Electrons for neutrinos

中微子是极难探测到的空灵粒子。它们在宇宙中的丰度很高(平均每秒大约有一万亿个中微子通过)，但它们的质量不到一个电子的百万分之一，而且只通过弱力和引力相互作用，它们相互作用的可能性非常小。事实上，中微子可以在没有相互作用的情况下穿过地球的整个直径！那么，人们如何在地球实验中观察和研究这些空灵粒子呢？解决方案是建造一个由尽可能大的材料体积组成的探测器，并产生具有令人难以置信的高通量的中微子光束。这样的设施已经投入使用，令人兴奋的新设施正在规划中。例如，在正在建设的深地下中微子实验(沙丘)中，费米实验室产生的中微子将穿过地幔800英里到达桑福德的另一个实验室，那里的探测器由40吨液体Argon组成！探测器中罕见的中微子相互作用是通过检查中微子与原子核相互作用时产生的反应产物(通常伴随着探测到被敲除的质子)而获得的。然后由核反应理论模型推断出产生反应的中微子的能量。问题源于这样一个事实，即用于这一目的的原子核需要很大，才能获得足够的中微子诱导事件；例如，沙丘的Argon的原子核中有40个质子和中子。大核是一个非常复杂的物体，反应过程的建模非常困难，有许多悬而未决的问题：我们能否压制被击倒的核子在离开原子核的途中散射的事件？当中微子被多个核子吸收，而我们只探测到一个核子时，我们能很好地抑制贡献吗？从核子中产生一个介子的概率与敲除类似，但我们能在多大程度上很好地抑制实验数据中的这些错误事件？我们能摆脱这样的过程吗？在这种过程中，一个(或多个)介子最初是在原子核中产生的，然后被重新吸收以敲除核子。如果我们在反应机制中激发一个核子，那么这种被激发的核子在原子核中的行为是怎样的？这些只是直接影响入射中微子能量和通量测定的一小部分悬而未决的问题。在我们的计划中，我们将使用与中微子-核相互作用类似的反应：我们将电子散射到原子核而不是中微子。这样做的好处是，我们可以获得更多数量级的事件来测试模型，而且非常重要的是，以一种受控的方式！通过知道入射电子能量，从核碎片到整个反应和大范围原子核的束能的所有假设都变得容易实现。为了减少理论模型中的误差，迫切需要这个数据集；这些误差通常会为中微子实验产生最大的系统误差。通过使用中微子和电子诱导反应的通用理论模型，我们可以挑战这些模型，并在以前不可能的细节水平上改进各种过程的建模。这大大减少了从下一代中微子设备提取物理时的任何系统误差。要取得进展，需要对杰斐逊实验室(美国)现有的以及计划进行的电子散射实验的主要方案进行分析，这些实验使用复杂的探测器系统。我们将构建一个新的分析框架，用来分析和归档数据，使其在今后几十年里随时可供核和粒子界使用。这项工作将作为包括麻省理工学院、ODU、杰斐逊实验室和特拉维夫大学的同事在内的新合作网络的一部分进行。

Neutrinos are ethereal particles which are extremely difficult to detect. They have a high abundance in the universe (around a trillion neutrinos pass through the average person every second), but having masses less than a millionth that of an electron and only interacting via the weak force and gravity, their probability of interaction is very small. In fact, neutrinos can pass through the entire diameter of the earth without interaction!So how one can observe and study these ethereal particles at all in terrestrial experiments? The solution is to construct a detector that is made up of as large a volume of material as possible and to produce neutrino beams with incredibly intense flux. Such facilities are operational and exciting new facilities are planned. For example, in the Deep underground Neutrino Experiment (DUNE) under construction, neutrinos produced at Fermilab will pass 800 miles through the earths mantle to another laboratory in Sanford, where detectors comprise 40 tonnes of liquid Argon! The rare interactions of neutrinos in detectors are obtained by examining the reaction products produced (usually with detection of a knocked-out proton) when a neutrino interacts with an atomic nucleus. The energy of the neutrino that produced the reaction is then inferred by nuclear reaction theoretical models. The problems stem from the fact that the atomic nuclei used for this purpose need to be large in order to get enough neutrino induced events; for example the Argon at Dune has 40 protons and neutrons in it's nucleus. A large nucleus is a very complicated object and the modelling of the reaction processes is very difficult with lots of outstanding questions: Can we suppress events where the knocked out nucleon scattered on it's way out of the nucleus? How well can we suppress contributions where the neutrino is absorbed on more than one nucleon and we only detect one? Producing a pion from a nucleon has a similar probability to knockout, but how well can we suppress these erroneous events in the experimental data? Can we get rid of processes where a pion (or pions) are initially produced in the nucleus and then reabsorbed to knock out nucleons? What about if we excite a nucleon in the reaction mechanism, and how do such excited nucleons behave in the nucleus? These are merely a small set of outstanding questions that directly impact the determination of the incident neutrino energy and flux.In our programme we will use an analogous reaction to the neutrino-nucleus interactions: we will scatter electrons off the nuclei rather than neutrinos. This has the advantage that we get many orders of magnitude more events to test the models and very importantly, in a controlled way! By knowing the incident electron energy, all the assumptions in getting from nuclear fragments to the beam energy for a whole host of reactions and a wide range of nuclei becomes accessible. This data set is urgently needed to reduce errors in the theoretical modelling; these errors typically produce the largest systematic error for the neutrino experiments. By using common theoretical models for the neutrino- and electron-induced reactions we can challenge the models and improve the modelling of various processes at a level of details that was previously impossible. This then reduces significantly any systematic errors in extracting physics from the next generation neutrino facilities. Progress requires a major programme of analysis of existing, as well as planned electron scattering experiments from nuclei with complex detector systems at Jefferson Laboratory (USA). We will construct a new analysis framework, which will be used to analyse and archive data in a form that makes it readily accessible and flexible for use by nuclear and particle communities for decades to come. The work will be carried out as part of a new collaborative network including colleagues at MIT, ODU, Jefferson Lab and Tel Aviv University.