China Jinping Underground Laboratory Research Articles

Status and plan of the PandaX experiment

PandaX is a large scale apparatus for the detection of rare events in particle physics and astrophysics with xenon. It is located in the China Jinping Underground Laboratory, which is the deepest underground laboratory all over the world to date. The first and second stages of the PandaX experiment utilize the dual phase xenon time projection chamber technology for the direct detection of one hypothetic candidate of dark matter, the weakly interacting massive particles (WIMPs), in the Galaxy halo. The technology makes it possible to measure both the scintillations and ionizations generated by the scattering of WIMP with nucleus, and thus backgrounds can be suppressed by the character of the measured signals. The aim of the first stage of PandaX, PandaX-I, is to test the possible signals reported by other experiments in low dark matter mass region with 120 kg of xenon. The first results of the PandaX-I experiment were released in August, 2014. No dark matter candidates were observed. Thus the 90% exclusion upper limits on the spin-independent WIMP-nucleon elastic scattering cross section were set. The results disfavored the WIMP interpretation of those signals and placed stringent limit to the property of dark matter below 10 GeV/ c 2. The final results of PandaX-I were released in 2015 based on 54×80.1 kg d of exposure with significantly improved analysis. The new analysis supported the conclusion of the first results and gave stringent limitation on the WIMP-nucleon cross section for WIMP mass smaller than 10 GeV, demonstrating the detection potential of xenon at low WIMP mass region. The second stage, PandaX-II, with an updated detector capable of containing 500 kg of xenon, is being tested and will start data taking in the end of 2015. The sensitivity of PandaX-II is expected to go far beyond the current limits being set by all the experiments so far. Unattainable parameter space of WIMPs will be explored by it. PandaX-III, the third stage, will be an experiment to detect the neutrinoless double beta decay with enriched high pressure 136Xe (to 90%) gaseous detector, so that the nature of neutrino is studied. The detector is able to measure the track of radiations inside it, helping to suppress the background by inspecting the different topological properties of desired signals and background. PandaX-III plans to build such a detector with 200 kg enriched 136Xe first, then build another 4 identical modules to reach ton scale. The initial design and prototype testing of PandaX-III is carrying out by different groups from the institutions all over the world. The first detector module of PandaX-III is expected to be finished construction and start operation in 2018. The success running of the PandaX experiments may lead to important discoveries in physics.

Point contact germanium detectors at 500 eVee threshold for light dark matter searches

Germanium detectors with sub-keV sensitivities can probe low-mass WIMP Dark Matter. This experimental approach is pursued at Kuo-Sheng Neutrino Laboratory (KSNL) in Taiwan and at China Jinping Underground Laboratory (CJPL) in China via TEXONO and CDEX programs, respectively. The highlights of R&D efforts on point contact germanium detectors and in particular the differentiation of surface and bulk events by pulse shape analysis are described. The latest results on WIMP-nucleon scattering cross-sections are also presented. Some of the allowed parameter space implied by other experiments are probed and excluded.

The electronics and data acquisition system for the PandaX-I dark matter experiment

We describe the electronics and data acquisition system used in the first phase of the PandaX experiment—a 120 kg dual-phase liquid xenon dark matter direct detection experiment in the China Jin-Ping Underground Laboratory. This system utilized 180 channels of commercial flash ADC waveform digitizers. During the entire experimental run, the system has achieved low trigger threshold (<1 keV electron-equivalent energy) and low deadtime data acquisition.

Recent status and prospect of CDEX and CJPL

The China Dark Matter Experiment (CDEX) aims at the direct searches of light Weakly Interacting Massive Particles (WIMPs) deploying point-contact germanium detector at the China Jinping Underground Laboratory (CJPL), which has about 2400 m of rock overburdened. Results on light WIMPs from the prototype CDEX-0 with a few gram mass and CDEX-1 with a 994 g mass are reported. The CDEX-10 experiment employed a germanium detector arrays and liquid argon anti-Compton is being constructed and tested. The multi-purpose experiment CDEX-1T and the expansion project of CJPL-II will also be discussed.

