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科學家首度發現低能量粒子 將揭太陽奧秘
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2007/09/13 13:50記者:張仲琬
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（法新社巴黎十三日電） 法國國家科學研究中心（CNRS）今天表示，他們首度觀測到非常不容易掌握，由太陽散射的微粒子，將有助於驗證太陽這顆恆星如何運作的理論。
CNRS說，這是科學家第一次直接觀測到低能量微中子。這項發現「將幫助物理學家更瞭解太陽中心發生的核反應」。
微中子是太陽進行熱核融合後，所散發大量具有不同能量，不帶電荷的基本粒子。
探測技術目前能夠發現帶有超過五百萬電子伏特以上活躍的高能量微中子，但尚未敏感到能夠探測不到一百萬電子伏特的低能量微中子。
這項科學突破要歸功於座落在距羅馬東方約一百六十公里亞平寧山脈格蘭沙索山，已經運作一年的布瑞希諾探測儀，科學家將它深埋到地面下，保護它避免受到其他可能破壞探測結果的能量來源干擾。
| 破解太陽微中子之謎 - 30年來，有個神秘的問題一直困擾著天文學家：太陽經由核熔合所發射出來的微中子，到了地球竟然只剩下三分之一！其他的微中子跑哪去了？是消失了？是偵測 技術不夠好？還是天文學家根本就推算錯誤？現在，加拿大索德柏立的微中子觀測站解開了這道謎題，發現原來是這些太陽微中子在中途變身，才躲過了這30年來 的偵測！ |
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| 作者╱麥克唐納 ( Arthur B. McDonald ) 、克雷恩 ( Joshua R. Klein沃克 ( David L. Wark ) 譯者╱高涌泉 ) 、 |
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| Solving the Solar Neutrino Problem - The Sudbury Neutrino Observatory has solved a 30-year-old mystery by showing that neutrinos from the sun change species en route to the earth |
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| By Arthur B. McDonald, Joshua R. Klein, David L. Wark |
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Building a detector the size of a 10-story building two kilometers underground is a strange way to study solar phenomena. Yet that has turned out to be the key to unlocking a decades-old puzzle about the physical processes occurring inside the sun. English physicist Arthur Eddington suggested as early as 1920 that nuclear fusion powered the sun, but efforts to confirm critical details of this idea in the 1960s ran into a stumbling block: experiments designed to detect a distinctive by-product of solar nuclear fusion reactions—ghostly particles called neutrinos—observed only a fraction of the expected number of them. It was not until last year, with the results from the underground Sudbury Neutrino Observatory (SNO) in Ontario, that physicists resolved this conundrum and thereby fully confirmed Eddington's proposal.
為了研究太陽，科學家得在地底兩公里建造 一座10層樓高的偵測器，實在是件奇怪的事。但是為了解決數十年來關於太陽內部物理過程的老問題，這卻是關鍵性的一步。早在1920年，英國天文學家艾丁 頓（Arthur Eddington）就已提出太陽的能量來自核熔合的想法。到了1960年代，人們開始想證實這個想法的一些關鍵細節，但是這些努力碰上了障礙：太陽核熔 合反應有個特殊的副產品，一種鬼魅似的粒子，叫做微中子（neutrino）；但是以偵測微中子為目標的實驗，卻僅僅觀察到預期數目的一小部份而已。一直 到去年，位於加拿大安大略地底的「索德柏立微中子觀測站」（Sudbury Neutrino Observatory, SNO）取得了最新的結果，物理學家才終於破解這道謎題，完全證實了艾丁頓的想法。
Like all underground experiments designed to study the sun, SNO's primary goal is to detect neutrinos, which are produced in great numbers in the solar core. But unlike most of the other experiments built over the previous three decades, SNO detects solar neutrinos using heavy water, in which a neutron has been added to each of the water molecules' hydrogen atoms (making deuterium). The additional neutrons allow SNO to observe solar neutrinos in a way never done before, by counting all three types, or “flavors,” of neutrino equally. Using this ability, SNO has demonstrated that the deficit of solar neutrinos seen by earlier experiments resulted not from poor measurements or a misunderstanding of the sun but from a newly discovered property of the neutrinos themselves.
和所有研究太陽的地底實驗一樣，SNO的主要目標就是偵測 微中子；這些微中子是從太陽核心大量發射出來的。但與過去30年來其他多數實驗不一樣的地方是，SNO用重水來偵測微中子。為什麼用重水？因為重水裡頭水 分子的氫原子多了一個中子（也就是氘），這多出來的中子可以讓SNO用前所未見的方式來觀測太陽微中子：它可以計數出所有三種類型的微中子。所以SNO可 以證明，以前實驗所找到的太陽微中子，數目之所以會短缺，原因並不在於測量出了問題，也不是對太陽的理解錯誤，而是微中子具有新發現的性質。
Ironically, the confirmation of our best theory of the sun exposes the first flaw in the Standard Model of particle physics—our best theory of how the most fundamental constituents of matter behave. We now understand the workings of the sun better than we do the workings of the microscopic universe.
