内容来源：https://www.quantamagazine.org/physicists-solve-a-muon-mystery-now-old-results-dont-add-up-20260729/

内容总结：

物理学重大谜题再现反转：旧实验数据与最新理论计算产生冲突，西伯利亚对撞机数据成关键线索

核心发现：物理学家曾因μ子（电子的“胖表亲”）在磁场中的异常摆动现象兴奋不已——这一“百万分之一”的微小偏差被视作可能存在未知新粒子的信号。但2021年，当研究团队采用全新的理论计算方法（格点QCD）后，计算结果竟与实验观测完美吻合，将偏差缩小至千亿分之一。然而，新的困惑随之而来：旧的理论计算本身似乎并无问题，但它们与新的计算结果却不一致。更关键的是，旧预测并非纯理论推导，而是基于其他实验数据反推得出。如果旧计算结果基于实验数据却与最新实验结果矛盾，那么是那些旧实验本身出了问题吗？

问题核心：μ子在磁场中摆动时，会短暂发射并重新吸收各种粒子（包括光子、夸克等），这些“量子涟漪”会微调其摆动幅度。这一“g-2”值被视为探测宇宙中所有粒子的“代理指标”。旧方法采用“数据驱动法”——通过分析电子-正电子对撞产生的夸克数量来间接推算μ子行为。而新方法“格点QCD”则采用空间网格模拟夸克间的强相互作用力。

关键转折点：位于西伯利亚新西伯利亚市的VEPP-2000对撞机在升级探测器后，于2023年公布了新的π介子产生率测量数据。令人震惊的是，这一数据与同一实验装置早年的结果出现“戏剧性偏离”，也与意大利、美国等其他实验的测量值相去甚远。2024年，宝马（BMW）团队的格点模拟计算结果与新数据大体吻合，而加州BABAR实验的早期数据分析却与旧数据保持高度一致。

物理学家现状：目前，学术界陷入了一场新的论战——到底是未知的新粒子在“捣乱”，还是实验中存在被忽视的系统性误差？伦敦大学学院高级研究员亚历克斯·凯沙瓦尔齐表示：“有长达四十年的测量数据，由不同团队、用不同方法、在不同实验中完成，却描绘出完全不同的图景。我们还有很多工作要做。”物理学家们正夜以继日地审查西伯利亚对撞机的数据，试图解开这一新的谜团。

中文翻译：

物理学家解开了一个巨大的量子谜团，但旧的计算结果却对不上了。

引言

25年来，物理学家一直被一个百万分之一量级的明显问题所困扰。他们对某些粒子在磁场中应如何摆动的预期，与他们在实验中观察到的现象存在冲突。这种差异是一个激动人心的线索，暗示他们可能看到了未知粒子的证据。

然而在2021年，这个线索似乎消失了。当研究人员更新他们的理论计算方法时，他们发现预测结果与实验数据的吻合程度比以前精确得多，达到了千亿分之一。

但这反过来又产生了另一个难题：旧的计算方法看起来完全有效。那么为什么它们与新的计算结果对不上呢？那些较早的预测并非纯粹基于理论；它们也是从其他实验中推断出来的。如果旧的计算结果与新的结果存在冲突，而旧的计算结果又是基于实验数据的，那么那些旧实验中是否出了什么奇怪的问题？

一个很有希望的线索来自西伯利亚的一个粒子对撞机，该对撞机最近发现其实验结果与它自身以及其他对撞机过去的观测结果出现了巨大差异。它的结果引发了一连串的活动，物理学家们正试图确定这些相互矛盾的测量结果是不同实验程序的副作用，还是毕竟有新粒子出现的迹象。

奇怪的摆动

处于这个谜团中心的粒子是μ子，它是电子的一个较重表亲。μ子的行为有点像一个小条形磁铁。让它在磁场中旋转一圈，磁力会使它摆动，画出它自己更小的圆圈。这些圆圈的大小由一个称为“g因子”的数字决定。

