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Search for Light Dark Matter with Ionization Signals in the PandaX-4T Experiment

Phys. Rev. Lett., Vol.130, 261001 (2023)

We report the search results of light dark matter through its interactions with shell electrons and nuclei, using the commissioning data from the PandaX-4T liquid xenon detector. Low energy events are selected to have an ionization-only signal between 60 to 200 photoelectrons, corresponding to a mean nuclear recoil energy from 0.77 to 2.54 $keV$ and electronic recoil energy from 0.07 to 0.23 $keV$. With an effective exposure of 0.55 tonne · year, we set the most stringent limits within a mass range from 40 $MeV/c^2$ to 10 $GeV/c^2$ for pointlike dark matter-electron interaction, 100 $MeV/c^2$ to 10 $GeV/c^2$ for dark matter-electron interaction via a light mediator, and 3.2 to 4 $GeV/c^2$ for dark matter-nucleon spin-independent interaction. For DM interaction with electrons, our limits are closing in on the parameter space predicted by the freeze-in and freeze-out mechanisms in the early Universe.


Limits on the luminance of dark matter from xenon recoil data


It is commonly conjectured that dark matter is a charge neutral fundamental particle. However, it may still have minute photon-mediated interactions through millicharge or higher-order multipole interactions, resulting from new physics at a high energy scale. Here we report a direct search for effective electromagnetic interactions between dark matter and xenon nuclei that produce a recoil of the latter from the PandaX-4T xenon-based detector. Using this technique, the first constraint on the charge radius of dark matter is derived with the lowest excluded value of $1.9 \times 10^{-10} fm^{2}$ for a dark matter mass of 40 giga electron volts per speed of light in a vaccum ($GeV / c^{2}$) more stringent than that for neutrinos by four orders of magnitude. Constraints on the magnitudes of millicharge, magnetic dipole moment, electric dipole moment and anapole moment are also improved substantially from previous searches, with corresponding tightest upper limits of $2.6 \times 10^{-11}e$, $4.8 \times 10^{-10}$ Bohr magnetons, $1.2 \times 10^{-23}$ ecm and $1.6 \times 10 ^{-33} cm^{2}$, respectively, for a dark matter mass of 20–40 $GeV / c^{2}$.


Search for Solar 8^B Neutrinos in the PandaX-4T Experiment Using Neutrino-Nucleus Coherent Scattering

Phys. Rev. Lett., Vol.130, 021802(2023)

A search for interactions from solar $^8B$ neutrinos elastically scattering off xenon nuclei using PandaX-4T commissioning data is reported. The energy threshold of this search is further lowered compared with the previous search for dark matter, with various techniques utilized to suppress the background that emerges from data with the lowered threshold. A blind analysis is performed on the data with an effective exposure of 0.48 tonne year, and no significant excess of events is observed. Among the results obtained using the neutrino-nucleus coherent scattering, our results give the best constraint on the solar $^8B$ neutrino flux. We further provide a more stringent limit on the cross section between dark matter and nucleon in the mass range from 3 to 9 $GeV/c^2$.


Measurement of double beta decay half-life of 136Xe with the PandaX-4T detector


Precise measurement of two-neutrino double beta decay (DBD) half-life is an important step for the searches of Majorana neutrinos with neutrinoless double-beta decay. We report the measurement of DBD half-life of ${}^{136}Xe$ using the PandaX-4T dual-phase Time Projection Chamber (TPC) with 3.7-tonne natural xenon and the first 94.9-day physics data release. The background model in the fiducial volume is well constrained in situ by events in the outer active region. With a ${}^{136}Xe$ exposure of 15.5 kg-year, we establish the half-life as 2.27±0.03(stat.)±0.10(syst.) $\times 10^{21}$ year. This is the first DBD half-life measurement with natural xenon and demonstrates the physics capability of a large-scale liquid xenon TPC in the field of rare event searches.


Search for light fermionic dark matter absorption on electrons in PandaX-4T

Physical Review Letters, Vol.129, 161804(2022)

We report a search on sub-MeV fermionic dark matter absorbed by electrons with an outgoing active neutrino using the $0.63$ tonne year exposure collected by the PandaX-4T liquid xenon experiment. No significant signals are observed over the expected background. The data are interpreted into limits to the effective couplings between such dark matter and the electron. For axial-vector or vector interactions, our sensitivity is competitive in comparison to existing astrophysical bounds on the decay of such a dark matter candidate into photon final states. In particular, we present the first direct detection limits for a vector (axial-vector) interaction which are the strongest in the mass range from $35$ to $55$ ($25$ to $45$) $keV/c^2$ in comparison to other astrophysical and cosmological constraints.


