Physicists Solve a Big Quantum Mystery. Now, Old Results Don’t Add Up.

Years after physicists at Fermi National Accelerator Laboratory in Illinois used a giant magnetic ring to measure precisely how much the muon wobbles, researchers are still puzzling over the result.

伊利诺伊州费米国家加速器实验室的物理学家使用巨型磁环精确测量μ子摆动的程度多年后，研究人员仍然对结果感到困惑。

For 25 years, physicists have been puzzled by an apparent one-part-in-a-million problem.

25 年来，物理学家一直对一个明显的百万分之一的问题感到困惑。

Their expectations of the way that certain particles should wobble in a magnetic field were clashing with what they saw in experiments.

他们对某些粒子在磁场中摆动方式的期望与他们在实验中看到的相冲突。

The discrepancy was an electrifying hint that they might be seeing evidence of unknown particles.

这种差异是一个令人震惊的暗示，表明他们可能看到了未知粒子的证据。

Then in 2021, that hint seemed to evaporate.

但到了 2021 年，这种暗示似乎消失了。

When researchers updated the way they did their theoretical calculations, they found that their predictions matched the experimental results much more precisely than before, to one part in 100 billion.

当研究人员更新他们的理论计算方式时，他们发现他们的预测与实验结果的匹配比以前更加精确，达到千亿分之一。

But that, in turn, has created another puzzle: The old calculations seem perfectly valid.

但这反过来又产生了另一个谜题：旧的计算似乎完全有效。

So why don’t they match the new calculations?

那么为什么它们与新的计算不匹配呢？

Those older predictions were not purely based on theory; they were also inferred from other experiments.

那些较早的预测并非纯粹基于理论；而是基于理论。它们也是从其他实验中推断出来的。

If the older calculations conflicted with newer results, and the older calculations were based on experimental data, was something strange going on in those old experiments?

如果旧的计算与新的结果相冲突，并且旧的计算是基于实验数据，那么那些旧的实验中是否发生了奇怪的事情？

One promising clue comes from a particle collider in Siberia, which has recently started seeing its experiments dramatically diverge from what it and other colliders saw in the past.

一个有希望的线索来自西伯利亚的一台粒子对撞机，该对撞机最近开始发现它的实验与它和其他对撞机过去所看到的有很大不同。

Its results have sparked a flurry of activity as physicists try to determine whether the conflicting measurements are a side effect of different experimental procedures, or a sign that new particles are popping up after all.

它的结果引发了一系列的活动，物理学家试图确定这些相互矛盾的测量结果是否是不同实验程序的副作用，或者到底是新粒子出现的迹象。

The particle at the center of the mystery is the muon, a heavier cousin of the electron.

这个谜团的核心粒子是μ子，它是电子的一个较重的表亲。

A muon behaves a bit like a tiny bar magnet.

μ子的行为有点像一个微小的条形磁铁。

Spin one in a circle inside a magnetic field and the magnetism will make it wobble, tracing out its own, smaller circles.

在磁场中绕一圈旋转，磁性会使它摆动，画出自己的更小的圆圈。

The sizes of these circles are determined by a number called a “ g -factor.”

这些圆圈的大小由称为“g 因子”的数字决定。

If the muon sat isolated from other particles, its g -factor would be exactly 2.

如果 μ 子与其他粒子隔离，它的 g 因子将恰好为 2。

But quantum theory requires that all other particles influence the g -factor.

但量子理论要求所有其他粒子都会影响 g 因子。

As the muon wobbles, it releases particles such as photons, which are too short-lived to show up in detectors.

当μ子摆动时，它会释放出光子等粒子，这些粒子的寿命太短，无法出现在探测器中。

These can release other particles, which can release still more particles.

它们可以释放其他颗粒，而其他颗粒又可以释放更多颗粒。

The muon quickly reabsorbs all these fleeting particles, and the only trace they leave behind is that the muon wobbles a little bit more.