如果μ子与其他粒子隔绝，它的g因子恰好是2。但量子理论要求所有其他粒子都影响g因子。当μ子摆动时，它会释放出光子等粒子，这些粒子寿命极短，无法在探测器中出现。这些粒子又可以释放出其他粒子，而其他粒子还能释放出更多的粒子。μ子迅速重新吸收所有这些转瞬即逝的粒子，它们留下的唯一痕迹就是μ子摆动得稍微多了一点。通过这些复杂的发射和再吸收链条，存在的每一个粒子都对μ子的运动产生一些微小的影响。

这使得额外摆动（即μ子的“g-2”）的精确大小，成为观察量子世界的一扇宝贵窗口。“测量μ子的g-2，就是间接揭示宇宙中存在多少粒子，”伦敦大学学院的高级研究员亚历克斯·凯沙瓦尔齐说。

因此，当长岛布鲁克海文国家实验室在2001年测量μ子的g因子时，物理学家们非常兴奋，因为它比预期的要大。对一些人来说，这暗示着新粒子——甚至可能是可以解释暗物质的粒子——在起作用。

物理学家们着手用更精确的测量来验证这个结果。2013年，布鲁克海文那个50英尺宽的磁环通过一系列精心安排的驳船和卡车运到了伊利诺伊州的费米国家加速器实验室（费米实验室），在那里，一个升级版的实验将收集更多数据。

为了准备这个新实验，物理学家们也付出了巨大努力来理解那个与数据不符的理论预测。他们面临的挑战是要极其详细地理解μ子的发射和再吸收链条。特别是，与自然界四种基本力相关的粒子在多大程度上参与了这些链条？

对于自然界四种力中的三种，计算是直截了当的。引力非常微弱，物理学家完全可以忽略它。电磁力和弱核力都可以用标准技术推导出来。

然而，强相互作用力就没那么容易处理了。这种力将称为夸克的粒子紧密地束缚成质子和中子这样的复合粒子。标准理论技术对强相互作用力不起作用。所以物理学家必须发挥创造力。

在2018年的博士论文中，凯沙瓦尔齐帮助完善了一种理解强相互作用力的替代方法，称为数据驱动法。在这种方法中，物理学家并不试图预测μ子发射和吸收夸克团的频率。他们直接去测量它。

实现这一点的主要方法是通过碰撞电子及其反物质伙伴——正电子。物质和反物质相互湮灭，产生其他粒子，包括夸克团。如果出现了大量夸克，物理学家就知道它们与电子和正电子等粒子有着紧密的量子联系。简而言之，在电子-正电子碰撞中出现的夸克越多，它们对μ子的影响就越强。

使用数据驱动法，物理学家着手计算μ子磁摆动的预期大小。这个于2020年6月发布的预测，与费米实验室在2021年4月公布的精确实验测量结果存在显著差异。这种差异如此之大，几乎超过了物理学家声称发现新粒子所需的严格阈值。

但另一种理论计算得出了不同的结论。

处理晶格

并非所有物理学家都采用数据驱动法来计算μ子的摆动。有些人正在研究一种更纯粹的理论技术来进行预测。

这种方法类似于天气预报。虽然原则上可以通过追踪大气中每一阵微风的确切轮廓来了解天气，但在实践中这项任务是荒谬的。相反，气象学家将大气分成大的方块——一个三维网格——并计算每个方块随时间变化的平均值。

同样，对物理学家来说，追踪每对夸克之间的每一次强相互作用力交互也太难了。因此，物理学家使用一种称为晶格量子色动力学（QCD，强相互作用力理论的简称）的技术，用一个大的网格来模拟夸克的整体行为。

2014年，一个由匈牙利布达佩斯、法国马赛和德国伍珀塔尔的研究人员组成的合作组——BMW小组——启动了一个项目，使用晶格QCD来计算μ子的g因子。

起初，他们的预测比数据驱动法的推断模糊10倍。低能量粒子倾向于扩散，因此捕获它们可能的位置需要一个巨大的晶格。高能量粒子需要一个相对较小的网格，但网格必须极其精细。“那时，很难想象有一天晶格能达到与数据驱动法相同的精度，”参与这项工作的伍珀塔尔大学教授卡尔曼·萨博说。