Low-mass dark matter search results from full exposure of the PandaX-I experiment

We report the results of a weakly-interacting massive particle (WIMP) dark matter search using the full 80.1\;live-day exposure of the first stage of the PandaX experiment (PandaX-I) located in the China Jin-Ping Underground Laboratory. The PandaX-I detector has been optimized for detecting low-mass WIMPs, achieving a photon detection efficiency of 9.6\%. With a fiducial liquid xenon target mass of 54.0\,kg, no significant excess event were found above the expected background. A profile likelihood analysis confirms our earlier finding that the PandaX-I data disfavor all positive low-mass WIMP signals reported in the literature under standard assumptions. A stringent bound on the low mass WIMP is set at WIMP mass below 10\,GeV/c$^2$, demonstrating that liquid xenon detectors can be competitive for low-mass WIMP searches.

Status and prospects of the China Dark Matter Experiment

The China Dark Matter Experiment (CDEX) pursues direct searches of light Weakly Interacting Massive Particles (WIMPs) at the China Jinping Underground Laboratory (CJPL), which is the deepest operating laboratory for astroparticle research in the world. Results from a prototype CDEX-0 20 g germanium detector array and CDEX-1 994 g pPCGe(p-type Point Contact Germanium) detector are reported. The new result from CDEX-1 pPCGe excludes the CoGeNT-2013 allowed region with an identical detector technique. CDEX-10 with a PCGearray of 10 kg target mass range enclosed in a liquid argon anti-Compton detector is being constructed and tested. The CDEX program evolves into the targets of "CDEX-1T Experiment". The CDEX-1T experiment will be located in CJPL-II which is under construction and will be finished by the end of 2016.

Measurement of intrinsic radioactive backgrounds from the 137Cs and U/Th chains in CsI(Tl) crystals**Supported by National Natural Science Foundation of China (11275107, 11175099)

The inorganic CsI(Tl) crystal scintillator is a candidate anti-compton detector for the China Dark matter Experiment. Studying the intrinsic radiopurity of the CsI(Tl) crystal is an issue of major importance. The timing, energy and spatial correlations, as well as the capability of pulse shape discrimination provide powerful methods for the measurement of intrinsic radiopurities. The experimental design, detector performance and event-selection algorithms are described. A total of 359×3 kg-days data from three prototypes of CsI(Tl) crystals were taken at China Jinping Underground Laboratory (CJPL), which offers a good shielding environment. The contamination levels of internal isotopes from 137Cs, 232Th and 238U series, as well as the upper bounds of 235U series are reported. Identification of the whole α peaks from U/Th decay chains and derivation of those corresponding quenching factors are achieved.

Study of the material photon and electron background and the liquid argon detector veto efficiency of the CDEX-10 experiment**Supported by National Natural Science Foundation of China (11175099, 10935005, 10945002, 11275107, 11105076) and State Key Development Program of Basic Research of China (2010CB833006)

The China Dark Matter Experiment (CDEX) is located at the China Jinping Underground Laboratory (CJPL) and aims to directly detect the weakly interacting massive particles (WIMP) flux with high sensitivity in the low mass region. Here we present a study of the predicted photon and electron backgrounds including the background contribution of the structure materials of the germanium detector, the passive shielding materials, and the intrinsic radioactivity of the liquid argon that serves as an anti-Compton active shielding detector. A detailed geometry is modeled and the background contribution has been simulated based on the measured radioactivities of all possible components within the GEANT4 program. Then the photon and electron background level in the energy region of interest (<10−2events·kg1·day−1·keV−1 (cpkkd)) is predicted based on Monte Carlo simulations. The simulated result is consistent with the design goal of the CDEX-10 experiment, 0.1cpkkd, which shows that the active and passive shield design of CDEX-10 is effective and feasible.

