From axion quality and naturalness problems to a high-quality <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi mathvariant="double-struck">Z</mml:mi><mml:mi>N</mml:mi></mml:msub></mml:math> QCD relaxion
Physical review. D/Physical review. D.