Observation of an isotriplet of excited charmed baryons decaying to Λ _c ⁺ π

We report the observation of an isotriplet of excited charmed baryons, decaying into Λ _c ⁺ π ^-, Λ _c ⁺ π ⁰, and Λ _c ⁺π ⁺. We measure the mass differences M(Λ _c ⁺ π) -M(Λ _c ⁺) and widths to be 515.4 _-3.1-6.0 ^+3.2+2.1 MeV/c ², 61 _-13-13 ⁺¹⁸⁺²² MeV for the neutral state; 505.4 _-4.6-2.0 ^+5.8+12.4 MeV/c ², 62 _-23-38 ⁺³⁷⁺⁵² MeV for the charged state; and 514.5 _-3.1-4.9 ^+3.4+2.8 MeV/c ², 75 _-13-11 ⁺¹⁸⁺¹² MeV for the doubly charged state, where the uncertainties are statistical and systematic, respectively. These results are obtained from a 281 fb ^-1 data sample collected with the Belle detector near the υ(4S) resonance, at the KEKB asymmetric energy e ⁺e ^- collider.