We report a new search for weakly interacting massive particles (WIMPs) using the combined low background data sets acquired in 2016 and 2017 from the PandaX-II experiment in China. The latest data set contains a new exposure of 77.1 live days, with the background reduced to a level of 0.8×10^{-3}  evt/kg/day, improved by a factor of 2.5 in comparison to the previous run in 2016. No excess events are found above the expected background. With a total exposure of 5.4×10^{4}  kg day, the most stringent upper limit on the spin-independent WIMP-nucleon cross section is set for a WIMP with mass larger than 100  GeV/c^{2}, with the lowest 90% C.L. exclusion at 8.6×10^{-47}  cm^{2} at 40  GeV/c^{2}.

We report the WIMP dark matter search results using the first physics-run data of the PandaX-II 500 kg liquid xenon dual-phase time-projection chamber, operating at the China JinPing underground Laboratory. No dark matter candidate is identified above background. In combination with the data set during the commissioning run, with a total exposure of 3.3×10 4 kg-day, the most stringent limit to the spin-independent interaction between the ordinary and WIMP dark matter is set for a range of dark matter mass between 5 and 1000 GeV/c 2 . The best upper limit on the scattering cross section is found 2.5 × 10 −46 cm 2 for the WIMP mass 40 GeV/c 2 at 90% confidence level.Weakly interacting massive particles, WIMPs in short, are a class of hypothetical particles that came into existence shortly after the Big Bang. The WIMPs could naturally explain the astronomical and cosmological evidences of dark matter in the Universe. The weak interactions between WIMPs and ordinary matter could lead to the recoils of atomic nuclei that produce detectable signals in deep-underground direct detection experiments. Over the past decade, the dual-phase xenon time-projection chambers (TPC) emerged as a powerful technology for WIMP searches both in scaling up the target mass, as well as in improving background rejection [1][2][3]. LUX, a dark matter search experiment with a 250 kg liquid xenon target, has recently reported the best limit of 6×10 −46 cm 2 on the WIMP-nucleon scattering cross section [4] The PandaX-II experiment, a half-ton scale dual-phase xenon experiment at the China JinPing underground Laboratory (CJPL), has recently reported the dark matter search results from its commissioning run (Run 8,19.1 live days) with a 5845 kg-day exposure [5]. The data were contaminated with significant 85 Kr background. After a krypton distillation campaign in early 2016, PandaX-II commenced physics data taking in March 2016. In this paper, we report the combined WIMP search results using the data from the first physics run from March 9 to June 30, 2016 (Run 9, 79.6 live days) and Run 8, with a total of 3.3×10 4 kg-day exposure, the largest reported WIMP data set among dual-phase xenon detectors in the world to date.The PandaX-II detector has been described in detail in Ref. [5]. The liquid xenon target consists of a cylindrical TPC with dodecagonal cross section (opposite-side distance 646 mm), confined by the polytetrafluoroethylene (PTFE) reflective wall, and a vertical drift distance of 600 mm defined by the cathode mesh and gate grid located at the bottom and top. For each physical event, the prompt scintillation photons (S1) and the delayed electroluminescence photons (S2) from the ionized electrons are collected by two arrays of 55 Hamamatsu R11410-arXiv:1607.07400v3 [hep-ex] Hamamatsu R8520-406 1-inch PMTs serving as an active veto. The γ background, which produces electron recoil (ER) events, can be distinguished from the dark matter nuclear recoil (NR) using the S2-to-S1 ratio. During the data taking period in Run 9, a few diffe...

The recent long-term shutdown of Japanese nuclear reactors has resulted in a significantly reduced reactor νe flux at KamLAND. This running condition provides a unique opportunity to confirm and constrain backgrounds for the reactor νe oscillation analysis. The data set also has improved sensitivity for other νe signals, in particular νe's produced in β-decays from 238 U and 232 Th within the Earth's interior, whose energy spectrum overlaps with that of reactor νe's. Including constraints on θ13 from accelerator and short-baseline reactor neutrino experiments, a combined three-flavor analysis of solar and KamLAND data gives fit values for the oscillation parameters of tan 2 θ12 = 0.436

We present results from the first phase of the KamLAND-Zen double-beta decay experiment, corresponding to an exposure of 89.5 kg yr of 136 Xe. We obtain a lower limit for the neutrinoless double-beta decay half-life of T 0ν 1/2 > 1.9 × 10 25 yr at 90% C.L. The combined results from KamLAND-Zen and EXO-200 give T 0ν 1/2 > 3.4 × 10 25 yr at 90% C.L., which corresponds to a Majorana neutrino mass limit of m ββ < (120 − 250) meV based on a representative range of available matrix element calculations. Using those calculations, this result excludes the Majorana neutrino mass range expected from the neutrinoless double-beta decay detection claim in 76 Ge, reported by a part of the Heidelberg-Moscow Collaboration, at more than 97.5% C.L. 21.10.Tg, 14.60.Pq, 27.60.+j Double-beta (ββ) decay is a rare nuclear process observable in even-even nuclei for which ordinary beta decay is energetically forbidden or highly suppressed by large spin differences. Standard ββ decay proceeds by a second-order weak interaction emitting two electron anti neutrinos and two electrons (2νββ). If, however, the neutrino is a massive Majorana particle, ββ decay might also occur without the emission of neutrinos (0νββ). Observation of such a process would demonstrate that lepton number is not conserved in nature. Moreover, if the process is mediated by the exchange of a light left-handed neutrino, its rate increases with the square of the effective Majorana neutrino mass m ββ ≡ Σ i U 2 ei m νi , and hence its measurement would provide information on the absolute neutrino mass scale. To date there has been only one claimed observation of 0νββ decay, in 76 Ge [1].At present there are several operating experiments performing 0νββ decay searches with design sensitivities sufficient to test the Majorana neutrino mass implied by the claim in [1] within a few years of running: GERDA with 76 Ge, CUORE-0 with 130 Te, and EXO-200 and KamLAND-Zen with 136 Xe. Among those experiments, KamLAND-Zen released its first 0νββ half-life limit, T 0ν 1/2 > 5.7 × 10 24 yr at 90% C.L., based on a 27.4 kg yr exposure [2]. Although the sensitivity of this result was impeded by the presence of an unexpected background peak just above the 2.458 MeV Q value of 136 Xe ββ decay, the Majorana neutrino mass sensitivity was similar to that in Ref. [1]. EXO-200 later improved on this limit by a factor of 2.8 [3], constraining the result in [1] for a number of nuclear matrix element (NME) calculations.As shown below, we have found the problematic background peak in the KamLAND-Zen spectrum to most likely come from metastable 110m Ag. We embarked recently on a purification campaign to remove this isotope. Doing so required extracting the Xe from the detector, thus marking the end of the first phase of KamLAND-Zen. In this Letter we report on the full data set from the first phase of KamLAND-Zen, corresponding to an exposure of 89.5 kg yr of 136 Xe. This represents a factor of 3.2 increase over KamLAND-Zen's first result [2], and is also the largest exposure for a ββ decay isot...