Observation of Λ_c<SUP>+</SUP> → Λa₀ (980)<SUP>+</SUP> and Evidence for Σ(1380)<SUP>+</SUP> in Λ_c<SUP>+</SUP> → Λπ<SUP>+</SUP>η

Based on 6.1 fb(-1) of ethorne- annihilation data collected at center-of-mass energies from 4.600 to 4.843 GeV with the BESIII detector at the BEPCII collider, a partial wave analysis of Lambda(+)(c) -> Lambda pi(+)eta is performed, and branching fractions and decay asymmetry parameters of intermediate processes are determined. The process Lambda(+)(c) -> Lambda a(0)(980)(+) is observed for the first time, and evidence for the pentaquark candidate Sigma(1380)(+) decaying into Lambda pi(+) is found with statistical significance larger than 3 sigma with mass and width fixed to theoretical predictions. The branching fraction product B[ Lambda(+)(c) -> Lambda a(0) (980)(+)]B[a(0)(980)(+) -> pi(+)eta] is determined to be (1.05 +/- 0.16(stat) +/- 0.05(syst) +/- 0.07(ext) )%, which is larger than theoretical calculations by 1-2 orders of magnitude. Here the third (external) systematic is from B(Lambda(+)(c) -> Lambda pi(+)eta). Finally, we precisely obtain the absolute branching fraction B(Lambda(+)(c) -> Lambda pi(+)eta) = (1.94 +/- 0.07(stat) +/- 0.11(syst))%.