[论文解读] Experimental search for the LSND anomaly with the ICARUS LAr TPC detector in the CNGS beam

M. Antonello, B. Baibussinov+55 more|arXiv (Cornell University)|Sep 1, 2012

Particle physics theoretical and experimental studies被引用 16

一句话总结

本研究利用ICARUS液氩时间投影室在730公里基线的CNGS中微子束中搜索LSND异常。在1091个中微子事例中，观测到2个电子样事例，而背景预期为3.7个，对于振荡概率大于5.4×10⁻³的情况，在90%置信水平下排除了LSND异常，显著限制了参数空间至Dm²≈0.5 eV²和sin²(2θ)≈0.05。

ABSTRACT

A search for a nu_e signal, due to a LSND anomaly, with ICARUS at LNGS exposed at the CNGS neutrino beam, is hereby presented. At a distance of 730 km and with the 0-30 GeV energy interval, such anomaly is characterised by fast energy oscillations with probability averaging to 0.5*sin^2(2 theta_new). This is compared with the small but significant backgrounds due to conventional neutrino sources. The ionization along the early part of e.m. showers, has been examined wire by wire to tag the presence of an electron emitted from the vertex of the neutrino event. This is powerful eliminator of gamma converting pairs, which are generally separated from the vertex and are double minimum ionizing. In order to simulate the LSND anomaly, Montecarlo nu_e events have been generated with the nu_mu energy spectrum. These events have been used to estimate the electron identification efficiency due to visibility cuts, found to be ~74%. The present ICARUS experimental sample is based on 1091 neutrino events, about 50% of the data collected up to now. Two events with a clear electron have been found, compared with the expectation of 3.7 events from conventional sources. Within the range of observations, the result is compatible with the absence of LSND anomaly. At 90 and 99% CL this corresponds to 3.41 and 7.13 events with oscillation probabilities 5.4 10^-3 and 1.1 x 10^-2 respectively. This result strongly limits the window for the LSND anomaly, reducing to a narrow region around Dm2=0.5 eV2 and sin^2(2 theta)=0.05, where there is an overall agreement (90 % CL) between the ICARUS limit, the published limit of KARMEN and the published positive signals of LSND and MiniBooNE collaborations. To complete the search of an LSND-like anomaly, especially in the surviving region, we intend to move the ICARUS detector to a much closer distance and to collect new data from a lower energy neutrino.

研究动机与目标

• 通过长基线中微子束中的电子中微子出现来检验LSND异常假说。
• 减少电子探测中传统中微子源带来的背景贡献。
• 利用逐线电离剖面提高电子识别能力，以抑制伽马射线引起的假阳性。
• 设定与LSND和MiniBooNE信号一致的惰性中微子振荡的严格限制。
• 通过识别未来实验可进一步研究的狭窄可行参数区域来指导后续研究。

提出的方法

• 使用蒙特卡洛模拟生成nu_e事例，基于nu_mu能量谱来模拟LSND样振荡。
• 对早期簇射发展过程中的电离沉积进行逐线分析，以识别电子顶点。
• 应用可见性判据以估算电子识别效率，结果约为74%。
• 量化并从观测事例样本中减去来自传统中微子源的背景。
• 统计分析比较了观测到的电子事例（2个）与预期的传统背景（3.7个），在90%和99%置信水平下进行检验。
• 建议未来将探测器移至更短基线，以增强对剩余异常区域的敏感度。

实验结果

研究问题

• RQ1 ICARUS探测器是否在CNGS束中观测到与LSND异常一致的电子样事例过剩？
• RQ2 使用逐线电离剖面对低能簇射进行电子识别的效率是多少？
• RQ3 传统中微子背景如何影响LSND参数空间中惰性中微子振荡的搜索？
• RQ4 在90%和99%置信水平下，LSND异常的振荡概率上限是多少？
• RQ5 基于ICARUS、KARMEN、LSND和MiniBooNE的综合约束，LSND异常的剩余参数空间位于何处？

主要发现

• 在ICARUS数据样本的1091个中微子事例中，观测到2个电子样事例，而传统源预期为3.7个。
• 该结果在90%置信水平下与LSND异常的缺失一致，对应上限为3.41个事例。
• 在99%置信水平下，上限为7.13个事例，对应最大振荡概率为1.1×10⁻²。
• LSND异常被强烈限制在Dm² = 0.5 eV²和sin²(2θ) = 0.05附近的狭窄区域。
• 该区域整体上与ICARUS限制、KARMEN限制以及LSND和MiniBooNE报告的正信号一致。
• 计划未来在更短基线运行ICARUS，以进一步探测剩余的可行参数空间。