Researchers from the Facility for Rare Isotope Beams (FRIB) Laboratory at Michigan State University (MSU) have taken a major step toward a theoretical first-principles description of neutrinoless double-beta decay.
來自美國密西根州立大學(MSU)，稀有同位素束設備(FRIB)實驗室的研究人員們，已經獲得邁向無中微子雙β衰變之理論性、首度原理描述的一大步。
圖1. 研究人員們已經研發出新方法來製作罕見的核變過程模型。在此圖中，粒子上的二進制代碼(1，0)代表，能被執行來更深入瞭解無中微子雙β衰變的電腦模擬。某些原子核衰變是藉由發出電子(e)及中微子(ν)。不過，已經被假設存在有無中微子的雙β衰變。
Observing this yet-unconfirmed rare nuclear process would have important implications for particle physics and cosmology. Theoretical simulations are essential to planning and evaluating proposed experiments. The research team presented their results in an article recently published in Physical Review Letters.
就粒子物理學及宇宙學而言，觀測上述尚未經證實之罕見核變過程，可能具有諸多重要意涵。理論模擬是計劃及評估，被提出之實驗所不可或缺的。在最近發表於《物理評論記事》期刊的一篇論文中，該研究團隊提出了，他們的研究結果。
FRIB theorists Jiangming Yao, research associate and the lead author of the study, Roland Wirth, research associate, and Heiko Hergert, assistant professor, are members of a topical collaboration on fundamental symmetries and neutrinoless double-beta decay. The U.S. Department of Energy Office of Science Office of Nuclear Physics is funding the topical collaboration.
稀有同位素束設備的理論學家，該項研究首要撰文人兼研究助理，Jiangming Yao、研究助理Roland Wirth及助理教授Heiko Hergert，是有關基本對稱性及無中微子雙β衰變的專題共同研究成員。該項專題共同研究是由，美國能源部科學局核子物理研究室所資助。
The theorists joined forces with fellow topical collaboration members from the University of North Carolina-Chapel Hill and external collaborators from the Universidad Autonoma de Madrid, Spain. Their work marks an important milestone toward a theoretical calculation of neutrinoless double-beta decay rates with fully controlled and quantified uncertainties.
此些理論學家與來自美國北卡羅萊納大學教堂山分校的同類專題共同研究成員，及來自西班牙馬德里自治大學的外部共同研究者們進行了合作。他們的研究標記了，邁向無中微子雙β衰變率，具有完全受控制及被量化之不確定性理論估算的重要里程碑。
The authors developed the In-Medium Generator-Coordinate Method (IM-GCM). It is a novel approach for modeling the interactions between nucleons that is capable of describing the complex structure of the candidate nuclei for this decay. The first application of IM-GCM to the computation of the neutrinoless double beta decay rate for the nucleus of calcium-48 sets the stage for explorations of the other candidates with controllable theoretical uncertainty.
此些撰文者們研發了，該種於介質內生成器坐標法(IM-GCM)。這是製作，能描述上述衰變候選原子核內，複雜結構之核子間交互作用模型的一種新方法。IM-GCM計算鈣48-原子核，無中微子雙β衰變率的首度應用，為探索其他具有可控制之理論上不確定性的候選物，建立了平台。
In neutrinoless double-beta decay, two protons simultaneously transform into neutrons without emitting the two neutrinos that appear in more typical weak-interaction processes. If it exists, this is an extremely rare decay that is expected to have a half-life greater than 10 septillion years (a 1 with 25 zeroes), which means that half of a sample of nuclei would have undergone neutrinoless double beta-decay in this extremely long period.
在無中微子雙β衰變中，兩個質子沒有發出，顯然在較典型之弱相互作用過程中的兩個中微子，而同時轉變成中子。倘若存在這種衰變，這是一種被預期，具有半衰期多於1025年的極度罕見衰變。這意味著，在此極長時期中，半數樣本原子核可能會經歷無中微子雙β衰變。
Its observation would demonstrate that neutrinos are their own antiparticles. Every subatomic particle has a corresponding antiparticle, which has the same mass but an equal and opposite charge. Particles and antiparticles can annihilate each other, leaving only energy. Hence, no neutrinos would be observed in neutrinoless double-beta decay.
此觀測可能證實，中微子是其自身的反粒子。所有亞原子粒子皆有，具相同質量、相等卻相反電荷的對應反粒子。粒子與反粒子會互相成對煙滅，僅留下能量。因此，在無中微子雙β衰變中，無法觀測到中微子
A neutrinoless double-beta decay observation would show that a fundamental law — the conservation of lepton number — is violated in nature. This could help explain why the universe contains more matter than antimatter, which consists of the aforementioned antiparticles. The observation would also direct efforts to complete the Standard Model of particle physics.
無中微子雙β衰變觀測可能證實，違反了自然界中，輕子(一種不參與強交互作用、自旋為1/2的基本粒子)數守恆的一項基本定律。這可能有助於解釋，為何宇宙具有比反物質(由上述反粒子組成)更多的物質。此觀測也可能引導諸多嘗試，來使粒子物理標準模型完備。
“The absence of neutrinos in this yet-unconfirmed decay makes it possible to determine the neutrino masses,” said Hergert. “These masses are an important parameter in models of the evolution of the universe. The theoretical decay rate is a key ingredient in the extraction of the neutrino masses from the measured lifetime, or at least provides new upper limits on these quantities.”
Hergert宣稱：「在這種尚未證實的衰變中，無中微子使其可能測定中微子質量。在諸多宇宙演變模型中，此些中微子質量是一項重要參數。在從測得的壽命來推算中微子質量中，該理論上的衰變率是一項關鍵要素。或則，至少為這些量提供了新的上限。」
Theoretical calculations like those presented by the authors will also help determine the size of the detectors needed for large-scale neutrinoless double-beta decay experiments.
類似由此些撰文者們提出的那些理論估算，也將有助於確定，大規模無中微子雙β衰變實驗，需要的探測器大小。
Developing and implementing tests of fundamental symmetries is an important element of FRIB’s mission. FRIB experiments explore the structure of neutrinoless double beta decay candidates and their neighboring isotopes, which affects the rate at which the decay might occur. The theoretical methods developed for this study can now be applied to other nuclei with complex structures that are studied at FRIB.
發展及實現基本對稱性的測試，是稀有同位素束設備(FRIB)任務，一項重要基本組成部分。FRIB的諸多實驗探索了，無中微子雙β衰變候選物，及其鄰近影響可能發生衰變之速率的同位素結構。目前，諸多為此研究開發的理論性方法能被應用於，具有在FRIB被研究之複雜結構的其他原子核。
原文網址：https://frib.msu.edu/news/2020/neutrinoless.html
翻譯：許東榮
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