Newsletter No. 422

4 No. 422, 4.9.2013 「空 即是色，我們的世界充滿幽靈（粒子）。以今晚演講廳為 例，每個座位的面積，平均每秒至少有一萬億粒幽靈粒 子穿過，它們會穿過整個地球，視地球如無物。」物理系 朱明中 教 授在8月24日舉行題為「幽靈粒子與宇宙演化」的講座中以此作開 場白，帶領三百多名師生校友及公眾人士走進神秘莫測的宇宙，為 這場中大五十周年校慶博文公開講座拉開序幕。 身兼大亞灣實驗香港合作組首席研究員的朱教授，多年來參與中微子（幽靈 粒子的一種）研究，為的是探尋萬物起源︰「根據大爆炸理論，一百三十八億 年前，宇宙只有能量，隨着宇宙膨脹，能量慢慢轉化為我們今天的物質。這 裏有一個很大的問題。按對稱原則，物質和反物質的產生應該是一一對應 的。但今天見到的宇宙絕大部分都是物質，極少數是反物質，反物質不知何 故不見了。這是宇宙學和粒子物理學的大懸案。」科學家推斷，宇宙早期的 物質比反物質多了一億分之一，憑着這些微的不對稱，卻造成了恆星、星系， 甚至有了生命和現在的我們。究竟因何有一億分之一的不對稱？這是至今未 解決的。 由於質量未明的中微子大量存在於宇宙中，而已知道它們不甚遵守對稱， 例如 楊振寧 和 李政道 兩位教授於1956年提出的宇稱不守恆現象（即左右 不對稱），因此它們可能是解開宇宙物質起源謎團的鑰匙。自然界有三類 中微子，每粒中微子穿越空間時會不斷在這三類之間轉變（稱為「振盪」 oscillation）。中微子振盪的頻率與中微子「質量差別」（mass splitting） 直接相關，而振盪幅度則是「混合角」（mixing angle）。透過觀察及測量 「質量差別」與「混合角」，有助窺探中微子的神秘面目。去年，大亞灣合作 組發現了一種新類型的中微子「振盪」，並獲權威國際學術期刊《科學》選 為2012年度十大科學突破之一。今年8月22日，合作組又公布最新成果，揭 示中微子的「振盪」現象與其能量變化的關係，測量出其中一個「質量差 別」，並為去年公布的「混合角」提供更準確數值，為解開宇宙初期組成之 謎邁進一步。  朱教授說，現時中微子成為熱門的研究課題，但下一步還需各國協作︰「在 大亞灣只能量度反中微子的振盪。美國、歐洲和日本現正計劃下一代的實 驗，利用加速器製造一些高能量的中微子，同時製造一些高能量的反中微 子，觀測它們在穿過長距離後的振盪有沒有不同。從而量度出一個與『物質 反物質不對稱』直接相關的參數。透過運算，看看結果是否能解釋那一億分 之一的不對稱。」 講座尾聲，有觀眾問：了解宇宙演化對我們日常生活到底有何作用？朱教授 坦言自己也反覆思考這個問題：「它有用嗎？老實說，沒有實際作用。但我深 信，很多人包括在座中許多發燒友仍有童真，想知道我們的出處。」要勘破宇 宙起源的奧秘，除了高端科研外，可能更需要一份赤子心。 追踪幽靈粒子 Tracing the Ghost Particles Back to Our Origin ‘T here is substance in the void. Countless ghost particles impinge on our world. As I speak, over a trillion ghost particles pass through each single seat in this lecture hall every second. They pass through us as if we don’t exist,’ thus began Prof. Chu Ming-chung of the Physics Department in the CUHK 50th Anniversary Public Lecture entitled ‘Ghost Particles and the Evolution of the Universe’ on 24 August. Professor Chu leads the Hong Kong team of the international Daya Bay Collaboration in studying neutrinos (one kind of ghost particles), in the hope of finding answers to some of the most fundamental questions related to the origin of the universe. ‘According to the Big Bang Theory, the universe was all energy 13.8 billions years ago. With the expansion of the universe, energy slowly became what is known as matter today. But a big question remains. The Big Bang should have produced the same amount of matter and antimatter. But what we can observe today is mostly matter and very little antimatter. Where have all the antimatter gone? It poses a cosmological and particle physics puzzle.’ Scientists infer that in the early phases of the universe, matter exceeded antimatter by a 100 millionth part. This minuscule asymmetry has produced stars, the galaxies, life and us. What is not known is why there is this asymmetry of a 100 millionth part. Due to the existence of large numbers of neutrinos and their renowned taste for breaking symmetries— such as the famous violation of left-right symmetry proposed by Prof. Yang Chen-ning and Prof. Lee Tsung-dao in 1956, understanding neutrinos may hold the key to the mystery of the origin of matter-antimatter asymmetry. The three types of neutrinos mix and change into each other when they travel through space (oscillations). The frequency of neutrino oscillation is directly related to the neutrino ‘mass splitting’, and ‘mixing angle’ represents the amplitude of the oscillation. Observing and measuring the ‘mass splitting’ and ‘mixing angle’ would help to elucidate the nature of neutrinos. The research group at Daya Bay discovered a new mode of neutrino oscillation last year, which was selected as one of the ‘Top Ten Scientific Breakthroughs’ of the year by Science . Building on that success, the group announced on 22 August their latest findings on the relationship between neutrino oscillations and energy changes, the measurement of a ‘mass splitting’. They also provided a more precise and refined value for the ‘mixing angles’ derived in the previous year. According to Professor Chu, the neutrino is being studied in many countries but taking the next step requires international collaborative effort. ‘The Daya Bay experiment can only measure the oscillations of anti-neutrinos. Researchers in the US, Europe and Japan are planning the next phase of experimentation: to produce high energy anti-neutrinos as well as high energy neutrinos with accelerators. Observing how these neutrinos and anti-neutrinos vary in their oscillations over long distances give us data that might explain the 100 millionth part asymmetry.’ Near the end of the lecture, a member of the audience asked what the use of getting to the bottom of the universe was. Professor Chu confessed that he had been thinking of the same question for a long time, ‘Honestly, I don’t see any practical uses. But I believe that, like me, many of you who came to this lecture today have this impish itch to know where we came from.’ In fathoming the origin of the universe, a child is perhaps as eligible as a scientist.