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

在宇宙射线被首次发现近一个世纪后，宇宙射线的起源仍然是一个深奥的谜。宇宙射线是一种不断从外太空落到我们身上的亚原子粒子。它们的能量远远超出了日内瓦欧洲核子研究组织国际实验室的巨型粒子加速器所能达到的范围。记录到的最高能量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1000万倍。我们很幸运，这些粒子不能直接击中我们！地球大气层就像一张巨大的毯子，吸收了它们。在这个过程中，进入地球的粒子会产生巨大的“空气簇射”--一个接近光速的等离子体圆盘--当它到达地球表面时，它会扩散到几十公里之外。此时，它的力量已经耗尽，但灵敏的电子探测器可以接收到信号，并告诉我们这颗“初级”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微小粒子加速到如此能量的呢？它们是来自潜伏在星系中心的巨大黑洞，吞噬恒星和As，并喷出巨大的等离子喷流，还是来自伽马射线爆发，这是宇宙中最大的爆炸，释放出相当于整个恒星质量的能量(E=mc^2！)在几秒钟到几分钟内。也许这些高能宇宙射线实际上是在大爆炸遗留下来的巨大粒子经历了一种缓慢的放射性衰变后产生的，大爆炸构成了宇宙的暗物质。追踪这些超高能粒子的来源将使我们能够回答这样的问题。为此，来自全球15个国家的200名科学家正斥资5500万美元在阿根廷马拉格安第斯山脉的阴影下建造“皮埃尔·奥格天文台”。当2005年建成时，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其中的许多事件。正如诺贝尔奖获得者物理学家吉姆·克罗宁所说：“这些高能射线的存在是一个谜，解开它的谜团将是新的基础物理学或天体物理学的发现。”换句话说，通过承担这项具有挑战性的任务，我们一定会了解到一些关于宇宙的深刻和有趣的东西。研究将亚原子粒子加速到如此巨大能量的宇宙引擎的另一种补充方式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，通常直接穿过地球，甚至不会注意到它的存在。尽管如此，科学家们已经学会了如何捕捉它们--通过建造充满水的巨大地下洞穴，由灵敏的电子眼观察。几乎所有撞击地球的中微子，例如来自太阳的中微子，都会直接通过探测器，但十亿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。对于预计来自宇宙加速器的超高能中微子，有必要大幅扩大探测器的尺寸。事实上，科学家们正计划利用南极冰盖下一公里大小的冰块来寻找这些中微子。这个耗资2.72亿美元的项目名为“冰立方”(IceCube)，将沉没装有探测器的“绳子”(使用5兆瓦的热水喷射！)被压缩到地下超过两公里的巨大压力下的晶莹剔透的冰层。这将使我们能够探测到来自非常遥远的中微子，并直视天空中最猛烈和令人敬畏的现象的核心。我们将打开一扇关于宇宙和新的

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