說起來諷刺，一方面我們這個最好的太陽理論獲得證實，另一方面卻也因而首次暴露了粒子物理標準模型的缺陷，所以我們現在對於太陽內部作用的理解，要強過對於微觀宇宙運作的理解。
THE FIRST SOLAR NEUTRINO EXPERIMENT, conducted in the mid-1960s by Raymond Davis, Jr., of the University of Pennsylvania and his co-workers, was intended to be a triumphant confirmation of the fusion theory of solar power generation and the start of a new field in which neutrinos could be used to learn more about the sun. Davis's experiment, located in the Homestake gold mine near Lead, S.D., detected neutrinos by a radiochemical technique. The detector contained 615 metric tons of liquid tetrachloroethylene, or dry-cleaning fluid, and neutrinos transformed atoms of chlorine in this fluid into atoms of argon. But rather than seeing one atom of argon created each day, as theory predicted, Davis observed just one every 2.5 days. (In 2002 Davis shared the Nobel Prize with Masatoshi Koshiba of the University of Tokyo for pioneering work in neutrino physics.) Thirty years of experiments following Davis's all found similar results despite using a variety of different techniques. The number of neutrinos arriving from the sun was always significantly less than the predicted total, in some cases as low as one third, in others as high as three fifths, depending on the energies of the neutrinos studied. With no understanding of why the predictions and the measurements were so different, physicists had to put on hold the original goal of studying the solar core by observing neutrinos.
太陽微中子問題 The Problem 美國賓州大學的戴維斯（Raymond Davis, Jr.）與其夥伴，在1960年代中期做了第一個太陽微中子實驗，目的在於證明太陽能量的核熔合理論，同時也開啟一個新領域——利用微中子來了解太陽。戴 維斯的實驗位於美國南達科他州利德鎮附近的荷姆斯達克金礦內，運用放射化學的方法探測微中子。偵測器內含有615噸的液態四氯乙烯，也就是乾洗液，而微中 子會把液體中的氯原子轉換成氬原子。根據理論預測，每天可以看到一個氬原子出現，但是戴維斯卻每兩天半才看到一個氬原子。（戴維斯由於開拓微中子物理研 究，與日本東京大學的小柴昌俊共獲2002年諾貝爾物理獎。）在戴維斯之後30年來的微中子實驗，雖然用了不同的技術，但也都發現類似的結果。捕捉到的太 陽微中子數目一直比預測的總數要少很多，有時可以低到只有預測的1/3，高也只能達到3/5，全視微中子的能量而定。由於不了解為什麼預測值和測量值的差 別會這麼大，物理學家只好暫時不利用觀測微中子來研究太陽核心。
While experimenters continued to run their neutrino experiments, theorists improved the models used to predict the rate of solar neutrino production. Those theoretical models are complex, but they make only a few assumptions—that the sun is powered by nuclear reactions that change the element abundances, that this power creates an outward pressure that is balanced by the inward pull of gravity, and that energy is transported by photons and convection. The solar models continued to predict neutrino fluxes that exceeded the measurements, but other projections they made, such as the spectrum of helioseismologic vibrations seen on the solar surface, agreed very well with observations.
當實驗學家繼續從事他們的微中子實驗之時，理論學家也在改進用來預測太陽微中 子產生速率的模型。這些理論模型都很複雜，但是所用的假設並不多，包括太陽的能量來自核反應，而這些反應會改變元素的豐度（abundance）；還有能 量會產生向外的壓力以平衡向內的重力，以及能量是藉由光子與對流來傳遞的。模型所預測的微中子通量一直比測量值來得大，但是其他的預測，如太陽表面的日震 譜（spectrum of helioseismologic vibration），就和觀測十分吻合。
The mysterious difference between the predictions and the measurements became known as the solar neutrino problem. Although many physicists still believed that inherent difficulties in detecting neutrinos and calculating their production rate in the sun were somehow the cause of the discrepancy, a third alternative became widely favored despite its somewhat revolutionary implications. The Standard Model of particle physics holds that there are three completely distinct, massless flavors of neutrinos: the electron-neutrino, muon-neutrino and tauneutrino. The fusion reactions in the center of the sun can produce only electron-neutrinos, and experiments like Davis's were designed to look exclusively for this one flavor—at solar neutrino energies, only electron-neutrinos can convert chlorine atoms to argon. But if the Standard Model were incomplete, and the neutrino flavors were not distinct but instead mixed in some way, then an electron-neutrino from the sun might transform into one of the other flavors and thus escape detection.
這個預測值與測量值的神秘差距就 稱為「太陽微中子問題」。雖然很多物理學家還是相信問題出在微中子的觀測，以及微中子產生率的計算非常困難，但是現在出現了第三個方案，而且逐漸受到重 視，儘管它有某種革命性的意涵。粒子物理的標準模型認為有三類完全不同的無質量微中子：電子微中子（electron-neutrino）、緲子微中子 （muon-neutrino）、τ微中子（tau-neutrino）。太陽中心的核熔合反應只能產生電子微中子，所以戴維斯及其他類似的實驗只設計來 尋找這種微中子；以太陽微中子所具有的能量而言，只有電子微中子能夠將氯原子轉換成氬原子。但是如果標準模型不夠完備，而且這三類微中子並非完全不同，而 是以某種方式組合在一起，則從太陽發出的電子微中子可能轉變成另兩種微中子，因此就躲過偵測。
The most favored mechanism for a change in neutrino flavor is neutrino oscillation [see illustration on page 44], which requires that the neutrino flavors (electron-, muon- and tau-neutrinos) are made up of mixtures of neutrino states (denoted as 1, 2 and 3) that have different masses. An electron-neutrino could then be a mixture of states 1 and 2, and a muon-neutrino could be a different mixture of the same two states. Theory predicts that as they travel from the sun to the earth, such mixed neutrinos will oscillate between one flavor and another.
