider well-motivated dimensional-four interactions consisting of couplings among vector and axial vector currents, as well as several typical dimension-five and dimension-six operators in effective field theory. The data set corresponding to a total exposure of 54-ton-day is reanalyzed, and limits on the effective couplings have been obtained as a function of the WIMP mass, both in isoscalar and isovector channels. We obtain the most stringent upper limits on spin-dependent WIMP-nucleon cross section of $9.0×10^{−42}cm^{2}$ for neutron-only coupling with mass of $40 GeV/c^{2}$ at 90% confidence level. Similarly, the minimum constraint on WIMP-proton cross section is $2.2×10^{−38}cm^{2}$.


Constraining Dark Matter Models with a Light Mediator at the PandaX-II Experiment

Physical Review Letters, Vol.121, 021304(2018)

We search for nuclear recoil signals of dark matter models with a light mediator in PandaX-II, a direct detection experiment in the China Jinping underground laboratory. Using data collected in 2016 and 2017 runs, corresponding to a total exposure of 54 ton day, we set upper limits on the zero-momentum dark matter-nucleon cross section. These limits have a strong dependence on the mediator mass when it is comparable to or below the typical momentum transfer. We apply our results to constrain self-interacting dark matter models with a light mediator mixing with standard model particles, and set strong limits on the model parameter space for the dark matter mass ranging from 5 GeV to 10 TeV.


Limits on Axion Couplings from the First 80 Days of Data of the PandaX-II Experiment

Physical Review Letters, Vol.119, 181806(2017)

We report new searches for solar axions and galactic axionlike dark matter particles, using the first low-background data from the PandaX-II experiment at China Jinping Underground Laboratory, corresponding to a total exposure of about $2.7×10^{4}$  kg day. No solar axion or galactic axionlike dark matter particle candidate has been identified. The upper limit on the axion-electron coupling ($g_{Ae}$) from the solar flux is found to be about $4.35×10^{−12}$ in the mass range from $10^{-5}$ to $1 keV/c^{2}$ with 90% confidence level, similar to the recent LUX result. We also report a new best limit from the ${}^{57}Fe$ deexcitation. On the other hand, the upper limit from the galactic axions is on the order of $10^{-13}$ in the mass range from 1 to $10 keV/c^{2}$ with 90% confidence level, slightly improved compared with the LUX.


Dark Matter Results From 54-Ton-Day Exposure of PandaX-II Experiment

Physical Review Letters, Vol.119, 181302(2017)

We report a new search for weakly interacting massive particles (WIMPs) using the combined low background data sets acquired in 2016 and 2017 from the PandaX-II experiment in China. The latest data set contains a new exposure of 77.1 live days, with the background reduced to a level of $0.8×10^{-3} evt/kg/day$, improved by a factor of 2.5 in comparison to the previous run in 2016. No excess events are found above the expected background. With a total exposure of $5.4×10^{4}kg day$, the most stringent upper limit on the spin-independent WIMP-nucleon cross section is set for a WIMP with mass larger than $100 GeV/c^{2}$, with the lowest 90% C.L. exclusion at $8.6×10^{-47}cm^{2}$ at $40 GeV/c^{2}$.


Explore the Inelastic Frontier with 79.6-day of PandaX-II Data

Phys. Rev. D, Vol.96, 102007(2017)

We report here the results of searching for inelastic scattering of dark matter (initial and final state dark matter particles differ by a small mass splitting) with a nucleon for the first 79.6 days of PandaX-II data (Run 9). We set the upper limits for the spin independent weakly interactive massive particle–nucleon scattering cross section up to a mass splitting of $300  keV/c^{2}$ at two benchmark dark matter masses of 1 and $10  TeV/c^{2}$.


