The search for dark matter has been blown wide open
The search for dark matter has been blown wide open
暗物质搜寻迎来全新局面
Underneath an Apennine massif, below the Jinping Mountains of Sichuan, and at the bottom of a South Dakota mine, there is a cosmic hunt afoot. Isolated deep beneath these rocky shields, massive detectors filled with liquid xenon aim to make the first direct detections of dark matter, the long-sought invisible substance whose gravity has sculpted our universe. 在亚平宁山脉深处、四川锦屏山之下，以及南达科他州的一座矿井底部，一场宇宙级的搜寻正在进行。在这些岩石屏障的深处，巨大的液氙探测器旨在首次直接探测到暗物质——这种人们苦苦追寻的隐形物质，正是其引力塑造了我们的宇宙。
The hope is that someday, a bit of dark matter called a weakly interacting massive particle (a WIMP, for short) will collide with a xenon atom, creating a burst of light and electric charge. After running for years, these experiments have recently begun seeing infrequent blips from a particle that glides ethereally through ordinary matter until it crashes into the detectors. 科学家们希望有一天，一种被称为“弱相互作用大质量粒子”（简称 WIMP）的暗物质能与氙原子发生碰撞，产生光和电荷的闪烁。经过多年的运行，这些实验最近开始观测到一些罕见的信号，这些信号来自一种能够轻盈地穿过普通物质，直到撞击探测器时才显露踪迹的粒子。
Unfortunately, the new signal is not produced by dark matter. Instead, the detectors are picking up on something similarly insubstantial but much more mundane: neutrinos, the featherweight subatomic particles that the sun and other stars produce in massive quantities. 遗憾的是，这些新信号并非由暗物质产生。相反，探测器捕捉到的是一种同样难以捉摸但更为常见的物质：中微子。这是一种由太阳和其他恒星大量产生的超轻亚原子粒子。
Physicists’ failure to find dark matter where they thought it was has led to a cornucopia of proposals for new ways to search: quantum sensors, liquid-helium-based detectors, searches in Jupiter’s atmosphere, and more. Physicists have known for decades that this neutrino background was there; they were just hoping to discover WIMP dark matter first. Now the chance is looking slim. 物理学家未能如预期般找到暗物质，这催生了大量新的搜寻方案：量子传感器、液氦探测器、木星大气搜寻等等。几十年来，物理学家一直知道这种“中微子背景”的存在，他们只是希望先发现 WIMP 暗物质。现在看来，这种可能性已经微乎其微。
Some of today’s WIMP detectors are simply so large and sensitive that they are entering the so-called “neutrino fog,” in which the ordinary particles are likely to drown out any signal from the main target. There is no shielding these detectors from neutrinos, which easily slip through the Earth itself. That means the next experiment to use this long-standing approach for seeking WIMP dark matter may be the last. 目前的一些 WIMP 探测器规模巨大且极其灵敏，以至于它们正进入所谓的“中微子迷雾”区域，在这种情况下，普通粒子很可能会淹没来自主要目标的任何信号。由于中微子能轻易穿透地球，探测器无法屏蔽它们。这意味着，下一次使用这种传统方法搜寻 WIMP 暗物质的实验，可能就是最后一次了。
Hitting the neutrino fog does not, however, mean an end to the search for dark matter. Researchers just have to shift the focus of their hunt. “We haven’t seen WIMP dark matter,” says Kathryn Zurek, a theoretical particle physicist at the California Institute of Technology. Nor, she says, have scientists found new particles in the Large Hadron Collider (LHC), the powerful proton-smashing facility that straddles the border between France and Switzerland. “And so people naturally broaden their scope,” Zurek says. As they do, there are plenty more candidates waiting in the wings. 然而，遭遇“中微子迷雾”并不意味着暗物质搜寻的终结。研究人员只是需要转移搜寻重点。加州理工学院的理论粒子物理学家凯瑟琳·祖雷克（Kathryn Zurek）表示：“我们还没有发现 WIMP 暗物质。”她说，科学家们在横跨法瑞边境的大型强子对撞机（LHC）这一强大的质子对撞设施中，也尚未发现新的粒子。“因此，人们自然会拓宽研究范围。”祖雷克说。随着范围的扩大，还有许多其他的候选者等待着被发掘。
In other words, the hunt is transforming from a narrow probe into a kind of free-for-all. It’s a big shift. Today, particle physicists are less sure about dark matter’s identity than when they began looking for it. They’ll freely admit that they cannot presume the basics—for example, if the stuff that makes up dark matter is heavier than the Earth or lighter than a radio wave, or if dark matter is one kind of particle or a dozen. 换句话说，这场搜寻正从单一的探测转向一种“百花齐放”的模式。这是一个巨大的转变。今天，粒子物理学家对暗物质本质的确定性，远不如他们刚开始搜寻时那样高。他们坦诚地承认，甚至无法预设其基本属性——例如，构成暗物质的物质是比地球重，还是比无线电波轻；或者暗物质究竟是一种粒子，还是由十几种粒子组成。
