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A bstract 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) × 10 − 3 mDRU for electron recoil and (2.8 ± 0.6) × 10 − 4 mDRU for nuclear recoil. In addition, nat Kr in the detector is estimated to be < 8 ppt.

A bstract Detailed studies of two-neutrino double electron capture (2 ν DEC) is a crucial step towards searching for the neutrinoless mode to explore the Majorana nature of neutrinos. We have measured precisely the half-life of the 2 ν DEC process in 124 Xe, utilizing a total exposure of 1.73 tonne year from the commissioning run and the first science run of the PandaX-4T experiment. A time-dependent background model in the O (10 keV) energy is constructed for the first time in PandaX-4T data. With an unbinned maximum likelihood fit, we determine the half-life of the 2 ν DEC process to be (1.03 ± 0.15 stat ± 0.08 sys ) × 10 22 yr. Furthermore, we have evaluated the capture fraction for both electrons captured from the K shell ( KK ) to be (65 ± 5)%, which aligns with the 124 Xe nuclear model calculations within 1.8 σ .

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 × 10 −5 kg −1 d −1 keV −1 . The nuclear recoil background in the same region is 2:8 × 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 × 10 −48 cm 2 at a dark matter mass of 40 GeV/ c 2 .

It is commonly conjectured that dark matter is a charge neutral fundamental particle. However, it may still have minute photon-mediated interactions through millicharge 1 , 2 or higher-order multipole interactions 3 – 10 , 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 11 , 12 . Using this technique, the first constraint on the charge radius of dark matter is derived with the lowest excluded value of 1.9 × 10 −10  fm 2 for a dark matter mass of 40 giga electron volts per speed of light in a vaccum squared (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 13 , 14 , with corresponding tightest upper limits of 2.6 × 10 −11  e, 4.8 × 10 −10  Bohr magnetons, 1.2 × 10 −23  ecm and 1.6 × 10 −33  cm 2 , respectively, for a dark matter mass of 20–40 GeV/ c 2 . A direct search for effective electromagnetic interactions between dark matter and xenon nuclei that produce a recoil of the latter is carried out and the first constraint on charge radius of dark matter is derived.

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/c 2 , providing a key test to the Weakly Interacting Massive Particle paradigm. By searching for the neutrinoless double beta decay of 136 Xe isotope in the detector, the effective Majorana neutrino mass can be measured to a 10–41 meV/c 2 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.

A bstract We perform a search of double beta decay of 136 Xe to the excited state, 0 1 + , of 136 Ba (2 νββ - 0 1 + ), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2 νββ - 0 1 + is observed, resulting in a lower limit on half-life of 7 . 5 × 10 22 yr at the 90% confidence level. This is the first experimental limit on such a rare decay in a natural xenon-based detector.

We perform a search of double beta decay of 136 Xe to the excited state,$$ {0}_1^{+} $$0 1 + , of 136 Ba (2 νββ -$$ {0}_1^{+} $$0 1 + ), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2 νββ -$$ {0}_1^{+} $$0 1 + is observed, resulting in a lower limit on half-life of 7 . 5 × 10 22 yr at the 90% confidence level. This is the first experimental limit on such a rare decay in a natural xenon-based detector.

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.

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 γ (s) in the ( α , 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.

Abstract We perform a search of double beta decay of 136Xe to the excited state, 0 1 + 0₁⁺ , of 136Ba (2νββ- 0 1 + 0₁⁺ ), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2νββ- 0 1 + 0₁⁺ is observed, resulting in a lower limit on half-life of 7.5 × 1022 yr at the 90% confidence level. This is the first experimental limit on such a rare decay in a natural xenon-based detector.