BMW小组花了十年时间开发巧妙的计算技术——并等待计算能力的提升——才处理好了既足够大又足够精细的网格。但他们确实做到了。2021年，就在费米实验室公布其更新的μ子g-2测量结果的同一天，BMW小组的研究结果发表在《自然》杂志上。

根据BMW小组的晶格计算，费米实验室的μ子摆动的方式完全符合预期。此后，独立的晶格研究小组发表了与之吻合的计算结果。

如今，许多物理学家认为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由已知粒子在已知力的已知定律支配下的发射和再吸收过程来解释。

那么，为什么数据驱动法显示的结果不同呢？

不一致的实验

为了弄清发生了什么，物理学家正在深入探究驱动数据驱动法的电子-正电子碰撞过程。这些碰撞本应是观察夸克行为的直接窗口，但基于这些数据的计算却与最新的实验结果以及BMW小组的预测都不一致。那么，在这些碰撞之后，真实情况究竟如何呢？

在西伯利亚南部的新西伯利亚市，VEPP-2000对撞机自世纪之交以来就断断续续地将电子撞向正电子。它是一个相对温和的对撞机，运行能量比日内瓦附近的欧洲核子研究中心大型强子对撞机低6000倍。

VEPP-2000配备了两个探测器，可以精确计数某些被称为π介子的夸克团从电子-正电子碰撞中出现的频率——物理学家一直利用这些数据来推断强相互作用力对μ子的影响程度。

2010年，物理学家安装了一个全新的探测器。然后他们用它来更精确地测量这个π介子产生率，并于2023年公布了结果。更新之后，他们发现这个比率发生了显著变化。

“这是一个意外。没人预料到会是那样，”英国利物浦大学的物理学家、团队成员费多尔·伊格纳托夫说。

物理学家此前已经看到了一些微弱的迹象，表明π介子比率有些奇怪。他们注意到，意大利和美国的实验对此的测量结果开始出现偏差。但新测量结果与探测器自身过去结果的巨大差异，加上该合作组声称的高精度，使得这种情况难以忽视。

物理学家们仔细审查了这个结果。“没有哪个测量结果受到过如此严格的审视，”凯沙瓦尔齐说。到目前为止，没有发现问题。

随后，BMW小组的成员在2024年使用晶格模拟预测了π介子比率。他们的计算结果与最新的结果基本一致。来自VEPP-2000对撞机另一个探测器的初步数据似乎也与新的比率相符。与此同时，2023年对加州BABAR对撞机另一个实验早期收集数据的分析，却与旧的比率惊人地吻合。

这一切让物理学家们想知道所有这些对撞机实验中到底发生了什么。这些差异要么指向未知粒子干扰夸克的迹象，要么指向被忽略的细节，这些细节造成了夸克行为异常的错觉。无论如何，粒子物理学家在解开新的电子-正电子之谜、弄清楚旧的π介子比率和新的π介子比率哪个正确之前，都不能停下脚步。

“在此之前有长达四十年的测量，它们用不同的方式完成，由不同的人完成，通过不同的实验完成，却都描绘了一幅完全不同的图景，”凯沙瓦尔齐说。“还有很多工作要做。”