μ 子很快重新吸收了所有这些转瞬即逝的粒子，它们留下的唯一痕迹就是 μ 子进一步摆动了一点。

Through these intricate chains of emission and reabsorption, every particle in existence has some small effect on the movement of the muon.

通过这些复杂的发射和重吸收链，存在的每个粒子都会对μ子的运动产生一些微小的影响。

Samuel Velasco, Mark Belan/ Quanta Magazine

塞缪尔·贝拉斯科 (Samuel Velasco)、马克·贝兰 (Mark Belan)/Quanta 杂志

That makes the precise size of the excess wobble, the muon’s “ g –2,” invaluable as a window into the quantum world.

这使得多余摆动的精确大小（μ子的“g –2”）作为进入量子世界的窗口变得非常宝贵。

“The measurement of muon g –2 is a proxy for saying how many particles exist in the universe,” said Alex Keshavarzi , a senior research fellow at University College London.

伦敦大学学院高级研究员 Alex Keshavarzi 表示：“μ介子 g –2 的测量可以作为衡量宇宙中存在多少粒子的指标。”

So when an experiment at Brookhaven National Laboratory on Long Island measured the muon’s g -factor in 2001, physicists were thrilled that it came out larger than expected.

因此，当 2001 年长岛布鲁克海文国家实验室的一项实验测量了 μ 子的 g 因子时，物理学家们非常兴奋，因为结果比预期的要大。

To some, it hinted that new particles — perhaps even particles that could account for dark matter — were at work.

对一些人来说，它暗示新粒子——甚至可能是可以解释暗物质的粒子——正在发挥作用。

Physicists set out to check the result with an even more precise measurement.

物理学家开始通过更精确的测量来检查结果。

In 2013, Brookhaven’s 50-foot-wide magnetic ring was moved via an elaborate series of barges and trucks to Fermi National Accelerator Laboratory (Fermilab) in Illinois, where an upgraded version of the experiment would take even more data.

2013 年，布鲁克海文的 50 英尺宽的磁环通过一系列精心设计的驳船和卡车运送到伊利诺伊州的费米国家加速器实验室 (Fermilab)，在那里该实验的升级版本将获取更多数据。

To prepare for that new experiment, physicists also made a huge effort to understand the theoretical prediction that disagreed with the data.

为了准备这项新实验，物理学家还付出了巨大的努力来理解与数据不一致的理论预测。

Their challenge was to understand the muon’s chains of emission and reabsorption in extreme detail.

他们面临的挑战是极其详细地了解 μ 子的发射和重吸收链。

In particular, how much do the particles associated with each of nature’s four fundamental forces participate in these chains?

特别是，与自然界四种基本力相关的粒子在这些链中的参与程度有多少？

Alex Keshavarzi, a physicist at University College London, helped refine a way to infer how the muon should wobble from certain collider experiments.

伦敦大学学院的物理学家亚历克斯·凯沙瓦齐 (Alex Keshavarzi) 帮助改进了一种方法，可以从某些对撞机实验中推断 μ 子应该如何摆动。

The calculation is straightforward for three of nature’s four forces.

对于自然四种力中的三种，计算很简单。

Gravity is so weak that physicists can ignore it outright.

引力是如此微弱，以至于物理学家可以完全忽略它。

And both the electromagnetic force and the weak nuclear force can be deduced using a standard technique.

电磁力和弱核力都可以使用标准技术推导出来。

The strong force, however, is not so easy to deal with.

然而，强大的力量却不是那么容易对付的。

That force tightly binds particles known as quarks into composite particles such as protons and neutrons.

这种力将夸克粒子紧密地结合成质子和中子等复合粒子。

Standard theoretical techniques don’t work on the strong force.

标准的理论技术不适用于强力。

So physicists have to get creative.

因此物理学家必须发挥创造力。

In his doctoral thesis in 2018, Keshavarzi helped hone an alternative way of understanding the strong force, called the data-driven method.

