The LHCb collaboration has observed the rare decay $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ for the first time, with a significance of 5.2σ. This decay was studied using pp collision data with an integrated luminosity of 1.0 fb$^{-1}$, collected with the LHCb detector. The measured branching fraction is $ (2.3 \pm 0.6) \, (stat.) \pm 0.1 \, (syst.) \times 10^{-8} $, and the ratio of the branching fractions of $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ and $ B^{+} \rightarrow K^{+}\mu^{+}\mu^{-} $ is $ 0.053 \pm 0.014 \, (stat.) \pm 0.001 \, (syst.) $. The decay $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ is a rare process that is sensitive to new physics beyond the Standard Model (SM). The ratio of the branching fractions $ R = \mathcal{B}(B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-}) / \mathcal{B}(B^{+} \rightarrow K^{+}\mu^{+}\mu^{-}) $ is predicted in the SM to be $ R = V^2 f^2 $, where $ V = |V_{td}| / |V_{ts}| $ and $ f $ is the ratio of form factors and Wilson coefficients. A deviation from this prediction would indicate a violation of the minimal flavour violation hypothesis. The analysis involved a detailed study of the decay modes and the use of a boosted decision tree (BDT) to separate signal candidates from background. The signal yield was determined through a simultaneous unbinned maximum likelihood fit to four invariant mass distributions. The branching fractions were calculated using the ratio of the yields of the $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ and $ B^{+} \rightarrow J/\psi K^{+} $ decays, with the latter serving as a reference. Systematic uncertainties were evaluated, including those from the efficiency ratios and the shape parameters of the decay distributions. The results are consistent with the SM predictions, and the measured ratio of branching fractions is compatible with previous determinations of $ |V_{td}| / |V_{ts}| $. The study provides important constraints on new physics beyond the SM.The LHCb collaboration has observed the rare decay $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ for the first time, with a significance of 5.2σ. This decay was studied using pp collision data with an integrated luminosity of 1.0 fb$^{-1}$, collected with the LHCb detector. The measured branching fraction is $ (2.3 \pm 0.6) \, (stat.) \pm 0.1 \, (syst.) \times 10^{-8} $, and the ratio of the branching fractions of $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ and $ B^{+} \rightarrow K^{+}\mu^{+}\mu^{-} $ is $ 0.053 \pm 0.014 \, (stat.) \pm 0.001 \, (syst.) $. The decay $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ is a rare process that is sensitive to new physics beyond the Standard Model (SM). The ratio of the branching fractions $ R = \mathcal{B}(B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-}) / \mathcal{B}(B^{+} \rightarrow K^{+}\mu^{+}\mu^{-}) $ is predicted in the SM to be $ R = V^2 f^2 $, where $ V = |V_{td}| / |V_{ts}| $ and $ f $ is the ratio of form factors and Wilson coefficients. A deviation from this prediction would indicate a violation of the minimal flavour violation hypothesis. The analysis involved a detailed study of the decay modes and the use of a boosted decision tree (BDT) to separate signal candidates from background. The signal yield was determined through a simultaneous unbinned maximum likelihood fit to four invariant mass distributions. The branching fractions were calculated using the ratio of the yields of the $ B^{+} \rightarrow \pi^{+}\mu^{+}\mu^{-} $ and $ B^{+} \rightarrow J/\psi K^{+} $ decays, with the latter serving as a reference. Systematic uncertainties were evaluated, including those from the efficiency ratios and the shape parameters of the decay distributions. The results are consistent with the SM predictions, and the measured ratio of branching fractions is compatible with previous determinations of $ |V_{td}| / |V_{ts}| $. The study provides important constraints on new physics beyond the SM.