Calculation of Nuclear Deeply Virtual Compton Scattering in HERMES Experiment

We investigate the possibility to acquire information of nuclear generalized parton distribution （GPD） H by studying the deeply virtual Compton scattering （DVCS） off several nuclear targets at the HERMES group （Hadron Electron Ring Accelerator Measurement of Spin）. Two different models are used and developed to demonstrate the leading asymmetry amplitude.A^sinФ LU for coherent-enriched and incoherent-enriched parts with both statistical and systematic uncertainties estimated. It is found that a clear enhancement of ratio of nuclear asymmetry A ^AsinФ to free proton asymmetry A^H,sinФ LU in the coherent-enriched region is expected by both models,and a decrease of the ratio in incoherent-enriched region; both give the information about nuclear modifications. It is also possible to distinguish between those two models even under the limited statistics.     

7Be半衰期的精确测量

用两套γ谱仪精确测量了注入到天然铍和天然金中的7Be发生电子俘获的半衰期.测得7Be在天然铍中半衰期为53.275(25)d,在天然Au中为53.270(19)d.该结果还表明,在这两种材料中7Be半衰期的相对变化小于0.12%. The half-life of 7Be implanted in natural beryllium and in natural gold has been measured with high precision using two high-purity germanium detectors. T_(1/2)=53.275(25) d in natural beryllium and T_(1/2)=53.270(19) d in natural gold is obtained. Within our experimental precision, the difference in the effect of host media beryllium and gold on the half-life of 7Be isn t observed and an upper limit 0.12% of this effect can be set.     

PandaX实验的进展与展望

PandaX是位于四川的中国锦屏地下实验室（CJPL）内的,利用氙作为工作物质,探测暗物质等其它极稀有事件的大型地下粒子探测实验计划。PandaX探测器中的氙工作在气液两相的模式。通过检测液相和气相中闪烁光强的比例,实现对目标粒子包括WIMP和Axion等疑似信号的鉴别。PandaX已顺利完成了有效区体积约为120 kg氙的一期计划,目前正在运行实施有效区体积约为500 kg氙的二期计划。在WIMP质量为45 GeV/c2附近,探测灵敏度达2.97×10-45 cm2（90%置信度）,没有发现WIMP疑似信号。本工作介绍了PandaX一期和二期的进展,以及后续升级计划,其中包括升级PandaX的时间投影室至4吨,建造用于屏蔽岩石放射性本底的水屏蔽,以及136Xe双贝塔衰变的研究计划等。PandaX is a xenon-based dark matter and other rare events searching project located at China JinPing underground Laboratory (CJPL), Sichuan province, China. Dual phases, liquid and gas phase of xenon are used in the detector. Particles, including Weak Interaction Massive Particles (WIMPs) or axions etc., may induce scintillating signals in both liquid (S1 signal) and gas (S2 signal) phases, and then are identified by analyzing the ratio of S1 and S2. The PandaX had completed its first stage, the 120 kg in fiducial volume, and now is running in its stage Ⅱ with fiducial volume of 500 kg. The running results show that the WIMP-matter cross section detecting sensitivity of about 2.97×10-45 cm2(90% C.L.) has been achieved at a WIMP mass of 44.7 GeV/c2, and no WIMP candidates are identified. The upgrading plan of the PandaX is discussed, which include to build a larger Time project chamber (TPC) of about 4 tons, a water tank for radiation shielding, and the plan of 136Xe double beta decay studies.     

高能电子束流引发的横向极化气体氢靶共振退极化研究

对于HERMES横向极化气体氢靶, 高能电子束流所激发的高频磁场会引发共振退极化. 本文就该极化度下降的产生机制进行研究, 并给出σ₂₄共振点发生位置的测量结果, 并且将极化靶磁场的强度稳定在远离共振点处, 挽回了数据获取时极化度近3%的共振损失.     

A Search for Solar Axions and Anomalous Neutrino Magnetic Moment with the Complete Panda X-Ⅱ Data

We report a search for new physics signals using the low energy electron recoil events in the complete data set from PandaX-Ⅱ,in light of the recent event excess reported by XENON1 T.The data correspond to a total exposure of 100.7 ton·day with liquid xenon.With robust estimates of the dominant background spectra,we perform sensitive searches on solar axions and neutrinos with enhanced magnetic moment.It is found that the axionelectron coupling gAe<4.6×10^-12 for an axion mass less than 0.1 keV/c² and the neutrino magnetic moment μv<4.9×10^-11μB at 90%confidence level.The observed excess from XENON1 T is within our experimental constraints.     

An artificial neural network for proton identification in HERMES data

The HERMES time-of-flight （TOF） system is used for proton identification, but must be carefully calibrated for systematic biases in the equipment. This paper presents an artificial neural network （ANN） trained to recognize protons from ∧^0 decay using only raw event data such as time delay, momentum, and trajectory. To avoid the systematic errors associated with Monte Carlo models, we collect a sample of raw experimental data from the year 2000. We presume that when for a positive hadron （assigned one proton mass） and a negative hadron （assigned one π^- mass） the reconstructed invariant mass lies within the ∧^0 resonance, the positive hadron is more likely to be a proton. Such events are assigned an output value of one during the training process; all others were assigned the output value zero.
The trained ANN is capable of identifying protons in independent experimental data, with an efficiency equivalent to the traditional TOF calibration. By modifying the threshold for proton identification, a researcher can trade off between selection efficiency and background rejection power. This simple and convenient method is applicable to similar detection problems in other experiments.