在 2018 年的博士论文中，Keshavarzi 帮助磨练了另一种理解强力的方法，称为数据驱动方法。

In this method, physicists don’t try to predict how often muons will emit and absorb groups of quarks.

在这种方法中，物理学家不会尝试预测μ子发射和吸收夸克群的频率。

They go out and measure it.

他们出去测量。

The main way that happens is by colliding electrons and their antimatter partners, positrons.

发生这种情况的主要方式是电子与其反物质伙伴正电子的碰撞。

The matter and antimatter annihilate each other, creating other particles, including bundles of quarks.

物质和反物质相互湮灭，产生其他粒子，包括夸克束。

If lots of quarks appear, physicists know they have a tight quantum link to particles such as electrons and positrons.

如果出现大量夸克，物理学家就知道它们与电子和正电子等粒子具有紧密的量子联系。

In short, the more quarks appear in electron-positron collisions, the more strongly they will affect the muon.

简而言之，电子-正电子碰撞中出现的夸克越多，它们对μ子的影响就越强烈。

Using the data-driven method, physicists set out to calculate the expected size of the muon’s magnetic wobble.

使用数据驱动的方法，物理学家开始计算 μ 子磁摆动的预期大小。

That prediction , which was released in June 2020, sharply differed from Fermilab’s precise experimental measurement , which came out in April 2021.

该预测于 2020 年 6 月发布，与费米实验室于 2021 年 4 月发布的精确实验测量存在巨大差异。

The discrepancy was so strong that it nearly crossed the stringent threshold required for physicists to claim they had discovered new particles.

这种差异是如此之大，以至于几乎超过了物理学家声称他们发现了新粒子所需的严格门槛。

But a different theoretical calculation would tell a different story.

但不同的理论计算会讲述不同的故事。

Not all physicists pursued the data-driven method to calculate the muon’s wobble.

并非所有物理学家都追求数据驱动的方法来计算 μ 子的摆动。

Some were working on a more purely theoretical technique to make their prediction.

有些人正在研究一种更纯粹的理论技术来做出他们的预测。

The approach resembles what happens in weather forecasting.

这种方法类似于天气预报中发生的情况。

While it is possible, in principle, to understand the weather by keeping track of the precise contour of every breeze in the atmosphere, in practice that task is absurd.

虽然原则上可以通过跟踪大气中每一次微风的精确轮廓来了解天气，但实际上这项任务是荒谬的。

Instead, meteorologists divide the atmosphere into big boxes — a 3D grid — and calculate how each box changes on average over time.

相反，气象学家将大气分成大盒子（3D 网格），并计算每个盒子随时间的平均变化。

Likewise, it’s too hard for physicists to keep track of every strong-force interaction between every pair of quarks.

同样，物理学家也很难跟踪每对夸克之间的每一次强力相互作用。

So physicists use a technique called lattice QCD (short for quantum chromodynamics, the theory of the strong force), to use a big grid to simulate the overall behavior of quarks.

因此，物理学家使用一种称为晶格QCD（量子色动力学的缩写，强力理论）的技术，使用大网格来模拟夸克的整体行为。

In 2014, a collaboration among researchers in Budapest, Hungary; Marseille, France; and Wuppertal, Germany — the BMW group — started on a project to use lattice QCD to calculate the muon g -factor.

2014年，匈牙利布达佩斯的研究人员合作；法国马赛；宝马集团和德国伍珀塔尔开始了一个使用晶格 QCD 来计算 μ 子 g 因子的项目。

In 2013, researchers carefully transported Brookhaven’s magnetic ring from New York to Illinois by sea, river, and road.

2013年，研究人员小心翼翼地将布鲁克海文的磁环通过海运、河流和公路从纽约运送到伊利诺伊州。

Brookhaven National Laboratory

布鲁克海文国家实验室

At first, their predictions were 10 times fuzzier than data-driven inferences.

起初，他们的预测比数据驱动的推论模糊十倍。

Low-energy particles tend to spread out, so capturing their possible positions requires using a huge lattice.

