[论文解读] Neutron lifetime, dark matter and search for sterile neutrino

A. П. Серебров, R. M. Samoilov+2 more|arXiv (Cornell University)|Feb 17, 2018

Atomic and Subatomic Physics Research参考文献 16被引用 9

一句话总结

该论文提出，中子寿命异常与反应堆反中微子异常可通过中子至暗物质中子振荡（n → n′）来解释，其中n′是一种质量比中子低约3 MeV的类惰性暗物质态。该模型预测质量差为mₙ − mₙ′ ≈ 3 MeV，但核裂变碎片的同位素产额分析排除了≥3 MeV的取值，仅剩下mₙ − mₙ′ ≤ 2 MeV是可行的。

ABSTRACT

A review is focused on experimental measurements on neutron lifetime. The latest measurements with a gravitational trap (PNPI NRC KI) and a magnetic trap (LANL, USA) confirmed PNPI result of 2005. The results of measurements with storage of ultra cold neutrons are in agreement, yet, there is discrepancy with a beam experiment by $3.5σ$ (1% of decay probability), which is discussed in literature as "neutron anomaly" along with the ideas of explaining it by decay into dark matter partially. The second part of the paper is devoted to so called "reactor antineutrino anomaly", which refers to deficiency of the measured flux of antineutrino from reactor in respect to the calculated flux by $3σ$(deviation by 6.6%). Specific feature of the proposal in this paper lies in the fact that both anomalies can be accounted for by one and the same phenomenon of oscillation in baryon sector between a neutron and a neutron of dark matter $n{ ightarrow}n'$ with mass $m_{n'}$, somewhat less than mass $m_n$ of an ordinary neutron. Calculations of the proposed model require one free parameter: mass difference $m_n-m_{n'}$ if one normalizes probability of oscillations for a free neutron on "neutron anomaly" 1%, then, having succeeded to interpret 6.6% of neutron anomaly in calculations, one can determine mass difference. According to preliminary estimations, the mass difference is $m_n-m_{n'}{\approx}$ 3 MeV. However, the analysis of cumulative yields of isotopes occurs in fission fragments was performed and it does not confirm possibility of existence of additional decay channel with emission of dark matter neutron with mass difference $m_n-m_{n'}{\approx}$ 3 MeV. The result of the analysis is the conclusion that for mirror neutrons the region of the mass difference $m_n-m_{n'} {\geq}$ 3 MeV is closed. The region of the mass difference $m_n-m_{n'}{\leq}$ 2 MeV turned out to be not closed.

研究动机与目标

• 解决使用捕获超冷中子与束流方法测量中子寿命之间存在的差异，即所谓的‘中子异常’。
• 解释反应堆反中微子通量相比理论预测出现6.6%的亏损，称为‘反应堆反中微子异常’。
• 在普通中子与一种暗物质中子态（n → n′）之间振荡的单一理论框架下统一解释这两个异常。
• 利用核裂变碎片产额的实验数据约束普通中子与暗物质中子之间的质量差。

提出的方法

• 提出一种重子扇区振荡模型，其中中子（n）可转变为质量mₙ′ < mₙ的暗物质中子态（n′）。
• 利用中子寿命测量中1%的差异（3.5σ）作为归一化条件，以标定振荡幅度。
• 将相同的振荡模型应用于解释反应堆反中微子通量中6.6%的亏损，假设相同的质量差mₙ − mₙ′。
• 分析核反应堆中裂变碎片的累积产额，以检验是否存在涉及n′发射的额外衰变通道。
• 将振荡模型下的同位素产额预测与实验数据进行比较，以约束mₙ − mₙ′。
• 利用裂变产额中未观测到偏差的事实，排除质量差≥3 MeV的可能性。

实验结果

研究问题

• RQ1 中子寿命异常（表现为捕获与束流实验间3.5σ的差异）能否通过向暗物质中子态的振荡来解释？
• RQ2 反应堆反中微子异常（表现为6.6%的通量亏损）能否由相同的振荡机制来解释？
• RQ3 在实验约束下，普通中子与暗物质中子之间的最大允许质量差mₙ − mₙ′是多少？
• RQ4 核反应堆中裂变碎片的实际产额是否支持或排除mₙ − mₙ′ ≈ 3 MeV的中子至暗物质中子振荡通道的存在？
• RQ5 在所有可用实验数据下，mₙ − mₙ′的上限是多少？

主要发现

• 该模型预测普通中子与暗物质中子态（n′）之间的质量差约为3 MeV，与中子寿命测量中1%的差异一致。
• 相同的振荡模型成功解释了反应堆反中微子通量中6.6%的亏损，将两个异常统一于同一机制之下。
• 对裂变碎片累积产额的分析排除了mₙ − mₙ′ ≥ 3 MeV的衰变通道的存在。
• mₙ − mₙ′ ≤ 2 MeV的区域仍为可行，因为裂变产额数据中未观测到实验偏差。
• 该模型仅在质量差低于3 MeV时与两个异常均保持一致，其上限由裂变数据约束。