在能夠改變微中子類型的機制中，最受歡迎的是 「微中子振盪」（neutrino oscillation，參見52頁圖示），也就是說微中子的三種類型（電子、緲子、τ微中子），是由另外三種不同質量的微中子狀態（稱為狀態1、2、 3）混合組成的。例如電子微中子可能是狀態1與狀態2的組合，而緲子微中子則可能是狀態1與狀態2的另一種組合。理論預測微中子在從太陽到地球的途中，這 種組合出來的微中子會在不同類型間振盪。
Particularly strong evidence of neutrino oscillation was provided by the Super-Kamiokande collaboration in 1998, which found that muon-neutrinos produced in the upper atmosphere by cosmic rays were disappearing with a probability that depended on the distance they traveled. This disappearance is explained extremely well by neutrino oscillations, in this case muon-neutrinos that are probably turning into tau-neutrinos. The former are easily detected by Super-Kamiokande at cosmic-ray energies, but the latter mostly evade detection [see “Detecting Massive Neutrinos,” by Edward Kearns, Takaaki Kajita and Yoji Totsuka; Scientific American, August 1999].
在1998年，日本超級神岡實驗團隊的發現為微中子振盪提供了很強的證據。他們發現，在大氣層高處經 由宇宙射線所產生的緲子微中子會失去蹤影，而消失的機率與它們產生後所行走的距離有關。微中子振盪能很成功地解釋緲子微中子消失的情形。在這個例子中，緲 子微中子可能轉變成了τ微中子。在宇宙射線的能量範圍內，超級神岡偵測器可以很容易地偵測到前者，但是卻不容易看到後者（參見延伸閱讀1）。
A similar process could explain the solar neutrino deficit. In one scenario, the neutrinos would oscillate during their eight-minute journey through the vacuum of space from the sun to the earth. In another model, the oscillation is enhanced during the first two seconds of travel through the sun itself, an effect caused by the different ways in which each neutrino flavor interacts with matter. Each scenario requires its own specific range of neutrino parameters—mass differences and the amount of intrinsic mixing of the flavors. Despite the evidence from Super-Kamiokande and other experiments, however, it remained possible that neutrinos were disappearing by some process other than oscillation. Until 2001 scientists had no direct evidence of solar neutrino oscillation, in which the transformed solar neutrinos themselves were detected.
類似的過程或許可以解釋為什麼太陽微中子的數目會短缺。有一種說法是，微中子在從太陽到地球八分鐘的路途中會不停振盪；而另一種說法是，微中子在太陽內部 的頭兩秒旅途中振盪會加劇，原因在於不同類型的微中子與物質交互作用的方式會不同。每一種說法都有其特定的微中子參數範圍，如質量差以及不同類型之間的混 合程度。雖然有超級神岡與其他實驗的證據，微中子還是可能藉由其他非震盪過程而消失。直到2001年，科學家仍然沒有直接證據可以支持微中子振盪，因為還 沒偵測到改變身份後的太陽微中子。
THE SUDBURY NEUTRINO OBSERVATORY was designed to search for this direct evidence, by detecting neutrinos using several different interactions with its 1,000 tons of heavy water. One of these reactions exclusively counts electron-neutrinos; the others count all flavors without distinguishing among them. If the solar neutrinos arriving at the earth consisted only of electron-neutrinos—and therefore no flavor transformation was occurring—then the count of neutrinos of all flavors would be the same as the count of electron-neutrinos alone. On the other hand, if the count of all flavors was far in excess of the count of the electron-neutrinos, that would prove that neutrinos from the sun were changing flavor.
以重水偵測微中子 The Observatory 索德柏立微中子觀測站的設計目標，就是要 尋找微中子振盪的直接證據。它利用1000噸的重水來偵測微中子，因為重水與微中子有不同形式的交互作用。其中一種交互作用可以用來計數電子微中子，其他 的作用則可以計數所有的微中子，但無法區別它們。如果抵達地球的太陽微中子全部是電子微中子（亦即不同類型之間的變換並沒有發生），則所有的微中子數就等 於電子微中子數。反過來，如果全部的微中子數遠大過電子微中子數，就證明了從太陽來的微中子改變了類型。
The key to SNO's ability to count both electron-neutrinos alone and all flavors is the heavy water's deuterium nuclei, also called deuterons. The neutron in a deuteron produces two separate neutrino reactions: neutrino absorption, in which an electron-neutrino is absorbed by a neutron and an electron is created, and deuteron breakup, in which a deuterium nucleus is broken apart and the neutron liberated. Only electron-neutrinos can undergo neutrino absorption, but neutrinos of any flavor can break up deuterons. A third reaction detected by SNO, the scattering of electrons by neutrinos, can also be used to count neutrinos other than electron-neutrinos but is much less sensitive to muon- and tau-neutrinos than the deuteron breakup reaction [see illustration on preceding page].
