A top view of the equipment used in the g-2 experiment at Fermilab. The experiment uses a muon beamline, electronic racks and superconducting magnetic storage ring cooled to minus 450 degrees Fahrenheit (minus 267 degrees Celsius) to study the wobble of muons.(Image credit: Reidar Hahn/Fermilab)

The results from one of the most hotly-anticipated experiments in particle physics are in, and they could be about to fulfill every researcher's wildest dreams: They maybe, perhaps, could break physics as we know it.

Evidence taken from the Fermi National Accelerator Laboratory near Chicago appears to point to a miniscule subatomic particle known as the muon wobbling far more than theory predicts it should. The best explanation, according to physicists, is that the muon is being pushed about by types of matter and energy completely unknown to physics.

If the results are true, the discovery represents a breakthrough in particle physics of a kind that hasn't been seen for 50 years, when the dominant theory to explain subatomic particles was first developed. The teeny-tiny wobble of the muon — created by the interaction of its intrinsic magnetic field, or magnetic moment, with an external magnetic field — could shake the very foundations of science.?

"Today is an extraordinary day, long awaited not only by us but by the whole international physics community," Graziano Venanzoni, co-spokesperson of the Muon g-2 experiment and physicist at the Italian National Institute for Nuclear Physics, said in a statement.

Sometimes known as "fat electrons," muons are similar to their more widely-known cousins but are 200 times heavier and radioactively unstable — decaying in mere millionths of a second into electrons and tiny, ghostly, chargeless particles known as neutrinos. Muons also have a property called spin which, when combined with their charge, makes them behave as if they were tiny magnets, causing them to wobble like little gyroscopes when plopped inside a magnetic field.

But today's results, which came from an experiment in which physicists sent muons whizzing around a superconducting magnetic ring, seem to show that the muon is wobbling far more than it should be. The only explanation, the study scientists said, is the existence of particles not yet accounted for by the set of equations that explain all subatomic particles, called the Standard Model — which has remained unchanged since the mid-1970s. Those exotic particles and the associated energies, the idea goes, would be nudging and tugging at the muons inside the ring.

The Fermilab researchers are relatively confident that what they saw (the extra wobbling) was a real phenomenon and not some statistical fluke. They put a number on that confidence of "4.2 sigma," which is incredibly close to the 5 sigma threshold at which particle physicists ?declare a major discovery. (A 5-sigma result would suggest there's a 1 in 3.5 million chance that it happened due to chance.)

"This quantity we measure reflects the interactions of the muon with everything else in the universe. But when the theorists calculate the same quantity, using all of the known forces and particles in the Standard Model, we don't get the same answer," Renee Fatemi, a physicist at the University of Kentucky and the simulations manager for the Muon g-2 experiment, said in a statement. "This is strong evidence that the muon is sensitive to something that is not in our best theory."

However, a rival calculation made by a separate group and published Wednesday (April 7) in the journal Nature could rob the wobble of its significance. According to this team's calculations, which give a much larger value to the most uncertain term in the equation that predicts the muon's rocking motion, the experimental results are totally in line with predictions. Twenty years of particle chasing could have all been for nothing.?

"If our calculations are correct and the new measurements do not change the story, it appears that we don't need any new physics to explain the muon's magnetic moment — it follows the rules of the Standard Model," Zoltan Fodor, a professor of physics at Penn State and a leader of the research team that published the Nature paper, said in a statement.

But Fodor added that, given that his group's prediction relied upon a totally different calculation with very different assumptions, their results were far from being a done deal. "Our finding means that there is a tension between the previous theoretical results and our new ones. This discrepancy should be understood," he said. "In addition, the new experimental results might be close to old ones or closer to the previous theoretical calculations. We have many years of excitement ahead of us."

In essence, physicists won't be able to conclusively say if brand-new particles are tugging on their muons until they can agree exactly how the 17 existing Standard Model particles interact with muons too. Until one theory wins out, physics is left teetering in the balance.

