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空间实验可能已经发现了一种构成暗物质的新粒子

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A space experiment may have identified a new particle that is the building block of dark matter, the mysterious stuff said to pervade a quarter of the universe that neither emits nor absorbs light.


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The results are based on a small amount of data and are far from definitive, scientists said Wednesday. Yet, they provide a provocative hint that the puzzle of dark matter -- a cosmic prize as eagerly sought as the now-discovered Higgs boson -- may also be on its way to being solved.


项空间实验可能已经发现了一种构成暗物质的新粒子。神秘的暗物质据说在宇宙中占据了四分之一的份额，它既不发光也不吸收光。



The results are the first obtained by a $2 billionparticle detector, known as Alpha Magnetic Spectrometer, or AMS, that is mounted on the exterior of the international space station. It collects and identifies charged cosmic rays arriving from the far reaches of space.


科学家们周三表示，上述结果是基于少量数据得出的，远非最终结论。但这一发现引人遐想地暗示，我们离暗物质谜题被最终解开的日子为时不远了。就像已经被发现的希格斯玻色子一样，暗物质是否存在也像一份有关宇宙的大奖，科学家都在试图赢得这一奖项。



The experiment is sponsored by the U.S. Department of Energy. It is led by Nobel laureate Samuel Ting of the Massachusetts Institute of Technology and involves hundreds of scientists from all over the world. The latest data will be published in the journal Physical Review Letters.


这是安装在国际空间站外部的一个造价20亿美元的粒子探测器首次发现的粒子。这个探测器名叫"阿尔法磁谱仪"(Alpha Magnetic Spectrometer)，它会收集和识别来自遥远空间的带电宇宙射线。



The AMS findings are consistent with particles that could be formed 'from the annihilation of dark matter particles in space, but not yet sufficiently conclusive to rule out other explanations,' according to a statement by the European particle-physics laboratory, CERN, in Geneva, which assembled the particle detector.


这项实验由美国能源部提供资金支持。实验由诺贝尔物理学奖得主、麻省理工学院(Massachusetts Institute of Technology)的丁肇中领导，数百名来自世界各地的科学家参与其中。最新的数据将发表在《物理评论快报》(Physical Review Letters)期刊上。



Figuring out what makes up dark matter is a big prize because it is the key to understanding the shape, size and even the fate of the universe.


位于日内瓦的欧洲粒子物理实验室CERN发布的一份声明说，阿尔法磁谱仪的发现和人们有关那些可能因宇宙中暗物质粒子的湮灭而形成的粒子的想法相符合，但据此还不足以确凿地排除其它解释。阿尔法磁谱仪正是由CERN组装的。



Knowing how much dark matter there is will tell us whether the universe will keep expanding; expand to a point and then collapse; or get bigger and bigger and then stop. It also can help predict how Earth's neighborhood, the Milky Way galaxy, formed and how it might evolve.


搞清楚构成暗物质的组成是科学界的一个"大奖"，因为这是理解宇宙形状、大小甚至是命运的关键。



Dark matter is invisible, yet its presence is felt by the immense gravitational tug it exerts on stars, galaxies and other cosmic bodies. What could this mysterious substance be made of? One of the leading candidates is a WIMP, or weakly interacting massive particle.


了解宇宙存在的暗物质数量将告诉我们宇宙是否会继续扩大。如果会，将扩大到什么程度？何时又会彻底毁灭？抑或宇宙会不断变大，然后停止膨胀？这也有助于预测地球的邻居银河系是如何形成的，它又可能会如何演变。



WIMPs are elusive. They rarely interact with normal matter such as atoms; indeed, billions of WIMPs may be darting right through the Earth every second without hitting anything.


暗物质是无形的，但人类可以通过其施加给恒星、星系和其他宇宙天体的巨大万有引力感知它的存在。这一神秘物质可能是由什么构成的？最有可能的一种解释是弱相互作用大质量粒子(WIMP)。



About 25% of the universe is believed to be dark matter, about 70% is the little-understood dark energy, and about 5% is ordinary matter made of atoms. Scientists have been looking for WIMPs in deep mines; in particle smash-ups in colliders; and, now, with detectors in space.


弱相互作用大质量粒子难以捕捉。它们很少与原子等普通物质发生相互作用。事实上，可能每秒钟都有数十亿弱相互作用大质量粒子穿过地球且没有击中任何东西。



'This is the decade of the WIMP,' said Michael Turner, a cosmologist at the University of Chicago. 'All of these experiments are zeroing in on the outrageous idea that most of the matter in the universe is' made up of WIMPs, a new form of matter.


据信宇宙约有25%是由暗物质构成的，约有70%是由人类几乎不理解的暗能量构成，只有约5%是由原子构成的普通物质组成。科学家们一直在深矿和大型强子对撞机中寻找弱相互作用大质量粒子，现在则用太空中的粒子探测器寻找。



In 1990, Dr. Turner and a colleague suggested a way in which WIMPs might be discovered in galaxies, which provided the theoretical underpinning for the AMS experiment. The idea is that when WIMPs crash into each other, they annihilate and produce two particles -- electrons and positrons -- which are much easier to detect than the WIMPs themselves.


芝加哥大学(University of Chicago)的宇宙学家特纳(Michael Turner)说，这是弱相互作用大质量粒子的10年。他说所有这些实验都在关注一种离谱的想法，即宇宙中的大部分物质由弱相互作用大质量粒子构成，这是一种新的物质形态。



The theory predicts two outcomes: that the annihilations should produce a large number of positrons; and that after the excess, there should be a sudden decline in positron production.


1990年，特纳博士和他的一名同事提出一种有可能在星系中发现弱相互作用大质量粒子的方法，这为阿尔法磁谱仪实验提供了理论基础。这一想法是，当弱相互作用大质量粒子相互碰撞后，会发生湮灭现象并形成两种粒子：电子和正电子。相比弱相互作用大质量粒子，后两种粒子更容易被探测到。



In its first 18 months, AMS analyzed 25 billionprimary cosmic ray events. It identified more than 400,000 positrons. The positron numbers then start to flatten out -- a possible sign that the hoped-for plunge in positron numbers could come next.


这一理论预测了两种结果：上述湮灭应该产生大量正电子；这之后正电子数量应该会突然下降。



So far, though, there isn't enough data to confirm that expected plunge. Physicists also need to ensure that the positrons they are seeing don't emanate from a pulsar, a type of star; that wouldn't be a finding about dark matter.


在服役后的头18个月里，阿尔法磁谱仪分析了250亿个初级宇宙射线事件。它识别了40多万个正电子。正电子的数量随后趋于稳定，这是前面预测的正电子数量骤降现象即将到来的可能迹象。



'What's tantalizing is that the positrons are leveling-off,' said Dr. Turner, who wasn't involved in the AMS experiment. 'But we're not there yet' because not enough data have been crunched. Subscribe to WSJ: http://online.wsj.com?mod=djnwires


然而到目前为止还没有足够数据能够证实人们预计的正电子数量骤降现象已经发生。物理学家还需要确保他们所看到的正电子不是来自脉冲星（一种恒星）。如果是，那么这就不是一个关于暗物质的发现。



Gautam Naik


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