SuperNEMO demonstrator module construction.

过去十年的实验已经证实，难以捉摸的中微子并不是像光子那样严格意义上的无质量粒子，但实际上它确实具有微小的非零质量。然而，这些测量并不能告诉我们质量是多少。因此，物理学家们正忙于设计新的方法来确定中微子的绝对质量。在一个引人入胜的理论转折中，可能会出现这样的情况，即非常小的中微子质量可以在更高的能量尺度（即大统一尺度）的物理学中自然地得到解释，这远远超出了可以通过实验直接获得的能量。这种机制要求中微子具有另一种奇异的性质：它们本身就是反粒子。在普通的β衰变中，当原子核中的一个中子转化为质子时，会发射出一个电子和一个中微子。只有在某些同位素中，两个这样的衰变可以同时发生，导致两个电子和两个中微子的发射。这一过程已在先前的许多实验中得到证实。然而，如果中微子是它们自己的反粒子，那么同样的衰变也可能发生，但没有中微子发射——这一过程被称为无中微子双β衰变。SuperNEMO实验旨在以前所未有的灵敏度寻找中微子双β衰变。SuperNEMO本质上是一个巨大的盖革计数器。然而，探测器将精确地重建发射电子的轨迹，并精确地测量它们的能量，而不是一次“咔哒”声表明放射性衰变的存在。这样，真正的双β衰变事件就可以与“背景”或假事件区分开来。电子能量的精确测量对于确定中微子双β衰变是至关重要的，因为在这些事件中，衰变的所有能量都由两个电子携带。相比之下，双β衰变中部分能量被未探测到的中微子带走，其特征是测量到的能量范围很广。在每个超级nemo模块的中心都有一层薄膜，其中包含双β衰变同位素，最初将是硒-82。围绕震源的是盖革跟踪探测器，该探测器正在英国开发和制造。然后，电子被塑料闪烁体吸收，产生的光被记录在光电倍增管中，从而估计出它们的能量。英国物理学家最近证明，使用这种技术的测量精度比以往任何时候都要高。英国也将参与开发优化最终探测器设计所需的模拟。这个项目的目标是建造一个演示模块，它本质上是最终SuperNEMO模块的原型。这将使我们能够证明建造最终探测器所涉及的所有步骤，并使我们能够证明我们设计的探测器确实具有所需的灵敏度。来自几个国家的大约90名物理学家将参与其中，但最大的贡献将来自法国和英国。演示模块最初将在“Nu-Lab”中组装和调试，这是最近在伦敦大学学院穆拉德空间科学实验室新建的一个实验室。从那里，它将被带到法国-意大利边境山脉深处的莫丹Souterrain de Modane实验室，在那里我们将能够评估探测器在非常低背景条件下的性能。该项目的成果将是未来SuperNEMO探测器的工作演示模块。我们将能够使用这个模块进行令人兴奋和有竞争力的物理测量，但更重要的是，我们将确切地知道如何建造更大的探测器，我们将需要它来发现中微子双β衰变。

Experiments over the last decade have confirmed that the elusive neutrino is not a strictly massless particle, like the photon, but does in fact possess a tiny non-zero mass. These measurements do not, however, tell us what that mass is. Physicists are therefore busy devising novel ways of pinning down the absolute mass of the neutrino. In a fascinating theoretical twist, it may be the case that the very small neutrino mass is naturally explained in terms of physics at a much higher energy scale known as the Grand Unification scale, which is far beyond the energies that can be directly accessed by experiments. Such a mechanism would require the neutrinos to have another bizarre property : they are their own anti-particles. In ordinary beta decay, an electron and a neutrino are emitted when a neutron in the nucleus converts into a proton. Very rarely and only in certain isotopes, two such decays can happen simultaneously, resulting in the emission of two electrons and two neutrinos. This process has been confirmed in a number of previous experiments. However, if neutrinos are their own anti-particles, it may be possible for the same decay to occur but with no neutrinos emitted - a process called neutrinoless double-beta decay. The SuperNEMO experiment is designed to search for neutrinoless double-beta decay with unprecedented sensitivity. SuperNEMO is, in essence, a giant Geiger counter. However, rather than a single 'click' indicating the presence of a radioactive decay, the detector will accurately reconstruct the trajectories of the emitted electrons and precisely measure their energies. This way, real double-beta decay events can be distinguished from 'background' or fake events. The precise measurement of the electron energies is crucial for the identification of neutrinoless double-beta decay, since in these events all the energy of the decay is carried by the two electrons. By contrast, double beta-decay in which part of the energy is carried away by the undetected neutrinos is characterised by a wide range of measured energies. At the centre of each SuperNEMO module sits a thin film containing the double beta-decay isotope, which to begin with will be Selenium-82. Surrounding the source is a Geiger tracking detector that is being developed and built in the UK. Electrons are then absorbed in blocks of plastic scintillator and the resulting light is recorded in photomultiplier tubes, giving an estimate of their energy. UK physicists have recently demonstrated a better measurement accuracy using this technique than has ever been achieved before. The UK will also be involved in developing the simulations that will be required to optimise the final detector design. The goal of this project is to build a demonstrator module, which is essentially a prototype of a final SuperNEMO module. This will enable us to prove all the steps involved in the construction of the final detector, and will enable us to demonstrate that the detector we have designed does have the required sensitivity. Approximately 90 physicists from several countries will be involved, but the largest contributions will come from France and the UK. The demonstrator module will initially be assembled and commissioned in the 'Nu-Lab', a new laboratory recently built at UCL's Mullard Space Science Laboratory. From there, it will be taken to the Laboratoire Souterrain de Modane, deep underneath the mountains on the French-Italian border, where we will be able to assess the performance of the detector under very low background conditions. The outcome of this project will be a working demonstrator module of the future SuperNEMO detector. We will be able to do exciting and competitive physics measurements using just this module, but more importantly we will know exactly how to build the much larger detector that we will need in order to discover neutrinoless double-beta decay.