[论文解读] Proposal: A Search for Sterile Neutrino at J-PARC Materials and Life Science Experimental Facility

Masahide Harada, S. Hasegawa+26 more|arXiv (Cornell University)|Oct 5, 2013

Neutrino Physics Research参考文献 7被引用 27

一句话总结

本文提出利用J-PARC材料与生命科学实验设施（MLF）的μ子衰变静止（DAR）中微子束，通过$\bar{\nu}_\mu \to \bar{\nu}_e$振荡搜索无菌中微子。实验利用脉冲质子束产生高强度、校准良好的$\bar{\nu}_\mu$，通过逆β衰变（$\bar{\nu}_e + p \to e^+ + n$）探测，并利用中子俘获伽马射线标记。关键贡献在于针对$\Delta m^2 \to 1~\text{eV}^2$区域的高灵敏度、低本底方法，未来可通过长基线远端探测器扩展至亚-eV$^2$区域。

ABSTRACT

We propose a definite search for sterile neutrinos at the J-PARC Materials and Life Science Experimental Facility (MLF). With the 3 GeV Rapid Cycling Synchrotron (RCS) and spallation neutron target, an intense neutrino beam from muon decay at rest (DAR) is available. Neutrinos come from μ+ decay, and the oscillation to be searched for is (anti νμ-> anti νe) which is detected by the inverse βdecay interaction (anti νe + p -> e+ + n), followed by a gamma from neutron capture. The unique features of the proposed experiment, compared with the LSND and experiments using horn focused beams, are; (1) The pulsed beam with about 600 ns spill width from J-PARC RCS and muon long lifetime allow us to select neutrinos from μDAR only. (2) Due to nuclear absorption of π- and μ-, neutrinos from μ- decay are suppressed to about the $10^{-3}$ level. (3) Neutrino cross sections are well known. The inverse βdecay cross section is known to be a few percent accuracy. (4) The neutrino energy can be calculated from positron energy by adding ~1.8 MeV. (5) The anti νμand νe fluxes have different and well defined spectra. This allows us to separate oscillated signals from those due to μ- decay contamination. We propose to proceed with the oscillation search in steps since the region of Δm^2 to be searched can be anywhere between sub-eV^2 to several tens of eV^2. We start to examine the large Δm^2 region, which can be done with short baseline at first. At close distance to the MLF target gives a high neutrino flux, and allows us to use relatively small detector. If no definitive positive signal is found, a future option exists to cover small Δm^2 region. This needs a relatively long baseline and requires a large detector to compensate for the reduced neutrino flux.

研究动机与目标

• 通过$\bar{\nu}_\mu \to \bar{\nu}_e$振荡搜索$\Delta m^2 \sim 1~\text{eV}^2$区域的无菌中微子，该区域由过去实验中的异常现象所提示。
• 利用J-PARC MLF独特的脉冲束流与长μ子寿命，从静止μ+衰变中分离出纯净的$\bar{\nu}_\mu$束流，最大限度减少来自π/ K衰变飞行的本底。
• 通过使用具有精确时间与顶点分辨率的分段液态闪烁体探测器，识别瞬发正电子与延迟中子俘获信号，实现高灵敏度。
• 通过使用长基线远端探测器与近-远探测器系统误差抵消机制，实现未来向更低$\Delta m^2$区域扩展的可能性。
• 通过束流门控、空间拒收与中子自屏蔽，减少μ-衰变与宇宙射线引起的本底。

提出的方法

• 利用J-PARC 3 GeV快速循环同步加速器（RCS）产生脉冲质子束，脉冲宽度约600 ns，实现对μ+衰变静止（DAR）中微子的选择性门控。
• 从$\mu^+ \to e^+ + \bar{\nu}_\mu + \nu_e$产生主要为$\bar{\nu}_\mu$与$\nu_e$的束流，通过核吸收将$\pi^-$与$\mu^-$诱导的中微子抑制至$\sim 10^{-3}$量级。
• 通过逆β衰变探测$\bar{\nu}_e$：$\bar{\nu}_e + p \to e^+ + n$，随后中子在质子上俘获产生延迟$\gamma$信号。
• 对瞬发正电子与延迟$\gamma$信号施加严格的时序与顶点相关性，以识别振荡产生的$\bar{\nu}_e$事例，并排除快中子引起的Michel电子。
• 采用带两端3英寸PMT的分段液态闪烁体模块，实现~5 cm顶点分辨率与~11.6%/$\sqrt{E}$能量分辨率，以抑制本底。
• 应用束流门控（脉冲开始后1 $\mu$s窗口）以排除与束流相关的快中子与本簇活动，减少来自$\mu^-$衰变与宇宙μ子的本底。

实验结果

研究问题

• RQ1 能否利用J-PARC MLF中μ+衰变静止产生的纯净高强度$\bar{\nu}_\mu$束流，实现对$\Delta m^2 \sim 1~\text{eV}^2$区域$\bar{\nu}_\mu \to \bar{\nu}_e$振荡的搜索？
• RQ2 J-PARC的脉冲束流结构与μ子寿命是否能有效抑制π/K衰变飞行与$\mu^-$诱导的本底？
• RQ3 具有精确时间与顶点分辨率的分段液态闪烁体探测器能否有效区分真实$\bar{\nu}_e$信号与Michel电子及其他本底？
• RQ4 所提出的17 m探测器对$\Delta m^2 \sim 1~\text{eV}^2$的灵敏度如何？与仅远端探测器或近-远端探测器配置相比有何差异？
• RQ5 通过近-远端探测器比较，系统误差是否能足够降低，从而在未来的长基线扩展中探测亚-eV$^2$区域？

主要发现

• 所提实验通过核吸收将来自$\mu^-$衰变的$\bar{\nu}_e$本底降至$\sim 10^{-3}$，显著减少污染。
• 探测器的5 cm顶点分辨率与350 ps时间分辨率可有效分离瞬发正电子与延迟中子俘获信号，降低误报率。
• 17 m基线探测器在1 MW束流功率下运行2年，预计可实现对$\Delta m^2 \sim 1~\text{eV}^2$的5$\sigma$灵敏度。
• 未来通过在60 m处部署1千吨探测器的长基线扩展，可探测亚-eV$^2$区域，且近-远端探测器间的系统误差抵消可进一步提升灵敏度。
• 远+近探测器配置在误差抵消后系统误差降至2%，而仅远端探测器配置则假设10%通量与50%本底误差。
• 探测器设计支持小模块的先导运行，可在全面部署前实现系统的早期测试与校准。