Observation of matter--anti-matter asymmetry in the B0 meson system

As far as one can see, our Universe is made of matter and no primordial anti-matter is evident. Whether this imbalance is a chance occurrence during the birth of the Universe or due to some fundamental difference between the behavior of matter and anti-matter under the charge-parity (CP) symmetry remains to be understood, and represents one of the biggest mysteries in Cosmology and Particle Physics.

Nature seems to always obey the CP symmetry with one known exception. A tiny but significant evidence of CP violation was observed in 1964, in sub-atomic particles called Kaons, and earned its discoverers the Nobel Prize in Physics. Following that discovery, tremendous efforts have gone into understanding the origin of this difference, and finding new experimental evidence in Nature. An elegant explanation of CP violation was proposed in 1973. In addition, this model predicted large CP violation in another type of sub-atomic particle called B0 meson.

In this thesis, we report on the observation of CP violation in the B0 meson system. The data for this analysis was produced, with a record-breaking pace, with the electron-positron collider PEP-II at SLAC, and recorded with the BaBar detector. After analyzing a sample of about 88 million pairs of B0--anti-B0 mesons, we have established that there is a large asymmetry in the decays of the B0 and anti-B0 particles. The result is in excellent agreement with the predictions of the model proposed in 1973 and represents a strong test of this theory.

The size of the observed asymmetry, however, is not sufficient to explain the disappearance of anti-matter since the early moments of our Universe. This is a hint of new Physics beyond our current understanding of Nature, and exciting years of investigation ahead of us.

This result was published on Physical Review Letters: Measurement of CP Asymmetry Amplitude sin2beta with B0 mesons Phys.Rev.Lett 89, 201802 (2002) (DOI: 10.1103/PhysRevLett.89.201802)