New work may solidify a critical benchmark
新的探索方向或許可以鞏固一個關鍵的基礎理論點。
Scientists love precision. They can measure the distance from Earth to the moon to within a couple of centimeters and the spins of far-off pulsars to fractions of a millisecond. When peering inside a nearby atom, however, that kind of precision is harder to come by. Consider protons, the positively charged chunks of matter found in every atomic nucleus. Physicists have been trying to pin down their size for more than half a century, but it has proved fiendishly difficult—and conflicting measurements have left researchers scratching their heads. Now an ultraprecise measurement at York University in Toronto may finally have tamed the proton.
科學家追求精確，他們將地球到月球測量的距離精確到了幾釐米的誤差內，並且將遙遠的脈衝星自轉精確到了十分之一毫秒。再將目標視角轉到原子時，然而這種精度卻很難達到。考慮質子的情況，在每個原子核中找到了帶正電荷的物質。物理學家開始嘗試了超過半個世紀去確定它們的大小，但是這被證明了極其困難，並且相悖的測量結果使研究人員絞盡腦汁。現在，在多倫多的約克大學中一個超精密的測量工具或許終於捕捉到了質子。
Protons are, of course, tiny—less than two trillionths of a millimeter across—so teasing out their radius requires exacting techniques. Researchers can fire a beam of electrons at a hydrogen atom, whose nucleus consists of a single proton; the angles at which the electrons bounce off the proton are determined by its size. Another strategy relies on spectroscopy, which measures the intensity of the radiation at various frequencies that an object emits. Scientists can excite a hydrogen atom’s electron so it jumps from one energy state to the next and then carefully track the frequency of the radiation needed to drive this transition.The size of the “gap” between the energy levels depends on the proton’s size.
當然，質子的直徑小於2萬億分之一毫米，所以確定它的半徑需要更加嚴格的技術。研究人員可以向氫原子發射一個電子束，氫原子的原子核由一個質子組成，電子與質子的反彈角度取決於質子的質量。而另一種方式則依賴於光譜學，它在在各種頻率下測量一個物體發出的輻射強度。科學家激發一個氫原子的電子，讓它從一個能態躍遷到下一個能態，並仔細跟蹤驅動放射物發生這種轉變所需要的頻率。能態躍遷時能級的改變強度取決於質子的大小。
Measurements dating back to the 1950s, from work using both methods, set the proton’s radius at an apparent 0.88 femtometer (a femtometer is 10–15 meter). In 2010 researchers led by Randolf Pohl, then at the Max Planck Institute for Quantum Optics in Garching, Germany, tried something differ ent. They used the spectroscopic method but with special “muonic” hydrogen: in stead of an electron, this atom containsa muon, a particle with about 200 times the mass of an electron. Because the muon hugs the proton more tightly than an elec tron would, its energy levels are more sensitive to proton size, promising more accurate results. Plus, the particular transition they studied (in which the muon jumps from its first excited state to its second) leads more directly to the proton radius than other transitions. Pohl and his team were surprised to find a lower value for the radius, pegging it at 0.84 femtometer—well outside the range of potential sizes established by earlier measurements.
這兩種方法的使用時間都可以追溯到1950年，將質子半徑設置爲0.88毫微微米，在2010年由Randolf Pohl領導的研究團隊，他們在在德國加興的馬克斯·普朗克量子光學研究所，嘗試了一些不同的東西。他們使用了光譜法，但是使用了特殊的u介子‘氫’代替了電子，這個原子所包含的u介子的質量是電子質量200倍，而且u介子比電子更加緊貼着質子，它的能級對質子大小更加敏感，需要更多準確的結果。另外，他們研究的特殊躍遷（從激發狀態的轉變導致u介子的躍遷）更爲直接的發現了質子的半徑對躍遷的影響。Pohl和它的團隊驚訝的發現半徑值更低，將其確定在0.84毫微微米，這大大超出了在早期的測量中確立的潛在半徑大小。
Pohl’s result sent the head scratching into high gear. Was something wrong with the earlier experiments? Or is there something peculiar about how protons interact with muons, compared with their behavior around electrons? That was the most intriguing possibility: that some as yet unknown physics, which might require a tweak to the socalled Standard Model, was at play.
Pohl’s的結果使人困惑。在一些早期實現中是否有一些問題？有一些關於質子和u介子結合後後的情況和與它周圍的電子反應相比，有什麼特殊之處？這是一個最有趣的可能性：這是至今物理學未知的問題，這可能需要對現有的物理標準模型進行調整後再進行探索。
“When there’s a discrepancy in the data, it really gets people excited,” says David Newell, a physicist at the National Institute of Standards and Technology in Gaithers burg, Md., whose work has focused on pinning down the value of Planck’s constant, another crucial parameter in atomic physics.
馬里蘭州蓋瑟斯堡國家標準與技術研究所的物理學家David Newell說道：“當試驗的結果與既定猜想的數據有差異時真的讓人感到興奮“，他的工作重點是確定普朗克常數（https://baike.baidu.com/item/%E6%99%AE%E6%9C%97%E5%85%8B%E5%B8%B8%E6%95%B0/812256）的值，而它是原子物理學中的另一個重要參數。
The discrepancy caught the attention of Eric Hessels, head of the York team, who a decade ago was at the workshop where Pohl first presented his results. Hessels took Pohl’s findings as something of a personal challenge and worked to replicate the experiment—right down to the particular energylevel transition—using regular instead of muonic hydrogen. This jump is known as the Lamb shift (for physicist Willis Lamb, who first measured it in the 1940s). A precise measurement of the Lamb shift in regular hydrogen seemed guaranteed to reveal something of interest. If it matched the earlier, larger value, it might point the way to new physics; if it matched the lower value, it would help pin down the size of the proton, solving a decade sold puzzle.