PandaX-I result sets a stringent limit for low-mass dark matter particles

Astrophysical and cosmological observations show that about 27% of the energy in our universe comes from dark matter (DM). What is DM is an outstanding puzzle in science in the 21st century. The weakly interacting massive particles (WIMPs) have been by far the favored candidate for DM, although they cannot be accommodated in the standard model of strong and electroweak interactions. DM theory asserts that our solar system is bathed in the DM halo of the MilkyWay,withmillions ofWIMPs passing throughour body every second. Interactions of DM with ordinary matter are exceedinglyweak andmay be detected by sensitive equipment through the energy deposited in a detector by elastic scattering of WIMPs with the nuclei of ordinary matter. There are many such directsearch experiments around the world, trying to catch the faint signals released in the form of photons, electrons or heat using different technologies. Several experiments, such as the Dark Matter/Large Sodium Iodide Bulk for Rare Processes experiment in Italy, theCoherentGermanium Neutrino Technology and Cryogenic Dark Matter Search (CDMS) experiments in the USA and the Germanled Cryogenic Rare Event Search with Superconducting Thermometers experiment, have in recent years reported positive signals, which could be interpreted as light WIMPs. The first results published in Sci China Phys Mech Astron this year by the PandaX collaboration, however, downgrade these claims [1]. The PandaX (Particle AND Astrophysical Xenon Detector) collaboration is an international team of about 40 scientists led by Professor Xiangdong Ji from Shanghai Jiao Tong University (also affiliated with the University of Maryland). This is the first large-scale xenon DM search experiment carried out in the newly established China Jinping Underground Laboratory (CJPL) in Sichuan, China. CJPL is among the deepest underground laboratories in the world, and being well shielded from the cosmic ray background. Xenon is a noble gas, which becomes liquid at about 110◦ below zero Celsius. The current experiment uses about 120 kg of liquid xenon as the active target for WIMPs. The detector employs a technology called the dual-phase xenon time-projection chamber (TPC), enabling a 3D reconstruction of the events. This technology was shown to be one of the more promising technologies based on the similar 100 kg XENON DM project (XENON100) and the Large Underground Xenon experiments. It now offers improved detection sensitivity over a wide range of DM mass by two orders of magnitude compared with other experiments. The first stage of PandaX operations, the PandaX-I, intends to examine the curious signals of WIMPs in the low-mass region reported by previous experiments. Its detector is a pancake-shaped TPC designed for high photon-collection efficiency. PandaX-I’s first results are based on the data collected in 17.4 live days, with more than 4 million raw events recorded. After removing the low-quality events and narrowing the energy search window to the regions of interest, 46 events are seen in the central fiducial volume with 37 kg of liquid xenon. These events are consistent with signals from the background radiation, the paper reports. Without candidate WIMP DM events, PandaX-I sets a stringent limit for low-mass DM particles and effectively rules out the interpretation of positive experimental results previously reported. Together with the results reported by the China Dark matter Experiment led by scientists at Tsinghua University (also at CJPL) and the SuperCDMS experiments in the USA, PandaX has set one of the best limits for WIMP DM with low mass, as can be seen from the Fig. 1. PandaX’s first result demonstrates that China has become a competitive player in the search for DM, a Science News article commented. ‘It is very exciting to see that our detector has such a capability’, said Jianglai Liu, who led the data analysis and also contributed significantly to the detector construction. Indeed, the PandaX collaboration has since collected more than 75 days of data, which will have five times better detection sensitivity. The PandaX experiment is also upgrading its detector mass to 500 kg, and will commence operations in 2015. Meanwhile, XENON1T, a similar experiment but with a 1.5-ton detector mass, developed by a US-Europe collaboration

China carves out larger role in underground science

As it is doing in so many areas of science, China is racing onto the world stage of underground astroparticle physics.

