T2K Experiment

在过去的15年里，物理学家发现了这种粒子的一种令人着迷的新特性，即中微子。这三种类型的中微子，即电子中微子、介子中微子和陶中微子，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它们的混合程度，可能会让我们在远远高于加速器可以探测的能量尺度上洞察潜在物理模型的结构。这些混合也可能与宇宙中物质比反物质多的根本谜团有关，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中的两种，而找出第三种中微子混合是否存在是该领域的主要目标之一。中微子振荡必须存在第三种类型的混合，才能违反时间反转的物理原理，也就是说，对于向前和向后的时间，振荡看起来是根本不同的。这一违反强调了中微子振荡与宇宙中物质过剩联系在一起的可能性，也是拟议中的中微子工厂设施的主要目标。因此，找到第三种类型的混合对该领域的进展至关重要。这第三种混合是T2K实验的目标。T2K是一项跨国合作，将世界上最大的中微子探测器与将成为地球上最高功率的脉冲质子束结合在一起。我们计划在日本东海岸的JPARC新设施中生产出有史以来最强的人工中微子束。中微子将直接穿过地球，对准295公里外靠近日本西海岸的超级神冈探测器。通过观察Super Kamiokande上的中微子类型的混合，并将它们与在JPARC上看到的“近”中微子探测器的混合进行比较，我们可以寻找这第三种中微子振荡的微妙信号，其灵敏度比现有实验高十倍以上。英国将提供Near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中微子光束线中最困难的部分-目标，在那里强大的质子束被转化为介子，然后衰变产生中微子束。因此，我们将在有望成为快速发展的中微子物理领域发展的下一个重要里程碑的过程中发挥中心作用。

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.