Mesons are particles composed by a quark and an anti-quark. For B-mesons one of these is the b-quark (or anti-quark). Rare decays of B-mesons are a promising place to search for New Physics. Here new particles can contribute as virtual particles, due to a quantum mechanics effect. An example is given in the figure below, where the Feynman diagram for the decay B0→K*μμ is shown. The left diagram shows how this decay happens in the Standard Model, the right one shows a possible contribution of a New Physics model (Super Symmetry). The external particles like the B0, the K*0 and the two muons are directly measurable, while the contribution of particles inside the diagram (virtual particles), like the W or Z bosons can only be inferred indirectly, measuring properties of the decay. Virtual particles can be very heavy, like the H0 in the right diagram. Therefore, studying the properties of rare decays allows us to test very high energies, orders of magnitudes higher than the energies we can reach directly at particle accelerators.
The LHCb experiment has been designed and optimized to measure these kind of decays. Since they are rare, we need at least billions of reconstructible B-mesons in order to have enough of these decays to make precise measurements.
Our group is involved in measurements of rare decays with electrons and muons in the final state.