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PandaX
The Particle and Astrophysical Xenon Detector, or PandaX, is a dark matter detection experiment at China Jinping Underground Laboratory (CJPL) in Sichuan, China.[1] The experiment occupies the deepest underground laboratory in the world, and is among the largest of its kind.
Participants
The experiment is run by an international team of about 40 scientists, led by researchers at China's Shanghai Jiao Tong University.[2] The project began in 2009 with researchers from Shanghai Jiao Tong University, Shandong University, the Shanghai Institute of Applied Physics (zh), and the Chinese Academy of Sciences.[3][4] Researchers from the University of Maryland, Peking University, and the University of Michigan joined two years later.[3] The PandaX team also includes members from the Ertan Hydropower Development Company.[5]
Design and construction
PandaX is a direct-detection experiment, consisting of a dual-phase xenon time projection chamber (TPC) detector.[1] The use of both liquid and gaseous phases of xenon, similarly to the XENON and LUX experiments, allows the location of events to be determined, and gamma ray events to be vetoed.[4] In addition to searching for dark matter events, PandaX is designed to detect Xe-136 double beta decay.[4]
Operational stages
Like most low-background physics, the experiment is constructing multiple generations of detectors, each serving as a prototype for the next. A larger size allows greater sensitivity, but this is only useful if unwanted "background events" can be kept from swamping the desired ones. Lessons learned in earlier generations are used to construct later ones.
The first generation, PandaX-I, operated until late November, 2014.[6]:15 It used 120 kg (260 lb) of xenon (of which 54 kg (119 lb) served as a fiducial mass)[7]:7,10 to probe the low-mass regime (<10 GeV) and verify dark matter signals reported by other detector experiments.[1][8] PandaX-I was the first dark matter experiment in China to use more than 100 kg of xenon in its detector, and its size was second only to the LUX experiment in the United States.[2]
PandaX-II, completed in March 2015 and commissioning in 2015 for physics operation in 2016–2017[7]:24, uses 500 kilograms (1,100 lb) of xenon (approximately 300 kg fiducual)[7]:24–25 to probe the 10–1,000 GeV regime.[1][8][9] The second stage of the experiment will reuse the shield, outer vessel, cryogenics, purification hardware, and general infrastructure from the first stage, but will require a larger cryostat and TPC.[4]
PandaX-II will be significantly more sensitive than both the XENON100 and LUX detectors.[7]:25 XENON100, in Italy has, in the three to four years prior to 2014, produced the highest sensitivities over a wide range of WIMP masses.[3][8]
The construction cost of PandaX is estimated at US$15 million, with an initial cost of $8 million for the first stage.[8][9]
A third generation PandaX-III detector, optimized for neutrinoless double beta decay detection, will be constructed in the second-phase CJPL-II laboratory during 2016, and a 20 tonne PandaX-IV dark matter detector is in the planning stages for c. 2020.[7]:7
Laboratory
PandaX is located at China Jinping Underground Laboratory (CJPL), the deepest underground laboratory in the world at more than 2,400 metres (1.5 mi) below ground.[2][9] The depth of the laboratory means the experiment is better shielded from cosmic ray interference than similar detectors, allowing the instrument to be scaled up more easily.[8] The muon flux at CJPL is 66 events per square meter per year, compared with 950 events/m2/year at the Homestake Mine, home of the LUX experiment, and 8,030 events/m2/year at the Gran Sasso lab in Italy, home to the XENON detector.[4] The marble at Jinping is also less radioactive than the rock at Homestake and Gran Sasso, further reducing the frequency of false detections.[4][9] Wolfgang Lorenzon, a collaborating researcher from the University of Michigan, has commented that "the big advantage is that PandaX is much cheaper and doesn't need as much shielding material" as similar detectors.[9]
Initial results
The majority of the PandaX experimental equipment was transported from Shanghai Jiao Tong University to China Jinping Underground Laboratory in August 2012, and two engineering test runs were conducted in 2013.[3] The initial data-collection run (PandaX-I) began in May 2014. Results from this run were reported in September 2014 in the journal Science China Physics, Mechanics & Astronomy. In the initial run, about 4 million raw events were recorded, with around 10,000 in the expected energy region for WIMP dark matter. Of these, only 46 events were recorded in the quiet inner core of the xenon target. These events were consistent with background radiation, rather than dark matter. The lack of an observed dark-matter signal in the PandaX-I run places strong constraints on previously-reported dark matter signals from similar experiments.[2]
Reception
Stefan Funk of the SLAC National Accelerator Laboratory has questioned the wisdom of having many separate direct-detection dark matter experiments in different countries, commenting that "spending all our money on different direct-detection experiments is not worth it."[8] Xiangdong Ji, spokesperson for PandaX and a physicist at Shanghai Jiao Tong University, concedes that the international community is unlikely to support more than two multi-tonne detectors, but argues that having many groups working will lead to faster improvement in detection technology.[8] Richard Gaitskell, a spokesperson for the LUX experiment and a physics professor at Brown University, commented, "I'm excited about seeing China developing a fundamental physics program."[9]
References
"PandaX Dark Matter Experiment". Shanghai Jiao Tong University.
"First dark matter search results from Chinese underground lab hosting PandaX-I experiment". Phys.org. September 30, 2014.
"Chinese scientists search for evidence of dark matter particles with new underground PandaX detector". Phys.org. July 23, 2014.
Ji, Xiangdong (June 5, 2013). "PandaX: Direct Dark Matter Search Experiment in China Jinping Underground Lab" (PDF). Shanghai Particle Physics and Cosmology Symposium 2013. Shanghai Jiao Tong University.
"PandaX Dark Matter Experiment: Team". Shanghai Jiao Tong University.
Giboni, Karl (15–17 December 2014). PandaX Results and Outlook (PDF). 7th symposium on large TPCs for low-energy rare event detection. Paris.
Liu, Jianglai (7–11 September 2015). The PandaX Experiment and the Results from the Full Exposure of PandaX-I (PDF). 14th International Conference on Topics in Astroparticle and Underground Physics. Torino.
Reich, Eugenie Samuel (February 20, 2013). "Dark-matter hunt gets deep". Nature (Nature Publishing Group) 494 (7437): 291–292. Bibcode:2013Natur.494..291S. doi:10.1038/494291a.
Strickland, Eliza (January 29, 2014). "Deepest Underground Dark-Matter Detector to Start Up in China". IEEE Spectrum. IEEE.
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