Spin-Dependent Weakly-Interacting-Massive-Particle–Nucleon Cross Section Limits from First Data of PandaX-II Experiment

Physical Review Letters, Vol.118, 071301(2017)

New constraints are presented on the spin-dependent weakly-interacting-massive-particle-(WIMP-)nucleon interaction from the PandaX-II experiment, using a data set corresponding to a total exposure of $3.3×10^{4}kg day$. Assuming a standard axial-vector spin-dependent WIMP interaction with ${}^{129}Xe$ and ${}^{131}Xe$ nuclei, the most stringent upper limits on WIMP-neutron cross sections for WIMPs with masses above $10 GeV/c^{2}$ are set in all dark matter direct detection experiments. The minimum upper limit of $4.1×10^{-41}cm^{2}$ at 90% confidence level is obtained for a WIMP mass of $40 GeV/c^{2}$. This represents more than a factor of 2 improvement on the best available limits at this and higher masses. These improved cross-section limits provide more stringent constraints on the effective WIMP-proton and WIMP-neutron couplings.


Krypton and radon background in the PandaX-I dark matter experiment

Journal of Instrumentation, Vol.12, T02002(2017)

We discuss an in-situ evaluation of the ${}^{85}Kr$, ${}^{222}Rn$, and ${}^{220}Rn$ background in PandaX-I, a 120-kg liquid xenon dark matter direct detection experiment. Combining with a simulation, their contributions to the low energy electron-recoil background in the dark matter search region are obtained.


Material screening with HPGe counting station for PandaX experiment

Journal of Instrumentation, Vol.11, T12002(2016)

A gamma counting station based on high-purity germanium (HPGe) detector was set up for the material screening of the PandaX dark matter experiments in the China Jinping Underground Laboratory. Low background gamma rate of 2.6 counts/min within the energy range of 20 to 2700 keV is achieved due to the well-designed passive shield. The sentivities of the HPGe detetector reach mBq/kg level for isotopes like K, U, Th, and even better for Co and Cs, resulted from the low-background rate and the high relative detection efficiency of 175%. The structure and performance of the counting station are described in this article. Detailed counting results for the radioactivity in materials used by the PandaX dark-matter experiment are presented. The upgrading plan of the counting station is also discussed.


Dark Matter Results from First 98.7 Days of Data from the PandaX-II Experiment

Physical Review Letters, Vol.117, 21303(2016)

We report the weakly interacting massive particle (WIMP) dark matter search results using the first physics-run data of the PandaX-II 500 kg liquid xenon dual-phase time-projection chamber, operating at the China JinPing underground laboratory. No dark matter candidate is identified above background. In combination with the data set during the commissioning run, with a total exposure of $3.3×10^{4} kg day$, the most stringent limit to the spin-independent interaction between the ordinary and WIMP dark matter is set for a range of dark matter mass between $5$ and $1000 GeV/c^{2}$. The best upper limit on the scattering cross section is found $2.5×10^{-46}cm^{2}$ for the WIMP mass $40 GeV/c^{2}$ at 90% confidence level.


The electronics and data acquisition system for the PandaX-I dark matter experiment

Journal of Instrumentation, Vol.11, T04002(2016)

We describe the electronics and data acquisition system used in the first phase of the PandaX experiment—a 120 kg dual-phase liquid xenon dark matter direct detection experiment in the China Jin-Ping Underground Laboratory. This system utilized 180 channels of commercial flash ADC waveform digitizers. During the entire experimental run, the system has achieved low trigger threshold (<1 keV electron-equivalent energy) and low deadtime data acquisition.


Performance of photosensors in the PandaX-I experiment

Journal of Instrumentation, Vol.11, T02005(2016)

We report the long term performance of the photosensors, 143 one-inch R8520-406 and 37 three-inch R11410-MOD photomultipliers from Hamamatsu, in the first phase of the PandaX dual-phase xenon dark matter experiment. This is the first time that a significant number of R11410 photomultiplier tubes were operated in liquid xenon for an extended period, providing important guidance to the future large xenon-based dark matter experiments.


