The Higgs at Last?


By Michael Riordan /Guido Tonelli/Sau Lan Wu M. リオーダン /G. トネッリ/S. L. ウー
English 日本語 日本語
Late on the evening of June 14, 2012, groups of graduate students and postdoctoral researchers working on the Large Hadron Collider began peering into a just opened data cache. This huge machine at CERN, the European laboratory for particle physics near Geneva, had been producing tremendous amounts of data in the months since it awoke from its winter-long slumber. But the more than 6,000 physicists who work on the LHC’s two largest experiments were wary of unintentionally adding biases to their analysis. They had agreed to remain completely unaware of the results—performing what are called “blind” analyses—until mid-June, when all would suddenly be revealed in a frenzy of nocturnal activity.  2012年6月14日の夜,大型ハドロン衝突型加速器LHCで研究している大学院生とポスドク研究員たちが封印を解かれたばかりのデータを調べ始めた。ジュネーブ近郊の欧州合同原子核研究機構(CERN)にあるこの巨大な装置は,一冬の眠りから目覚めた春以降,膨大な量のデータを生み出していた。だが,LHCで行われている2つの主要実験に携わる6000人を超える物理学者たちは解析に意図せぬバイアスが加わることを恐れ,データを完全に秘匿したまま,いわゆる「ブラインド解析」をすることに決めたのだった。そして6月中旬,その夜の熱狂のなかで全データが封印を解かれた。
Many of the young scientists worked through that night to untangle the newly freed threads of evidence. Although the LHC is a giant collider feeding multiple experiments, only the two largest ones—ATLAS and CMS—had been tasked with finding the Higgs boson, the long-sought particle that would complete the Standard Model of particle physics, the theoretical description of the subatomic world. Each massive detector records the subatomic debris spewing relentlessly from proton collisions in its midst; a detailed, independent accounting of these remnants can reveal fleeting new phenomena, including perhaps the elusive Higgs boson. Yet the detectors have to sift through the particle tracks and energy deposits while enduring a steady siege of low-energy background particles that threaten to swamp potentially interesting signals. It is like drinking from a fire hose while trying to ferret out a few tiny grains of gold with your teeth.  多くの若手科学者が夜通しで新データの解析に取り組んだ。LHCでは数多くの実験が行われているが,ヒッグス粒子の探索に取り組んでいるのはATLASとCMSという2つの実験グループだけだ。探索努力が長らく続いてきたヒッグス粒子が見つかれば,素粒子の世界を理論的に記述する「標準モデル」が完結することになる。
Fortunately, the scientists knew what they were looking for. After the LHC’s disastrous start—an electrical splice between two magnets warmed and melted just nine days after the LHC came online in 2008, triggering a powerful spark that punctured the surrounding vessel, released tons of helium and ripped scores of costly superconducting magnets from their mounts—the collider had been collecting reams of data during 2011, enough to pick up an early hint of a Higgs signal.  幸い,何を見つけたいのかははっきりしている。LHCは2008年の稼働からわずか9日後に大事故に見舞われたが(2個の磁石の間の電気接続部が過熱・溶融,強いスパークが飛んで周囲の容器に穴が開いて大量の液体ヘリウムが流出し,多数の高価な超電導磁石が台座から引きちぎられた),復旧後は順調に運転を続け,2011年に膨大な量のデータを集めてヒッグス粒子の兆候を示す信号をつかんだ。
After that run ended in October for its scheduled winter shutdown, Fabiola Gianotti, spokesperson for ATLAS, and one of us (Tonelli), then spokes­person for CMS, delivered a special seminar to an overflowing audience in the main CERN auditorium. Both detectors independently found suggestive bumps in the data.  冬季休止のため予定通り2011年10月に運転を終えた後,ATLAS実験の代表を務めるジャノッティ(Fabiola Gianotti)と当時のCMS実験代表者トネッリ(著者の1人)はCERNの主講堂に超満員の聴衆を集め,合同セミナーを開いた。両検出器はそれぞれ独立に,観測データにヒッグス粒子存在を示唆するピークを見いだしていた。
What’s more, these telltale hints of a Higgs boson corroborated one another. Both ATLAS and CMS reported several dozen events above the expected background in which two photons came blazing out with combined energies of 125 billion electron volts, or 125 GeV. (GeV is the stand­ard unit of mass and energy in particle physics, about equal to a proton mass.) If proton collisions had created short-lived Higgs bosons, they could have decayed into these photons. Each experiment also found a few surplus events in which four charged leptons (electrons or muons) carried off similar total energies. These could also have been the result of a Higgs [see box on next page]. Such a concurrence of signals was unprecedented. It suggested that something real was beginning to appear in the data.  また,それらの興味深い兆候は互いを補強しあうものだった。ATLASもCMSも,合計エネルギーが125GeVの2個の光子が飛び出してくる事象を数十例報告した〔GeV(10億電子ボルト)は素粒子物理学で質量とエネルギーを表す標準的な単位で,1GeVは陽子の質量に相当〕。陽子どうしの衝突でヒッグス粒子が生じた場合,すぐに崩壊してこれらの光子になると考えられる。両実験チームはこのほか,同じ約125GeVのエネルギーを4個の荷電レプトン(電子やミュー粒子)が運び去る現象もいくつか発見した。これらもヒッグス粒子による可能性がある。こうした事象が同時に起こっているのが確認されたのは初めてだった。データは何らかの確かな事実が現れつつあることを示している。