1974年11月：粲夸克的发现

Quarks are just one branch of the family of particles in the Standard Model. But it was the discovery of the charmed quark in particular—discovered almost simultaneously by two different teams at two different accelerators, using different approaches and led by two very different men—that launched a series of breakthroughs collectively dubbed the “November Revolution.”

夸克只是标准模型中粒子家族的一个分支。但尤其是粲夸克的发现——由两个不同的团队在两个不同的加速器上几乎同时发现，使用不同的方法并由两个截然不同的人领导——引发了一系列统称为“十一月革命”的突破。

By the early 1930s, physicists thought they had a complete picture of the constituents of matter: electrons, protons, neutrons, neutrinos, and their corresponding antiparticles. But Nature threw them a curveball in 1936 with the discovery of the muon, a heavier version of the electron. It was so unexpected that I.I. Rabi famously declared, “Who ordered that?” As physicists continued to collide particles at ever higher energies, they discovered more and more particles.

到 20 世纪 30 年代初，物理学家认为他们已经掌握了物质组成的完整图像：电子、质子、中子、中微子及其相应的反粒子。但 1936 年，随着 μ 子（电子的较重版本）的发现，《自然》杂志给了他们一个难题。实在是太出乎我的意料了。拉比曾说过一句著名的话：“谁下令的？”随着物理学家继续以更高的能量碰撞粒子，他们发现了越来越多的粒子。

It wasn’t until 1964 that a theoretical solution emerged. That was the year Murray Gell-Mann and George Zweig proposed that all these new particles were actually made up of different combinations of even smaller, more fundamental particles, dubbed “quarks” after a famous nonsense line in James Joyce’s Finnegan’s Wake. (“Three quarks for Muster Mark!”) They speculated about a possible fourth quark, but in 1970, Sheldon Glashow, John Iliopoulos, and Luciano Maiani made a specific prediction for its existence, to explain the absence of an expected particle interaction. This set the stage for the experimental discoveries to come.

直到 1964 年，理论解决方案才出现。那一年，默里·盖尔曼和乔治·茨威格提出，所有这些新粒子实际上都是由更小、更基本的粒子的不同组合组成的，这些粒子以詹姆斯·乔伊斯的《芬尼根守灵夜》中一句著名的废话而被称为“夸克”。（“穆斯特·马克的三夸克！”）他们推测可能存在第四夸克，但在 1970 年，谢尔登·格拉肖 (Sheldon Glashow)、约翰·伊利奥普洛斯 (John Iliopoulos) 和卢西亚诺·麦亚尼 (Luciano Maiani) 对第四夸克的存在做出了具体预测，以解释预期粒子相互作用的缺失。这为后来的实验发现奠定了基础。

At Brookhaven National Laboratory in New York, C.C.Ting was heading up a particle-hunting experiment that involved shooting high-speed protons into a beryllium target to produce showers of new particles, and then using a mix of magnetic fields and detectors to filter out interesting results.

在纽约布鲁克海文国家实验室，丁肇中正在领导一项粒子搜寻实验，其中包括将高速质子射入铍靶中，以产生新的粒子簇射，然后使用磁场和探测器的组合来过滤掉感兴趣的粒子结果。

Ting’s Chinese parents were visiting Ann Arbor, Michigan when their son was born prematurely, making him a U.S. citizen. They returned to China, but the Japanese invasion interrupted his education. He was largely home schooled until the age of 12, under the watchful eye of his grandmother. At 20, he returned to the U.S. to attend the University of Michigan, completing his Ph.D. in 1962. He became a professor at Columbia University, and then joined the faculty at MIT. His group used the Alternating Gradient Synchrotron (AGS) at Brookhaven for their experiments, since its accelerator could produce higher energies.

丁肇中的中国父母在访问密歇根州安娜堡时，他们的儿子早产，使他成为美国公民。他们回到中国，但日本的入侵中断了他的学业。12 岁之前，他大部分时间都是在祖母的注视下在家接受教育。20岁时，他回到美国，就读密歇根大学，完成博士学位。1962年，他成为哥伦比亚大学教授，随后加入麻省理工学院任教。他的团队使用布鲁克海文的交替梯度同步加速器（AGS）进行实验，因为它的加速器可以产生更高的能量。

Meanwhile, across the country at Stanford University, Burton Richter headed up the team hunting for quarks. Born in 1931 in Queens, New York, Richter began his undergraduate studies at MIT unsure whether he wanted to study physics or chemistry, but soon chose the former. He stayed at MIT for his graduate studies, and eventually ended up in particle physics. After completing his Ph.D., he joined the faculty of Stanford University, where he soon became involved in building an 80-meter diameter accelerator called SPEAR (Stanford Positron Electron Accelerating Ring). Completed in 1973, it was capable of accelerating counter-rotating electron and positron beams up to four billion electron volts.

与此同时，在全国各地的斯坦福大学，伯顿·里克特 (Burton Richter) 领导着寻找夸克的团队。里克特 1931 年出生于纽约皇后区，在麻省理工学院开始本科学习时，他不确定自己是想学习物理还是化学，但很快选择了前者。他留在麻省理工学院攻读研究生，最终进入粒子物理学领域。完成博士学位后，他加入了斯坦福大学，并很快参与了直径 80 米的加速器 SPEAR（斯坦福正电子加速环）的建造工作。它于 1973 年竣工，能够将反向旋转电子束和正电子束加速至 40 亿电子伏特。

The accelerator at Brookhaven accelerated protons. But protons are not fundamental particles; electrons are, and Stanford physicists thought they would make for a much better probe. A carousel-like storage ring accelerated a stream of electrons and a stream of positrons in opposite directions and then made them collide to produce showers of new particles from the energy of the collision. The beam was directed to what was basically a scaled-up version of Ernest Rutherford’s original scattering experiments, in this case using liquid hydrogen and deuterium as targets.