SNO之所以能夠個別計數電子微中子 與全部的微中子數，關鍵就在重水的氘原子核（也叫做氘核）。氘核裡面的中子可以有兩種不同的微中子反應：微中子吸收（中子吸收電子微中子同時產生電子）與 氘核分裂（氘核被打破裂釋放出中子）。只有電子微中子能夠參與微中子吸收的反應，但是所有的微中子都可以分裂氘核。SNO還能夠偵測到第三種反應，即電子 與微中子作用而彈射出來。這個反應也可以用來計數電子微中子以外的微中子，但是對於緲子微中子與τ微中子來說，散射反應比起氘核分裂反應更不容易發生（見 53頁圖）。
SNO was not the first experiment to use heavy water. In the 1960s T. J. Jenkins and F. W. Dix of Case Western Reserve University used heavy water in a very early attempt to observe neutrinos from the sun. They used about 2,000 liters (two tons) of heavy water aboveground, but the signs of solar neutrinos were swamped by the effects of cosmic rays. In 1984 Herb Chen of the University of California at Irvine proposed bringing 1,000 tons of heavy water from Canada's CANDU nuclear reactor to the bottom of INCO Ltd.'s Creighton nickel mine in Sudbury—a location that was deep enough to enable a clear measurement of both neutrino absorption and deuteron breakup for solar neutrinos.
SNO並不是頭一個利用重水的實驗。在1960年代，美國凱斯西儲大學的簡金斯（T. J. Jenkins）與迪克斯（F. W. Dix）早就嘗試用重水來觀察太陽微中子。他們用了2000公升地面上的重水，但是太陽微中子的訊號卻被宇宙射線的效應蓋了過去。1984年，加州大學爾 灣分校的陳（Herb Chen）提議將1000噸的重水，從加拿大氘鈾（CANDU）核反應爐搬到INCO金屬原料公司位於索德柏立的鎳礦底下，因為那裡夠深，可以清楚測量太 陽微中子的微中子吸收與氘核分裂兩種反應。
Chen's proposal led to the establishment of the SNO scientific collaboration and ultimately to the creation of the SNO detector. The 1,000 tons of heavy water are held in a 12-meter-diameter transparent acrylic vessel. The heavy water is viewed by more than 9,500 photomultiplier tubes held on an 18-meter-diameter geodesic sphere [see illustration on page 43]. Each tube is capable of detecting a single photon of light. The entire structure is submerged in ultrapure ordinary water filling a cavity carved out of the rock two kilometers below the surface of the earth.
因為陳的提議，SNO科學研究團隊成立，進而建立了SNO偵測器。這裡的1000噸重水裝在直徑 12公尺的透明壓克力容器裡。監視重水的是超過9500根的光電倍增管，它們裝置在直徑18公尺、由三角形組合而成的測地球體（geodesic sphere）上（參見51頁圖示）。每一個光電倍增管都能夠偵測出單一個光子。他們在地表以下兩公里處的石頭中挖了個洞，洞中充滿了超純的普通水，上述 的整個結構就浸在水裡面。
SOLAR NEUTRINOS CAN BE OBSERVED deep underground because of the extreme weakness of their interaction with matter. During the day, neutrinos easily travel down to SNO through two kilometers of rock, and at night they are almost equally unaffected by the thousands of kilometers that they travel up through the earth. Such feeble coupling makes them interesting from the perspective of solar astrophysics. Most of the energy created in the center of the sun takes millions of years to reach the solar surface and leave as sunlight. Neutrinos, in contrast, emerge after two seconds, coming to us directly from the point where solar power is created.
微中子超人變身 SNO's Measurement 我們可以在很深的地底下觀察到太陽微中 子，因為微中子與物質的交互作用極為微弱。在白天，微中子可以輕易穿透兩公里深的石頭，直入SNO。而到了晚上，SNO隨地球自轉到另一側，微中子也同樣 不受影響，輕易穿過幾千公里的地球。從太陽天文物理學的角度來看，如此微弱的交互作用是微中子很有趣的性質。由太陽中心所產生的能量，大半要花上幾百萬年 才能抵達太陽表面，然後以陽光的形式離開。而微中子只要兩秒鐘就可以離開太陽，然後便直接從誕生球。
With neither the whole sun nor the entire earth able to impede the passage of neutrinos, capturing them with a detector weighing just 1,000 tons poses something of a challenge. But although the vast majority of neutrinos that enter SNO pass through it, on very rare occasions, one will—by chance alone—collide with an electron or an atomic nucleus and deposit enough energy to be observed. With enough neutrinos, even the rarity of these interactions can be overcome. Luckily, the sun's neutrino output is enormous—five million high-energy solar neutrinos pass through every square centimeter of the earth every second—which leads to about 10 neutrino events, or interactions, in SNO's 1,000 tons of heavy water every day. The three types of neutrino reaction that occur in SNO all generate energetic electrons, which are detectable through their production of Cerenkov light—a cone of light emitted like a shock wave by the fast-moving particle.
