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The proton spin crisis (sometimes called the "proton spin puzzle") is a theoretical crisis precipitated by an experiment in 1987 which tried to determine the spin configuration of the proton. The experiment was carried out by the European Muon Collaboration (EMC).〔 〕 Physicists expected that the quarks carry all the proton spin. However, not only was the total proton spin carried by quarks far smaller than 100%, these results were consistent with almost zero proton spin being carried by quarks. This surprising and puzzling result was termed the "proton spin crisis".〔 〕 The problem is considered one of the important unsolved problems in physics. ==Background== A key question is how the nucleon's spin is distributed amongst its constituent partons (quarks and gluons). Physicists originally expected that quarks carry all of the nucleon spin. According to quantum chromodynamics, the proton is built from two ''up'' and one ''down'' quark, gluons and possibly additional pairs of quark and anti-quark.〔 〕 The ruling assumption was that since the proton is stable, then it exists in the lowest possible energy level. Therefore, it was expected that the quark's wave function is the spherically symmetric s-wave with no spatial contribution to angular momentum. The proton is, like each of its quarks, a spin-1/2 particle. Therefore, it was assumed that two of the quarks have opposite spins and the spin of the third quark is parallel to the proton spin. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Proton spin crisis」の詳細全文を読む スポンサード リンク
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