Measuring the neutron lifetime with ultracold neutrons

点击翻译按钮获取中文摘要

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.