低能粒子往往会扩散，因此捕获它们可能的位置需要使用巨大的晶格。

High-energy particles need a comparatively smaller grid, but one with an extremely fine mesh.

高能粒子需要相对较小的网格，但网格必须非常细。

“Back then, it was unimaginable that one day lattice would reach the same precision” as the data-driven method, said Kalman Szabo , a professor at Wuppertal who was involved in the effort.

参与这项工作的伍珀塔尔大学教授卡尔曼·萨博 (Kalman Szabo) 表示，“当时，难以想象有一天格子能够达到与数据驱动方法相同的精度”。

It took a decade of developing clever computational techniques — and waiting for increased computing power — for the BMW group to wrangle grids that were both sufficiently big and sufficiently detailed.

宝马集团花了十年时间开发巧妙的计算技术，并等待计算能力的提高，才解决了足够大和足够详细的网格问题。

But wrangle them they did.

但他们确实跟他们争吵过。

In 2021, on the same day that Fermilab released its updated muon g –2 measurement, the BMW group’s result appeared in the journal Nature .

2021年，就在费米实验室发布更新后的μ介子g-2测量结果的同一天，宝马集团的结果发表在《自然》杂志上。

According to the BMW group’s lattice calculation, Fermilab’s muons were wobbling exactly as they should.

根据宝马集团的晶格计算，费米实验室的μ介子完全按照其应有的方式摆动。

Since then, independent lattice groups have published matching calculations.

此后，独立晶格组发表了匹配计算。

Today, many physicists believe the muon mystery is no more: According to the lattice simulations, the muon’s extra wobble can be explained entirely by the emission and reabsorption of known particles obeying the known laws of the known forces.

如今，许多物理学家相信μ子之谜已不复存在：根据晶格模拟，μ子的额外摆动完全可以通过遵守已知力的已知定律的已知粒子的发射和重吸收来解释。

So why does the data-driven method indicate otherwise?

那么为什么数据驱动方法却显示出相反的结果呢？

To figure out what’s going on, physicists are drilling into the electron-positron collisions driving the data-driven method.

为了弄清楚到底发生了什么，物理学家正在深入研究驱动数据驱动方法的电子-正电子碰撞。

These collisions are supposed to be a direct window into quark behavior, but calculations based on this data disagree with both the latest experimental results and BMW’s prediction.

这些碰撞被认为是了解夸克行为的直接窗口，但基于这些数据的计算与最新的实验结果和宝马的预测不一致。

So what’s really going on in the aftermath of those collisions?

那么这些碰撞之后到底发生了什么？

In the city of Novosibirsk in southern Siberia, the VEPP-2000 collider has been crashing electrons into positrons on and off since the turn of the millennium.

在西伯利亚南部的新西伯利亚市，自世纪之交以来，VEPP-2000 对撞机一直断断续续地将电子碰撞为正电子。

It’s a relatively gentle collider, operating at 6,000 times lower energy than CERN’s Large Hadron Collider, near Geneva.

这是一个相对温和的对撞机，其运行能量比日内瓦附近的 CERN 大型强子对撞机低 6,000 倍。

The VEPP-2000 features two detectors that precisely count how often certain bundles of quarks, known as pions, pop out of the electron-positron crashes — data that physicists have been using to infer how much the strong force was messing with muons.

VEPP-2000配备了两个探测器，可以精确计算某些夸克束（称为π介子）从电子-正电子碰撞中弹出的频率——物理学家一直用这些数据来推断强力对μ介子的干扰程度。

In 2010, physicists installed a completely new detector.

2010年，物理学家安装了一个全新的探测器。

They then used it to more precisely measure this pion production rate , which they published in 2023.

然后他们用它来更精确地测量 π 介子生产率，并于 2023 年发表。

After the refresh, they found that the rate changed significantly.