Low-mass dark matter search results from full exposure of the PandaX-I experiment

Physical Review D, Vol.92, 052004(2015)

We report the results of a weakly interacting massive particle (WIMP) dark matter search using the full 80.1 live-day exposure of the first stage of the PandaX experiment (PandaX-I) located in the China Jin-Ping Underground Laboratory. The PandaX-I detector has been optimized for detecting low-mass WIMPs, achieving a photon detection efficiency of 9.6%. With a fiducial liquid xenon target mass of 54.0 kg, no significant excess events were found above the expected background. A profile likelihood ratio analysis confirms our earlier finding that the PandaX-I data disfavor all positive low-mass WIMP signals reported in the literature under standard assumptions. A stringent bound on a low-mass WIMP is set at a WIMP mass below $10  GeV/c^{2}$, demonstrating that liquid xenon detectors can be competitive for low-mass WIMP searches.


First dark matter search results from the PandaX-I experiment

Science China Physics, Mechanics & Astronomy, Vol.57, 2024(2017)

We report on the first dark-matter (DM) search results from PandaX-I, a low threshold dual-phase xenon experiment operating at the China JinPing Underground Laboratory. In the 37-kg liquid xenon target with 17.4 live-days of exposure, no DM particle candidate event was found. This result sets a stringent limit for low-mass DM particles and disfavors the interpretation of previously-reported positive experimental results. The minimum upper limit, $3.7 × 10^{-44}cm^{2}$, for the spin-independent isoscalar DM-particle-nucleon scattering cross section is obtained at a DM-particle mass of $49GeV/c^{2}$ at 90% confidence level.


PandaX: a liquid xenon dark matter experiment at CJPL

Science China Physics, Mechanics & Astronomy, Vol.57, 1476(2014)

PandaX is a large liquid-xenon detector experiment usable for direct dark-matter detection and ${}^{136}Xe$ double-beta decay search. The central vessel was designed to accommodate a staged target volume increase from initially 120 kg (stage I) to 0.5 t (stage II) and eventually to a multi-ton scale. The experiment is located in the Jinping Deep-Underground Laboratory in Sichuan, China. The detector operates in dual-phase mode, allowing detection of both prompt scintillation, and ionization charge through proportional scintillation. In this paper a detailed description of the stage I detector design and performance as well as results established during the commissioning phase are presented.


Development of the liquid level meters for the PandaX dark matter detector

Chinese Physics C, Vol.38, 056002(2014)

The two-phase xenon detector is at the frontier of dark matter direct search. This kind of detector uses liquid xenon as the sensitive target and is operated in two-phase (liquid/gas) mode, where the liquid level needs to be monitored and controlled in sub-millimeter precision. In this paper, we present a detailed design and study of two kinds of level meters for the PandaX dark matter detector. The long level meter is used to monitor the overall liquid level while short level meters are used to monitor the inclination of the detector. These level meters are cylindrical capacitors that are custom-made from two concentric metal tubes. Their capacitance values are read out by a universal transducer interface chip and are recorded by the PandaX slow control system. We present the developments that lead to level meters with long-term stability and sub-millimeter precision. Fluctuations (standard deviations) of less than 0.02 mm for the short level meters and less than 0.2 mm for the long level meter were achieved during a few days of test operation.


The cryogenic system for the Panda-X dark matter search experiment

Journal of Instrumentation, Vol.8, p01002(2013)

Panda-X is a liquid xenon dual-phase detector for the Dark Matter Search. The first modestly-sized module will soon be installed in the China JinPing Deep Underground Laboratory in Sichuan province, P.R. China. The cryogenic system is designed to handle much larger detectors, even the final version in the ton scale. Special attention has been paid to the reliability, serviceability, and adaptability to the requirements of a growing experiment. The system is cooled by a single Iwatani PC150 Pulse Tube Refrigerator. After subtracting all thermal losses, the remaining cooling power is still 82 W. The fill speed was 0.75 g/s, but could be boosted by $LN_{2}$ assisted cooling to 3.3 g/s. For the continuous recirculation and purification through a hot getter, a heat exchanger was employed to reduce the required cooling power. The recirculation speed is limited to 2.9 g/s by the gas pump. At this speed, recirculation only adds 18.5 W to the heat load of the system, corresponding to a 95.2 % efficiency of the heat exchanger.