Amplitude analysis of $D_{s}^{+} \rightarrow \pi^{+}\pi^{0}\eta$ and first observation of the pure $W$-annihilation decays $D_{s}^{+} \rightarrow a_{0}(980)^{+}\pi^{0}$ and $D_{s}^{+} \rightarrow a_{0}(980)^{0}\pi^{+}$

Abstract

We present the first amplitude analysis of the decay Ds+→π+π0η. We use an e+e− collision data sample corresponding to an integrated luminosity of 3.19fb−1 collected with the BESIII detector at a center-of-mass energy of 4.178 GeV. We observe for the first time the pure W-annihilation decays Ds+→a0(980)+π0 and Ds+→a0(980)0π+. We measure the absolute branching fractions B(Ds+→a0(980)+(0)π0(+),a0(980)+(0)→π+(0)η)=(1.46±0.15stat.±0.23sys.), which is larger than the branching fractions of other measured pure W-annihilation decays by at least one order of magnitude. In addition, we measure the branching fraction of Ds+→π+π0η with significantly improved precision.