這個矛盾的試驗數據引起了約克大學負責人Eric Hessels的注意，十年前Eric Hessels參加了Pohl首次發表了它的結果的研討會。Hessels說Pohl’s的發現對一些人來說是一個挑戰，並對結果進行了重複試驗論證-一直到特定的能級躍遷-使用了常規的電子而不是u介子氫。這個躍遷叫做蘭姆位移（https://baike.baidu.com/item/%E5%85%B0%E5%A7%86%E7%A7%BB%E4%BD%8D/2685520）（由物理學家Willis Lamb在1940年測量出）。對常規氫中蘭姆位移的精確測量似乎肯定能從裏面發現一些有趣的東西。如果這個結果比之前更早，更大的值匹配，它或許會爲一種新物理指明一條道路。如果這個值與預期的更小，它將有助於確定質子的大小，解決一個物理學家困惑十年的難題。
It took Hessels eight years to find the answer. “It was a more difficult measure ment than I anticipated,” he says, “and more difficult than any other measurement that we’ve taken on in our lab.” He used radio frequency radiation to excite hydrogen atoms, noting the precise frequency at which the radiation drove the electron energy jump associated with the Lamb shift. In the end, his team determined that the proton’s radius is 0.833 femtometer, plus or minus 0.010 femtometer—which agrees with Pohl’s measurement. Science published the results in September.
Hessels花費了8年時間去尋早答案，他說：“實驗並沒有預期的簡單，比我們在實驗室進行的其他任何測量都要困難”。他使用射頻輻射去激發氫原子，注意到輻射驅使的氫原子與蘭姆位移相關的電子能量躍遷的精確頻率，最後，他的團隊確定了質子的半徑是0.833±0.01毫微微米-這個結果與Pohl’s的測量結果一致。《科學》雜誌在9月發佈了這項成果。
In an age of “big science”—think of the Large Hadron Collider and its tunnel’s 27kilometer circumference—physicists may take some comfort in the fact that such important results can still be obtained with tabletop experiments. Hessels’s setup fit in a
single room on York’s campus.
在現在這個“科學迸發”的時代-大型強子對撞機和它長達27千米的圓周隧道。而Hessels’s在約克大學的一個房間裏就完成了試驗設備的安裝和調試，並得到最終的測量數據，如此重要的結果仍然可以通過相對簡易的方式獲得，或許會給物理學家一些寬慰。
It is unclear why previous experiments produced a larger value for the proton’s radius. Errors in experimental design are one possibility, researchers suggest. Another possibility—seemingly less likely, in light of Hessels’s measurement—is that unknown physics still skews the results.
目前還不清楚爲什麼以前的實驗會產生更大的質子半徑值。研究人員認爲實驗設計可能是一種錯誤。而另一種看起來更小的可能：根據Hessels’s的測量是一個未知的物理現象導致了結果的偏差。
The York finding’s precision and closeness to the 2010 figure suggest a consensus forming around the lower value for the proton radius.“There are now a number of measurements, and they’re starting to line up with the muonic hydrogen measurement,” Hessels says. “So the controversy is starting to diminish.”
約克大學試驗結果的精度和接近於2010年的結果數值和質子半徑低值數據一致。Hessels 說：“現在有一些新的測量方法，然後他們開始與u介子-氫的測量方法一致，所以現在爭議開始變少了”
Diminish but not disappear: As good as Hessels’s result is—it is one of the best spectroscopic measurements achieved with normal hydrogen—Pohl’s measurement is more precise because of the greater sensitivity of the muonic hydrogen method. This finding means there is room for even more sensitive experiments, researchers say.
爭議變得減少但並不意味着消失：實際上Hessels’s 的研究結果是：它是以普通氫完成的最適合光譜學的測量之一，Pohl的測量因爲使用了更敏感的u介子氫方法使得結果更加精確 。研究人員說：這個發現意味着這個測量結果還可以進行更加敏感測試的空間。
Meanwhile there are other secrets the proton has yet to give up. For starters, we know protons and neutrons both consist of three quarks bound by the strong nuclear force—but the exact nature of that binding is poorly under stood, says Nilanga Liyanage, a physicist at the University of Virginia.
Virginia大學的物理學家Nilanga Liyanage說：與此同時科學家仍然在探索着質子的其他奧祕。首先，我們知道質子和中子都是由被強核力束縛的3個夸克組成，但是人們對這種約束原因知之甚少。
“Protons are the stuff we’re made of,” says Liyanage, who has tackled the proton radius puzzle through electron scattering experiments at the Jefferson Lab in Virginia. And “99.9 percent of our mass-of ourselves, of everything in the universe—comes from protons and neutrons.” The proton radius is a critical benchmark quantity, he adds: “It’s a very important particle, and we need to understand it.” —Dan Falk
Liyanage說：質子是組成我們的物質，她在Virginia大學的Jefferson實驗室使用電子散射試驗解決了質子半徑謎題。我們自身質量和在宇宙中所有物質的99.9%都來自於質子和中子。質子的半徑是一個關鍵的基準量（單位），它是一個很重要的粒子，我們需要對它研究透徹。—Dan Falk
