Dark matter

寻寻觅觅暗物质

Fractional distillation

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The hunt for the missing 85% of matter in the universe is closing in on its quarry

搜寻现已逼近猎物:占宇宙总物质量85%的暗物质,你还要藏到哪一天?

IF YOU thought the Higgs boson was elusive, consider the case of dark matter. The Higgs—the particle that gives other subatomic species mass—was predicted in 1964 but actually nabbed only last year. That 48-year hunt, though, was a breeze compared with the one for dark matter. Physicists have known the stuff must exist since 1933, when Fritz Zwicky, a Swiss astro-physicist, coined the term to describe a substance which cannot be seen but without which visible galaxies would fly apart as they rotate. The latest results from the European Space Agency’s Planck satellite suggest it makes up 85% of all the matter in the universe (up from an earlier estimate of around 80%).

如果你认为希格波色子已经够扑朔迷离的了,那你不妨再想想暗物质的情况。赋予其他亚原子粒子质量的希格子是人们在1964年预言的,但只在去年才真正找到了它们。不过,跟对暗物质的搜寻相比,这48年的搜寻只不过是小巫见大巫而已。自1933年起,物理学家们就知道这种物质一定存在;当时,瑞士天体物理学家弗瑞兹•兹维基注1(Fritz Zwicky)创造了暗物质这一术语,用以描述一种人们无法目睹、但如果它不存在则肉眼可见的星系就会在旋转时分崩离析、四散纷飞的物质。来自欧洲航天局(European Space Agency)“普朗克”(Planck)号卫星的最新结果表明,暗物质所占宇宙全部物质总量的比例多于早些时约80%的估计,达到了 85%。

Like the Higgs boson, though, the actual particles of which dark matter is composed have proved elusive. Eight decades after Zwicky’s observations, and dozens of experiments later, they remain undetected. But on April 3rd an experiment called the Alpha Magnetic Spectrometer (AMS) offered the most tantalising hints yet.

但人们发现,实际构成暗物质的粒子也跟希格波色子一样神秘莫测。在兹维基预言其存在后的80年间,尽管人们进行了几十次实验,但它依旧鸿飞冥冥,“芳”踪杳然。然而,4月3日,一项人称阿尔法磁谱探测(AMS)的实验给出了迄今为止最为撩人心弦的线索。

Although Samuel Ting, the Nobel laureate who heads the effort, presented the findings at CERN (Europe’s, and the world’s, principal particle-physics laboratory), they did not stem from CERN’s own accelerators hidden beneath the Franco-Swiss countryside outside Geneva. In fact, they did not hail from Earth at all, for AMS sits on board the International Space Station (ISS), and is arguably the only piece of scientifically useful kit ever to grace that $100 billion contraption.

主持这一项目的是诺贝尔奖金得主丁肇中注2(Samuel Ting);尽管他是在欧洲核子研究委员会(CERN)(欧洲与全球首屈一指的粒子物理实验室)发布这一结果的,但这些成果并不是在CERN自己的粒子加速器(深藏在日内瓦城外、法国-瑞士交界的乡村地下)内完成的。而且,这些结果实际上根本就不是地球上的产物,因为AMS现在正搭乘国际空间站飞行——有人认为,迄今为止,这台科学上用处良多的设备是唯一一套能让耗资1000亿美元的奇妙空间站增色的装置。

Like its ground-based cousins at CERN, AMS consists of a large magnet and an array of sensors to track a charged particle’s path. Unlike them, the particles it tracks are not the product of smashing things together in the Large Hadron Collider (LHC), humanity’s biggest particle accelerator. Instead, AMS uses the most powerful accelerator of all: the universe itself.

跟它设置在CERN的那些地球同侪们一样,AMS也是由一大块磁铁和一个追踪带电粒子轨迹的传感器阵列组成的。但与它们不同的是,AMS追踪的并不是在人类最大的粒子加速器——大型强子对撞机(Large Hadron Collider (LHC))中粒子对撞的产物;它所使用的,是一切加速器中最为强大的一台:宇宙本身。

A matter of fact?

事实果真如此吗?

