Dark matter
Bent light and galaxy motions reveal gravity that known stars and gas cannot fully explain. What supplies the additional mass?
Particle physics
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Quantum gravity
Gravity describes a changing spacetime. Quantum physics describes matter and its possibilities. How should they work together in a single physical account?
Matter–antimatter asymmetry
Matter and antimatter can annihilate each other. Yet the observable universe contains enough leftover matter to make stars, planets, and people. What produced that imbalance?
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?
Yang–Mills existence and mass gap
A successful framework for particle physics still needs a rigorous four-dimensional construction with a positive gap above its vacuum energy.
Hierarchy problem
The Higgs field sets an important scale in particle physics. Why is that scale so much smaller than other scales that may enter the underlying theory?
Absolute neutrino mass
Neutrino oscillations show that neutrinos have mass differences, but they do not tell us the absolute masses. How light are these elusive particles?
Majorana nature of neutrinos
A neutrino might be its own antiparticle. Establishing whether that is true would change how we understand neutrino mass and a basic particle-number symmetry.
Proton decay
Protons appear stable in ordinary observations. Could they eventually decay through interactions too rare to have been detected yet?
Neutrino mass ordering
Neutrinos have three mass states. Which is heavier: the close pair or the third state?
Leptonic CP violation
Do neutrinos and antineutrinos change flavour differently in a way that violates charge–parity symmetry?
Neutron lifetime puzzle
Proton-counting beam measurements and stored-neutron measurements give incompatible neutron lifetimes.
QCD critical point
Does hot, baryon-rich nuclear matter have a critical endpoint where the character of its phase transition changes?
Neutron-star equation of state
How does pressure depend on density inside a cold neutron star?
Diffuse astrophysical neutrino sources
Which populations produce the diffuse high-energy neutrino flux measured by IceCube?
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.
Gallium anomaly
Gallium detectors exposed to intense laboratory neutrino sources make fewer germanium atoms than standard capture calculations predict. Is the missing yield a nuclear-calculation problem, a measurement effect or new neutrino physics?