Measuring the neutron lifetime with ultracold neutrons

中子构成了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，即自由中子在衰变为质子、电子和反中微子之前存活的时间，影响着宇宙中元素的形成，尤其是最丰富的元素之一氦的形成；宇宙中近四分之一的质量是由氦原子产生的，其余的几乎都是氢原子。利用中子寿命和天体物理观测，可以测试大爆炸元素产生模型。测定中子寿命的两种主要方法束法和瓶法之间存在较大的差异。波束法预测更长的寿命，但只对上述衰减通道敏感。瓶子实验测量的寿命较短，并且包括所有可能的中子衰变通道。最近，这引发了关于暗衰变通道是否存在的热烈讨论，暗衰变通道也有助于解决关于宇宙的一个重大谜团：宇宙中的暗物质比普通物质多五倍以上，但我们还没有发现它。本提案所要求的支持对慕尼黑工业大学（TUM）开发和建造的质子萃取中子寿命精密实验（PENeLOPE）有重要贡献。该实验旨在将中子寿命的精度提高一个数量级，达到0.1秒的水平。大量的超冷中子（UCN）将被储存在磁引力阱中。磁场是由24个超导线圈产生的。中子衰变产生的质子将被实时检测，从而直接反映陷阱中中子的指数衰减率。利用磁存储技术，PENeLOPE避免了材料壁阱实验中固有的（异常）中子吸收和上散射所带来的不确定性。在我在TUM工作期间，在磁体的概念设计和优化阶段之后，TUM团队专注于超导磁体的工程和建设。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-朗格万研究所（ILL）的第一次数据收集活动做准备。这项建议所要求的加拿大的贡献对于完成中子电子学的发展、将质子探测器纳入实验以及收集和分析数据是至关重要的。ILL活动的目标是达到前所未有的0.3秒的测量精度。为了推动0.1秒，PENeLOPE将安装在下一代UCN源，如即将在TRIUMF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.