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 T$0v\atop{1/2}$(0+ →21+) > 2.6×1025 yr and T$2v\atop{1/2}$ (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$0v\atop{1/2}$(0+ →01+) > 2.4×1025 yr and T$2v\atop{1/2}$(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$0v\atop{1/2}$(0+ →22+) > 2.6×1025 yr and T$2v\atop{1/2}$(0+ →22+) > 9.0×1023 yr (90% C.L.). Finally, these results are compared to recent theoretical predictions.