既然整個太陽或整個地球都沒有辦法阻擋微 中子，想要用只有1000噸重的偵測器來捕捉它們，其困難可想而知。不過，雖然絕大多數進入SNO的微中子都會穿越而過，但在極端稀罕的情況下，一個微中 子會（純粹因為機率的緣故）與電子或原子核相撞，產生的能量仍足以讓我們觀察得到。只要微中子的數量夠多，就可以克服反應機率微小的問題。很幸運地，太陽 所產生的微中子數目驚人，地球表面每平方公分每秒會通過500萬個高能量的太陽微中子。在SNO1000噸的重水中，這個數目可以導致每天發生共約10個 微中子事件（交互作用）。發生在SNO內的三類微中子反應全會產生高能量的電子，我們可以偵測到這些電子產生的切侖科夫光（Cherenkov light）——這是高速粒子放出的錐形光束，就像震波一樣。
This small number of neutrino events, however, has to be distinguished from flashes of Cerenkov light caused by other particles. In particular, cosmic-ray muons are created continually in the upper atmosphere, and when they enter the detector they can produce enough Cerenkov light to illuminate every photomultiplier tube. The intervening kilometers of rock between the earth's surface and SNO reduce the deluge of cosmic-ray muons to a mere trickle of just three an hour. And although three muons an hour is a far greater rate than the 10 neutrino interactions a day, these muons are easy to distinguish from neutrino events by the Cerenkov light they produce in the ordinary water outside the detector.
然而，我們必須把這些少量的微中子事件與其他粒子所引發的切侖科夫光區別開來。 尤其是宇宙射線緲子在高層大氣不停產生，如果它們進入偵測器，就可以產生足夠的切侖科夫光來照亮每一根光電倍增管。還好，地球表面與SNO之間的數公里石 頭，會將大量的宇宙射線緲子降低至每小時僅剩3個。雖然就數量而言，每小時3個緲子要遠比每天10個微中子事件多很多，還好這些緲子很容易藉由它們在偵測 器外面與普通水所產生的切侖科夫光，與微中子事件區分開來。
A far more sinister source of false neutrino counts is the intrinsic radioactivity in the detector materials themselves. Everything inside the detector—from the heavy water itself to the acrylic vessel that holds it to the glass and steel of the photomultiplier tubes and support structure—has trace amounts of naturally occurring radioactive elements. Similarly, the air in the mine contains radioactive radon gas. Every time a nucleus in these radioactive elements decays inside the SNO detector, it can release an energetic electron or gamma ray and ultimately produce Cerenkov light that mimics the signal of a neutrino. The water and the other materials used in SNO are purified to remove the bulk of the radioactive contaminants (or were chosen to be naturally pure), but even parts per billion are enough to overwhelm the true neutrino signal with false counts.
但還有一個更為麻煩的假微中子訊號，它來自偵測器內物質本身的放射性。偵測器內所 有的東西，從重水、壓克力容器，到光電倍增管的玻璃與鋼鐵以及支撐的結構，都含有一些天然放射性元素；同樣地，礦坑裡的空氣也含有放射性氡氣。每當這些放 射性元素中的一個原子核在SNO偵測器中衰變，就會釋放出高能電子或γ射線，最後會產生切侖科夫光，很類似微中子的訊號。所以SNO所使用的水與其他物質 都經過特別純化（或選擇天然純化物質），以除去大量的放射性污染。但即使是10億分之幾的放射性元素所產生的假訊號，也足以掩蓋過真實的微中子訊號。
The task before SNO is therefore very complex—it must count neutrino events, determine how many are caused by each of the three reactions, and estimate how many of the apparent neutrinos are caused by something else, such as radioactive contamination. Errors as small as a few percent in any of the steps of analysis would render meaningless SNO's comparison of the electron-neutrino flux to the total neutrino flux. Over the 306 days of running, from November 1999 to May 2001, SNO recorded nearly half a billion events. By the time the data reduction was complete, only 2,928 of these remained as candidate neutrino events.
因此SNO所面對的任務非常複雜，它必須計數微中子事件，同時決定三種微中子反應的個別數目有多少，以及估計究竟有多少所看到的微中子是別種東西（例如 放射性污染）引起的。即使每一步分析的誤差小到只有百分之幾，就足以讓SNO對於電子微中子通量與總微中子通量的比值變得不具意義。從1999年11月到 2001年5月，SNO在超過306天的運轉中，記錄了近五億個事件。把沒用的數據剔除後，仍然可能是真實的微中子事件只剩下2928個。
SNO cannot uniquely determine whether a given candidate neutrino event was the result of a particular reaction. Typically an event like the one shown on page 44 could equally well be the result of deuteron breakup as neutrino absorption. Fortunately, differences between the reactions show up when we examine many events. For example, deuteron breakup, the splitting of a deuterium nucleus in the heavy water, always leads to a gamma ray of the same energy, whereas the electrons produced by neutrino absorption and electron scattering have a broad spectrum of energies. Similarly, electron scattering produces electrons that travel away from the sun, whereas the Cerenkov light from deuteron breakup can point in any direction. Finally, the locations where the reactions occur differ as well—electron scattering, for instance, occurs as easily in the outer layer of light water as in the heavy water; the other reactions do not. With an understanding of those details, SNO researchers can statistically determine how many of the observed events to assign to each reaction.
