What is the nature of the strong CP problem?
Category: Physics
Status: Queued
QCD allows a CP-violating term proportional to a parameter θ. Experimentally, θ is constrained to be less than 10^-10, but the Standard Model offers no reason for it to be so small.
The leading explanation invokes a Peccei-Quinn symmetry whose breaking produces an axion. Decade-long searches for axions are ongoing (ADMX, IAXO).
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