1993: precise results on matter–antimatter asymmetry

Matter and antimatter were created in equal amounts at the Big Bang, yet today we seem to live in a Universe made entirely of matter.

Are there large regions of space made of antimatter? Or does nature have a preference for matter? The latter seems to be the case, with just one particle of matter surviving for every billion particles of matter and antimatter to have annihilated with each other in the wake of the Big Bang.

Experiments in high-altitude balloons, aboard satellites and on the Space Shuttle have all looked for atoms of antimatter in space. So far, nothing has been seen. Here on Earth, experiments studying the decays of particles of matter and antimatter have recorded a tiny difference in the way matter and antimatter behave, in a phenomenon called CP symmetry breaking. Although not nearly enough to account for the apparent matter–antimatter imbalance in the Universe, these observations are the first indication that Nature really does have a preference at the level of particle interactions.

In 1993, the NA31 experiment at CERN published the first precise results on what is known as ‘direct’ CP symmetry breaking, which indicates more clearly the physics underlying the phenomenon. This measurement was further refined by the subsequent NA48 experiment, which announced its final result on the matter in 2001. However, matter–antimatter asymmetry remains on the agenda at CERN, in particular with the LHCb experiment at the Large Hadron Collider.