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2013-01-27

Breakthrough of the Year
The Discovery of the Higgs Boson
年度进展
发现希格斯波色子

Adrian Cho

Science has chosen the finding of the Higgs boson as its Breakthrough of the Year.

《科学》杂志将找到希格斯波色子作为它的年度突破。

No recent scientific advance has generated more hoopla than this one. On 4 July, researchers working with the world's biggest atom smasher—the Large Hadron Collider (LHC) in Switzerland—announced that they had spotted a particle that appears to be the long-sought Higgs boson, the last missing piece in physicists' standard model of fundamental particles and forces. The seminar at which the results were presented turned into a media circus, and the news captured the imagination of people around the world. “[H]appy ‘god particle’ day,” tweeted will.i.am, the singer for pop group The Black Eyed Peas, to his 4 million Twitter followers.

近期没有哪个科学进展引起比它更大的关注。2012年7月4日，使用世界上最大的原子对撞机——瑞士的大型强子对撞机（LHC）的研究人员宣称他们发现了一种粒子，正是人们追寻已久的希格斯波色子，也就是物理学家们的基本粒子和力标准模型的最后一部分。展示这一结果的学术会变成了一场媒体的狂欢，这个新闻引发了全世界人的想象。“上帝粒子日快乐”——流行乐团黑眼豆豆的歌手will.i.am通过推特向他的400万粉丝这样祝贺。

Yet, for all the hype, the discovery of the Higgs boson easily merits recognition as the breakthrough of the year. Hypothesized more than 40 years ago, the Higgs boson is the key to physicists' explanation of how other fundamental particles get their mass. Its observation completes the standard model, perhaps the most elaborate and precise theory in all of science. In fact, the only big question hanging over the advance is whether it marks the beginning of a new age of discovery in particle physics or the last hurrah for a field that has run its course.

在喧嚣的褒奖声中，希格斯波色子的发现轻易获得了年度科学进展的美誉。自40年前被假设存在，希格斯波色子是物理学家解释其他基本粒子如何产生质量的关键。它的观察发现完善了标准模型这一可能是所有科学中最完备最精确的模型。事实上，前进路上最大的问题是：这标志着一个粒子物理发现新时代的开始，还是一个领域跑完了它这一程的最后一声喝彩。

The Higgs solves a basic problem in the standard model. The theory describes the particles that make up ordinary matter: the electrons that whiz around in atoms, the up quarks and down quarks that make up the protons and neutrons in atomic nuclei, the neutrinos that are emitted in a type of radioactivity, and two sets of heavier cousins of these particles that emerge in particle collisions. These particles interact by exchanging other particles that convey three forces: the electromagnetic force; the weak nuclear force, which spawns neutrinos; and the strong nuclear, which binds quarks.

希格斯解决了标准模型中的一个基本问题，这个理论描述了构成物质的那些粒子：原子中，电子咻咻地在绕圈，上夸克和下夸克组成了原子核中的质子和中子，中微子以辐射的形式发射出来，两组更重的同类粒子在粒子碰撞的时候出现。这些粒子通过与其他粒子交换来互相作用传递三种力：电磁力，产生中微子的弱核力以及连接夸克的强核力。

But there's a catch. At first blush, the standard model appears to be a theory of massless particles. That's because simply assigning masses to the particles makes the theory go haywire mathematically. So mass must somehow emerge from interactions of the otherwise massless particles themselves.

但存在这样一个困境：乍一看，标准模型看起来似乎是个描述无质量粒子的理论。因为简单地为这些粒子赋予质量会导致这个理论在数学上如同一团乱麻。因此质量必须通过这些本没有质量的粒子们自己相互作用的时候不知怎么地就搞出来了。

That's where the Higgs comes in. Physicists assume that empty space is filled with a “Higgs field,” which is a bit like an electric field. Particles interact with the Higgs field to acquire energy and, hence, mass, thanks to Albert Einstein's famous equivalence of the two, encapsulated in the equation E = mc2. Just as an electric field consists of particles called photons, the Higgs field consists of Higgs bosons woven into the vacuum. Physicists have now blasted them out of the vacuum and into brief existence.

That feat marks an intellectual, technological, and organizational triumph. To produce the Higgs, researchers at the European particle physics laboratory, CERN, near Geneva, built the $5.5 billion, 27-kilometer-long LHC. To spot the Higgs, they built gargantuan particle detectors—ATLAS, which is 25 meters tall and 45 meters long, and CMS, which weighs 12,500 tonnes. The ATLAS and CMS teams boast 3000 members each. More than 100 nations have a hand in the LHC.

Perhaps most impressive is the fact that theorists predicted the existence of the new particle and laid out its properties, right down to the rates at which it should decay into various combinations of other particles. (To test whether the particle really is the Higgs, researchers are measuring those rates now.) Physicists have made such predictions before. In 1970, when only three types of quarks were known, theorists predicted the existence of a fourth, which was discovered 4 years later. In 1967, they predicted the existence of particles that convey the weak force, the W and Z bosons, which were found in 1983.

Particle theorists offer various explanations of their knack for prognostication. Particle collisions are inherently reproducible and free of contingency, theorists say. Whereas no two galaxies are exactly the same, all protons are identical. So when smashing them, physicists need not worry about the peculiarities of this proton or that proton because there are none. Moreover, theorists say, in spite of its mathematical complexity, the standard model is conceptually simple—a claim that nonphysicists might not buy.

The standard model ultimately owes its predictive power to the fact that the theory is based on the notion of mathematical symmetry, some theorists say. Each of the three forces in the standard model is related to and, in some sense, necessitated by a different symmetry. The Higgs mechanism itself was invented to preserve such symmetry while giving mass to force-carrying particles like the W and the Z. Simply put, symmetry arguments are powerful predictive tools.

No matter the reason for particle physicists' predictive prowess, with the Higgs boson apparently in the bag, they have no similar prediction to test next. They have plenty of reason to think the standard model is not the final word on fundamental physics. The theory is obviously incomplete, as it doesn't incorporate the force of gravity. And the theory itself suggests that interactions between the Higgs and other particles ought to make the Higgs hugely heavy. So physicists suspect that new particles lurking in the vacuum may counteract that effect. But those arguments aren't nearly as precise as the one necessitating the Higgs boson.

In fact, scientists have no guarantee that any new physics lies within the reach of the LHC or any conceivable collider. The standard model could be all of the inner workings of the universe that nature is willing to reveal. The discovery of the Higgs is a breakthrough. Will particle physicists ever score a similar breakthrough again?

Science 21 December 2012: Vol. 338 no. 6114 pp. 1524-1525

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