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Search for Light Dark Matter-Electron Scatterings in the PandaX-II Experiment

Physical Review Letters(2021)

We report constraints on light dark matter through its interactions with shell electrons in the PandaX-II liquid xenon detector with a total 46.9 tonne⋅day exposure. To effectively search for these very low energy electron recoils, ionization-only signals are selected from the data. 1821 candidates are identified within ionization signal range between 50 to 75 photoelectrons, corresponding to a mean electronic recoil energy from 0.08 to 0.15 $keV$. The 90% C.L. exclusion limit on the scattering cross section between the dark matter and electron is calculated based on Poisson statistics. Under the assumption of point interaction, we provide the world's most stringent limit within the dark matter mass range from 15 to 30 $MeV/c^2$, with the corresponding cross section from $2 \times 10^{-38}$ to $3 \times 10^{−37} cm^2$.


Determination of responses of liquid xenon to low energy electron and nuclear recoils using the PandaX-II detector

Chinese Physics C(2021)

We report a systematic determination of the responses of PandaX-II, a dual phase xenon time projection chamber detector, to low energy recoils. The electron recoil (ER) and nuclear recoil (NR) responses are calibrated, respectively, with injected tritiated methane or ${}^{220}Rn$ source, and with ${}^{241}Am-Be$ neutron source, within an energy range from 1-25 keV (ER) and 4-80 keV (NR), under the two drift fields of 400 and 317 V/cm. An empirical model is used to fit the light yield and charge yield for both types of recoils. The best fit models can well describe the calibration data. The systematic uncertainties of the fitted models are obtained via statistical comparison against the data.


The Cryogenics and Xenon handling system for the PandaX-4T Experiment

Journal of Instrumentation(2021)

PandaX-4T, the new generation of the PandaX detector, is a xenon dual-phase detector with about six tons of liquid xenon as target for dark matter search. A cryogenics and xenon handling system is designed to liquify and purify this large detector. In this paper, the results on the commission of the cryogenics and gas handling system are reported. The maximum cooling power of ~ 580 W at 178 K with three cooperating coldheads has been achieved. The filling rate with an assisted liquid nitrogen cooling can reach ~ 700 kg/day. The average rate of recuperation by liquid nitrogen is around 440 kg/day. The maximum total purification speed of two circulation loops is up to ~ 155 slpm. Each loop is using a large heat exchanger with a measured heat exchange efficiency of ~ 97.5+-0.5%.


An improved design of the readout base board of the photomultiplier tube for future PandaX dark matter experiments

Journal of Instrumentation, Vol.15, T12006(2020)

The PandaX project consists of a series of xenon-based experiments that are used to search for dark matter (DM) particles and to study the fundamental properties of neutrinos. The next DM experiment PandaX-4T will be using 4 ton liquid xenon in the sensitive volume, which is nearly a factor of seven larger than that of the previous experiment PandaX-II. Due to the increasing target mass, the sensitivity of searching for both DM and neutrinoless double-beta decay ($0\nu\beta\beta$) signals in the same detector will be significantly improved. However, the typical energy of interest for $0\nu\beta\beta$ signals is at the MeV scale, which is much higher than that of most popular DM signals. In the baseline readout scheme of the photomultiplier tubes (PMTs), the dynamic range is very limited. Signals from the majority of PMTs in the top array of the detector are heavily saturated at MeV energies. This deteriorates the $0\nu\beta\beta$ search sensitivity. In this paper we report a new design of the readout base board of the PMTs for future PandaX DM experiments and present its improved performance on the dynamic range.


A search for solar axions and anomalous neutrino magnetic moment with the complete PandaX-II data

Chinese Physics Letters, Vol.38, 011301(2020)

We report a search for new physics signals using the low energy electron recoil events in the complete data set from PandaX-II, in light of the recent event excess reported by XENON1T. The data correspond to a total exposure of 100.7 ton-day with liquid xenon. With robust estimates of the dominant background spectra, we perform sensitive searches on solar axions and neutrinos with enhanced magnetic moment. We find that the axion-electron coupling $g_{Ae} \lt 4.6\times 10^{-12}$ for an axion mass less than $\rm 0.1~keV/c^2$ and the neutrino magnetic moment $μ_ν \lt 3.2\times 10^{-11}μ_{B}$ at 90% confidence level. The observed excess from XENON1T is within our experimental constraints.