The Second-phase Development of the China JinPing Underground Laboratory

During 2013-2015 an expansion of the China JinPing underground Laboratory (CJPL) will be undertaken along a main branch of a bypass tunnel in the JinPing tunnel complex. This second phase of CJPL will increase laboratory space to approximately 96,000 m3, which can be compared to the existing CJPL-I volume of ∼ 4,000 m3. One design configuration has eight additional hall spaces, each over 60 m long and approximately12 m in width, with overburdens of about 2.4km of rock, oriented parallel to and away from the main water transport and auto traffic tunnels. There are additional possibilities for further expansions at a nearby second bypass tunnel and along the entrance and exit branches of both bypass tunnels, potentially leading to an expanded CJPL comparable in size to Gran Sasso. Concurrent with the excavation activities, planning is underway for dark matter and other rare-event detectors, as well as for geophysics/engineering and other coupled multi-disciplinary sensors. In the town meeting on 8 September, 2013 at Asilomar, CA, associated with the 13th International Conference on Topics in Astroparticle and Underground Physics (TAUP), presentations and panel discussions addressed plans for one-ton expansions of the current CJPL germanium detector array of the China Darkmatter EXperiment (CDEX) collaboration and of the duel-phase xenon detector of the Panda-X collaboration, as well as possible new detector initiatives for dark matter studies, low-energy solar neutrino detection, neutrinoless double beta searches, and geoneutrinos. JinPing was also discussed as a site for a low-energy nuclear astrophysics accelerator. Geophysics/engineering opportunities include acoustic and micro-seismic monitoring of rock bursts during and after excavation, coupled-process in situ measurements, local, regional, and global monitoring of seismically induced radon emission, and electromagnetic signals. Additional ideas and projects will likely be developed in the next few years, driven by China's domestic needs and by international experiments requiring access to very great depths.

Status and Prospects of CJPL and the CDEX Experiment

The China Dark Matter Experiment (CDEX) pursues the direct detection of light WIMPs towards the goal of a ton-scale germanium detector array at the China Jinping Underground Laboratory (CJPL) located in Sichuan, China. This facility having about 2400 m of rock overburden is the deepest operational underground laboratory in the world. Results on light WIMPs from the CDEX-0 and CDEX-1 employed a germanium detector array with a total mass of 20g and a crystal mass of 994g pPCGe detector respectively were reported. We highlight the status and perspectives of the dark matter programs at CJPL, in particular the project of CDEX-10.

Limits on light weakly interacting massive particles from the CDEX-1 experiment with ap-type point-contact germanium detector at the China Jinping Underground Laboratory

We report results of a search for light Dark Matter WIMPs with CDEX-1 experiment at the China Jinping Underground Laboratory, based on 53.9 kg-days of data from a p-type point-contact germanium detector enclosed by a NaI(Tl) crystal scintillator as anti-Compton detector. The event rate and spectrum above the analysis threshold of 475 eVee are consistent with the understood background model. Part of the allowed regions for WIMP-nucleus coherent elastic scattering at WIMP mass of 6-20 GeV are probed and excluded. Independent of interaction channels, this result contradicts the interpretation that the anomalous excesses of the CoGeNT experiment are induced by Dark Matter, since identical detector techniques are used in both experiments.

First dark matter search results from the PandaX-I experiment

We report on the first dark-matter (DM) search results from PandaX-I, a low threshold dual-phase xenon experiment operating at the China Jinping Underground Laboratory. In the 37-kg liquid xenon target with 17.4 live-days of exposure, no DM particle candidate event was found. This result sets a stringent limit for low-mass DM particles and disfavors the interpretation of previously-reported positive experimental results. The minimum upper limit, $3.7\times10^{-44}$\,cm$^2$, for the spin-independent isoscalar DM-particle-nucleon scattering cross section is obtained at a DM-particle mass of 49\,GeV/c$^2$ at 90\% confidence level.