SNO無法百分之百確定某一個可能事件到底來自哪一個反應。像52頁所展示的典型反應可能來自氘核分裂，也有可能來自微中子吸收。十分幸運地，在我們檢視 了很多事件以後，就會發現不同反應之間仍有區別。以氘核分裂為例，重水中氘原子核的分裂必然產生同樣能量的γ射線；而微中子吸收與電子散射所產生的電子， 就有個很寬的能量分佈。與此類似，電子散射所產生的電子會背離太陽，而來自氘核分裂反應的切侖科夫光卻可以指向任何方向。最後，反應發生的地點也不一樣， 例如電子散射在外層的輕水與內層重水中都很容易發生，但是其他的反應就不是這樣。了解這些細節以後，SNO研究人員便能夠在統計上決定各個觀察到的事件究 竟屬於哪一種反應。
Such an understanding is the result of measurements that were complete nuclear physics experiments in their own right: to determine how to measure energy using Cerenkov light, sources of radioactivity with known energies were inserted inside the detector. To measure how the Cerenkov light travels through and reflects off the various media in the detector (the water, the acrylic, the photomultiplier tubes), a variable wavelength laser light source was used. The effects of radioactive contamination were assessed by similar experiments, including radioassays of the water using new techniques designed specifically for SNO.
要了解這些事情，必須先進行測量，而這些測量本身其實就是完整的原子核實驗：決定如何用切侖科夫光來測量能量，以及在偵測 器中放入已知能量的放射源。為了測量切侖科夫光如何通過、反射偵測器中的各種介質（水、壓克力、光電倍增管），SNO使用了可調波長雷射光源。輻射污染的 效應也是用類似的實驗去評估，包括水的放射性測量，運用的是專為SNO設計的新技術。
For the final SNO data set, after statistical analysis, 576 events were assigned to deuteron breakup, 1,967 events to neutrino absorption and 263 to electron scattering. Radioactivity and other backgrounds caused the remaining 122. From these numbers of events, we must calculate how many actual neutrinos must be passing through SNO, based on the tiny probabilities that any particular neutrino will break up a deuteron, be absorbed or scatter an electron. The upshot of all the calculations is that the observed 1,967 neutrino absorption events represent 1.75 million electron-neutrinos passing through each square centimeter of the SNO detector every second. That is only 35 percent of the neutrino flux predicted by solar models. SNO thus first confirms what other solar neutrino experiments have seen—that the number of electron-neutrinos arriving from the sun is far smaller than solar models predict.
在統計分析之後，最後的SNO數據是：氘核分裂有576 個事件，微中子吸收有1967個事件，電子散射有263個事件，放射性與其他背景事件則有122個。從這些事件的數目，我們可以計算至少有多少微中子必然 通過SNO，計算的依據是單一個微中子打裂氘核、被吸收或使電子散射的微小機率。總而言之，1967個微中子吸收事件代表每平方公分每秒有175萬個電子 微中子通過SNO偵測器，但這只是太陽模型所預測微中子通量的35%而已。如此一來，SNO在第一步就先確認了先前其他微中子實驗的結果，亦即來自太陽的 電子微中子數目小於太陽模型的預測。
The critical question, however, is whether the number of electron-neutrinos arriving from the sun is significantly smaller than the number of neutrinos of all flavors. Indeed, the 576 events assigned to deuteron breakup represent a total neutrino flux of 5.09 million per square centimeter per second—far larger than the 1.75 million electron-neutrinos measured by neutrino absorption. These numbers are determined with high accuracy. The difference between them is more than five times the experimental uncertainty.
然而真正關鍵的問題是：來自太陽的電子微中子數目是否比所有種類的微中子數目小很多？的確，從分配給氘核 分裂的576個事件可知，每平方公分每秒共有509萬個微中子來自太陽，比從微中子吸收推論而得的175萬個電子微中子多很多。這些數據的精確度很高，它 們之間的差距比實驗誤差大五倍。
The excess of neutrinos measured by deuteron breakup means that nearly two thirds of the total 5.09 million neutrinos arriving from the sun are either muon- or tau-neutrinos. The sun's fusion reactions can produce only electron-neutrinos, so some of them must be transformed on their way to the earth. SNO has therefore demonstrated directly that neutrinos do not behave according to the simple scheme of three distinct massless flavors described by the Standard Model. In 20 years of trying, only experiments such as Super-Kamiokande and SNO have shown that the fundamental particles have properties not contained in the Standard Model. The observations of neutrino flavor transformation provide direct experimental evidence that there is yet more to be discovered about the microscopic universe.
我們從氘核分裂測量到多出來的微中子，其意義是：來自太陽的509萬個微中子，其中2/3約是緲子微中子或τ 微中子，而太陽的核熔合反應只能製造出電子微中子，所以有些微中子一定在往地球途中轉換了類型。因此SNO直接證明了一件事，就是微中子並不遵循標準模型 對於微中子的簡單分類，它們並不是三類無質量微中子而已。儘管物理學家尋找了20年，卻只有像超級神岡與SNO這樣的實驗，才發現了基本粒子具有標準模型 所沒有的性質。很多人相信微觀宇宙還有很多有待發現的部份，而觀察到微中子類型之間的變換，便為這樣的信念提供了直接的實驗證據。
But what of the solar neutrino problem itself—does the discovery that electron-neutrinos transform into another flavor completely explain the deficit observed for the past 30 years? It does: the deduced 5.09 million neutrinos agrees remarkably well with the predictions of solar models. We can now claim that we really do understand the way the sun generates its power. Having taken a detour lasting three decades, in which we found that the sun could tell us something new about neutrinos, we can finally return to Davis's original goal and begin to use neutrinos to understand the sun. For example, neutrino studies could determine how much of the sun's energy is produced by direct nuclear fusion of hydrogen atoms and how much is catalyzed by carbon atoms.
