Search for double-beta decay of ¹³⁶Xe to excited states of ¹³⁶Ba with the KamLAND-Zen experiment

A search for double-beta decays of ¹³⁶Xe to excited states of ¹³⁶Ba has been performed with the first phase data set of the KamLAND-Zen experiment. The 0₁ ⁺, 2₁ ⁺ and 2₂ ⁺ transitions of 0νββ decay were evaluated in an exposure of 89.5 kg⋅yr of ¹³⁶Xe, while the same transitions of 2νββ decay were evaluated in an exposure of 61.8 kg⋅yr. No excess over background was found for all decay modes. The lower half-life limits of the 2₁ ⁺ state transitions of 0νββ and 2νββ decay were improved to T1/20ν(0⁺→2₁ ⁺)>2.6×10²⁵ yr and T_1/2 ^2ν(0⁺→2₁ ⁺)>4.6×10²³ yr (90% C.L.), respectively. We report on the first experimental lower half-life limits for the transitions to the 0₁ ⁺ state of ¹³⁶Xe for 0νββ and 2νββ decay. They are T_1/2 ^0ν(0⁺→0₁ ⁺)>2.4×1025 yr and T_1/2 ^2ν(0⁺→0₁ ⁺)>8.3×1023 yr (90% C.L.). The transitions to the 2₂ ⁺ states are also evaluated for the first time to be T_1/2 ^0ν(0⁺→2₂ ⁺)>2.6×10²⁵ yr and T_1/2 ^2ν(0⁺→2₂ ⁺)>9.0×10²³ yr (90% C.L.). These results are compared to recent theoretical predictions.