A 1000km3 Ultra-High Energy Neutrino Acoustic Detector

近年来，科学家们扩大了天文学的视野，他们首先用波长越来越小的光子观察宇宙，然后求助于光子以外的粒子。我们工具箱中的最新成员是中微子，这是一种质量很小的中性粒子，比电子轻50万倍。虽然小质量的中微子已经让它们变得相当“奇异”，但现在有可能，从外太空雨点般落到地球上的超高能中微子将有助于我们理解宇宙中的一些灾难性事件。我们确实知道动能在1018 eV及以上的粒子(可能是质子)确实存在于宇宙辐射中，事实上，一些科学家声称观察到了能量在1020 eV以上的宇宙射线。对于单个基本粒子来说，这是一个巨大的能量，相当于一本中等大小的书从桌子上掉到地上的动能！相比之下，我们最强大的粒子加速器只能将质子加速到1012 eV。我们还不完全了解什么样的过程可以产生如此高能量的粒子，但如果也能产生如此高能量的中微子，我们将有一条重要的线索。这种高能粒子发出的信号足以被传统的粒子探测器记录下来，然而，这种高能中微子落在地球上的速度预计会如此之低，以至于需要数千立方公里的活性物质才有机会探测到一些。一个合适的探测器不能用螺母和螺栓建造，但它必须利用现有的材料！海水在地球表面非常常见，在美国国家科学基金会的支持下，我们小组正在安装一个系统，该系统将展示通过超高能中微子相互作用并停在海水中时预计会产生的声学噪声来探测宇宙辐射中的超高能中微子的可能性。事实上，它们的巨大动能预计会转化为热量，使海水膨胀，产生一种特殊的声音。在我们最初的研究中，这种声音将由位于佛罗里达州海岸外的一组水听器(水下麦克风)探测到，美国海军将使用该水听器进行海军演习。我们与海军达成了一项协议，将允许我们安装一个特殊的数据采集系统，使我们能够在由人类活动和不同种类的海洋生物引起的大型海洋背景中找到中微子相互作用的微小脉冲。这个项目几乎完全是由本科生发起的。最初的可行性研究是由现就读于康奈尔大学的斯坦福大学本科生沙菲克·亚当完成的。斯坦福大学本科生贾斯汀·范登布鲁克完成了对海军阵列的第一次数据采集和相关数据分析，他现在作为伯克利的研究生在南极洲从事NSF阿曼达/冰立方探测器的工作。斯坦福大学的研究生仓桥直子(Naoko Kurahashi)正在牵头安装目前的系统，而斯坦福大学的另一名本科生杰森·克温(Jason Kerwin)正在建造一种校准装置的部件，我们希望在这项研究中使用该装置。

In recent years scientists have expanded the horizon of astronomy by observing the Universe first with photons of increasingly smaller wavelengths and then by resorting to particles other than photons. The latest additions to our toolkit are neutrinos, neutral particles with a tiny mass, more than 500,000 times lighter than electrons. While the small mass of neutrinos make them already rather "exotic", it is now possible that ultra-high-energy neutrinos raining down to Earth from the outer space will help us understanding some catastrophic events in the universe. We do know that particles (possibly protons) of kinetic energies 1018 eV and above do occur in cosmic radiation and, in fact, some scientists claim to have observed cosmic rays with energies above 1020 eV. This is a tremendous energy for a single elementary particle, equivalent to the kinetic energy of a medium-sized book falling to the floor from a desk! As a comparison our most powerful particle accelerator can only accelerate protons to 1012 eV. We do not fully understand what kind of process can produce such high energy particles but, if also neutrinos of such high energies were to be produced we would have an important clue. The signals from such energetic particles would be dramatic enough to be recorded in conventional particle detector however the rate at which such energetic neutrinos would rain on Earth is expected to be so low that thousands of cubic kilometers of active material would be needed to have the chance of detecting some. A suitable detector cannot be built with nuts and bolts but it has to take advantage of an existing body of material! Ocean water is very common on the Earth surface and our group, with NSF support, is installing a system that will demonstrate the possibility of detecting ultra-high-energy neutrinos in cosmic radiation by the acoustic noise they are expected to produce when they interact and stop in sea water. Indeed their tremendous kinetic energy is expected to be converted into heat that would make the sea-water expand, producing a peculiar sound. The sound will be detected, in our initial study, by an array of hydrophones (underwater microphones) located off the coast of Florida and used by the US Navy for naval exercises. We have an agreement with the Navy that will allow us to install a special data acquisition system that will allow us to find the tiny pulses characteristic of the neutrino interaction in the large sea background caused by human activities and different sort of marine creatures. This project was initiated almost exclusively with undergraduate students. The initial feasibility study was done by Stanford Undergraduate Shaffique Adam, now at Cornell. The first data taking at the Navy array and the relative data analysis was performed by Stanford undergraduate Justin Vandenbroucke who is now working on the NSF Amanda/Ice Cube detectors in Antarctica as a Berkeley graduate student. A Stanford graduate student, Naoko Kurahashi, is leading the installation of the present system, while another Stanford undergraduate, Jason Kerwin, is building parts of a calibration device that we hope to use in the study.