PandaX实验介绍和进展

Status and plan of the PandaX experiment

Panda X是一个利用惰性元素"氙"作为探测介质进行粒子及天体物理稀有事件探测的大型实验装置.Panda X的一期和二期实验主要利用"二相型氙"时间投影室技术来进行弱相互作用暗物质粒子的直接探测.其中一期实验使用了120 kg氙,目标是在低质量暗物质区域检验以往其他实验所发现的疑似信号.一期实验的结果不支持这些疑似信号,并对10 Ge V/c2以下质量的暗物质性质给出了严格的限制.二期实验升级后的探测器使用了500 kg的氙.二期实验目前正在进行调试和试运行,它将在2015年末正式开始数据采集.Panda X第三期实验将采用高压的富氙-136(90%富集度)气体探测器进行无中微子双贝塔衰变探测,以研究中微子的本质.三期实验正在进行概念设计和原型测试.

Panda X 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 Panda X 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 Panda X, Panda X-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 Panda X-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 Ge V/c2. The final results of Panda X-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 Ge V, demonstrating the detection potential of xenon at low WIMP mass region. The second stage, Panda X-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 Panda X-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. Panda X-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. Panda X-III plans to build such a detector with 200 kg enriched 136 Xe first, then build another 4 identical modules to reach ton scale. The initial design and prototype testing of Panda X-III is carrying out by different groups from the institutions all over the world. The first detector module of Panda X-III is expected to be finished construction and start operation in 2018. The success running of the Panda X experiments may lead to important discoveries in physics.