Last year, physicists reported seeing tantalizing experimental traces of the axion, a hypothetical subatomic particle that's been mentioned as a possible constituent of cosmic dark matter. But the axion was showing up where theory said it shouldn't be. It now looks as if it wasn't there after all.
The axion sprang from an attempt to explain certain differences between the strong and weak nuclear forces. Cosmologists seized on the axion because its properties made it a plausible component of dark matter, the unseen material that far outweighs ordinary matter in the universe.
In 2000, Giovanni Cantatore and his colleagues at the Italian National Institute of Nuclear Physics in Legnaro were investigating the behavior of photons by shining a laser beam through a strong magnetic field. They noticed that the light's polarization shifted slightly after it went through the field—not the effect they were looking for.