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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 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 .

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.

Radon and its progenies are significant sources of background in rare event detection experiments, including dark matter searches like the PandaX-4T experiment and other rare decay studies such as neutrinoless double beta decay (NLDBD). In order to measure and control radon emanation for these experiments, we have developed two specialized radon measurement systems: a radon emanation measurement system suitable for small-sized samples with a blank rate of \\(0.03 0.01\\) mBq in the 12.3 L counting chamber, and a radon trap system designed for large-volume samples using low-temperature radon trapping techniques, which improves the sensitivity by a factor of 30 with 1 standard liter per minute (slpm) gas flow and 6 hours trapping time. To boost the detection sensitivity, various surface treatments of the chambers were investigated, including mechanical polishing, electrochemical polishing, and mirror polishing, which reveals that smoother surfaces lead to lower radon emanation rates. In addition, treatments such as applying epoxy coating and covering with aluminized Mylar to stainless steel chambers can also reduce the radon emanation by (\\(90 7)\\%\\) and (\\(60 12)\\%\\), respectively.

Direct detection of light dark matter (DM) is generally difficult due to its small recoil energy. The inelastic scattering of DM can produce unique signatures in the DM direct detection experiments. Using the low-energy unpaired ionization data from PandaX-4T, we newly analyze the probe of the exothermic inelastic dark matter (ineDM). We demonstrate that PandaX-4T can probe the ineDM mass-splitting down to 0.05 keV and probe the ineDM mass to the sub-GeV range. For the ineDM with a dark photon mediator, we use the PandaX-4T data to impose stringent bounds on the mixing parameter between the dark photon and photon.

We customized a laser calibration system and four radioactive \\(\\gamma\\)-ray calibration sources for the Jiangmen Underground Neutrino Observatory (JUNO), a 20-kton liquid scintillator-based neutrino detector. The laser source system was updated to realize the isotropic light emission timing within \\(\\pm0.25\\)~nsec level and to allow the tuning of the laser intensity covering more than four orders of magnitude. In addition, methods to prepare four different radioactive sources (\\(^{18}{\\rm F}\\), \\(^{40}{\\rm K}\\), \\(^{226}{\\rm Ra}\\), and \\(^{241}{\\rm Am}\\)), covering energies from O(10)~keV to O(1)~MeV, for the JUNO detector were established in this study. The radioactivity of each source and the risk of impurities leaking into the detector from the source were confirmed to meet the experimental requirements.

Photomultiplier tubes (PMTs) are essential in xenon detectors like PandaX, LZ, and XENON experiments for dark matter searches and neutrino properties measurement. To minimize PMT-induced backgrounds, stringent requirements on PMT radioactivity are crucial. A novel 2-inch low-background R12699 PMT has been developed through a collaboration between the PandaX team and Hamamatsu Photonics K.K. corporation. Radioactivity measurements conducted with a high-purity germanium detector show levels of approximately 0.08 mBq/PMT for \\(^60Co\\) and 0.06~mBq/PMT for the \\(^238U\\) late chain, achieving a 15-fold reduction compared to R11410 PMT used in PandaX-4T. The radon emanation rate is below 3.2 \\( \\)Bq/PMT (@90\\% confidence level), while the surface \\(^210Po\\) activity is less than 18.4 \\(\\)Bq/cm\\(^2\\). The electrical performance of these PMTs at cryogenic temperature was evaluated. With an optimized readout base, the gain was enhanced by 30\\%, achieving an average gain of \\(4.23 10^6\\) at -1000~V and -100~\\(^\\)C. The dark count rate averaged 2.5~Hz per channel. Compactness, low radioactivity, and robust electrical performance in the cryogenic temperature make the R12699 PMT ideal for next-generation liquid xenon detectors and other rare event searches.

Photomultiplier tubes (PMTs) are essential in xenon detectors like PandaX, LZ, and XENON experiments for dark matter searches and neutrino properties measurement. To minimize PMT-induced backgrounds, stringent requirements on PMT radioactivity are crucial. A novel 2-inch low-background R12699 PMT has been developed through a collaboration between the PandaX team and Hamamatsu Photonics K.K. corporation. Radioactivity measurements conducted with a high-purity germanium detector show levels of approximately 0.08 mBq/PMT for \\(\\rm^{60}Co\\) and 0.06~mBq/PMT for the \\(\\rm^{238}U\\) late chain, achieving a 15-fold reduction compared to R11410 PMT used in PandaX-4T. The radon emanation rate is below 3.2 \\(\\rm \\mu\\)Bq/PMT (@90\\% confidence level), while the surface \\(\\rm^{210}Po\\) activity is less than 18.4 \\(\\mu\\)Bq/cm\\(^2\\). The electrical performance of these PMTs at cryogenic temperature was evaluated. With an optimized voltage distribution, the gain was enhanced by 30\\%, achieving an average gain of \\(4.23 \\times 10^6\\) at -1000~V and -100~\\(^{\\circ}\\)C. The dark count rate averaged 2.5~Hz per channel. Compactness, low radioactivity, and robust electrical performance in the cryogenic temperature make the R12699 PMT ideal for next-generation liquid xenon detectors and other rare event searches.

The proton-proton (\\(pp\\)) fusion chain dominates the neutrino production from the Sun. The uncertainty of the predicted \\(pp\\) neutrino flux is at the sub-percent level, whereas that of the best measurement is \\(\\mathcal{O}(10\\%)\\). In this paper, we present the first result to measure the solar \\(pp\\) neutrinos in the electron recoil energy range from 24 to 144 keV, using the PandaX-4T commissioning data with 0.63 tonne\\(\\times\\)year exposure. The \\(pp\\) neutrino flux is determined to be $(8.0 \\pm 3.9 \\,{\\rm{(stat)}} \\pm 10.0 \\,{\\rm{(syst)}} )\\times 10^{10}\\, $$\\rm{s}^{-1} \\rm{cm}^{-2}\\(, consistent with Standard Solar Model and existing measurements, corresponding to a flux upper limit of \\)23.3\\times 10^{10}\\, $$\\rm{s}^{-1} \\rm{cm}^{-2}$ at 90\\% C.L..

We report the first search for the elastic scatterings between cosmic-ray boosted sub-MeV dark matter and electrons in the PandaX-4T liquid xenon experiment. Sub-MeV dark matter 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\\(\\cdot\\)year exposure, we set new constraints on DM-electron scattering cross sections for DM masses ranging from 10~eV/\\(c^2\\) to 3~keV/\\(c^2\\).