Editing Strong CP problem (section)

{{Short description|Question of why quantum chromodynamics does seem to not break CP-symmetry}}
The '''strong CP problem''' is a question in [[particle physics]], which brings up the following quandary: why does [[quantum chromodynamics]] (QCD) seem to preserve [[CP violation#CP-symmetry|CP-symmetry]]?

In particle physics, '''CP''' stands for the combination of [[C-symmetry]] (charge conjugation symmetry) and [[Parity (physics)|P-symmetry]] (parity symmetry). According to the current mathematical formulation of quantum chromodynamics, a [[CP violation|violation of CP-symmetry]] in [[strong interaction]]s could occur. However, no violation of the CP-symmetry has ever been seen in any experiment involving only the strong interaction. As there is no known reason in QCD for it to necessarily be conserved, this is a "[[Fine-tuning (physics)|fine tuning]]" problem known as the '''strong CP problem'''.

The strong CP problem is sometimes regarded as an [[List of unsolved problems in physics|unsolved problem in physics]], and has been referred to as "the most underrated puzzle in all of physics."<ref>{{cite conference |first=T. |last=Mannel |title=Theory and Phenomenology of CP Violation |book-title=Nuclear Physics B
|volume=167 |pages=170–174 |publisher=Elsevier |conference=The 7th International Conference on Hyperons, Charm, and Beauty Hadrons (BEACH 2006) |date=2–8 July 2006 |location=Lancaster |url=https://indico.cern.ch/event/427023/session/6/contribution/43/attachments/912026/1288208/Lancester-Mannel-Proc.pdf |doi=10.1016/j.nuclphysbps.2006.12.083 |access-date=15 Aug 2015 |bibcode=2007NuPhS.167..170M}}</ref><ref>{{Cite web | url=https://www.forbes.com/sites/startswithabang/2019/11/19/the-strong-cp-problem-is-the-most-underrated-puzzle-in-all-of-physics |title = The 'Strong CP Problem' is the Most Underrated Puzzle in All of Physics| website=[[Forbes]] }}</ref> There are several proposed solutions to solve the strong CP problem. The most well-known is [[Peccei–Quinn theory]],<ref>{{Cite journal|author1=Peccei, R.D. |author-link1=Roberto Peccei |author2=Quinn, H.R. |author-link2=Helen Quinn|year=1977|title=''CP'' conservation in the presence of pseudoparticles|url=https://www.researchgate.net/publication/248549883|journal=[[Physical Review Letters]]|volume=38|issue=25|pages=1440–1443|bibcode=1977PhRvL..38.1440P|doi=10.1103/PhysRevLett.38.1440}}</ref> involving new [[pseudoscalar]] particles called [[axion]]s.