Antonio Pich
Professor of Theoretical Physics at the Institute of Corpuscular Physics (IFIC), University of Valencia - CSIC
In the Big Bang, the explosion that formed the early universe, matter and antimatter were created in equal proportions. However, all observations show that our current universe contains only matter (the traces of antimatter observed are completely negligible). After the Big Bang, matter and antimatter annihilated each other, giving rise to radiation, until the antimatter was exhausted. Fortunately, a tiny excess of matter remained, giving rise to the universe we live in, where we observe barely one particle of matter (protons, neutrons, electrons) for every billion particles of radiation (photons). Our very existence is due to this insignificant excess of matter, but how did it come about?
The physical laws of electromagnetism, strong (nuclear) interaction and gravitation are identical for particles of matter and antimatter. They are distinguished only by weak interaction. However, in order to explain the asymmetry observed between matter and antimatter, the physical laws must also change when a ‘CP transformation’ (charge plus parity inversion) is performed: a universe of antimatter observed through a mirror must be distinguishable from the universe of matter. Although the current theory of fundamental interactions (the Standard Model) satisfies this condition, we know that it does so insufficiently. Therefore, matter-antimatter asymmetry originates from some additional unknown interaction that is not symmetrical under CP transformation. Hence the enormous interest in experimentally searching for violations of CP symmetry.
CP symmetry violations measured in the laboratory are consistent with the predictions of the Standard Model, but until now they had only been observed in the decays of mesons, particles made up of a quark and an antiquark. However, ordinary matter is made up of particles composed of three quarks, called baryons (protons and neutrons), and electrons. The importance of the results announced by the LHCb experiment lies in the fact that this is the first time a CP violation has been observed in baryons. Specifically, a significant difference has been observed between the decays of the Lambda_b baryon and its corresponding antibaryon. The Lambda_b particle (a ‘b d u’ state) is like a neutron (‘d d u’) in which a light quark “d” has been replaced by a heavy quark ‘b’. The fact that the first observation was made in a heavy baryon is in line with expectations in the Standard Model, but it is still too early to draw conclusions.
We hope that this interesting measurement will be followed by other observations of CP-violating phenomena, providing us with valuable information about unknown physics beyond the Standard Model.