First search for the absorption of fermionic dark matter with the PandaX-4T experiment

Physical Review Letters, Vol.129, 161803(2022)

Compared to the signature of dark matter elastic scattering off nuclei, absorption of fermionic dark matter by nuclei opens up a new searching channel for light dark matter with a characteristic mono-energetic signal. In this letter, we explore the 95.0-day data from PandaX-4T commissioning run and report the first dedicated searching results of the fermionic dark matter absorption signal through neutral current process. No significant signal was found and the lowest limit on the dark matter-nucleon interaction cross section is set to be $ 1.5 \times 10^{-50} cm^{2} $ for a fermionic dark matter mass of $40 MeV / c^{2}$ with 90% confidence level.


Constraints on the axial-vector and pseudo-scalar mediated WIMP-nucleus interactions from PandaX-4T experiment

Physics Letters B(2022)

We present the constraints on the axial-vector and pseudo-scalar mediated WIMP-nucleus interactions from the PandaX-4T experiment, using the data set corresponding to a total exposure of 0.63 tonne⋅year. No significant signal excess is observed, and the most stringent upper limits to date on the spin-dependent WIMP-neutron scattering cross section are set at 90% confidence level with the minimum WIMP-neutron scattering cross section of $5.8\times 10^{-42}cm^{2}$ for WIMP mass of 40 $GeV/c^{2}$. Exclusion limits on the axial-vector and pseudo-scalar simplified models are also derived.


A search for two-component Majorana dark matter in a simplified model using the full exposure data of PandaX-II experiment

Physics Letters B, Vol.832, 137254(2022)

In the two-component Majorana dark matter model, one dark matter particle can scatter off the target nuclei, and turn into a slightly heavier component. In the framework of a simplified model with a vector boson mediator, both the tree-level and loop-level processes contribute to the signal in direct detection experiment. In this paper, we report the search results for such dark matter from PandaX-II experiment, using total data of the full 100.7 tonne⋅day exposure. No significant excess is observed, so strong constraints on the combined parameter space of mediator mass and dark matter mass are derived. With the complementary search results from collider experiments, a large range of parameter space can be excluded.


Study of background from accidental coincidence signals in the PandaX-II experiment

Chinese Physics C, Vol.46, 103001(2022)

Study of background from accidental coincidence signals in the PandaX-II experiment

DOI: 10.1088/1674-1137/ac7cd8

Neutron-induced nuclear recoil background in the PandaX-4T experiment

Chinese Physics C(2022)

Neutron-induced nuclear recoil background is critical to the dark matter searches in the PandaX-4T liquid xenon experiment. This paper studies the feature of neutron background in liquid xenon and evaluates their contribution in the single scattering nuclear recoil events through three methods. The first method is fully Monte Carlo simulation based. The last two are data-driven methods that also use the multiple scattering signals and high energy signals in the data, respectively. In the PandaX-4T commissioning data with an exposure of 0.63 tonne⋅year, all these methods give a consistent result that there are 1.15±0.57 neutron-induced background in dark matter signal region within an approximated nuclear recoil energy window between 5 and 100 keV.


Low radioactive material screening and background control for the PandaX-4T experiment

Journal of High Energy Physics, Vol.06, 147(2022)

PandaX-4T is a ton-scale dark matter direct detection experiment using a dual-phase TPC technique at the China Jinping Underground Laboratory. Various ultra-low background technologies have been developed and applied to material screening for PandaX·4T, including HPGe gamma spectroscopy, ICP-MS, NAA, radon emanation measurement system, krypton assay station, and alpha detection system. Low background materials were selected to assemble the detector. Surface treatment procedures were investigated to further suppress radioactive background. Combining measured results and Monte Carlo simulation, the total material background rates of PandaX-4T in the energy region of $1–25$ $keV_{ee}$ are estimated to be $(9.9 ± 1.9)$ $\times$ $10^{−3} $mDRU$ for electron recoil and $(2.8 ± 0.6)$ $\times$ $10^{−4}$ $mDRU$ for nuclear recoil. In addition, ${}^{nat}Kr$ in the detector is estimated to be < $8$ $ppt$.


Search for Cosmic-Ray Boosted Sub-GeV Dark Matter at the PandaX-II Experiment

Physical Review Letters, Vol.128, 171801(2022)

We report a novel search for the cosmic-ray boosted dark matter using the 100 tonne · day full dataset of the PandaX-II detector located at the China Jinping Underground Laboratory. With the extra energy gained from the cosmic rays, sub-GeV dark matter particles can produce visible recoil signals in the detector. The diurnal modulations in rate and energy spectrum are utilized to further enhance the signal sensitivity. Our result excludes the dark matter–nucleon elastic scattering cross section between 10−31 and 10−28 cm2 for dark matter masses from 0.1 MeV/c2 to 0.1 GeV/c2, with a large parameter space previously unexplored by experimental collaborations.


