Proton spin crisis

Unsolved problem in physics:
How do the quarks and gluons carry the spin of protons?
<a href="/facts/List_of_unsolved_problems_in_physics/vJ3d1A83">(more unsolved problems in physics)</a>

<p>The proton spin crisis (or proton spin puzzle) is a theoretical crisis precipitated by a 1987 experiment by the <a href="/facts/European_Muon_Collaboration/cI3RL1La">European Muon Collaboration</a> (EMC),
which tried to determine the distribution of spin within the <a href="/facts/Proton/clCjepXP">proton</a>.
</p><p>Physicists expected that the <a href="/facts/Quark/hwuuVkV5">quarks</a> carry all a proton's <a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a>. However, not only was the total proton spin carried by quarks far smaller than 100%, these results were consistent with almost zero (4–24%) 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 <a href="/facts/Unsolved_problems_in_physics/vJ3d1A83">unsolved problems in physics</a>.
</p>

Proton spin crisis open-in-new

Proton spin crisis