The ARGUS experiment was the first experiment that observed the mixing of the B mesons into its antiparticle, the anti-B meson; this was done in 1987. This observation led to the conclusion that the second-heaviest quark – the bottom quark – could under certain circumstances convert into a different, hitherto unknown quark, which had to have a huge mass. This quark, the top quark, was discovered in 1995 at Fermilab.