BESIII

Isospin symmetry, to a good approximation, is preserved by the strong interaction due to the small mass difference between up and down quarks with respect to the mass of hadrons. Therefore, isospin-breaking decays are believed to be sensitive probes that can be used to access, for example, the up- and down-quark mass differences. Moreover, isospin breaking effects are considered as a signature for identifying exotic states of matter, such as the $X(3872)$. Charmonium is an excellent system to study pure hadronic effects that lead to isospin breaking, since the contribution of electromagnetic processes is shown to be small compared to the quark-mass difference. We will report on systematic studies of isospin-suppressed transitions in charmonium performed by the BESIII collaboration. Measurements of the branching fractions of transitions $\psi(2S) \rightarrow \pi^{0} J/\psi (h_{c})$ and $\chi_{c0,2}\rightarrow\pi^{0} \eta_{c}$, will be presented and interpreted using different theoretical approaches.