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

自由中子的衰变是最简单的核β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。标准模型的几个内部一致性测试需要一个精确的中子寿命数值。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类之间的相互比较为BBN理论提供了一个关键的一致性检验。本实验将使用磁捕获超冷中子(UCN)来测量中子寿命。磁捕获器是通过从超流氦中散射一束低能量的中子束来“填充”的。一些中子失去了足够的能量而被困住，剩下的则离开了囚禁区域。然后关闭束流，当这些被俘获的中子经历贝塔衰变时，测量中子衰减率，从而测量它们的寿命。这项技术应该会使研究人员大大提高已知中子平均寿命的精度。使用磁捕获法，利用现有的和新一代的中子源，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. Theparameters that describe the decay of the free neutron, especially theneutron lifetime, provide essential inputs to all investigations thatinvolve the weak interaction. A precise value for the neutron lifetimeis required for several internal consistency tests of the StandardModel. The neutron lifetime is also an essential parameter in thetheory of Big Bang Nucleosynthesis (BBN). At present, the uncertaintyin the neutron lifetime currently provides the largest contribution tothe uncertainty in the value of the helium to hydrogen ratio predictedby BBN theory. Intercomparison between the neutron lifetime, theobserved cosmic light element abundances and the known number of lightneutrino species provides a critical consistency check for BBN theory.This experiment will measure the neutron lifetime using magneticallytrapped ultracold neutrons (UCN). A magnetic trap is ``filled'' byscattering a beam of low-energy neutrons from superfluid helium. Someneutrons lose enough energy to become trapped, with the remainderleaving the trapping region. The beam is then turned off and the rateof neutron decay, and thus their lifetime, is measured as these trappedneutrons undergo beta-decay. This technique should allow theresearchers to substantial improve the precision that the neutron betadecay lifetime is known. Using the method of magnetic trapping, itshould be possible to improve the accuracy of the neutron lifetime by afactor of between 2 and 50 using existing and next-generation neutronsources.