T2K Experiment

在过去的15年里，物理学家发现了一种被称为中微子的粒子的迷人的新特性。三种类型的中微子，称为电子中微子，μ子中微子和τ中微子，被认为是完全不同的，但我们现在知道它们可以在传播过程中相互转化。这种现象被称为中微子振荡，意味着中微子有质量，通过对中微子振荡的详细测量来测量它们的质量和混合程度，可能会让我们深入了解能量尺度远高于加速器所能探测到的能量尺度的基础物理模型的结构。这些混合也可能与为什么宇宙中的物质比反物质多的基本奥秘有关，因此更好地理解它们是粒子物理学和天文学非常感兴趣的话题。到目前为止，我们已经成功地观察到了三种可能的中微子混合类型中的两种，而找出第三种混合类型是否存在是该领域的主要目标之一。第三种类型的混合必须存在，中微子振荡才能违反时间反演的物理原理，即，让振荡在时间向前和向后的过程中看起来完全不同。这种违反的基础是中微子振荡与宇宙中物质过剩有关的可能性，并且是拟议中的中微子工厂设施的主要目标。因此，找到这第三种类型的混合对该领域的进展至关重要。第三次混合是T2 K实验的目标。T2 K是一项跨国合作，将世界上最大的中微子探测器与地球上最高功率的脉冲质子束相结合。我们计划在日本东海岸的JPARC新设施中制造有史以来最强的人造中微子束。中微子将直接穿过地球，瞄准295公里外日本西海岸附近的超级神冈探测器。通过观察超级神冈的中微子类型的混合，并将它们与JPARC在“近”中微子探测器中看到的混合进行比较，我们可以寻找这第三种类型振荡的微妙信号，其灵敏度比现有实验高出十倍以上。英国将提供“近”探测器的关键部件，卢瑟福实验室的工程师还将帮助设计JPARC中微子束线中最困难的部分-目标，在那里强大的质子束被转换为π介子，随后衰变产生中微子束。因此，我们将在迅速发展的中微子物理学领域的下一个重要里程碑中发挥核心作用。

Over the last 15 years physicists have discovered a fascinating new property of the particles known as neutrinos. The three types of neutrinos, called electron, muon, and tau neutrinos, were thought to be completely distinct, but we now know that they can change into each other as they propagate. This phenomenon, called neutrino oscillations, implies that neutrinos have mass, and measurements of their masses and the degree of their mixings by detailed measurements of neutrino oscillations can potentially give us insight into the structure of the underlying physical models at energy scales far above what can be probed with accelerators. These mixings may also be related to the fundamental mystery of why there is more matter than anti-matter in the Universe, and hence better understanding of them is a topic of tremendous interest to particle physics and astronomy. So far we have managed to observe two of the possible three types of neutrino mixings, and finding out whether the third type of mixing exists or not is one of the main targets of the field. This third type of mixing must exist for neutrino oscillations to violate the physical principle of time reversal, i.e., for oscillations to look fundamentally different for time going forward and time going back. This violation underlies the possibility that neutrino oscillations are linked the matter excess in the Universe, and is the main target of the proposed Neutrino Factory facility. Finding this third type of mixing is therefore essential to progress in the field. This third mixing is the target of the T2K experiment. T2K is a multinational collaboration combining the worlds largest neutrino detector with what will be the highest power pulsed proton beam on the planet. We plan to produce the most intense beam of artificial neutrinos ever made at the new JPARC facility on the east coast of Japan. The neutrinos will be aimed straight through the earth at the Super Kamiokande detector, 295 km away near the west coast of Japan. By observing the mix of neutrino types at Super Kamiokande and comparing them to the mix seen at JPARC in a 'near' neutrino detector we can look for the subtle signals of this third type of oscillation with a sensitivity more than ten times greater than existing experiments. The UK will provide critical parts of the 'near' detector, and engineers from the Rutherford Lab will also help design the most difficult part of the neutrino beamline at JPARC - the target, where the powerful proton beam is converted into the pions which subsequently decay to produce the neutrino beam. We will therefore play a central role in what promises to be the next important milestone in the development of the rapidly evolving field of neutrino physics.