Parameters
Strong CP problem:
QCD allows θ term: L⊃θ(g²/32π²)G̃G which violates CP. But neutron EDM bounds say |θ|<10⁻¹⁰. Why?
Peccei-Quinn:
Global U(1)_PQ symmetry → θ dynamical: a/f_a. Potential V(a) from instantons makes ⟨a⟩=0.
Axion mass:
m_a ≈ (5.7 μeV)(10¹² GeV/f_a) × (Λ_QCD/T)⁴
Dark matter:
Misalignment mechanism: θ_i ≠ 0, oscillates as DM relic.
QCD allows θ term: L⊃θ(g²/32π²)G̃G which violates CP. But neutron EDM bounds say |θ|<10⁻¹⁰. Why?
Peccei-Quinn:
Global U(1)_PQ symmetry → θ dynamical: a/f_a. Potential V(a) from instantons makes ⟨a⟩=0.
Axion mass:
m_a ≈ (5.7 μeV)(10¹² GeV/f_a) × (Λ_QCD/T)⁴
Dark matter:
Misalignment mechanism: θ_i ≠ 0, oscillates as DM relic.