The uncertainty can be frustrating, even humbling. “The potential range where the candidates could be is so enormous that the odds of any one small experiment finding it are very, very small,” says Hugh Lippincott, a dark matter experimentalist at the University of California, Santa Barbara. But physicists’ failure to find dark matter where they thought it was has also led to a cornucopia of proposals for new ways to search: quantum sensors, liquid-helium-based detectors, searches in Jupiter’s atmosphere, and more. 这种不确定性令人沮丧，甚至让人感到谦卑。加州大学圣塔芭芭拉分校的暗物质实验物理学家休·利平科特（Hugh Lippincott）说：“候选物质可能存在的范围极其巨大，任何单一小型实验发现它的几率都非常、非常小。”但物理学家在预期地点未能找到暗物质，也促使他们提出了大量新的搜寻方案：量子传感器、液氦探测器、木星大气搜寻等。
“Now there’s a great deal of excitement. And finally, there’s technology there,” says Gray Rybka, a University of Washington physicist who co-leads an experiment looking for axions, an ultra-lightweight dark matter candidate. Still, with so many places to look, where does it make sense for physicists to begin again? “现在大家非常兴奋。而且，技术终于跟上了，”华盛顿大学物理学家格雷·雷布卡（Gray Rybka）说。他共同领导着一项寻找轴子（一种超轻暗物质候选者）的实验。然而，面对如此多的搜寻方向，物理学家从哪里重新开始才最合理呢？
Astronomical ignorance
天文学上的未知
For starters: the birth of the universe. Dark matter has been with us since the beginning, and there’s much to learn from those early eons. Maps of the cosmic microwave background—the first light from the universe’s early years—are full of fluctuations caused by the clumpiness of underlying matter. Reading these cosmic dregs, researchers can tell that only 17% of the matter in the universe is made of ordinary particles like protons and neutrons. The remaining 83% is dark matter, which has little to no interaction with light or ordinary matter other than through gravity. 首先是宇宙的诞生。暗物质从宇宙之初就伴随着我们，从那些早期的纪元中，我们还有很多东西需要学习。宇宙微波背景图——来自宇宙早期的第一缕光——充满了由底层物质团块引起的波动。通过解读这些宇宙残迹，研究人员可以断定，宇宙中只有 17% 的物质是由质子和中子等普通粒子组成的。其余 83% 都是暗物质，除了引力之外，它与光或普通物质几乎没有相互作用。
We can tell quite a bit about dark matter from those gravitational effects. We know that the Milky Way contains a halo of the stuff. Our own solar system orbits the galactic center far too quickly to be bound by the tug of ordinary matter alone: without dark matter’s gravitational tether, we would be flung off into intergalactic space. We can also see how the heft of a galaxy’s dark matter bends the path of light as it makes its way to Earth’s telescopes. And on the grandest scale, we can see how superclusters of galaxies are distributed in space like dewdrops on a spiderweb. 我们可以通过这些引力效应了解暗物质的许多信息。我们知道银河系中存在一个暗物质晕。我们的太阳系绕银河系中心公转的速度太快，仅靠普通物质的引力无法将其束缚：如果没有暗物质的引力牵引，我们早就被甩入星系际空间了。我们还能观察到星系的暗物质质量如何在其光线射向地球望远镜的过程中弯曲光路。在最宏大的尺度上，我们能看到星系超星系团在空间中的分布，就像蜘蛛网上的露珠一样。
No cosmological theory without dark matter can explain all these phenomena. But all the astronomical and cosmological evidence has little to say about what dark matter is actually made of. “It does not tell you anything about the individual constituents. It just tells you the effect of a bunch of them together,” says Lippincott, who has led the LZ experiment, a WIMP dark matter detector currently in operation at the former Homestake Mine in South Dakota. 任何没有暗物质的宇宙学理论都无法解释所有这些现象。但所有的天文和宇宙学证据都无法说明暗物质究竟是由什么构成的。“它无法告诉你关于单个成分的任何信息。它只能告诉你它们聚集在一起产生的效果，”利平科特说。他曾领导 LZ 实验，这是一个目前在南达科他州前霍姆斯特克矿运行的 WIMP 暗物质探测器。
The idea of WIMPs emerged during the 1980s. At the time, theorists were exploring add-ons to the standard model, the overarching theory of particle physics that describes all the universe’s fundamental particles and their interactions. The standard model is powerful but doesn’t account for everything—notably, it omits gravity—so some adjustments seemed necessary. The most popular idea, a class of theories called supersymmetry (SUSY, informally), called for pairing each known particle type in the universe with an as-yet-unseen “superpartner.” To have avoided detection, superpartners would have to have a lot of mass (putting them outside the reach of existing colliders) and be weakly. WIMP 的概念出现于 20 世纪 80 年代。当时，理论家们正在探索标准模型的补充方案。标准模型是描述宇宙所有基本粒子及其相互作用的粒子物理学核心理论。标准模型虽然强大，但无法解释一切——特别是它忽略了引力——因此一些调整似乎是必要的。最流行的观点是一类被称为“超对称”（非正式称为 SUSY）的理论，它要求将宇宙中每种已知的粒子类型与一种尚未发现的“超对称伙伴”配对。为了逃避探测，超对称伙伴必须具有很大的质量（使其超出了现有对撞机的探测范围），并且相互作用微弱。