The beauty of the rare

The decay of a B meson (a subatomic particle) into two muons is the rarest of the particle processes ever measured in the Large Hadron Collider (LHC) at CERN, the world's largest and most powerful particle accelator. This decay is as rare as finding a needle in 10.000 haystacks, making its detection a big challenge for particle physicists. Apart from its rareness, the relative rate at which such a process occurs is very sensitive to new and maybe unexpected fundamental particles or forces. This quality makes it a good probe of the current theory describing elementary particle physics, known as the Standard Model, and of physics lying beyond it. This thesis provides the measurement of the decay rate of the B meson decaying into two muons using the data gathered by the LHCb detector, one of the detectors in the LHC ring. The results obtained are world-leading precision results of such decay rates, providing some more insights into the laws ruling the inner working of nature at its most fundamental level.