We present the first search for the semileptonic decay $D_s^+\to \pi^0 e^+\nu_e$ using a data sample of electron-positron collisions recorded with the BESIII detector at center-of-mass energies between 4.178 and 4.226~GeV, corresponding to an integrated luminosity of 6.32~fb$^{ -1}$. This decay is expected to be sensitive to $\pi^0$--$\eta$ mixing. No significant signal is observed. We set an upper limit of $6.4 \times 10^{ -5}$ on the branching fraction at the $90\%$ confidence level.
Categories: hep-ex