Determination of Absolute Neutrino Mass Using Quantum Technologies

中微子是宇宙中最丰富的物质粒子，但我们不知道它有多重。我们知道，这种粒子携带着超新星爆炸释放的99%的能量，在早期宇宙的演化中发挥了重要作用，它的质量异常小，但我们也知道它不可能是零。因此，我们必须测量这一粒子物理标准模型中最后一个未知质量。我们不能在实验室里直接测量中微子的质量。相反，我们试图尽可能精确地限制在核衰变等过程中产生中微子的能量。爱因斯坦著名的方程告诉我们如何计算中微子的质量。由于中微子没有被探测到，所以测量最小中微子能量的实验任务实际上是测量所有其他粒子携带的最大能量。最有希望使用的系统是氚，其中一个正常氢原子核内的质子伴随着两个中子。氚的半衰期为12.3年，衰变能量非常小，只有18.6千电子伏特；这种衰变能量如此之小的事实使得它对微小的中微子质量非常敏感。我们将需要开发捕获大量氚的技术，并以极高的灵敏度测量β衰变电子的能量。作为第一步，我们将使用氘，它比放射性的氚更容易处理。我们将把氘束流磁减速到特征非常明显的磁阱中。阱内产生的电子将作圆周运动，并在此过程中发出微波辐射。我们将开发能够探测以这种方式产生的消失的低功率信号的量子传感器。这个项目的最终目的是表明，原则上，我们已经拥有了对所有可能的中微子质量值都敏感的更大实验所需的技术。这样的实验或许可以在英国进行，那里的Culham聚变能源中心（Culham Centre for Fusion Energy）拥有世界领先的处理大量氚库存的设施，并且正在进一步开发中。

The neutrino is the most abundant matter particle in the universe, and yet we do not know how much it weighs. We know that this particle, which carries 99% of the energy released in supernova explosions and has played an important role in the evolution of the early universe, has an anomalously small mass but we also know that it cannot weigh nothing. It is therefore imperative that we measure this, the last unknown mass in the Standard Model of particle physics.We cannot measure the neutrino mass directly in the laboratory. Rather, we try to constrain as precisely as possible the energy that has gone into creating the neutrino in processes such as nuclear beta-decay. Einstein's famous equation then tells us how to calculate the neutrino mass. Since the neutrino escapes undetected, the experimental task involved in measuring the minimum neutrino energy is actually to measure the maximum energy carried by all of the other particles. The most promising system to use is tritium, in which the proton inside a normal hydrogen nucleus is accompanied by two neutrons. Tritium beta-decays with a half-life of 12.3 years and a very small decay energy of 18.6 kilo-electron-volts; the fact that this decay energy is so small makes it uniquely sensitive to the tiny neutrino mass.We will need to develop techniques for trapping very large populations of tritium and measuring with exquisite sensitivity the energy of beta-decay electrons. As a first step we will use deuterium, which is much easier to handle than radioactive tritium. We will magnetically decelerate beams of deuterium into very well characterised magnetic traps. Electrons generated inside the trap will undergo circular motion and in so doing will emit microwave radiation. We will develop the quantum sensors that are capable of detecting the vanishingly low-power signals that are generated in this way. The ultimate aim of this project is to show that we have, in principle, the technologies required for a much larger experiment that would have sensitivity to all possible values of the neutrino mass. Such an experiment could perhaps be hosted in the UK where, at the Culham Centre for Fusion Energy, world-leading facilities for handling large tritium inventories exist and are being further developed.