超级神冈中微子实验 陈少敏 清华大学 2019/4/27

Super-Kamiokande detector A 50k tons water Č detector located at 1k m underground 41.4 m 39.3 m 2019/4/27

Physics topics in Super-Kamiokande Nucleon decay Solar neutrino Atmospheric neutrino Neutrinos from supernova Long baseline neutrino oscillation Gamma ray burst Dark matter search … 2019/4/27

Super-Kamiokande collaboration Initially (1992): Japan, USA Later: Korea, Poland Now: China ~ 140 Scientists and ~ 35 Institutions 2019/4/27

History of Super-Kamiokande # of PMTs 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Start Threshold Achievement Discovery of atmosphere  oscillation 11,146 (40%) SK-I 5 MeV Discovery of Solar  oscillation Accident Partial reconstruction Discovery of Atmosphere  L/E effect 5,182 (19%) SK-II 7 MeV Full reconstruction 11,129 (40%) SK-III 4 MeV(plan) 2019/4/27

From SK-II to SK-III 5,182 PMTs SK-I in 2001 accidence Partial reconstruction SK-II in 2002 Full reconstruction 11,129 PMTs PMT with FRP mask SK-III in 2006 2019/4/27

Detector goals in SK-III Lower energy threshold Extend energy range Special trigger logic? Change electronic threshold? Lower water temperature from 13°C to 10°C? Adding Gd in water? … Down to 4 MeV Improved from 0 – 300 p.e.s/PMT to 0 – 1250 p.e.s./PMT with newly designed electronics. Up to multi TeV scale 2019/4/27

清华中微子物理小组在超级神冈的物理目标 超新星中微子 中微子直接振荡证据 需要对探测器进行改进 利用可能的超新星爆发测量绝对中微子质量 观测超新星遗迹中微子 需要对探测器进行改进 利用高能大气中微子观测 2019/4/27

1987年发现的超新星爆发事件 After Before SN1987A 2019/4/27

超新星爆发中微子  距离 L 因中微子质量造成的时间延迟 t 1987A 超新星距地球 52±5 kpc 结合引力波实验测量中微子到达的时间差测量中微子质量。  当能量大于10MeV时,存在大量非弹散射过程, 严重影响SN的方向及最初到达时刻的确定。 2019/4/27

反衰变事例对超新星爆发中微子的影响 2019/4/27

超新星遗迹中微子与反  衰变 Current SK result limited by cosmic ray  background Possible improvement: to tag inverse  decay SK upper limit: <1.2 /cm2/sec 2019/4/27

Invisible soft muon background Post-activity Evis  Pre-activity Without neutron tag: S/B ~0.1 With neutron tag: S/B~1 Evis =15~30MeV 2019/4/27

Inverse beta decay n Hardly seen by SK-I/II trigger system Prompt e+  Delayed  (2.2MeV~7PMT hits) Thermal neutron free mean path ~50cm Tagging method ~200s Trigger low events with prompt e+ and delayed  at the same location. Reaction threshold: Can we make it possible in SK-III? Hardly seen by SK-I/II trigger system 2019/4/27

目前正在研究的两种探测方法 Possibility 2 Possibility 1 n+p→d + g 2.2MeV g n+Gd →~8MeV g’s DT = several 10th msec DT = ~ 200 msec Add 0.2% GdCl3 in water # of hit PMT = 7 @ SK-III A simple & intermediate solution but with efficiency problem Complicated & still understudy project but with good efficiency 清华大学 UCI 2019/4/27

2.2 MeV  满足不了神冈的物理触发阈 由于能量太低无法通过 SK的物理触发, 只有采 用强制触发记录可能的 2.2MeV光子事例 # of PMT hits 由于能量太低无法通过 SK的物理触发, 只有采 用强制触发记录可能的 2.2MeV光子事例 2019/4/27

离线分析中采用时间符合降低本底的影响 Because of time-of-flight difference to individual PMT, the PMT timings of 2.2MeV  can not form a peak against BG. 2.2MeV  # of hits Averaged BG PMT time # of hits Averaged BG PMT time Without time-of-flight correction, 2.2MeV gamma looks no more difference from the low energy background in SK. 2019/4/27

离线数据分析准备工作 Forced trigger event = Random trigger data + 2.2 MeV  MC 2.2 MeV  signal from simulation, taking into account neutron walk time # of PMT hits time Noise from SK-I random trigger data Approach to mimic the real forced trigger events In total, we get 500 + 500 forced trigger events for both training and test samples. 2019/4/27

利用神经网络寻找信号 目前估计在效率为40%的条件下,本底可降低10倍 Both NN training and testing curves merge and converge 目前估计在效率为40%的条件下,本底可降低10倍 2019/4/27

触发电子学硬件准备工作 Ready for installation next month 6 LED counter NIM power FPGA+MCU Level convert 8 NIM Input 8 NIM Output FPGA configure port 2019/4/27

在日本神冈的清华学生 Around ICRR research building “Kenkyu-tou” Inside the mine 2019/4/27

总结与展望 经过五年的准备，超级神冈探测器开始新一轮的数据采集 新实验朝降低能量阈和提高可测能量范围进行努力 清华大学目前的重点放在改进超新星中微子探测方面，欢迎各位老师与同学加入我们的团队 2019/4/27