Scientific Publications

We propose a major upgrade to the existing PandaX-4T experiment at the China Jinping Underground Laboratory. The new experiment, PandaX-xT, will be a multi-ten-tonne liquid xenon, ultra-low background, and general-purpose observatory. The full-scaled PandaX-xT contains a 43-t liquid xenon active target. Such an experiment will significantly advance our fundamental understanding of particle physics and astrophysics. The sensitivity of dark matter direct detection will be improved by nearly two orders of magnitude compared to the current best limits, approaching the so-called “neutrino floor” for a dark matter mass above 10 GeV/c², providing a key test to the Weakly Interacting Massive Particle paradigm. By searching for the neutrinoless double beta decay of ¹³⁶Xe isotope in the detector, the effective Majorana neutrino mass can be measured to a 10–41 meV/c² sensitivity, providing a key test to the Dirac/Majorana nature of neutrinos. Astrophysical neutrinos and other ultra-rare interactions can also be measured and searched for with an unprecedented background level, opening up new windows of discovery. Depending on the findings, PandaX-xT will seek the next stage upgrade utilizing isotopic separation of natural xenon.

The PandaX-4T liquid xenon detector at the China Jinping Underground Laboratory is used to measure the solar ⁸B neutrino flux by detecting neutrinos through coherent scattering with xenon nuclei. Data samples requiring the coincidence of scintillation and ionization signals (paired), as well as unpaired ionization-only signals (US2), are selected with energy threshold of approximately 1.1 keV (0.33 keV) nuclear recoil energy. Combining the commissioning run and the first science run of PandaX-4T, a total exposure of 1.20 and 1.04 tonne·year are collected for the paired and US2, respectively. After unblinding, 3 and 332 events are observed with an expectation of 2.8±0.5 and 251±32 background events, for the paired and US2 data, respectively. A combined analysis yields a best-fit ⁸B neutrino signal of 3.5 (75) events from the paired (US2) data sample, with ∼37% uncertainty, and the background-only hypothesis is disfavored at 2.64σ significance. This gives a solar  ⁸B neutrino flux of (8.4±3.1)×10⁶ cm^-2 s^-1, consistent with the standard solar model prediction. It is also the first indication of solar B8 neutrino “fog” in a dark matter direct detection experiment.

We report the first search for the elastic scatterings between cosmic-ray boosted sub-MeV dark matter (DM) and electrons in the PandaX-4T liquid xenon experiment. Sub-MeV DM particles can be accelerated by scattering with electrons in the cosmic rays and produce detectable electron recoil signals in the detector. Using the commissioning data from PandaX-4T of 0.63  tonne·year exposure, we set new constraints on DM-electron scattering cross sections for DM masses ranging from 10  eV/c² to 3  keV/c².

The proton-proton (pp) fusion chain dominates the neutrino production in the Sun. The uncertainty of the predicted pp neutrino flux is at the sub-percent level, whereas that of the best measurement is O(10%). In this study, for the first time, we measure solar pp neutrinos in the electron recoil energy range from 24 to 144 keV using the PandaX-4T commissioning data with 0.63 tonne × year exposure. The pp neutrino flux is determined as (8.0 ± 3.9 (stat) ± 10.0 (syst)) × 10¹⁰ s^-1 cm^-2 , which is consistent with the Standard Solar Model and existing measurements, corresponding to an upper flux limit of 23.3×10¹⁰ s^-1cm^-2 at 90% C.L..

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.

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.

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.

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.

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.

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 $GeV/c^2$.

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.

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 $3.1 \times 10^{−38} cm^2$.

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 $GeV / c^{2}$.

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.

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.

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.

The PandaX-4T experiment, a 4-ton scale dark matter direct detection experiment, is being planned at the China Jinping Un- derground Laboratory. In this paper we present a simulation study of the expected background in this experiment. In a 2.8-ton fiducial mass and the signal region between 1-10 keV electron equivalent energy, the total electron recoil background is found to be $4.9 \times 10^{5} kg^{-1}d^{-1}keV^{-1}$. The nuclear recoil background in the same region is $2.8 \times 10^{-7} kg^{-1}d^{-1}keV^{-1}$. With an exposure of 5.6 ton-years, the sensitivity of PandaX-4T could reach a minimum spin-independent dark matter-nucleon cross section of $6 \times 10^{-48}cm^{2}$ at a dark matter mass of 40 $GeV/c^{2}$.

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.

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.

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 $10  TeV/c^{2}$.

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.