A tiny preference for matter

The NA31 and NA48 experiments

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. Nature appears to have a tiny preference, with just one particle of matter having survived for every billion particles of matter and antimatter that annihilated with each other after the Big Bang.

One clue as to how this happened may come from experiments that study a very small difference in the way that particles of matter and antimatter behave, in a phenomenon called CP symmetry breaking. This difference appears in the decays of certain particles of matter and antimatter, in particular, electrically neutral versions of particles called kaons.

The NA31 experiment at CERN was conceived to observe what is known as ‘direct’ CP symmetry breaking, using a beam from the Super Proton Synchrotron (SPS) to create the neutral kaons. In 1993 it published the first precise results on this ‘direct’ effect, which is very difficult to observe, but 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.