Reconstruction and classification of tau lepton decays with ILD

Tau-lepton decays with up to two ${\pi }^0$’s in the final state, $\tau^+ \to \pi^+ \bar{\nu}\tau$,$\rho^+ (\pi^+\pi^0) \bar{\nu}\tau$,$a^+1 (\pi^+\pi^0\pi^0) \bar{\nu}\tau$, are used to study the performance of the barrel part of the silicon-tungsten electromagnetic calorimeter (Si-W ECAL) of the International Large Detector (ILD) at the future $e^{+}-e^{-}$ International Linear Collider. A correct reconstruction of the tau decay mode is crucial for constraining the tau spin state and measuring the Higgs boson CP state in $H\to \tau^+\tau^-$decays\mathrm{.}About95$\pi^+ \bar{\nu}\tau$and90$\rho^+\bar{\nu}\tau$and$a^+1\bar{\nu}\tau$decaysfrom$e^+e^-\to Z^0\to \tau^+\tau^-$reactionat$e^\pm$-beam energy of 125 GeV are correctly reconstructed. In a smaller ILD detector, with Si-W ECAL radius reduced by about 20% these numbers degrade by at most 2%. The ${\pi }^0$ mass resolution stays below 10%. Since the failures in the tau-lepton reconstruction are mainly due to photons, the increase of the ILD magnetic field from 3.5 T to 4 T does not bring any significant improvement.