布鲁克海文的加速器加速质子。但质子并不是基本粒子，而电子是基本粒子。斯坦福大学的物理学家认为它们可以制造出更好的探测器。一个类似旋转木马的存储环以相反的方向加速电子流和正电子流，然后使它们碰撞，从碰撞的能量中产生新的粒子流。光束被引导到基本上是欧内斯特·卢瑟福原始散射实验的放大版本，在这种情况下使用液氢和氘作为目标。

Over the summer of 1974, both teams independently spotted their prey. Since quarks cannot exist on their own, the discovery came in the form of a meson comprising a charm and anti-charm quark. At SLAC, Richter’s team spotted a massive spike (resonance) in the data indicating the presence of a new particle—the charm quark. And it had a much longer lifetime than expected.
1974 年夏天，两支队伍独立发现了猎物。由于夸克不能单独存在，这一发现以介子的形式出现，介子由粲夸克和反粲夸克组成。在 SLAC，Richter 的团队在数据中发现了一个巨大的尖峰（共振），表明存在一种新粒子——粲夸克。而且它的使用寿命比预期的要长得多。

Richter and Ting compared notes at a November meeting at SLAC and realized their respective teams had both discovered a fourth flavor of quark. They quickly made a joint announcement, and the two teams published papers a week or so later detailing their respective discoveries. Richter had dubbed his the “psi particle, since its decay pattern involved four particles curving in the magnetic field to form something that looked like the Greek letter psi. Ting called his the J particle,” since that letter resembled the Chinese symbol for his name. They compromised and combined the two, so the charm quark is officially the J/psi particle.
Richter 和 Ting 在 SLAC 11 月的一次会议上交换了意见，并意识到他们各自的团队都发现了第四种夸克。他们很快发表了联合声明，两个团队在大约一周后发表了论文，详细介绍了各自的发现。里克特将他的粒子称为“psi粒子”，因为它的衰变模式涉及四个粒子在磁场中弯曲，形成看起来像希腊字母psi的东西。丁称他为“J 粒子”，因为这个字母类似于他名字的中文符号。他们妥协并将两者结合起来，因此粲夸克正式成为 J/psi 粒子。

Richter and Ting shared the 1976 Nobel Prize in physics, “for their pioneering work in the discovery of a heavy elementary particle of a new kind.” With this discovery, there were now two complete generations of particles: the first—the electron and the up and down quarks; the second—the short-lived muon, charm, and strange quarks.
里希特和丁分享了 1976 年诺贝尔物理学奖，“表彰他们在发现新型重基本粒子方面的开创性工作”。有了这一发现，现在就有了两代完整的粒子：第一代是电子和上下夸克；第二代是电子和上下夸克；第二代是电子和上下夸克。第二个——短命的介子、粲子和奇怪的夸克。

But nature had a few more surprises in store. Within a few years, SLAC scientists discovered the tau lepton, setting off a search for two more flavors of quark. Fermilab’s Tevatron accelerator found the bottom quark by colliding protons with a stationary target at even higher energies, and examining their statistical data for telltale “bumps” indicating the presence of an upsilon (made up of bottom and anti-bottom quarks). They succeeded in 1977. But it would take nearly 20 years for Fermilab’s scientists to produce the elusive top quark: that didn’t happen until 1995, and it proved much more massive than originally expected—as heavy as a gold nucleus.
但大自然还给我们带来了更多惊喜。几年之内，SLAC 的科学家发现了 tau 轻子，从而开始寻找另外两种夸克。费米实验室的 Tevatron 加速器通过将质子与更高能量的固定目标碰撞来发现底夸克，并检查其统计数据以寻找表明 upsilon（由底夸克和反底夸克组成）存在的明显“碰撞”。他们在 1977 年取得了成功。但费米实验室的科学家们花了近 20 年的时间才产生了难以捉摸的顶夸克：直到 1995 年才实现，而且事实证明它比最初预期的要大得多——与金核一样重。

As for the charm quark, it continues to surprise scientists. In 2002, Fermilab’s SELEX collaboration announced they’d detected a singly charged, doubly charmed particle, made up of a down quark and two charm quarks. The catch: other experiments have since failed to produce any more such particles. And just this year the LHCb detector at the Large Hadron Collider in Switzerland discovered a rare combination of particles: a doubly charged, doubly charmed Xi particle, comprising an up quark and two charm quarks. LHCb’s particle also has significantly higher mass than that detected by SELEX. Either one result is wrong—and both analyses were very careful and clean—or it may be that some theoretical tweaking is in order to account for the discrepancy. Who knows what other secrets the charm quark might be hiding?
至于粲夸克，它继续让科学家们感到惊讶。 2002 年，费米实验室的 SELEX 合作宣布他们发现了一种单电荷、双粲粒子，由一个下夸克和两个粲夸克组成。问题是：此后其他实验均未能产生更多此类粒子。就在今年，瑞士大型强子对撞机的 LHCb 探测器发现了一种罕见的粒子组合：一种双电荷、双粲 Xi 粒子，由一个上夸克和两个粲夸克组成。 LHCb 粒子的质量也明显高于 SELEX 检测到的质量。要么一个结果是错误的——而且这两种分析都非常仔细和干净——或者可能是为了解释这种差异而进行了一些理论上的调整。谁知道粲夸克还隐藏着什么秘密呢？