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Charge conjugation and parity violation in beauty Physics

Final Activity Report Summary - B PHYSICS (Charge conjugation and parity violation in beauty physics)

The combination of Charge conjugation and parity symmetry (CP) is fundamental in particle physics. Consider a given reaction, for instance the decay of a particle into two daughter particles, and apply symmetry transformations to the theory that describe this decay. The reaction is invariant under these transformations only if the symmetries are not broken. In short, if the particle that decays is replaced by its antiparticle using charge conjugation C and at the same time one applies a parity P transformation to this reaction, which is tantamount to observing the reaction in a mirror, we expect this decay reaction to occur at the same rate. However, already in the 1950s a famous experiment conducted by C. S. Wu, proposed by the theorists T. D. Lee and C. N. Yang, revealed that the weak interaction force, responsible for nuclear decays, does not conserve parity: the final particles in a decay are produced in a preferred direction, that is the weak interactions differentiate the right from the left. Later on in 1964, Christenson, Cronin, Fitch and Turlay showed in kaon- decay experiments at Brookhaven National Lab that weak interactions violate not only the charge-conjugation symmetry between particles and antiparticles as well as parity, but also the combined CP symmetry. The discovery stunned the particle physics community and opened the door to questions still at the core of particle physics and of cosmology today. This is due to the discrepancy between the amount of matter and antimatter in our universe. It is experimentally observed that matter and antimatter cannot coexist, and particle-antiparticle collisions results in their mutual annihilation. If both, matter and antimatter, were present in exactly the same amount, life based on matter, as we know today, would simply not be possible. We also know CP-violation to be a key element in creating an inbalance of matter and antimatter in the first few seconds after the Big Bang.

In 2001, a new generation of experiments, including the BaBar Experiment at the Stanford Linear Accelerator Center (United States) and the Belle Experiment at the High Energy Accelerator Research Organisation (KEK, Japan), have observed CP violation using Beauty (or simply B) mesons. These mesons, like the just mentioned kaons, are particles made of fundamental constituents of nature, namely quarks. More precisely, there are three 'families' of quarks and meson consist of a quark from one and an antiquark from another family. Before these experiments, it was a possibility that all CP violation was confined to kaon physics. These experiments dispelled any doubt that the interactions of the Standard Model of particles violated CP.

In our work, we were concerned with theoretical studies of B decays into two or three much lighter mesons than the B. The BaBar and Belle Collaborations have observed significant CP violations in these decays. We worked within the Standard Model of particle physics, which organises all known particles and their different types of interactions, namely the strong and electroweak ones described by quantum chromodynamics and the electroweak theory respectively, in a systematic manner. In principle, the Standard Model allows one to calculate the B meson decay rates from the fundamental elements of this theory. Therefore, if theoretical predictions are in contradiction with the current experiments, this would strongly indicate that our knowledge of particle physics is incomplete and requires an extension beyond the Standard Model, which is also suggested by theoretical considerations. Practically, though, physicists have thus far struggled with many theoretical limitations and uncertainties, as certain components of the decay rates are not calculable a priori with standard techniques. In order to still make contact with experimental results, one relies to a certain extent on models inspired by the underlying theory. We made progress in improving such models to describe strong interaction effects that occur between the final particles in B decays. In doing so, we integrated knowledge from earlier experiments that measured the 'strength' of the interaction between these particles in another reaction.

Assuming a universal behaviour of the interaction, this method allowed us to determine with higher precision the strong interaction in B decays. Furthermore, based on a relativistic quark model, we also determined a so far poorly known element in the theoretical expressions of the decay rate, referred to as transition form factor, which describes parts of the evolution from the initial B meson to one of the final decay particles. Its precise calculation is extremely important. All these efforts aim at limiting theoretical uncertainties in any calculations and at a better understanding of the experimentally observed CP violations. We were able to contribute to these endeavours and succeeded in decreasing parts of the uncertainties that were previously not even considered.