刷新后，他们发现速率发生了显着变化。

Fedor Ignatov, a physicist at the University of Liverpool in the UK, was a member of the team that measured a mysterious new rate of pion production at the VEPP-2000 collider.

英国利物浦大学的物理学家费多尔·伊格纳托夫 (Fedor Ignatov) 是测量 VEPP-2000 对撞机神秘的新π介子产生率的团队成员之一。

“It was a surprise.

“这是一个惊喜。

No one expected it to be like that,” said Fedor Ignatov , a physicist at the University of Liverpool in the UK and member of the team.

没有人预料到会是这样。”该团队成员、英国利物浦大学物理学家费多尔·伊格纳托夫 (Fedor Ignatov) 说道。

Physicists had seen faint hints that something strange was going on with the pion rate.

物理学家已经看到了微弱的迹象，表明π介子率正在发生一些奇怪的事情。

They noticed that measurements of it from experiments in Italy and the United States were starting to drift apart.

他们注意到意大利和美国的实验测量结果开始出现偏差。

But the dramatic divergence of the new measurement from the detector’s own past results, along with the collaboration’s claim of high precision, made the situation hard to ignore.

但新的测量结果与探测器过去的结果存在巨大差异，再加上合作组织声称的高精度，使得这种情况难以忽视。

Physicists have pored over the result.

物理学家仔细研究了结果。

“No measurement has been scrutinized more,” Keshavarzi said.

“没有任何测量结果受到如此严格的审查，”克沙瓦尔齐说。

So far, no problems have been found.

到目前为止，还没有发现任何问题。

Recent lattice-based simulations align with the newly measured rate.

最近基于晶格的模拟与新测量的速率一致。

And preliminary data from the other detector at the VEPP-2000 collider also seem to match.

VEPP-2000 对撞机的另一个探测器的初步数据似乎也匹配。

Meanwhile, a 2023 analysis of data collected earlier at yet another experiment at a collider, BABAR in California, sits in striking agreement with the older rate.

与此同时，对 2023 年加利福尼亚州 BABAR 对撞机实验中收集的数据进行的分析与之前的数据惊人一致。

All of this leaves physicists wondering what’s really going on in all these collider experiments.

所有这些都让物理学家想知道所有这些对撞机实验中到底发生了什么。

The discrepancies point either to signs of unknown particles meddling with the quarks, or to overlooked details generating the mistaken impression that quarks are misbehaving.

这些差异要么表明存在未知粒子干扰夸克的迹象，要么表明被忽视的细节产生了夸克行为不端的错误印象。

Either way, particle physicists can’t rest until they have solved the new electron-positron mystery, and figured out whether the old pion rate, or the new pion rate, is the right one.

无论哪种方式，粒子物理学家都不能休息，直到他们解决了新的电子-正电子之谜，并弄清楚旧的 π 介子速率或新的 π 介子速率是否正确。

“There are four decades of measurements that preceded that, that were all done in different ways, that were all done by different people, that were all done by different experiments, that all paint a completely different picture,” Keshavarzi said.

“在此之前有四十年的测量，都是以不同的方式完成的，都是由不同的人完成的，都是通过不同的实验完成的，都描绘了完全不同的画面，”克沙瓦尔齐说。

“There is so much still left to do.”

“还有很多事情要做。”

Correction: July 30, 2026 The original version of this article stated that the BMW group carried out simulations that directly predict the new pion rate.

更正：2026 年 7 月 30 日 本文原始版本称，宝马集团进行了直接预测新π介子率的模拟。

BMW’s calculations support the new rate, but only indirectly.

宝马的计算支持了新的利率，但只是间接的。

Other lattice groups have more directly gone after the pion rate.

其他晶格群更直接地追求π介子率。

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《Quanta》杂志对评论进行审核，以促进知情、实质性、文明的对话。

Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected.

辱骂、亵渎、自我推销、误导、语无伦次或偏离主题的评论将被拒绝。

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版主在正常工作时间（纽约时间）有工作人员，并且只能接受英文评论。