The axion is a hypothetical elementary particle postulate by Peccei-Quinn theory in 1977 to resolve the strong-CP problem in quantum chromodynamics (QCD). In 2005, an experimental search by the PVLAS collaboration reported results suggesting axion detection ; however new experiments performed by the PVLAS team exclude this result . The 2005 PVLAS results were problematic because compatibility with the negative results of other searches, such as CAST, as well as astrophysical limits, ruled out standard axion scenarios, while alternative hypotheses have been postulated by other researchers. .