Space may look empty, but it is in fact abuzz with particles produced in an assortment of astrophysical processes, and known collectively as cosmic rays. One process of particular interest to dark-matter hunters involves hypothetical particles called neutralinos. These are predicted by supersymmetry, a theory which removes mathematically inelegant fiddle factors from the Standard Model, the reigning rule book of particle physics, by doubling the number of species in the particle zoo.

宇宙空间看上去或许空空荡荡,但实际上,其中却充斥着形形色色的天体物理过程产生的粒子,人们将之统称为宇宙射线。那些追寻暗物质的人们对之特别有兴趣的一个过程涉及中轻微子,一种假定存在的粒子。这些粒子是超对称理论预言的,该理论通过将粒子“动物园”中的“物种”数加倍,而以数学方法从“标准模型”中剥离那些粗糙的非真因素;而标准模型则是粒子物理学家奉为经典的准则。

Neutralinos are the lightest of the predicted supersymmetric beasts, with a mass equivalent to that of a few hundred protons (the Higgs, by comparison, has a mass of about 124 protons). They do not interact with light, and are therefore invisible. They are also stable enough to stick around in space for a long time. Just the sort of properties, in other words, that dark matter is thought to possess.

中轻微子是人们预言的超对称粒子中最轻的一个,其质量相当于几百个质子的质量;与此相比,希格子的质量大约为124个质子的质量。中轻微子不与光发生作用,因此是看不见的。它们也足够稳定,能够在空间长期存在。换言之,这正是人们认为暗物质应该有的性质。

To physicists’ chagrin, attempts to conjure neutralinos from the LHC have failed. But if they do exist—and make up the bulk of dark matter in the cosmos—they ought to leave traces that AMS can detect.

让物理学家们沮丧的是,他们在LHC中创造中轻微子的尝试全部以失败告终。但如果中轻微子果真存在并且组成了宇宙中的大部分暗物质,它们就应该留下AMS能够检测得到的痕迹。

When two neutralinos bump into each other, the theory goes, they should annihilate one another and produce in their stead an electron and its antimatter equivalent, a positron. Since, as Albert Einstein showed, mass and energy are one and the same, and because electrons and positrons are equal and opposite, each carries precisely as much energy as one neutralino has mass. It is these high-energy electrons and positrons that AMS is on the lookout for.

超对称理论预言,两个中轻微子在碰撞时会湮灭,并代之以一个电子和一个电子的反粒子——正电子。之所以如此,就是阿尔伯特•爱因斯坦所告诉我们的:质量与能量其实完全是一回事,而电子与正电子的各项性质成镜像(即恰恰相反),而它们中的每一个所带有的能量都跟中轻微子拥有的质量相等。AMS寻找的正是这些高能电子和正电子。

The problem is spotting them against a backdrop of electrons from other cosmic sources, which are much more common than positrons are. To get round this, AMS examines how the ratio of positrons to electrons varies with the particles’ energy. At low energies, cosmic-ray electrons from other sources dominate. If high-energy cosmic positrons do indeed come mainly from dark-matter annihilation, however, then the “positron fraction” should rise with energy, and peak when it reaches the mass of a neutralino. Beyond that peak, the fraction should plummet, because few high-energy positrons from other sources would be expected to exist, whereas energetic electrons are abundant.

麻烦的事情是要在来自其他宇宙来源的电子背景下找出这些电子与正电子,而这些背景电子要比正电子常见得多。为克服这一困难,AMS检查的是正电子与电子间的比例随粒子能量变化的规律。当能量低时,来自其他来源的宇宙射线电子占绝大多数。但如果宇宙中的高能正电子确实主要来自暗物质湮灭,“正电子比例”就应该随能量增加而增大,而在能量为中轻微子的质量时达到最高点。这一比例应在超过这一峰值后急剧下降,因为人们认为,尽管高能电子数量不少,但来自其他来源又有这么高能量的正电子极少。

In the 18 months following AMS’s delivery to the ISS by the space shuttle Endeavour in May 2011, it recorded the passage of 30 billion cosmic rays. These included 6.4m electrons and 400,000 positrons that had energies ranging from 0.5 to 350 giga-electron-volts (GeV), measured in the esoteric units particle physicists like to use. These data show that the positron fraction does indeed rise with energy, just as theory predicts. As important, the same pattern is visible wherever AMS happened to be pointing as it orbited Earth.