Light yield and field dependence measurement in PandaX-II dual-phase xenon detector

Journal of Instrumentation, Vol.17, 01008(2022)

The dual-phase xenon detector is one of the most sensitive detectors for dark matter direct detection, where the energy deposition of incoming particles can be converted into light and electrons through xenon excitation and ionization. The detector response to signal energy deposition varies significantly with the electric field in liquid xenon . We study the detector light yield and its dependence on the electric field in PandaX-II dual-phase detector containing 580 kg liquid xenon in the sensitive volume. From measurement, the light yield at electric field from 0 V/cm to 317 V/cm is obtained for energy deposition up to 236 keV.


${}^{83}Rb$/$ {}^{83m}Kr$ production and cross-section measurement with 3.4 MeV and 20 MeV proton beams

Physical Review C, Vol.105, 014604(2022)

${}^{83m}Kr$ with a short lifetime is an ideal calibration source for liquid xenon or liquid argon detector. The ${}^{83m}Kr$ isomer can be generated through the decay of ${}^{83}Rb$ isotope, and ${}^{83}Rb$ is usually produced by proton beams bombarding natural krypton atoms. In this paper, we report a successful production of ${}^{83}Rb$/$ {}^{83m}Kr$ with $3.4$ $MeV$ proton beam energy and measure the production rate with such low proton energy for the first time. Another production attempt was performed with newly available 20 MeV proton beam in China, the production rate is consistent with our expectation. The produced ${}^{83m}Kr$ source has been successfully injected into PandaX-II liquid xenon detector and yielded enough statistics for detector calibration.


Dark Matter Search Results from the PandaX-4T Commissioning Run

Physical Review Letters, Vol.127(2021)

We report the first dark matter search results using the commissioning data from PandaX-4T. Using a time projection chamber with 3.7-tonne of liquid xenon target and an exposure of 0.63~tonne⋅year, 1058 candidate events are identified within an approximate electron equivalent energy window between 1 and 30 $keV$. No significant excess over background is observed. Our data set a stringent limit to the dark matter-nucleon spin-independent interactions, with a lowest excluded cross section (90% C.L.) of $3.3 \times 10^{−47}cm^2$ at a dark matter mass of 30 $GeV/c^2$.


PandaX-4T cryogenic distillation system for removing krypton from xenon

Review of Scientific Instruments, Vol.92, 123303(2021)

An efficient cryogenic distillation system was designed and constructed for the PandaX-4T dark matter detector based on the McCabe–Thiele method and the conservation of mass and energy. This distillation system is designed to reduce the concentration of krypton in commercial xenon from $5$ $\times$ $10^{−7}$ to $\sim$ $10^{−14}$ $mol/mol$ with $99\%$ xenon collection efficiency at a maximum flow rate of $10$ $kg/h$. The offline distillation operation has been completed and $5.75$ tons of ultra-high purity xenon was produced, which is used as the detection medium in the PandaX4T detector. The krypton concentration of the product xenon is measured with an upper limit of $8.0$ $ppt$. The construction, operation, and stable purification performance of the cryogenic distillation system are studied with the experimental data, which is important for theoretical research and distillation operation optimization.


A 500 MS/s waveform digitizer for PandaX dark matter experiments

Journal of Instrumentation(2021)

Waveform digitizers are key readout instruments in particle physics experiments. In this paper, we present a waveform digitizer for the PandaX dark matter experiments. It supports both external-trigger readout and triggerless readout, accommodating the needs of low rate full-waveform readout and channel-independent low threshold acquisition, respectively. This digitizer is a 8-channel VME board with a sampling rate of 500 MS/s and 14-bit resolution for each channel. A digitizer system consisting of 72 channels has been tested in situ of the PandaX-4T experiment. We report the system performance with real data.


Performance of cryogenic demountable indium seal at high pressures

Review of Scientific Instruments, Vol.92, 093905(2021)

An essential challenge in seal design is to provide an ultra-low leak rate at cryogenic temperatures and high pressures. In this paper, the performance of demountable indium seals under a charging pressure of 8.5 MPa A and at cryogenic temperatures down to −190 °C was investigated. Three indium seal structures with a diameter of 30 mm were specifically designed and tested. All three structures went through both room temperature and cryogenic temperature tests in cycles with a pressure of up to 8.5 MPa A. In addition, leak rate experiments regarding the creep relaxation effect of the indium ring were conducted. The results showed that the leak rates of all three structures were lower than $1 \times 10^{-10} Pa\thinspace m^{3} s^{-1}$ at both room temperature and cryogenic temperature with the pressure up to 8.5 MPa A when the torque was 8 or 12 N m. It was concluded that the linear loads for achieving a successful indium seal were 163, 171, and $220 N mm^{-1}$ alongside its circumference for the 2 mm indium M-T structure, the 3 mm indium M-T structure, and the Z-shaped seal structure, respectively. Furthermore, although the torque slightly dropped after the assembly due to the creep relaxation effect, the leak rates of the structure were still lower than $1 \times 10^{-10} Pa\thinspace m^{3} s^{-1}$ three days after the assembly. The present work is helpful for designing ultra-low leak rate demountable indium seals at cryogenic temperatures and high pressures.


