Measuring the neutron lifetime with ultracold neutrons

中子占宇宙中普通物质的八分之一以上。它们的基本性质影响了宇宙大爆炸之后的宇宙历史。它15分钟的β衰变寿命，即一个自由中子在衰变成质子、电子和反中微子之前的生存时间，影响着宇宙中元素的产生，特别是最丰富的元素之一氦;宇宙中几乎四分之一的质量是由氦原子产生的，氢原子几乎占了其余的全部。利用中子寿命和天体物理观测，可以检验大爆炸元素产生模型。中子寿命的两种主要测定方法--束流法和瓶法之间存在着较大的差异。束方法预测更长的寿命，但只对上述衰变通道敏感。瓶实验测量较短的寿命，包括所有可能的中子衰变通道。这最近引发了关于暗衰变通道存在的热烈讨论，这也有助于解决宇宙中的一个大谜团：宇宙中的暗物质是普通物质的五倍多，但我们还没有发现它。本提案所要求的支持对慕尼黑技术大学开发和建造的质子提取中子寿命操作精密实验作出了重大贡献。该实验旨在将中子寿命的精度提高一个数量级，比以前的实验提高到0.1秒的水平。大量的超冷中子（UCN）将被储存在磁引力阱中。磁场由24个超导线圈产生。来自中子衰变的质子将在真实的时间内被探测到，从而直接反映出阱中中子的指数衰变率。使用磁存储技术，PENeLOPE避免了由于（异常）中子吸收和上散射的不确定性，这是固有的实验与材料壁陷阱。在我在TUM期间领导的磁体的概念设计和优化阶段之后，TUM小组专注于超导磁体的工程和建造。它于2020年交付，低温调试和线圈培训将于年底完成。下一步是为2022年在法国劳厄-朗之万研究所（ILL）的首次数据收集活动做准备。本提案中要求加拿大提供的捐助对于完成中子学的开发、将质子探测器纳入实验以及数据收集和分析至关重要。ILL活动的目标是前所未有的0.3秒测量精度。为了达到0.1 s，PENeLOPE将安装在下一代UCN源上，例如TRIUMF即将推出的TUCAN UCN源。

Neutrons make up more than one eighth of the ordinary matter of the universe. Their fundamental properties have influenced the history of the universe starting right after the Big Bang. Its 15-min beta-decay lifetime, the time a free neutron survives before decaying into a proton, an electron and an anti-neutrino, impacts element creation in the universe, especially of one of the most abundant elements, helium; almost a quarter of the mass in the universe is generated by helium atoms with hydrogen making up nearly all the rest. Using the neutron lifetime together with astrophysical observations, Big Bang element creation models can be tested. There is a large discrepancy between the two major methods to determine the neutron lifetime, the beam method and the bottle method. The beam method predicts a longer lifetime but is only sensitive to the decay channel mentioned above. Bottle experiments measure a shorter lifetime and are inclusive of all possible neutron decay channels. This has recently sparked lively discussion about the existence of dark decay channels that could also help solving one of the big mysteries about the universe: there is more than five times more dark matter in the universe, than ordinary matter, but we have not found it, yet. The support requested in this proposal contributes significantly to the Precision Experiment on the Neutron Lifetime Operating with Proton Extraction (PENeLOPE) which has been developed and constructed at the Technical University of Munich (TUM). The experiment is aiming to improve the precision of the neutron lifetime by an order of magnitude over previous experiments to a level of 0.1 s. A large number of ultracold neutrons (UCN) will be stored in a magneto-gravitational trap. The magnetic field is generated by 24 superconducting coils. The protons from neutron decay will be detected in real time, thus directly reflecting the exponential decay rate of the neutrons in the trap. Using the technique of magnetic storage, PENeLOPE avoids the uncertainties due to (anomalous) neutron absorption and up-scattering that are inherent in experiments with material-wall traps. After a conceptual design and optimization phase of the magnet, which I spearheaded during my time at TUM, the TUM group focused on the engineering and construction of the superconducting magnet. It was delivered in 2020 with cryogenic commissioning and coil training to be completed by the end of the year. The next step is to prepare the experiment for its first data-collection campaign at the Institut Laue-Langevin (ILL), France in 2022. The Canadian contribution requested in this proposal is vital to complete the development of the neutronics, integrate the proton detector into the experiment and for data collection and analysis. The goal of the ILL campaign is an unprecedented measurement precision of 0.3 s. To push for 0.1 s, PENeLOPE will be installed at a next generation UCN source, such as the upcoming TUCAN UCN source at TRIUMF.