Strong CP problem
The theory of the strong nuclear force permits a kind of asymmetry that experiments find to be extraordinarily small. Why is nature so close to the symmetric case?
Quantum chromodynamics
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QCD critical point
Does hot, baryon-rich nuclear matter have a critical endpoint where the character of its phase transition changes?
EMC effect
Quark distributions change when protons and neutrons are bound in nuclei. What physical mechanism causes those changes?
Proton spin
A proton’s total spin is known. How is it shared among quark spin, gluon spin and their orbital motion?
Hadronic vacuum polarization
Different methods still disagree on parts of the hadronic contribution to the muon’s magnetic moment. Which data or corrections explain the differences?
Collectivity in small collision systems
Particles from some proton collisions have coordinated motion. Which interactions create those patterns in such small systems?
Chiral magnetic effect in nuclear collisions
Hot quark matter may separate electric charges along a magnetic field. Similar particle patterns can also arise from ordinary collision dynamics.