Auger and IceCube: Cosmic Ray and Neutrino Probes of the High Energy Universe

宇宙射线是一种亚原子粒子，从外太空不断向我们倾泻而下，在宇宙射线首次被发现近一个世纪后，它的起源仍然是一个深刻的谜。它们的能量远远超出了日内瓦国际实验室欧洲核子研究中心（CERN）的巨型粒子加速器所能达到的水平。记录到的能量最高的宇宙射线的能量相当于皮特·桑普拉斯发球的能量，但携带的粒子比网球小1000亿倍。幸运的是，这些粒子不会直接撞击我们！地球的大气层就像一个巨大的毯子，吸收了它们。在这个过程中，进入地球的粒子产生了一个巨大的“空气阵雨”——一个以接近光速运动的等离子体圆盘——当它到达地球表面时，它可以传播到几十公里外。到这个时候，它的力量已经耗尽，但敏感的电子探测器可以接收到信号，并告诉我们“主”粒子的能量有多大，以及它来自天空的哪个方向。然而，这样的事件相当罕见，平均每个世纪每平方公里只有一次。大自然是如何创造条件将一个微小的粒子加速到这样的能量？它们是来自潜伏在星系中心的巨大黑洞，吞噬恒星并喷出巨大的等离子体射流，还是来自伽马射线爆发，宇宙中最大的爆炸，在几秒钟到几分钟内释放出相当于整个恒星质量的能量（E = mc^2!）也许这些高能宇宙射线实际上是由大爆炸遗留下来的大质量粒子产生的，这些粒子构成了宇宙中的暗物质，经历了一种缓慢的放射性衰变。追踪这些超高能粒子的来源将使我们能够回答这些问题。为此，来自世界15个国家的200名科学家正在阿根廷马拉格的安第斯山脉阴影下建造耗资5500万美元的“皮埃尔·奥格天文台”。当2005年完工时，这个电子探测器阵列将覆盖兰开夏郡大小的区域，使科学家能够捕捉到许多这样的事件。正如诺贝尔物理学奖得主吉姆·克罗宁所指出的那样：“这些高能射线的存在是一个谜，解决它的方法将是发现新的基础物理学或天体物理学。”换句话说，通过这项具有挑战性的任务，我们肯定会学到一些关于宇宙的深刻而有趣的东西。另一种互补的方法是研究宇宙引擎，它将亚原子粒子加速到如此惊人的能量，那就是研究在这个过程中产生的被称为中微子的幽灵粒子。与其他形式的辐射不同，这些中微子与物质的相互作用非常弱，通常会穿过地球，甚至不会注意到它的存在。即便如此，科学家们已经学会了如何捕捉它们——建造巨大的地下洞穴，装满水，用灵敏的电子眼观察。几乎所有撞击地球的中微子，例如来自太阳的中微子，它们是由太阳核心的核聚变产生的，都会直接穿过探测器，但万亿分之一的中微子可能会相互作用，从而发出可以探测到的微弱闪光。对于预期从宇宙加速器中产生的高能中微子，有必要大幅扩大探测器的规模。事实上，科学家们正计划利用南极冰盖深处一公里大小的冰块来寻找这些中微子。这个耗资2.72亿美元的项目被称为“冰立方”，它将把携带探测器的“绳子”（使用5兆瓦的热水射流！）沉入地下两公里处被压缩到巨大压力下的清澈的冰中。这将使我们能够探测到从很远的地方来的中微子，并直接窥视到天空中最暴力、最可怕的现象的核心。我们将打开一个关于宇宙的新“窗口”

Nearly a century after they were first discovered, the origin of cosmic rays, the sub-atomic particles that constantly rain down on us from outer space, remains a profound mystery. Their energies extend far beyond what can be achieved even in the giant particle accelerators at the international laboratory CERN in Geneva. The most energetic cosmic ray recorded has an energy equivalent to that of a Pete Sampras serve -but carried on a particle ten million million times smaller than a tennis ball. We are fortunate that such particles cannot hit us directly! The Earth's atmosphere acts as a giant blanket which absorbs them. In the process the incoming particle generates a huge 'air shower' - a disc of plasma travelling at nearly the speed of light - which can spread to several tens of kilometres by the time it hits the Earth's surface. By this time its force is spent but sensitive electronic detectors can pick up the signals and tell us how energetic the 'primary' particle was and which direction in the sky it came from. Such events are however rather rare, on average only one per square kilometre, per century. How does nature create the conditions to accelerate a tiny particle to such an energy? Are they coming from the giant black holes that lurk at the centres of galaxies, gobbling up stars and as and spewing out huge jets of plasma, or are they from gamma-ray bursts, the biggest explosions in the Universe, which release an energy equivalent to the mass of an entire star (E = mc^2!) within a few seconds to minutes. Perhaps these energetic cosmic rays are in fact created when massive particles left over from the Big Bang, which make up the dark matter of the Universe, undergo a sort of slow radioactive decay. Tracking these ultrahigh-energy particles back to their sources will enable us to answer such questions. To this end, two hundred scientists from 15 countries around the world are building the $55 million 'Pierre Auger Observatory' in the shadow of the Andes mountains in Malargue, Argentina. When completed in 2005, this array of electronic detectors will cover an area the size of Lancashire, allowing scientists to catch many of these events. As Nobel Prize winning physicists Jim Cronin has noted: 'The existence of these high energy rays is a puzzle, the solution of which will be the discovery of new fundamental physics or astrophysics'. In other words, we are sure to learn something profound and interesting about the Universe by undertaking this challenging task. Another, complementary, way to investigate the cosmic engines that accelerate sub-atomic particles to such stupendous energies is to study ghostly particles called neutrinos which are created in the process. Unlike other forms of radiation these neutrinos interact very weakly with matter and usually travel right through the Earth without even noticing it is there. Even so scientists have learnt how to catch them - by building giant underground caverns filled with water, watched by sensitive electronic eyes. Nearly all the neutrinos hitting the Earth, for example from the Sun where they are created by nuclear fusion in its core, pass straight through the detector, but one in a trillion may interact thus emitting a faint flash of light which can be detected. For the very high energy neutrinos expected from cosmic accelerators it is necessary to scale up the size of the detector considerably. In fact scientists are planning to use a kilometre size block of ice deep under the South Polar cap to look for these neutrinos. This $272 million project, called 'IceCube', will sink 'strings' carrying detectors (using a five MW jet of hot water!) into the crystal clear ice compressed to enormous pressures over two kilometres underground. This will allow us to detect neutrinos coming form very far away and peer straight into the heart of the most violent and awesome phenomena in the heavens. We will open a new 'window' on the Universe and new