在“奋进号”航天飞机于2011年5月将AMS运送到ISS之后的18个月间,它记录了300亿道宇宙射线的踪迹,其中包括640万个电子和40万个正电子的踪迹,它们的能量在0.5至350万亿电子伏特(GeV)之间,这一玄奥的能量单位是粒子物理学家圈子内喜欢用的。这些数据表明,正电子的比例确实如理论预测的那样随能量增加而增大。同样重要的是,在AMS环绕地球飞行期间,它向任何方向的探测都观察到了同样的变化规律。

This sits nicely with the notion that dark matter is strewn more or less evenly across the universe. At the same time it excludes another possible source of the particles: random cosmic events like exploding stars, which would not be so uniformly distributed, at least not over a period as cosmically brief as 1½ years.

这与“暗物质大致平均分布于宇宙之中”这一见解吻合良好。这同时也排除了这些粒子的另一个可能来源:即恒星爆炸这类宇宙随机事件,因为这种事件不会均匀分布,至少在1年半这样一个时期之内不会如此,因为在宇宙的长河中,这一时间区间只不过是短暂的一瞬。

Unfortunately, more data are needed to rule out a third possibility: that the observed particles were created by pulsars, the remnants of these stellar explosions. At the moment, AMS has not seen enough electrons and positrons with energies above 350GeV to draw meaningful conclusions about them. If pulsars are responsible, theory predicts that the positron fraction should decline steadily at energies above this value. If neutralinos are responsible, though, at some point—corresponding to the energy equivalent of their mass—the positron fraction will fall off a cliff. Approximately, one proton mass corresponds to one GeV, so this could happen soon. But the theory of supersymmetry does not vouchsafe exactly what a neutralino’s mass should be, so it might not. The “few hundred” protons may turn out to be nearer 1,000. Collecting enough high-energy electrons and positrons to test that will take quite a long time.

遗憾的是,人们还需要更多的数据才能排除第三种可能性:即人们观察到的粒子是由脉冲星产生的,后者是恒星爆炸的遗留物。到现在为止,AMS还没有观察到足够数量的能量高于350GeV的电子和正电子,因此尚不足以对它们做出任何有意义的结论。按照理论预测,如果脉冲星确实是造成这一观察结果的原因,当能量超过上述数值时,正电子的比例应该平缓下降。但如果原因在于中轻微子,则正电子比例会在超过中轻微子质量的对应能量后急剧下降。1个质子的质量大约相当于1GeV,所以这种情况会很快得到证实。但超对称理论并没有准确认定中轻微子的质量应该是多少,所以急剧下降也可能不会在略高于350GeV的情况下就发生。最终人们或许会证实,所谓“几百个”质子其实更接近1000个。收集能量足够高的电子与正电子来证实这一点需要相当长的时间。

Fortunately, AMS is in it for the long term. It is designed to last for another 20 years or so. That means (assuming the space station is not, as is currently planned, abandoned as being too costly to maintain) it may still be delivering results on the centenary of Zwicky’s discovery. But if neutralinos take that long to find, the hunt for the Higgs really will look like a doddle by comparison.

幸运的是,AMS会长期执行这一任务。它的设计寿命还将有20年左右。这就是说(假定空间站不会像现在计划的那样,会因日常费用过高而放弃),到兹维基发现的百周年纪念日时它可能还会继续发回结果。但如果人们需要这么长的时间才能找到中轻微子,与此相比,对希格子的追寻看上去就真的是小菜一碟了。

[注1] 弗里茨•兹维基(1898 - 1974):瑞士天文学家,曾在超新星、星系团、暗物质等方面做出了重要贡献。

[注2] 丁肇中(Samuel Chao Chung Ting )(1936年1月27日-),美籍实验物理学家,汉族,祖籍山东省日照市,现任美国麻省理工学院教授,曾获得1976年诺贝尔物理学奖。他曾发现了一种新的基本粒子,并以和自己中文姓氏“丁”类似的英文字母“J”将其命名为“J粒子”。