A search for sub-dominant neutrino oscillations and measurement of the third neutrino mixing angle, theta13, with the Double Chooz experiment

什么是中微子？中微子是自然界的基本粒子，尽管是宇宙中最丰富的粒子之一，但中微子的第一次观测仅在50年前。中微子的这种高度难以捉摸的性质来自于它们几乎不与周围的任何东西相互作用的事实。道格拉斯亚当斯曾经描述过，中微子在穿过地球时发生相互作用的几率“大致相当于从一架巡航的波音747上随机扔下一个滚珠，然后击中一个鸡蛋”。中微子的重量至少比电子小1/4百万倍，电子是下一个最轻的粒子，而且它们可能还要轻得多。人们普遍认为，关于中微子还有许多有趣的事实有待发现。这项研究计划是关于试图理解和了解更多关于中微子的特殊性质。有三种类型的中微子已经直接看到，但可能有更多。在过去的十年里，科学家们有了一个惊人的发现：一个中微子可以从一种中微子转变为另外两种中微子中的一种或两种。但真正有趣的是，中微子会不断变化，然后又变回来。这种来回的变化被称为中微子振荡，因为它像老爷钟的钟摆一样有规律地摆动。此外，事实证明，自然界只允许一定比例的中微子在任何时候经历这些变化。无论是50%的中微子在这三种类型之间来回振荡，还是只有5%，这都是自然界的一个基本参数，而这项研究计划的目的就是测量这个数字。中微子的一个强源是核电站，你可以在很远的地方探测到中微子。发电站的墙壁对中微子几乎是完全透明的，它们可以直接穿过。你可以想象从发电站发出的中微子就像从玻璃灯泡里发出的光一样：它向各个方向发光。发电站释放出大量的中微子：每秒大约产生10000亿亿个中微子。这项研究计划中的实验将位于距离发电站约1公里的地方，它将在每24小时通过的数十亿中微子中检测到约100个中微子。如果在这个实验中发现的中微子正在改变类型，那么这将是一个非常重要的发现。这是真的，不仅因为这将是第一次测量到自然界的一个基本常数，而且它还将告诉我们中微子是否可能是一个长期悬而未决的谜团的答案，即宇宙中所有的反物质都去了哪里。今天的宇宙充满了物质和非常少的反物质。但是当它在大爆炸中形成时，产生了等量的物质和反物质。所有反物质究竟发生了什么是一个有趣的谜题，通过了解中微子，我们可能会找到答案。如果这个实验看到来自发电站的中微子振荡，那么这将告诉我们中微子和反中微子的行为是否可能不同。因此，通过研究中微子，我们可能会对宇宙的一个大谜团有一个答案。

What are neutrinos? Neutrinos are fundamental particles of Nature and despite being one of the most abundant particles in the Universe, the first observation of a neutrino was only 50 years ago. This highly elusive nature of neutrinos comes from the fact that they hardly ever interact with anything around them. Douglas Adams once described the chance of a neutrino interacting as it passes through the Earth as being 'roughly comparable to that of dropping a ball bearing at random from a cruising 747 and hitting, say, an egg sandwich'. Neutrinos weigh at least a 1/4 of a million times less than the electron, which is the next lightest particle and they could be much lighter still. It is widely believed that there are many fascinating facts about neutrinos yet to be discovered. This research proposal is about trying to understand and learn more about the special nature of neutrinos. There are three types of neutrino that have been directly seen but there could be more. In the last decade scientists have made a remarkable discovery: that a neutrino born as one type of neutrino can change in to one or both of the other two types of neutrino. But what's really interesting is that the neutrinos change, and then change back, repeatedly. This changing back and forth has been dubbed neutrino oscillations because it happens regularly like the swinging of the pendulum in a grandfather clock. Furthermore, it turns out that Nature only allows a certain fraction of the neutrinos to undergo these changes at any one time. Whether it's 50% of the neutrinos oscillating back and forth between the three types or just 5%, that is a fundamental parameter of Nature and it is the aim of this research proposal to measure that number. A strong source of neutrinos is a nuclear power station and you can detect the neutrinos from a long way away. The walls of the power station are almost completely transparent to the neutrinos and they pass straight through. You can imagine the neutrinos coming from the power station much like the light coming from inside a glass light bulb: it shines out in all directions. Vast numbers of neutrinos are emitted by the power station: around 1000 billion billion are produced every second. The experiment in this research proposal will be located about 1 km from the power station and it will detect only about one hundred neutrinos out of the billions passing through every 24 hours. If the neutrinos detected in this experiment were found to be changing type then it would be a tremendously important discovery. This is true not only because a fundamental constant of Nature would have been measured for the first time but it would also tell us whether neutrinos might hold the answers to a long outstanding mystery, namely where all the anti-matter in the Universe has gone. The Universe today is filled with matter and very little antimatter. But when it was formed in the Big Bang, equal amounts of matter and antimatter were made. Exactly what happened to all the antimatter is an intriguing puzzle and it is possible that by learning about neutrinos we will find the answer. If this experiment sees neutrinos from the power station oscillating then that will tell us whether it's possible for the behaviour of neutrinos and anti-neutrinos to be different. Thus, through studying neutrinos, we may have the beginnings of an answer to one of the big mysteries of the Universe.