Limits on light WIMPs with a germanium detector at 177 eVee threshold at the China Jinping Underground Laboratory

The China Dark Matter Experiment reports results on light WIMP dark matter searches at the China Jinping Underground Laboratory with a germanium detector array with a total mass of 20 g. The physics threshold achieved is 177 eVee ("ee" represents electron equivalent energy) at 50% signal efficiency. With 0.784 kg-days of data, exclusion region on spin-independent coupling with the nucleon is derived, improving over our earlier bounds at WIMP mass less than 4.6 GeV.

The characteristics of a low background germanium gamma ray spectrometer at China JinPing underground laboratory

A low background germanium gamma ray spectrometer, GeTHU, has been installed at China JinPing Underground Laboratory (CJPL). The integral background count rate of the spectrometer was 0.629cpm between 40 and 2700keV, the origins of which were studied by Monte Carlo simulation. Detection limits and efficiencies were calculated for selected gamma peaks. Some samples of rare event experiments were measured and 137Cs contamination was found in boric acid. GeTHU will be mainly used to measure environmental samples and screen materials in dark matter and double beta decay experiments.

Environmental gamma background measurements in China Jinping Underground Laboratory

To determine the environmental gamma background levels which affects rare events experiments, we measured in situ gamma spectrum at four locations in the China Jinping Underground Laboratory. The integral background count rates (40–2,700 keV) varied from 3.76 to 74.1 cps. The average count rate of the measurements inside the CJPL was 73.4 cps. The spectrometer was calibrated with a 152Eu point source and Monte Carlo simulation to obtain the activity conversion factors for the rock and the air, respectively. The rocks that surrounded the CJPL was characterized by very low activity concentrations of 238U (3.69–4.21 Bq kg−1), 232Th (0.52–0.64 Bq kg−1) and 40K (4.28 Bq kg−1).

CDEX-1 1 kg point-contact germanium detector for low mass dark matter searches

The CDEX collaboration has been established for direct detection of light dark matter particles, using ultra-low energy threshold point-contact p-type germanium detectors, in China JinPing underground Laboratory (CJPL). The first 1 kg point-contact germanium detector with a sub-keV energy threshold has been tested in a passive shielding system located in CJPL. The outputs from both the point-contact P+ electrode and the outside N+ electrode make it possible to scan the lower energy range of less than 1 keV and at the same time to detect the higher energy range up to 3 MeV. The outputs from both P+ and N+ electrode may also provide a more powerful method for signal discrimination for dark matter experiment. Some key parameters, including energy resolution, dead time, decay times of internal X-rays, and system stability, have been tested and measured. The results show that the 1 kg point-contact germanium detector, together with its shielding system and electronics, can run smoothly with good performances. This detector system will be deployed for dark matter search experiments.

First results on low-mass WIMPs from the CDEX-1 experiment at the China Jinping underground laboratory

The China Dark matter Experiment collaboration reports the first experimental limit on WIMP dark matter from 14.6 kg-day of data taken with a 994 g p-type point-contact germanium detector at the China Jinping underground Laboratory where the rock overburden is more than 2400 m. The energy threshold achieved was 400 eVee. According to the 14.6 kg-day live data, we placed the limit of N= 1.75 * 10^{-40} cm^{2} at 90% confidence level on the spin-independent cross-section at WIMP mass of 7 GeV before differentiating bulk signals from the surface backgrounds.

Measurement of cosmic ray flux in the China JinPing underground laboratory

The China JinPing underground Laboratory (CJPL) is the deepest underground laboratory running in the world at present. In such a deep underground laboratory, the cosmic ray flux is a very important and necessary parameter for rare-event experiments. A plastic scintillator telescope system has been set up to measure the cosmic ray flux. The performance of the telescope system has been studied using the cosmic rays on the ground laboratory near the CJPL. Based on the underground experimental data taken from November 2010 to December 2011 in the CJPL, which has an effective live time of 171 days, the cosmic ray muon flux in the CJPL is measured to be (2.0±0.4)×10−10/(cm2·s). The ultra-low cosmic ray background guarantees an ideal environment for dark matter experiments at the CJPL.