一個微小的、晃動的μ子撼動了粒子物理學的核心

文章作者是本杰明?特納，出版于2021年4月8日

但這不是故事的結局。

這是費米實驗室中g-2實驗用到的設備俯視圖。該實驗使用μ子的粒子束、電子機架和零下450華氏度(零下267攝氏度）的超導磁存儲環(huán)來研究μ子的晃動。

(圖源:雷達爾·哈恩/費米實驗室)

粒子物理學中最值得期待的一項實驗結果已經公布，讓每一位研究人員得以實現他們最偉大的夢想。實驗結果很有可能會打破我們已知的物理學。

從芝加哥附近的費米國家加速器實驗室中得到的證據似乎指出，一種被稱為μ子的微小亞原子粒子的擺動幅度比理論預測的要大得多。據物理學家所說，對此最好的解釋是μ子的擺動是由物理學中全然未知的各種物質和能量所導致的。

如果結果屬實，那么這項發(fā)現是自解釋亞原子粒子的主導理論首次創(chuàng)立以來的50年里，在粒子物理學中一項前所未有的突破。μ子微小的波動是由其內在磁場中的相互作用所導致的，或者是由外在磁場的磁矩所導致的，這可能動搖科學的基礎。

μ子 g-2實驗的聯合發(fā)言人、意大利國家核物理研究所的物理學家Graziano Venanzoni在一份聲明中說:“今天是意義非凡的一天，為了這一天的到來，不僅我們等待已久，整個國際物理界都等待已久?！?/p>

μ子有時候被稱為“胖電子”，雖然與更廣為人知的同類介子相似，其重量卻是它們的兩百倍，而且放射性不穩(wěn)定——在百萬分之一秒內就會衰變成電子和微小的、幽靈般的無電荷粒子，即中微子。μ子同樣也有一個叫做自旋的特性，當它們與電荷相結合時，會使它們表現得就像微小的磁鐵，當它們在磁場中被擊中時，會像小型陀螺儀一樣旋轉。

但是如今，物理學家讓μ子圍繞超導磁環(huán)旋轉的這一項實驗結果似乎表明：μ子的晃動程度遠遠超過它應有的程度。實驗研究的科學家表示，對這個現象的唯一解釋是：存在一些未被一組稱之為“標準模型”的方程式所解釋的粒子。自20世紀70年代中期以來，標準模型一直沒有變化過。這個解釋說明，這些奇異的粒子和與其相關的能量，會對環(huán)內的μ子產生推拉作用。

費米實驗室的研究人員相當自信，認為他們看到的μ子額外的晃動是一個真實的現象而不是僥幸得到的統(tǒng)計結果。他們在“4.2sigma”的可信度上增加了一個數字，非常接近粒子物理學家宣布的重大發(fā)現“5sigma”。(一個“5sigma”的結果將表明這發(fā)生的概率是三百五十萬分之一）

肯塔基大學的物理學家、μ子g-2實驗的模擬經理Renee Fatemi在一份聲明中說:“我們測量的這個量反映了μ子與宇宙中其它一切物質的相互作用。但是當理論家們用標準模型中所有已知的力和粒子來計算相同的量時，我們得到的答案并不一樣。這有力地證明了μ子對我們最佳理論中不存在的東西很敏感?！?/p>

然而，另一個競爭團隊的測量結果于周三(4月7日）發(fā)布在《自然》期刊上，這可能會改變μ子晃動的意義。該團隊的計算給預測μ子晃動運動公式中的最不確定項賦予了一個大得多的值，實驗結果和預測完全一致。這說明過去十二年的粒子探索之路上的努力可能全部付諸東流。

賓夕法尼亞州立大學物理學教授、在《自然》雜志上發(fā)表論文的研究團隊負責人Zoltan Fodor在一份聲明中說:“如果我們的計算是正確的而新的測量數據不會改變什么，那么我們不需要任何新的物理學來解釋μ子的磁矩是否遵循了標準模型的規(guī)則?！?/p>

但是Fodor還表示，如果他的團隊的預測是依賴于不同假設下的不同計算，那么他們的實驗結果是遠遠不能確定的。他說:“這個發(fā)現意味著先前的理論結果和我們現在得出的結果之間有矛盾，這個矛盾是可以理解的。另外，新的實驗結果可能接近于原有的實驗結果或者更接近于原有的理論計算。激動人心的時刻還在后頭等著我們呢，研究之路任重而道遠?！?/p>

實際上，只有物理學家們就17種現有的標準模型粒子與μ子的相互作用達成確切一致時，才能得出結論，證明有全新的粒子在影響著μ子的運動。除非有一種理論勝出，否則物理學會搖擺不定而失去平衡。

BY:Ben Turner

FY:Katrina

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