Editing Axion (section)

=== Searches <span class="anchor" id="K-S-V-Z-vs-D-F-S-Z-anchor"></span> ===
The axion models originally proposed by Wilczek and by Weinberg chose axion coupling strengths that were so strong that they would have already been detected in prior experiments. It had been thought that the [[Peccei–Quinn theory|Peccei–Quinn mechanism]] for solving the [[strong CP problem]] required such large couplings. However, it was soon realized that "invisible axions" with much smaller couplings also work. Two such classes of models are known in the literature as {{nowrap|KSVZ}} ([[Kim Jihn-eui|Kim]]–[[Mikhail Shifman|Shifman]]–[[Arkady Vainshtein|Vainshtein]]–{{nowrap|{{abbr|Zakharov|Valya Zakharov}})<ref name=Kim-1979>{{cite journal |last1=Kim |first1=J. E. |year=1979 |title=Weak-interaction singlet and strong CP invariance |journal=Physical Review Letters |volume=43 |issue=2 |pages=103–107 |bibcode=1979PhRvL..43..103K |doi=10.1103/PhysRevLett.43.103}}</ref><ref name=Shifman-etal-1980>{{cite journal |last1=Shifman |first1=M. |last2=Vainshtein |first2=A. |last3=Zakharov |first3=V. |year=1980 |journal=Nuclear Physics&nbsp;B |volume=166 |issue=3 |pages=493–506 |title=Can confinement ensure natural CP invariance of strong interactions? |doi=10.1016/0550-3213(80)90209-6 |bibcode=1980NuPhB.166..493S}}</ref>}} and {{nowrap|DFSZ}} ([[Michael Dine|Dine]]–[[Willy Fischler|Fischler]]–{{abbr|Srednicki|Mark Srednicki}}–{{nowrap|{{abbr|Zhitnitsky|Ariel R. Zhitnitsky}}).<ref>{{cite journal |last1=Dine |first1=M. |last2=Fischler |first2=W. |last3=Srednicki |first3=M. |year=1981 |journal=Physics Letters B |volume=104 |issue=3 |pages=199–202 |title=A simple solution to the strong CP problem with a harmless axion |doi=10.1016/0370-2693(81)90590-6 |bibcode=1981PhLB..104..199D}}</ref><ref>{{cite journal |last1=Zhitnitsky |first1=A. |year=1980 |journal=Soviet Journal of Nuclear Physics |language=en |volume=31 |page=260 |title=On possible suppression of the axion–hadron interactions |url= https://www.inp.nsk.su/images/preprint/1979_081.pdf}}</ref>}}

The very weakly coupled axion is also very light, because axion couplings and mass are proportional. Satisfaction with "invisible axions" changed when it was shown that any very light axion would have been overproduced in the early universe and therefore must be excluded.<ref name="auto"/><ref name="A cosmological bound on the invisib"/><ref name="The not-so-harmless axion"/>