The Neutron Enigma


By Geoffrey L. Greene /Peter Geltenbort G. L. グリーン /P. ゲルテンボルト
English 日本語 日本語
Luckily for life on Earth, most matter is not radioactive. We take this fact for granted, but it is actually somewhat surprising because the neutron, one of the two components of atomic nuclei (along with the proton), is prone to radioactive decay. Inside an atomic nucleus, a typical neutron can survive for a very long time and may never decay, but on its own, it will transform into other particles within 15 minutes, more or less. The words “more or less” cover a disturbing gap in physicists’ understanding of this particle. Try as we might, we have not been able to accurately measure the neutron lifetime.  地球上の生命にとって幸運なことに物質の多くは放射線を出さない。私たちはこの事実を当然のように思っているが,陽子とともに原子核を構成する中性子が簡単に放射性崩壊を起こす粒子であることを考えれば,これは驚きに値するのではないだろうか。
This “neutron lifetime puzzle” is not just embarrassing for us experimentalists; resolving it is vital for understanding the nature of the universe. The neutron decay process is one of the simplest examples of the nuclear “weak” interaction—one of nature’s four fundamental forces. To truly understand the weak force, we must know how long neutrons live. Furthermore, the survival time of the neutron determined how the lightest chemical elements first formed after the big bang. Cosmologists would like to calculate the expected abundances of the elements and compare them with astrophysical measurements: agreement would confirm our theoretical picture, and discrepancy could indicate that undiscovered phenomena affected the process. To make such a comparison, however, we need to know the neutron lifetime.  この「中性子の寿命の謎」は私たち実験家の面目をつぶすだけではすまない。中性子の正確な寿命を知ることは宇宙の性質を理解するために不可欠なのだ。中性子の崩壊プロセスは,原子核で働く「弱い力」の作用で生じる最も単純な例の1つだ。弱い力とは自然界に存在する基本的な4つの力の1つ。その力を本当に理解するには中性子の寿命を知る必要がある。
More than 10 years ago two experimental groups, one a Russian-led team in France and the other a team in the U.S., attempted separately to precisely measure the lifetime. One of us (Geltenbort) was a member of the first team, and the other (Greene) was a member of the second. Along with our colleagues, we were surprised and somewhat disturbed to find that our results disagreed considerably. Some theoreticians suggested that the difference arose from exotic physics—that some neutrons in the experiments might have transformed into particles never before detected, which would have affected the different experiments in divergent ways. We, however, suspected a more mundane reason—perhaps one of our groups, or even both, had simply made a mistake or, more likely, had overestimated the accuracy of its experiment.  10年以上前,2つの実験グループが別々に中性子の寿命の精密測定を試みた。一方はロシア人研究者をリーダーにフランスで,もう一方は米国で実験を行った。著者の1人(ゲルテンボルト)は前者の,もう1人(グリーン)は後者のメンバーだった。両者の測定結果が大幅に食い違うことを知った私たちを含む両実験グループのメンバーは驚き,いささか困惑した。