Editing Axion (section)

=== Strong CP problem ===
As shown by [[Gerard 't Hooft]],<ref>{{cite journal |last='t Hooft |first=Gerard |year=1976 |title=Symmetry breaking through Bell-Jackiw anomalies |journal=Physical Review Letters |volume=37 |issue=1}}{{cite journal |last='t Hooft |first=Gerard |year=1976 |title=Computation of the quantum effects due to a four-dimensional pseudo-particle |journal=Physical Review D |publisher=APS |volume=14 |issue=12 |pages=3432–3450 |bibcode=1976PhRvD..14.3432T |doi=10.1103/PhysRevD.14.3432}}</ref> [[strong interaction]]s of the Standard Model, QCD, possess a non-trivial vacuum structure{{efn|This non-trivial vacuum structure solves a problem associated to the U(1) axial symmetry of QCD<ref>{{cite journal |last1=Katz |first1=Emanuel |last2=Schwartz |first2=Matthew D |title=An eta primer: solving the U(1) problem with AdS/QCD |journal=Journal of High Energy Physics |date=28 August 2007 |volume=2007 |issue=8 |pages=077 |doi=10.1088/1126-6708/2007/08/077 |arxiv=0705.0534 |bibcode=2007JHEP...08..077K |s2cid=119594300 }}</ref><ref>{{cite web |url=https://www.classe.cornell.edu/~pt267/files/documents/A_instanton.pdf |title='t Hooft and η'ail Instantons and their applications | first=Flip | last=Tanedo | publisher=Cornell University | access-date=2023-06-20}}</ref>}} that in principle permits violation of the combined symmetries of [[C-symmetry|charge conjugation]] and [[parity (physics)|parity]], collectively known as CP. Together with effects generated by [[weak interaction]]s, the effective periodic strong CP-violating term, {{overline|Θ}}, appears as a [[Standard Model]] input – its value is not predicted by the theory, but must be measured. However, large CP-violating interactions originating from QCD would induce a large [[Neutron electric dipole moment|electric dipole moment (EDM) for the neutron]]. Experimental constraints on the unobserved EDM implies CP violation from QCD must be extremely tiny and thus {{overline|Θ}} must itself be extremely small. Since {{overline|Θ}} could have any value between 0 and 2{{math|π}}, this presents a "naturalness" problem for the Standard Model. Why should this parameter find itself so close to zero? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the [[strong CP problem]].{{efn|<!-- This same paragraph appears in [[strong CP problem]]. Does it need to be repeated here? --> One simple solution to the [[strong CP problem]] exists: If at least one of the [[quarks]] of the Standard Model is massless, CP-violation becomes unobservable. However, empirical evidence strongly suggests that none of the quarks are massless. Consequently, particle theorists sought other resolutions to the problem of inexplicably conserved CP.}}