# Determining the Mass of the $K_S^0$, $\pi^+$ and $\pi^-$ Mesons Using the Theil Index

Jovan Markov

Mentori: Vladimir Gligorov, DiegoMartinez Santos, Miriam Lucioano Martinez

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The aim of this project is to determine the mass of the $K_S^0$, $\pi^+$ and $\pi^-$ mesons observed in the decay $K_S^0 \to \pi^+ \pi^-$ using the Theil index and to see how precise this method will be. Filtered collision data from the LHCb detector at CERN was provided along with the information concerning the momenta of the particles listed above. The Armenteros-Podolanski plot wass used to display the data. Real data was overlapped individually with multiple pseudo datasets generated using theoretical predictions. The overlaps were compared to each other using the Theil index. The overlap with the largest Theil index was the one using the pseudo data with the best estimates for the particle mass values. The mass uncertainties were calculated using toy datasets. The calculated invariant meson masses are: $m_{K_S^0} = 497±3 MeV/c^2$ and $m_{\pi^+}=138.8±1.8 MeV/c^2$.