素粒子「Ｗボソン」質量　標準理論との顕著な「ずれ」最新研究

【ＡＦＰ＝時事】素粒子の一種「Ｗボソン」が、理論値を著しく上回る質量を持つとする研究論文が７日、発表された。約１０年に及ぶ精密な測定に基づくもので、宇宙の仕組みに関する理解の根幹を揺るがす研究結果だ。（写真は米イリノイ州バタビア郊外にある、周長６．４キロのテバトロン粒子加速器。資料写真）
　宇宙を理解する際の基礎となっているのは、素粒子物理学の「標準理論」だ。標準理論は、宇宙の最も基本的な構成要素とそれらをどのような力が支配しているかを最もよく説明する科学的理論とされる。
　自然界に存在する基本的な四つの力（相互作用）の一つ、「弱い力」を媒介するボース粒子（ボソン）のうち電荷を持つのがＷボソンで、標準理論の柱の一つとなっている。
　だが、米科学誌サイエンスに掲載された最新の論文によると、これまでで最も高精度なＷボソンの測定値は、標準理論の規則と真っ向から相反する。
　研究を主導した米デューク大学の物理学者、アシュトシュ・コトワル氏によると、今回の結果は科学者４００人以上が１０年あまりを費やし、「約４５０兆回の衝突のデータセット」を記録し、詳細に分析して得られた。粒子衝突実験には、米フェルミ国立加速器研究所のテバトロン加速器が使用された。
　テバトロン加速器を使った「ＣＤＦ衝突実験」の研究チームによると、Ｗボソンの質量を０．０１％の精度で測定できた。これは従来の測定実験の２倍の精度だという。
　そして、Ｗボソンの質量の測定値と標準理論の予測とでは、実験誤差を表す標準偏差（シグマ）の７倍のずれがあることを明らかにした。
　欧州合同原子核研究機構（ＣＥＲＮ）の世界最大の粒子加速器「大型ハドロン衝突型加速器（ＬＨＣ）」で研究を行っている英ケンブリッジ大学の粒子物理学者、ハリー・クリフ氏は、「まぐれでシグマの５倍の結果が得られる確率は３５０万分の１だ」と説明する。
　「この結果が本当だとすると、何らかのシステム的な偏りや計算方法の誤解でもないのであれば、これは大変なことだ。これまでに未発見の新たな宇宙の基本構成要素が存在することを意味するからだ」とクリフ氏は述べている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/04/18-08:40）

After a decade of meticulous measurements, scientists announced Thursday that a fundamental particle -- the W boson -- has a significantly greater mass than theorised, shaking the foundations of our understanding of how the universe works.
Those foundations are grounded by the Standard Model of particle physics, which is the best theory scientists have to describe the most basic building blocks of the universe, and what forces govern them.
The W boson governs what is called the weak force, one of the four fundamental forces of nature, and therefore a pillar of the Standard Model.
However new research published in the Science journal said that the most precise measurement ever made of the W Boson directly contradicts the model's prediction.
Ashutosh Kotwal, a physicist at Duke University who led the study, told AFP that the result had taken more than 400 scientists over 10 years to scrutinise four million W boson candidates out of a dataset of around 450 trillion collisions.
These collisions -- made by smashing particles together at mind-bending speeds to study them -- were done by the Tevatron collider in the US state of Illinois.
It was the world's highest-energy particle accelerator until 2009, when it was supplanted by the Large Hadron Collider near Geneva, which famously observed the Higgs boson a few years later.
The Tevatron stopped running in 2011, but the scientists at the Collider Detector at Fermilab (CDF) have been crunching numbers ever since.
- 'Fissures' in the model -
Harry Cliff, a particle physicist at Cambridge University who works at the Large Hadron Collider, said the Standard Model is probably the most successful scientific theory that has ever been written down.
It can make fantastically precise predictions, he said. But if those predictions are proved wrong, the model cannot merely be tweaked.
It's like a house of cards, you pull on one bit of it too much, the whole thing comes crashing down, Cliff told AFP.
The standard model is not without its problems.
For example, it doesn't account for dark matter, which along with dark energy is thought to make up 95 percent of the universe. It also says that the universe should not have existed in the first place, because the Big Bang ought to have annihilated itself.
On top of that, a few fissures have recently been exposed in the model, physicists said in a companion Science article.
In this framework of clues that there are missing pieces to the standard model, we have contributed one more, very interesting, and somewhat large clue, Kotwal said.
Jan Stark, physicist and director of research at the French CNRS institute, said this is either a major discovery or a problem in the analysis of data, predicting quite heated discussions in the years to come.
He told AFP that extraordinary claims require extraordinary evidence.
- 'Huge deal' -
The CDF scientists said they had determined the W boson's mass with a precision of 0.01 percent -- twice as precise as previous efforts.
They compared it to measuring the weight of a 350-kilogram (800-pound) gorilla to within 40 grams (1.5 ounces).
They found the boson was different than the standard model's prediction by seven standard deviations, which are also called sigma.
Cliff said that if you were flipping a coin, the chances of getting a five sigma result by dumb luck is one in three and a half million.
If this is real, and not some systematic bias or misunderstanding of how to do the calculations, then it's a huge deal because it would mean there's a new fundamental ingredient to our universe that we haven't discovered before, he said.
But if you're going to say something as big as we've broken the standard model of particle physics, and there's new particles out there to discover, to convince people of that you probably need more than one measurement from more than one experiment.
CDF co-spokesperson David Toback said that it's now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery.
And after a decade of measurements, Kotwal isn't done yet.
We follow the clues and leave no stone unturned, so we'll figure out what this means.