愛麗絲夢遊仙境2 通往暗物質世界的鑰匙

A sublime coincidence slipped by unnoticed last month. While audiences were thrilling to premieres of Tim Burton’s film Alice Through the Looking Glass, in which Alice steps into a strange world where everything is “contrariwise”, physicists in Hungary announced they had possibly found the key to an unseen portion of the universe.

今年5月，一個令人歎爲觀止的巧合被世人忽視了。就在觀衆們爲蒂姆•波頓(Tim Burton)的電影《愛麗絲夢遊仙境2：鏡中奇遇記》(Alice Through the Looking Glass)興奮不已的同時，匈牙利物理學家宣稱，他們可能發現了通往宇宙不可見部分的鑰匙。在《愛麗絲》一片中，愛麗絲進入了一個奇異的世界，那裏的一切東西都是“反的”。

The results, which are a talking point in laboratories the world over, are remarkable because they might point to the existence of a fifth fundamental force. The current model for nature’s physical laws allows for four forces: the electromagnetic force, the strong nuclear interaction, the weak nuclear interaction, and gravitation. A fifth would, literally, be a force for revolutionary change.

匈牙利物理學家的研究結果正成爲世界各地實驗室的熱門話題。這些成果之所以引人注目，是因爲它們可能指向第五種基本作用力的存在。自然界物理定律的當前模型允許存在四種作用力：電磁力、強相互作用、弱相互作用和萬有引力。毫不誇張地說，第五種基本力會成爲一場革命性變革的推動力。

In truth, a revolution is a little overdue. The reason is so-called dark matter, the hidden stuff that makes up more than 80 per cent of the mass of the universe. Astronomers know it is there because of the gravitational effect it exerts on celestial bodies, but it behaves differently from the matter that makes up the visible world.

說實在的，革命來得晚了一些。原因在於所謂的暗物質，這種隱藏的物質佔了宇宙質量的80%以上。天文學家知道暗物質的存在，因爲它對天體產生了引力效應。不過，暗物質的行爲方式與組成可見世界的物質大不相同。

One way of investigating matter is to engineer collisions between particles, and then detect the particles that flutter out of the debris; this is how the elementary

有一種研究物質的方法，是想辦法讓粒子相互碰撞，然後檢測從殘骸中飛出的粒子——這正是歐洲粒子物理研究中心(CERN)預言希格斯玻色子這種基本粒子的辦法。爲此，匈牙利核物理研究所(Institute for Nuclear Physics)的奧蒂洛•克勞斯瑙霍爾考伊(Attila Krasznahorkay)用質子轟擊鋰同位素，產生了不穩定的鈹原子核。這些鈹原子核接着就像預期的那樣，衰變成了以不同角度飛離彼此的電子-正電子偶。

Higgs boson particle was conjured up at Cern. To this end, Attila Krasznahorkay at Hungary’s Institute for Nuclear Physics, fired protons at lithium isotopes, which produced unstable beryllium nuclei. These beryllium nuclei then decayed, as expected, into electron-positron pairs that flew away from each other at various angles.

然而，不同尋常的是，這些電子-正電子偶似乎喜歡以140度角彼此散開。對此，最簡單的解釋是這一放射性衰變組合中有一種新的中介粒子。計算似乎表明，這種粒子的質量爲17兆電子伏(MeV)，大約是希格斯波色子的七千分之一。

But, anomalously, these electron-positron pairs seemed to have a fondness for shooting away from each other at 140 degrees. The simplest explanation was a new, intermediate particle in the radioactive decay mix. Calculations suggested a mass of 17 megaelectronvolts (MeV), around 7,000 times lighter than the Higgs boson.

儘管科學家曾預言過希格斯波色子的存在，但這種新的活躍粒子沒有被預言過。出於這個原因，該團隊在公諸於衆之前用了三年時間檢驗自己的結果。正如愛麗絲在被紅桃皇后(Red Queen)迷惑時的想法，這完全是一道沒有答案的謎題。

While the Higgs was forecast to exist, this new nimble particle was not. That is why the team spent three years checking their results before going public. It is, as Alice muses while being bamboozled by the Red Queen, exactly like a riddle with no answer.

有一種激動人心的可能性是，這種新粒子正是我們熟悉的物質世界與我們看不見的暗物質世界之間缺失的一環。有一種理論認爲，新粒子是一種“暗光子”，是一種可能與暗物質有關的尚不爲人知的作用力的載體。由加州大學歐文分校(UCI)的馮孝仁(Jonathan Feng)領隊開展的重新分析，並不完全支持這種“暗光子”的提議，但確實支持新粒子代表在超短程作用的第五種力的構想。

One exciting possibility is that the new particle is a missing link between our familiar world of matter and the unseen world of dark matter. One theory is that it is a “dark photon”, the carrier of an as yet unidentified force that might be connected with dark matter. A reanalysis led by Jonathan Feng at the University of California, Irvine, does not quite support the “dark photon” suggestion but does back the idea that it represents a fifth force operating over super-short distances.

這一發現也引起了“暗光”(DarkLight)項目實施者的好奇心。他們正在探索包括暗物質和暗能量在內的“黑暗地帶”，目前正在尋找10到100兆電子伏之間的暗光子。如今，他們重點關注17兆電子伏，試圖揭開相同粒子的面紗。

The finding has also piqued the curiosity of those on the DarkLight project, who are exploring the “dark sector” (both dark matter and dark energy), and are looking for dark photons at between 10 and 100 MeV. They will now target 17 MeV as a priority, in an attempt to unmask the same particle.

如果說這種確認將令人興奮不已，那就說得太輕了。物理學理論也許十分優美，然而它們並不完善，只能解釋可觀測宇宙的一小部分。發現一種橫跨可見物質世界和不可見暗物質世界的新粒子，會開闢物理學的新天地。

To say that confirmation would be thrilling is an understatement. The theories of physics may be elegant but they are incomplete, capable of explaining only a small fraction of the observable universe. To uncover a new particle that straddles our visible world and the invisible world of dark matter would unlock a new realm of physics.

即便如此，對於我們已知存在於宇宙中的事物，可見物質和暗物質加起來也只能解釋不到一半。最大的組成部分據信是暗能量，而暗能量反而可能與暗物質毫無關係。我們瞭解得越多，真相就越發弔詭。那位自傲於能在早餐前相信六件不可能的事的白皇后，必然會開心地拍起手來。