[论文解读] Ionization cooled ultra pure beta-beams for long distance neu-e to neu-mu transitions, theta13 phase and CP-violation

Carlo Rubbia|ArXiv.org|Sep 22, 2006

Neutrino Physics Research被引用 15

一句话总结

本文提出了一种新型的离子束冷却β-束概念，利用完全电离的B-8离子在紧凑环形装置中储存，以产生超纯的电子中微子束，用于长基线中微子振荡实验。结果表明，该中微子源结合50–100千吨液氩时间投影室（LAr TPC）探测器，可实现对sin²(2θ₁₃)的无背景灵敏度，低至6.0 × 10⁻⁴，显著优于当前限制，从而实现对CP破坏和θ₁₃相位的精确测量。

ABSTRACT

The key process is the observation of tiny oscillation mixing between numu and nue related to so far unknown theta13 amplitude, at distances corresponding to an invariant neutrino flight path around about 2.0 MeV/km. Zucchelli [12] has proposed the production of very pure neue beams (beta beams), in which relativistic radio-nuclides are stored in a high energy storage ring and decay in a long straight section pointing toward the neutrino detector far away. This method produces ultra pure anti-neue (He-6) and neue (Ne-18) with a negligible numu contamination (10-5). A novel kind of beta element production has been recently proposed [15] in which slow (v about 0.1 c) fully ionized ions are stored in a very small storage ring. Products like the isomeric doublet Li-7(d,p) Li-8 and Li-6(He3,n) B-8 are stopped in some foils few micron thick, from which they quickly diffuse as neutral atoms. The improvements are shown to be considerable. An ion source, a chain of several accelerators in cascade and a high energy storage/decay ring can produce a B-8 induced nue spectrum which has as much as 7.7 times higher energy than the one from He-6 induced anti-nue for a given magnetic rigidity of the storage ring. At a given neutrino energy, the CC cross sections for nue are about 3 times larger than the ones of anti-nue while the NC/CC inelastic pion background faking muons is about 3 times smaller. An optimal neu-e source might be fully ionized B-8 from the 120 GeV Main Energy Injector at FNAL, followed by a decay storage ring and a neutrino fly path of about 700-800 km. As detector we consider a LAr TPC with a fiducial mass of 50 to 100 kton. Such a technology should permit to detect a background free signal for sin2(2theta13) as small as about 6.0 x 10-4. By comparison, the present experimental limit is < 0.14.

研究动机与目标

• 开发一种高纯度、长基线中微子束源，能够以高灵敏度探测难以捉摸的θ₁₃混合角。
• 通过减少中微子束污染并提高信噪比，克服现有β-束设计的局限性。
• 通过实现对极小sin²(2θ₁₃)值的灵敏度，实现对中微子味中CP破坏的精确测量。
• 探索在紧凑储存环中使用离子束冷却的完全电离B-8离子以增强中微子通量和能量谱的可行性。
• 证明700–800公里基线搭配50–100千吨液氩时间投影室（LAr TPC）探测器可实现对振幅低于0.1%的振荡信号的无背景探测。

提出的方法

• 利用小型高能储存环中储存的完全电离B-8离子，通过β⁻衰变产生纯正的电子中微子束。
• 采用离子束冷却技术约束并聚焦离子束，降低发射度，从而实现高强度、低发散角的中微子束。
• 依赖环形装置长直段中储存B-8离子的衰变，实现700–800公里基线上的定向中微子束。
• 利用νₑ的带电流截面（≈比反νₑ大3倍）和中性流非弹性π介子背景（≈比反νₑ小3倍）的优势，提高信号可见度。
• 通过加速器级联和高能储存环，实现中微子能量谱比相同磁刚度下He-6基β-束高出最多7.7倍。
• 采用具有50–100千吨有效质量的液氩时间投影室（LAr TPC），实现对νₑ → νμ转变的近无背景探测。

实验结果

研究问题

• RQ1 完全电离的B-8 β-束源是否能实现足够的纯度和强度，以探测sin²(2θ₁₃) < 0.14的νₑ → νμ振荡？
• RQ2 与传统设计相比，离子束冷却和紧凑储存环的使用如何提升β-束中微子源的性能？
• RQ3 在该新型β-束构型下，使用无背景LAr TPC探测器可实现的sin²(2θ₁₃)最小可探测值是多少？
• RQ4 与He-6基中微子谱相比，B-8基中微子谱在能量、通量和信噪比方面有何差异？
• RQ5 该装置是否能实现对中微子混合矩阵中CP破坏相位的高精度测量？

主要发现

• 所提出的B-8 β-束源产生的中微子能量谱，在相同磁刚度下，能量最高可达He-6基源的7.7倍。
• νₑ的带电流截面约为反νₑ的3倍，显著提升了信号探测效率。
• 中性流非弹性π介子背景（可能模拟μ子信号）对νₑ而言约为反νₑ的1/3，有效降低了误报率。
• 在50–100千吨液氩TPC探测器下，该系统可实现对sin²(2θ₁₃)低至6.0 × 10⁻⁴的νₑ → νμ转变的无背景探测。
• 当前实验对sin²(2θ₁₃)的限制为< 0.14，表明该装置的灵敏度提升超过一个数量级。
• 识别出一条可行路径：利用费米实验室120 GeV主注入器，随后连接衰变储存环和700–800公里基线，可实现高精度中微子振荡测量。