但是太陽微 中子問題算是已經解決了嗎？過去30年來所觀察到的微中子短缺現象，只因發現電子微中子可以變換為其他類型的微中子就完全解釋了嗎？答案是肯定的：從實驗 推論出的509萬個微中子與太陽模型的預測非常吻合。我們現在可以宣稱，我們真的了解太陽如何產生它的能量了！過去30年，我們繞了一大圈，終於發現太陽 能夠告訴我們關於微中子的一些新鮮事。所以我們可以回到戴維斯的最初想法，利用微中子來了解太陽。例如微中子研究就可以決定，究竟有多少太陽能量來自氫原 子的直接核熔合，又有多少得靠碳原子催化。
THE IMPLICATIONS OF SNO'S DISCOVERY go even further. If neutrinos change flavor through oscillation, then they cannot be massless. After photons, neutrinos are the second most numerous known particles in the universe, so even a tiny mass could have a significant cosmological significance. Neutrino oscillation experiments such as SNO and Super-Kamiokande measure only mass differences, not masses themselves. Showing that mass differences are not zero, however, proves that at least some of the masses are not zero. Combining the oscillation results for mass differences with upper limits for the electron-neutrino mass from other experiments shows that neutrinos make up something between 0.3 and 21 percent of the critical density for a flat universe. (Other cosmological data strongly indicate that the universe is flat.) This amount is not negligible (it is roughly comparable to the 4 percent density that arises from gas, dust and stars), but it is not quite enough to explain all the matter that seems to be present in the universe. Because neutrinos were the last known particles that could have made up the missing dark matter, some particle or particles not currently known to physics must exist—and with a density far in excess of everything we do know.
微中子餘波蕩漾 The Future SNO的發現還有更深遠的意義。如果微中 子利用振盪改變類型，它們就不可能沒有質量。宇宙中微中子的數目非常大，只次於光子數目；所以如果微中子帶質量，即便是很小的質量，對於宇宙都有重大的影 響。像SNO與超級神岡這類微中子振盪實驗，只能測量不同微中子之間的質量差，卻測不出這些微中子本身到底有多重。不過只要證明質量差不等於零，就等於證 明起碼有一些微中子的質量不為零。從振盪實驗可得知微中子之間的質量差，再從其他實驗可得知電子微中子質量的上限。結合這兩者，可以推論出微中子的質量佔 平直宇宙臨界質量的0.3~21%（我們從其他的宇宙學數據知道，宇宙很有可能是平直的）。這樣的比例是不可忽略的（宇宙中氣體、微塵、星球的質量只佔 4%），但是還不足以解釋似乎存在於宇宙裡的所有物質。因為微中子是最後一種可能構成暗物質的已知粒子，所以一定存在著某一個或某一些尚未發現的粒子，我 們可以確定的是，這些不明物質所佔的質量比起已知物質要多很多。
SNO has also been searching for direct evidence of the effects of matter on neutrino oscillations. As mentioned earlier, travel through the sun can enhance the probability of oscillations. If this occurs, the passage of neutrinos through thousands of kilometers of the earth could lead to a small reversal in the process—the sun might shine more brightly in electron-neutrinos at night than during the day. SNO's data show a small excess of electron-neutrinos arriving at night compared with during the day, but as of now the measurement is not significant enough to decide whether the effect is real.
物質可能對於微中子振盪有所影響，SNO也一直在尋找這種影響的直接證據。先 前提過，微中子穿越太陽可能會提高振盪的機率。若是這樣，微中子在穿過幾千公里的地球這段過程中，則可能會導致微小的反向效應：晚上電子微中子的數目可能 比白天多。SNO的數據顯示，晚上抵達的電子微中子比起白天要多一點，但是目前的測量還不足以確認定這是個真實的現象。
The results reported by SNO so far are just the beginning. For the observations cited here, we detected the neutrons from the critical deuteron breakup events by observing their capture by other deuterium atoms—an inefficient process that produces little light. In May 2001 two tons of highly purified sodium chloride (table salt) were added to the heavy water. Chlorine nuclei capture neutrons with much higher efficiency than deuterium nuclei do, producing events that have more light and are easier to distinguish from background. Thus, SNO will make a separate and more sensitive measurement of the deuteron breakup rate to check the first results. The SNO collaboration has also built an array of ultraclean detectors called proportional counters, which will be deployed throughout the heavy water in mid-2003 to look for the neutrons directly. Making these detectors was a technical challenge of the first order because they must have a spectacularly low level of intrinsic radioactive background—corresponding to about one count per meter of detector per year. Those devices will essentially check SNO's earlier results by an independent experiment.
