Collaborative Research: Determination of the Neutron Lifetime Using Magnetically Trapped Neutrons

自由中子的衰变是最简单的核衰变。描述自由中子衰变的参数，特别是中子寿命，为所有涉及弱相互作用的研究提供了必要的输入。标准模型的若干内部一致性试验需要中子寿命的精确值。中子寿命也是大爆炸核合成（BBN）理论中的一个重要参数。目前，中子寿命的不确定性对BBN理论预测的氦氢比值的不确定性贡献最大。中子寿命、观测到的宇宙轻元素丰度和已知的轻中微子种数之间的相互比较为BBN理论提供了关键的一致性检验。该实验将使用磁捕获超冷中子（UCN）来测量中子寿命。通过从超流氦散射一束低能中子来“填充”磁阱。一些中子失去了足够的能量被捕获，剩下的离开了捕获区。然后关闭束流，测量中子衰变的速率，从而测量中子的寿命，因为这些被捕获的中子经历了β衰变。这项技术将使研究人员能够大大提高中子衰变寿命已知的精度。利用磁捕获的方法，利用现有的和下一代的中子源，应该可以将中子寿命的精度提高2到50倍。

The decay of the free neutron is the simplest nuclear beta decay. The parameters that describe the decay of the free neutron, especially the neutron lifetime, provide essential inputs to all investigations that involve the weak interaction. A precise value for the neutron lifetime is required for several internal consistency tests of the Standard Model. The neutron lifetime is also an essential parameter in the theory of Big Bang Nucleosynthesis (BBN). At present, the uncertainty in the neutron lifetime currently provides the largest contribution to the uncertainty in the value of the helium to hydrogen ratio predicted by BBN theory. Intercomparison between the neutron lifetime, the observed cosmic light element abundances and the known number of light neutrino species provides a critical consistency check for BBN theory.This experiment will measure the neutron lifetime using magnetically trapped ultracold neutrons (UCN). A magnetic trap is "filled" by scattering a beam of low-energy neutrons from superfluid helium. Some neutrons lose enough energy to become trapped, with the remainder leaving the trapping region. The beam is then turned off and the rate of neutron decay, and thus their lifetime, is measured as these trapped neutrons undergo beta-decay. This technique should allow the researchers to substantial improve the precision that the neutron beta decay lifetime is known. Using the method of magnetic trapping, it should be possible to improve the accuracy of the neutron lifetime by a factor of between 2 and 50 using existing and next-generation neutron sources.