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In the past decade, the precise measurement of the lastly known neutrino mixing angle θ 13 has enabled the resolution of neutrino mass hierarch (MH) at medium-baseline reactor neutrino oscillation (MBRO) experiments. Recent calculations of the reactor neutrino flux predict percent-level sub-structures in the ν ¯ e spectrum due to Coulomb effects in beta decay. Such fine structure in the reactor spectrum has been an issue of concern for efforts to determine the neutrino MH for the MBRO approach, the concern being that the sub-dominant oscillation pattern used to discriminate different hierarchies will be obscured by fine structure. The energy resolutions of current reactor experiments are not sufficient to measure such fine structure, and therefore the size and location in energy of these predicted discontinuities has not been confirmed experimentally. There has been speculation that a near detector is required with sufficient energy resolution to resolve the fine structure. This article studies the impact of fine structure on the resolution of MH, based on predicted reactor neutrino spectra, using the measured spectrum from Daya Bay as a reference. We also investigate how a near detector could improve the sensitivity of neutrino MH resolution based on various assumptions of near detector energy resolution.

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 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 .

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.

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.

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 $2.27 \\\pm 0.03 (stat.)\\\pm 0.10 (syst.)\\\times 10^{21}$ years. 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.

We examine the potential of the future medium-baseline reactor neutrino oscillation (MBRO) experiments in studying neutrino wave-packet impact. In our study, we treat neutrinos as wave packets and use the corresponding neutrino flavor transition probabilities. The delocalization, separation and spreading of the wave packets lead to decoherence and dispersion effects, which modify the plane-wave neutrino oscillation pattern, by amounts that depend on the energy uncertainties in the initial neutrino wave packets. We find that MBRO experiments could be sensitive to the wave-packet impact, since the baseline is long enough and also the capability of observing small corrections to the neutrino oscillations due to excellent detector energy resolution. Besides studying the constraints on the decoherence parameter, we also examine the potential wave-packet impacts on the precision of measuring \\(\\theta_{12}\\) and other oscillation parameters in the future medium-baseline reactor neutrino oscillation experiments. Moreover, we also probe the potential benefits of an additional detector for studying such exotic neutrino physics.

In the past decade, the precise measurement of the lastly known neutrino mixing angle \\(_13\\) has enabled the resolution of neutrino mass hierarchy (MH) at medium-baseline reactor neutrino oscillation (MBRO) experiments. On the other hand, recent calculations of the reactor neutrino flux predict percent-level sub-structures in the \\(_e\\) spectrum due to Coulomb effects in beta decay. Such fine structure in the reactor spectrum could be an important issue for the determination of neutrino MH for the MBRO approach since they could affect the sub-dominant oscillation pattern used to discriminate different hierarchies. Inconveniently, the energy resolutions of current reactor experiments are not sufficient to measure such fine structure, and therefore the size and location in energy of these predicted discontinuities has not been confirmed experimentally. There has been speculation that a near detector is required with sufficient energy resolution to resolve the fine structure such that it can be accounted for in any analysis which attempts to discriminate the MH. This article studies the impact of fine structure on the resolution of MH, based on the predicted reactor neutrino spectra, using the measured spectrum from Daya Bay as a reference. We also investigate whether a near detector could improve the sensitivity of neutrino MH resolution using various assumptions of near detector energy resolution.