Anti-xi-zero.

One of the predictions of quantum theory, the branch of physics that deals with the smallest elements in the universe, was the existence of some seventeen basic particles of matter and antimatter. One fundamental bit of antimatter, the infamous anti-xi-zero, was difficult to identify for several reasons, chief among them being that it has no charge. In the usual bubble chambers where particles are studied, anti-xi-zero would leave no track, even if it were present. Physicists faced the dual challenge of finding the right combinations of particles to combine in order to produce it as a residual and of recognizing it when it was found. They also did not know what properties anti-xi-zero should have, such as how it would affect other atoms or particles during collisions.