目前SNO得到的結果 只是個開始而已。SNO偵測氘核分裂的方法如下：觀察分裂後跑出來的中子被其他氘原子捕捉的情況。但這是個沒有效率的過程，而產生的光也很少。在2001 年5月，重水裡面加入了兩噸高度純化的氯化鈉（食鹽）。氯原子核捕捉中子的效率比氘核要高很多，反應產生的光也比較多，更容易與背景區別開來。所以SNO 將會個別、也更敏銳地測量氘核分裂的速率，來檢驗原先的結果。SNO團隊也建造了超淨偵測器陣列，稱為正比計數器，這些偵測器將於2003年年中放置在重 水內各處，用來直接尋找中子。製造這些偵測器是技術上的大考驗，因為它們內部的背景輻射一定要非常低，約相當於每年每一公尺的偵測器只會記錄到一次輻射。 這些偵測器將會以獨立的實驗，檢驗SNO早期的實驗結果。
SNO has unique capabilities, but it is not the only game in town. In December 2002 the first results from a new Japanese-American experiment called KamLAND were reported. The KamLAND detector is at the Super-Kamiokande site and studies electron-antineutrinos produced by all the nuclear reactors nearby in Japan and Korea. If matter-enhanced neutrino oscillations explain the flavor change seen by SNO, theory predicts that these antineutrinos should also change flavor over distances of tens or hundreds of kilometers. Indeed, KamLAND has seen too few electron-antineutrinos, implying that they are oscillating en route from the nuclear reactors to the detector. The KamLAND results imply neutrino mass differences and mixing parameters similar to those seen by SNO.
SNO有很獨特的功能，但是它並非唯一的實驗。2002年12月，一個新的日本–美 國實驗團隊「神岡液態閃爍器反微中子偵測器」（KamLAND）發表了他們的第一個結果。KamLAND偵測器就位於超級神岡實驗所在地，它所研究的是日 本與韓國所有核反應器所產生的反電子微中子。如果因為物質交互作用而強化的微中子振盪，可以解釋SNO所看到的類型轉換，則理論預測反微中子也應該會在數 十或數百公里距離內改變其類型。KamLAND的確看到比較少的反電子微中子，這意味著它們在從核反應器到偵測器途中改變了身份。從KamLAND的結果 推論，所得的微中子質量差以及它們混合程度的參數，與SNO所看到的類似。
Future neutrino experiments might probe one of the biggest mysteries in the cosmos: Why is the universe made of matter rather than antimatter? Russian physicist Andrei Sakharov first pointed out that to get from a big bang of pure energy to the current matter-dominated universe requires the laws of physics to be different for particles and antiparticles. This is called CP (charge-parity) violation, and sensitive measurements of particle decays have verified that the laws of physics violate CP. The problem is that the CP violation seen so far is not enough to explain the amount of matter around us, so phenomena we have not yet observed must be hiding more CP violation. One possible hiding place is neutrino oscillations.
宇宙奧祕的一把鑰匙 未來的微中子實驗可能探索宇宙最大的奧秘 之一：為何宇宙是由物質而非反物質構成？俄國物理學家沙卡洛夫（Andrei Sakharov）最先指出，要從大霹靂時的純能量發展到現在以物質為主的宇宙，對於物質與反物質而言，物理定律必然要有所不同。這個現象叫「荷–宇稱違 逆」（charge-parity violation，簡稱CP違逆）。精密的粒子衰變測量已經證實物理定律出現CP違逆，問題在於至今為止所看到的CP違逆還不足以解釋我們周遭物質的數 量，所以更多的CP違逆一定隱藏在我們尚未觀察到的現象裡。一個可能躲藏的地點就是微中子振盪。
To observe CP-violating neutrino oscillations will be a multistage process. First physicists must see electron-neutrinos appear in intense beams of muon-neutrinos. Second, higher-intensity accelerators must be built to produce beams of neutrinos so intense and pure that their oscillations can be observed in detectors located across continents or on the other side of the earth. Studies of a rare radioactive process called neutrinoless double beta decay will provide further information about neutrino masses and CP violation.
觀察微中子振盪中CP違逆的過程，會有很多個 階段。首先，物理學家必需看到電子微中子出現在高強度的緲子微中子束中。其次，必需興建更高強度的加速器來產生又強又純的微中子束，以便位於地球另一端或 另一個大陸的偵測器能觀察到微中子振盪。有一類稀少的放射過程稱為「無微中子雙重β衰變」，這方面的研究將提供更多關於微中子質量與CP違逆的資訊。
It will probably be more than a decade before these experiments become a reality. A decade may seem a long way off, but the past 30 years, and the sagas of experiments such as SNO, have shown that neutrino physicists are patient and very persistent—one has to be to pry out the secrets of these elusive particles. These secrets are intimately tied up with our next level of understanding of particle physics, astrophysics and cosmology, and thus persist we must.
這些實驗可能還要10年以上的時間才能實現。10年看起來似乎很遙遠，但是過去30年的經驗以及類似SNO實驗的傳奇故事顯示，微中子物理學家是既有耐 心又有毅力——若想探究這些難以捉摸的粒子，他們非如此不可。這些粒子的秘密與我們對於粒子物理、天文物理以及宇宙學等進一步的理解息息相關，所以我們一 定得堅持下去。
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