The Coming Revolutions in Particle Physics

ヒッグスだけじゃない LHCが変える素粒子物理学

By Chris Quigg C. クイッグ
English 日本語 日本語
When physicists are forced to give a single-word answer to the question of why we are building the Large Hadron Collider (LHC), we usually reply “Higgs.” The Higgs particle―the last remaining undiscovered piece of our current theory of matter―is the marquee attraction. But the full story is much more interesting. The new collider provides the greatest leap in capability of any instrument in the history of particle physics. We do not know what it will find, but the discoveries we make and the new puzzles we encounter are certain to change the face of particle physics and to echo through neighboring sciences. 大型ハドロン衝突型加速器LHC(Large Hadron Collider)の建設目的を一語で答えよ──あえてそう問われたなら,物理学者たちはふつう「ヒッグス」と答える。ヒッグス粒子は現在の理論が存在を予測する粒子のうち未発見のまま残っている唯一の粒子であり,LHCの“呼び物”だ。しかし,LHCが探るストーリーの全貌はさらに興味深い。
In this new world, we expect to learn what distinguishes two of the forces of nature―electromagnetism and the weak interactions―with broad implications for our conception of the everyday world. We will gain a new understanding of simple and profound questions: Why are there atoms? Why chemistry? What makes stable structures possible? 自然界の力のうち2つ,「電磁気力」と「弱い相互作用」を異なるものにしているのは何かがわかり,日常世界に関する概念に大きな影響が及ぶだろう。基本的にして深遠ないくつかの疑問について,新たな理解が得られるだろう。なぜ原子が存在するのか? なぜ化学的現象が生じるのか? 安定な構造が存在しうるのはなぜなのか──といった問いだ。
The search for the Higgs particle is a pivotal step, but only the first step. Beyond it lie phenomena that may clarify why gravity is so much weaker than the other forces of nature and that could reveal what the unknown dark matter that fills the universe is. Even deeper lies the prospect of insights into the different forms of matter, the unity of outwardly distinct particle categories and the nature of spacetime. The questions in play all seem linked to one another and to the knot of problems that motivated the prediction of the Higgs particle to begin with. The LHC will help us refine these questions and will set us on the road to answering them. ヒッグス粒子の探索は重要なステップだが,最初の一歩にすぎない。重力が自然界の他の力に比べてこれほど弱いのはなぜか,宇宙を満たしている暗黒物質(ダークマター)の正体は何かといった事柄を明らかにする現象は,その先にある。さらに先を探れば,別形態の物質や,大きく異なるように見える粒子分類の統合,時空の本質について,手がかりを得られる期待もある。これらの疑問はどれも互いに関連しているようだし,また,そもそもヒッグス粒子の存在を予言するきっかけになった諸問題にもつながっている。LHCはこれらの問いをより洗練した形に整理する助けとなり,謎解きに向けた道に導いてくれるだろう。
The Matter at Hand
What physicists call the “Standard Model” of particle physics, to indicate that it is still a work in progress, can explain much about the known world. The main elements of the Standard Model fell into place during the heady days of the 1970s and 1980s, when waves of landmark experimental discoveries engaged emerging theoretical ideas in productive conversation. Many particle physicists look on the past 15 years as an era of consolidation in contrast to the ferment of earlier decades. Yet even as the Standard Model has gained ever more experimental support, a growing list of phenomena lies outside its purview, and new theoretical ideas have expanded our conception of what a richer and more comprehensive worldview might look like. Taken together, the continuing progress in experiment and theory point to a very lively decade ahead. Perhaps we will look back and see that revolution had been brewing all along.
Our current conception of matter comprises two main particle categories, quarks and leptons, together with three of the four known fundamental forces, electromagnetism and the strong and weak interactions. Gravity is, for the moment, left to the side. Quarks, which make up protons and neutrons, generate and feel all three forces. Leptons, the best known of which is the electron, are immune to the strong force. What distinguishes these two categories is a property akin to electric charge, called color. (This name is metaphorical; it has nothing to do with ordinary colors.) Quarks have color, and leptons do not. 物質に関する現在の理解は,「クォーク」と「レプトン」という大きく2つのカテゴリーに分けられる粒子と,自然界の4つの基本的力のうち3つ,つまり「電磁気力」と「強い相互作用(強い力)」「弱い相互作用(弱い力)」からなっている。重力はとりあえず除外しておく。
The guiding principle of the Standard Model is that its equations are symmetrical. Just as a sphere looks the same whatever your viewing angle is, the equations remain unchanged even when you change the perspective from which they are defined. Moreover, they remain unchanged even when the perspective shifts by different amounts at different points in space and time. 標準モデルは「物理の方程式は対称的である」ということを指導原理としている。球はどの方向から見ても同じく球に見えるように,物理の方程式は,それを定義した視点と別の視点に移っても,不変のままだ。さらに,視点が時空の各点で異なる分量だけ変わったとしても,方程式は不変だ。
Ensuring the symmetry of a geometric object places very tight constraints on its shape. A sphere with a bump no longer looks the same from every angle. Likewise, the symmetry of the equations places very tight constraints on them. These symmetries beget forces that are carried by special particles called bosons [see “Gauge Theories of the Forces between Elementary Particles,” by Gerard ’t Hooft; Scientific American, June 1980, and “Elementary Particles and Forces,” by Chris Quigg; Scientific American, April 1985]. 物体が幾何学的に対称であるには,その形状に非常に強い制約が生じる。球にこぶが1つ加わっただけで,それはもはや「どの方向から見ても同じ」ではなくなる。同様に,方程式の対称性も非常にきつい制約となる。この制約のため,ボース粒子(ボソン)と総称されるタイプの粒子によって媒介される力がいくつか生じるのだ(G. トフーフト「ゲージ理論」サイエンス1980年8月号,C. クイッグ「素粒子と自然界の4つの力」サイエンス1985年6月号)。