Design study of the SuperNEMO experiment

在这项提案中，我们请求支持对一种新的大型探测器SuperNEMO进行研究，该探测器将搜索无中微子的双贝塔衰变。尽管这个过程没有中微子，但这项实验实际上帮助我们回答了有关中微子性质的基本问题。中微子是非常常见的基本粒子。每秒都有几万亿个中微子经过你的手指。尽管如此，我们对它们知之甚少。我们不知道的一件事是，中微子是否有像电子和正电子一样的反粒子，称为反中微子。电子和中微子是轻子家族的一部分，它们和夸克家族一起构成了宇宙中的物质。最近，实验证明，中微子的质量很小，但我们不知道这些质量有多大。为什么这很重要？这一次，因为我们周围有太多的中微子(你的手指！)即使是很小的质量也会对宇宙中包含的质量产生很大的影响。还有许多其他悬而未决的问题：为什么与其他粒子相比，中微子的质量这么小？为什么我们观察到的所有过程中的轻子数都是守恒的？尼莫实验正试图通过寻找一种以前从未观察到的非常罕见的过程来回答这些问题：无中微子的双贝塔衰变。放射性核β衰变是一种非常普遍的现象，它发生在核反应堆或岩石中。在这种衰变过程中，原子核中的一个中子转变为质子，同时发射出一个电子和一个反中微子(一个轻子和一个反轻子，因此轻子总数为零)。对于某些特殊的原子核，能量守恒定律禁止这一过程，但允许两个中子同时衰变。两个电子和两个反中微子在这次“双β衰变”中被发射出来。这项拟议的实验的目标是在不发射反中微子的情况下寻找双β衰变。在这个过程中，两个中子通过发射两个电子而不发射中微子而衰变为质子。中微子仍然在那里，但在一个中子的贝塔衰变中发射的中微子立即被另一个中子的贝塔衰变吸收。只有当中微子有质量并且是它自己的反粒子时，这才是可能的！在这个过程中，轻子的数量是不守恒的。这是从未被观察到的。超NEMO实验通过将由可能经历无中微子双贝塔衰变的材料组成的薄片放入一个寻找发射电子的大探测器中来执行搜索无中微子双贝塔衰变的任务。实验包括两个步骤，首先我们寻找由两个电子在大氦体积中产生的两个轨道。这些轨道是用一组巨大的导线来测量的，它们的工作原理类似于经典的盖革计数器。然后，电子被量热计探测器吸收，这个吸收过程产生的光用于测量电子的能量。由于双β衰变过程非常罕见，探测器必须很大，来自其他过程的背景必须被很好地抑制。这些背景要么来自探测器内部的放射性(普通的β衰变)，要么来自外部来源，如宇宙射线。通过将探测器放置在地下深处，例如在隧道或矿山中，使用不含放射性的非常干净的材料，以及通过长时间测量(通常是数年)，可以降低外部背景，因为这一过程非常罕见。但回报将是非常高的：拟议中的超NEMO实验将通过探测低于0.05 eV的中微子的质量范围来解决有关自然的基本问题，中微子的质量比电子的质量小1000万倍！

In this proposal we ask for support to do research on a big new detector, SuperNEMO, which will search for neutrino-less double-beta decay. Even though the process is 'neutrino-less', the experiment actually helps us to answer fundamental questions about the nature of neutrinos. Neutrinos are very common elementary particles. Several trillion neutrinos pass through your finger every second. Still, we know very little about them. One thing we don't know is whether neutrinos have anti-particles, called anti-neutrinos, like the electron and the positron. Electrons and neutrinos are part of the families of leptons which together with the families of quarks make up the matter in the Universe. Very recently it was experimentally proven that neutrinos have a very small mass but we don't know how large these masses are. Why is this important ? For once because there are so many neutrinos around us (your finger!) that even a tiny mass would make a big difference for the amount of mass contained in the universe. The are many other unanswered questions: why are the neutrino masses so small in comparison to other particles ? Why is the number of leptons in all processes we observe conserved ? The NEMO experiment is trying to answer these questions by searching for a very rare process which has never been observed before: neutrino-less double-beta decay. Beta decay of radioactive nuclei is a very common phenomenon, it occurs in nuclear reactors or in rocks. During this decay a neutron in the nucleus is turned into a proton, while an electron and an anti-neutrino are emitted (one lepton and one anti-lepton, so the total number of leptons is zero). For some special nuclei this process is forbidden by the energy conservation law, but the decay of two neutrons simultaneously is allowed. Two electrons and two anti-neutrinos are emitted in this 'double beta decay'. The goal of the proposed experiment is to search for double beta decay without the emission of anti-neutrinos. In this process two neutrons decay into protons by emitting two electrons and no neutrinos. The neutrino is still there, but the neutrino emitted in the beta decay of one neutron is immediately absorbed in the beta decay of the other. This is only possible if the neutrino has mass and if it is its own anti-particle! The number of leptons is not conserved in this process. This has never been observed. The SuperNEMO experiment performs the search for neutrino-less double beta decay by putting foils consisting of material which could undergo neutrino-less double-beta decay in a big detector which looks for the emitted electrons. The experiment consists of two steps, first we look for two tracks produced by the two electrons in a large Helium volume surrounding the foils. The tracks are measured using a huge assembly of wires which work similar to a classical Geiger counter. The electrons are then absorbed in a calorimeter detector, this absorption process produces light which is used to measure the energy of the electrons. Since the double-beta decay process is very rare, the detectors must be large and backgrounds from other processes must be very well suppressed. These backgrounds are either internal from the radioactivity of the detector (ordinary beta decay) or come from outside source such as cosmic rays. The external backgrounds are reduced by putting the detector deep underground, for example in tunnels or mines, and by using very clean materials, containing no radioactivity, and by measuring for a long time, usually for years, because the process is so rare. But the reward will be very high: The proposed SuperNEMO experiment will address fundamental questions about nature by probing a mass range for neutrinos below 0.05 eV, which is 10,000,000 times less than the mass of the electron !