英文来源：

Physicists Solve a Big Quantum Mystery. Now, Old Results Don’t Add Up.
Introduction
For 25 years, physicists have been puzzled by an apparent one-part-in-a-million problem. Their expectations of the way that certain particles should wobble in a magnetic field were clashing with what they saw in experiments. The discrepancy was an electrifying hint that they might be seeing evidence of unknown particles.
Then in 2021, that hint seemed to evaporate. 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.
But that, in turn, has created another puzzle: The old calculations seem perfectly valid. So why don’t they match the new calculations? Those older predictions were not purely based on theory; they were also inferred from other experiments. 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?
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. 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.
Weird Wobbles
The particle at the center of the mystery is the muon, a heavier cousin of the electron. A muon behaves a bit like a tiny bar magnet. Spin one in a circle inside a magnetic field and the magnetism will make it wobble, tracing out its own, smaller circles. The sizes of these circles are determined by a number called a “g-factor.”
If the muon sat isolated from other particles, its g-factor would be exactly 2. But quantum theory requires that all other particles influence the g-factor. As the muon wobbles, it releases particles such as photons, which are too short-lived to show up in detectors. These can release other particles, which can release still more particles. The muon quickly reabsorbs all these fleeting particles, and the only trace they leave behind is that the muon wobbles a little bit more. Through these intricate chains of emission and reabsorption, every particle in existence has some small effect on the movement of the muon.
That makes the precise size of the excess wobble, the muon’s “g–2,” invaluable as a window into the quantum world. “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.
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. To some, it hinted that new particles — perhaps even particles that could account for dark matter — were at work.
Physicists set out to check the result with an even more precise measurement. 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.
To prepare for that new experiment, physicists also made a huge effort to understand the theoretical prediction that disagreed with the data. Their challenge was to understand the muon’s chains of emission and reabsorption in extreme detail. In particular, how much do the particles associated with each of nature’s four fundamental forces participate in these chains?
The calculation is straightforward for three of nature’s four forces. Gravity is so weak that physicists can ignore it outright. And both the electromagnetic force and the weak nuclear force can be deduced using a standard technique.
The strong force, however, is not so easy to deal with. That force tightly binds particles known as quarks into composite particles such as protons and neutrons. Standard theoretical techniques don’t work on the strong force. So physicists have to get creative.
In his doctoral thesis in 2018, Keshavarzi helped hone an alternative way of understanding the strong force, called the data-driven method. In this method, physicists don’t try to predict how often muons will emit and absorb groups of quarks. They go out and measure it.
The main way that happens is by colliding electrons and their antimatter partners, positrons. The matter and antimatter annihilate each other, creating other particles, including bundles of quarks. If lots of quarks appear, physicists know they have a tight quantum link to particles such as electrons and positrons. In short, the more quarks appear in electron-positron collisions, the more strongly they will affect the muon.
Using the data-driven method, physicists set out to calculate the expected size of the muon’s magnetic wobble. That prediction, which was released in June 2020, sharply differed from Fermilab’s precise experimental measurement, which came out in April 2021. The discrepancy was so strong that it nearly crossed the stringent threshold required for physicists to claim they had discovered new particles.
But a different theoretical calculation would tell a different story.
Wrangling Lattices
Not all physicists pursued the data-driven method to calculate the muon’s wobble. Some were working on a more purely theoretical technique to make their prediction.
The approach resembles what happens in weather forecasting. 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. Instead, meteorologists divide the atmosphere into big boxes — a 3D grid — and calculate how each box changes on average over time.
Likewise, it’s too hard for physicists to keep track of every strong-force interaction between every pair of quarks. 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.
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.
At first, their predictions were 10 times fuzzier than data-driven inferences. Low-energy particles tend to spread out, so capturing their possible positions requires using a huge lattice. High-energy particles need a comparatively smaller grid, but one with an extremely fine mesh. “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.
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. But wrangle them they did. 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.
According to the BMW group’s lattice calculation, Fermilab’s muons were wobbling exactly as they should. Since then, independent lattice groups have published matching calculations.
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.
So why does the data-driven method indicate otherwise?
Inconsistent Experiments
To figure out what’s going on, physicists are drilling into the electron-positron collisions driving the data-driven method. 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. So what’s really going on in the aftermath of those collisions?
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. It’s a relatively gentle collider, operating at 6,000 times lower energy than CERN’s Large Hadron Collider, near Geneva.
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.
In 2010, physicists installed a completely new detector. They then used it to more precisely measure this pion production rate, which they published in 2023. After the refresh, they found that the rate changed significantly.
“It was a surprise. 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.
Physicists had seen faint hints that something strange was going on with the pion rate. They noticed that measurements of it from experiments in Italy and the United States were starting to drift apart. 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.
Physicists have pored over the result. “No measurement has been scrutinized more,” Keshavarzi said. So far, no problems have been found.
Members of the BMW group then used a lattice simulation to predict the pion rate in 2024. Their calculation largely agrees with the most recent results. And preliminary data from the other detector at the VEPP-2000 collider also seem to match the new rate. 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.
All of this leaves physicists wondering what’s really going on in all these collider experiments. 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. 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.
“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. “There is so much still left to do.”