BambooMC -- A Geant4-based simulation program for the PandaX experiments

Journal of Instrumentation(2021)

The purpose of the PandaX experiments is to search for the possible events resulted from dark matter particles, neutrinoless double beta decay or other rare processes with xenon detectors. Understanding the energy depositions from backgrounds or calibration sources in these detectors is very important. The program of BambooMC is created to perform the Geant4-based Monte Carlo simulation, providing reference information for the experiments. We introduce the design and features of BambooMC in this report. The running of the program depends on a configuration file, which combines different detectors, event generators, physics lists and analysis packs together in one simulation. The program can be easily extended and applied to other experiments.


Constraining self-interacting dark matter with the full dataset of PandaX-II

SCIENCE CHINA Physics, Mechanics & Astronomy, Vol.64, 111062(2021)

Self-interacting dark matter (SIDM) is a leading candidate proposed to solve discrepancies between predictions of the prevailing cold dark matter theory and observations of galaxies. Many SIDM models predict the existence of a light force carrier that mediates strong dark matter self-interactions. If the mediator couples to the standard model particles, it could produce characteristic signals in dark matter direct detection experiments. We report searches for signals of SIDM models with a light mediator using the full dataset of the PandaX-II experiment, basing on a total exposure of 132 tonne-days. No significant excess over background is found, and our likelihood analysis leads to a strong upper limit on the dark matter-nucleon coupling strength. We further combine the PandaX-II constraints and those from observations of the light element abundances in the early universe, and show that direct detection and cosmological probes can provide complementary constraints on dark matter models with a light mediator.


Design and commissioning of the PandaX-4T cryogenic distillation system for krypton and radon removal

Journal of Instrumentation, Vol.16, 07046(2021)

An online cryogenic distillation system for the removal of krypton and radon from xenon was designed and constructed for PandaX-4T, a highly sensitive dark matter detection experiment. The krypton content in a commercial xenon product is expected to be reduced by $7$ orders of magnitude with $99\%$ xenon collection efficiency at a flow rate of $10$ $kg/h$ by design. The same system can reduce radon content in xenon by reversed operation, with an expected radon reduction factor of about $1.8$ in PandaX-4T under a flow rate of $56.5$ $kg/h$. The commissioning of this system was completed, with krypton and radon operations tested under respective working conditions. The krypton concentration of the product xenon was measured with an upper limit of $8.0$ $ppt$.


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

Journal of Instrumentation, Vol.16(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%.

DOI: 10.1088/1748-0221/16/06/T06007

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

Physical Review Letters, Vol.126, 211803 (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 tonnes⋅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 an ionization signal range between 50 and 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 with systematic uncertainties properly taken into account. 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 $2.5 \times 10^{-37}$ to $3.1 \times 10^{−38} cm^2$.


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.


Diurnal effect of sub-GeV dark matter boosted by cosmic rays

PHYSICAL REVIEW LETTERS, Vol.126, 091804(2021)

We point out a new type of diurnal effect for the cosmic ray boosted dark matter (DM). The DM-nucleon interactions not only allow the direct detection of DM with nuclear recoils but also allow cosmic rays to scatter with and boost the nonrelativistic DM to higher energies. If the DM-nuclei scattering cross sections are sufficiently large, the DM flux is attenuated as it propagates through the Earth, leading to a strong diurnal modulation. This diurnal modulation provides another prominent signature for the direct detection of boosted sub-GeV DM, in addition to signals with higher recoil energy.


Internal calibration of the PandaX-II detector with radon gaseous sources

Journal of Instrumentation, Vol.15, 12038(2020)

We have developed a low-energy electron recoil (ER) calibration method with ${}^{220}Rn$ for the PandaX-II detector. ${}^{220}Rn$, emanated from natural thorium compounds, was fed into the detector through the xenon purification system. From 2017 to 2019, we performed three dedicated calibration campaigns with different radon sources. We studied the detector response to $\alpha$, $\beta$, and $\gamma$ particles with focus on low energy ER events. During the runs in 2017 and 2018, the amount of radioactivity of ${}^{222}Rn$ were on the order of $1\%$ of that of ${}^{220}Rn$ and thorium particulate contamination was negligible, especially in 2018. We also measured the background contribution from ${}^{214}Pb$ for the first time in PandaX-II with the help from a ${}^{222}Rn$ injection. Calibration strategy with ${}^{220}Rn$ and ${}^{222}Rn$ will be implemented in the upcoming PandaX-4T experiment and can be useful for other xenon-based detectors as well.


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.