Results of Dark Matter Search using the Full PandaX-II Exposure

Chinese Physics C, Vol.44, 125001(2020)

We report the dark matter search results using the full 132 ton⋅day exposure of the PandaX-II experiment, including all data from March 2016 to August 2018. No significant excess of events were identified above the expected background. Upper limits are set on the spin-independent dark matter-nucleon interactions. The lowest 90% confidence level exclusion on the spin-independent cross section is $2.0 \times 10^{−46} cm^{2}$ at a WIMP mass of 15 $GeV / c^{2}$.


An Improved Evaluation of the Neutron Background in the PandaX-II Experiment

SCIENCE CHINA Physics, Mechanics & Astronomy(2019)

In dark matter direct detection experiments, neutron is a serious source of background, which can mimic the dark matter-nucleus scattering signals. In this paper, we present an improved evaluation of the neutron background in the PandaX-II dark matter experiment by a novel approach. Instead of fully relying on the Monte Carlo simulation, the overall neutron background is determined from the neutron-induced high energy signals in the data. In addition, the probability of producing a dark-matter-like background per neutron is evaluated with a complete Monte Carlo generator, where the correlated emission of neutron(s) and $\gamma$(s) in the ($\alpha$, n) reactions and spontaneous fissions is taken into consideration. With this method, the neutron backgrounds in the Run 9 (26-ton-day) and Run 10 (28-ton-day) data sets of PandaX-II are estimated to be 0.66±0.24 and 0.47±0.25 events, respectively.


Searching for neutrino-less double beta decay of 136Xe with PandaX-II liquid xenon detector

Chinese Physics C, Vol.43, 113001(2019)

We report the Neutrino-less Double Beta Decay (NLDBD) search results from PandaX-II dual-phase liquid xenon time projection chamber. The total live time used in this analysis is 403.1 days from June 2016 to August 2018. With NLDBD-optimized event selection criteria, we obtain a fiducial mass of 219 kg of natural xenon. The accumulated xenon exposure is 242 kg $\cdotp$ yr, or equivalently 22.2 kg $\cdotp$ yr of ${}^{136}Xe$ exposure. At the region around ${}^{136}Xe$ decay Q-value of 2458 keV, the energy resolution of PandaX-II is 4.2%. We find no evidence of NLDBD in PandaX-II and establish a lower limit for decay half-life of 2.1$\times 10^{23}$ yr at the 90% confidence level, which corresponds to an effective Majorana neutrino mass $m_{\beta \beta}\lt$ (1.4 - 3.7) eV. This is the first NLDBD result reported from a dual-phase xenon experiment.


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.


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.


PandaX-II Constraints on Spin-Dependent WIMP-Nucleon Effective Interactions

Physics Letters B, Vol.792, 193-198(2018)

We present PandaX-II constraints on candidate WIMP-nucleon effective interactions involving the nucleon or WIMP spin, including, in addition to standard axial spin-dependent (SD) scattering, various couplings among vector and axial currents, magnetic and electric dipole moments, and tensor interactions. The data set corresponding to a total exposure of 54-ton-days is reanalyzed to determine constraints as a function of the WIMP mass and isospin coupling. We obtain WIMP-nucleon cross section bounds of $1.6\times 10^{-41}cm^2$ and $9.0\times10^{-42}cm^2$ (90% c.l.) for neutron-only SD and tensor coupling, respectively, for a mass $M_{WIMP} ∼ 40 GeV/c^2$. The SD limits are the best currently available for $M_{WIMP} > 40 GeV/c^2$. We show that PandaX-II has reached a sensitivity sufficient to probe a variety of other candidate spin-dependent interactions at the weak scale.


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.


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 $10  TeV/c^{2}$.


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 $40 GeV/c^{2}$.


Update of the trigger system of the PandaX-II experiment

Journal of Instrumentation, Vol.12, T08004(2017)

PandaX-II experiment is a dark matter direct detection experiment using about half-ton of liquid xenon as the sensitive target. The electrical pulses detected by photomultiplier tubes from scintillation photons of xenon are recorded by waveform digitizers. The data acquisition of PandaX-II relies on a trigger system that generates common trigger signals for all waveform digitizers. Previously an analog device-based trigger system was used for the data acquisition system. In this paper we present a new FPGA-based trigger system. The design of this system and trigger algorithms are described. The performance of this system on real data is presented.


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.


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.


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.