Search for double beta decay of ¹³⁶Xe to the 01+ excited state of ¹³⁶Ba with PandaX-4T

We perform a search of double beta decay of ¹³⁶Xe to the excited state, 01+, of ¹³⁶Ba (2νββ-01+), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2νββ-01+ is observed, resulting in a lower limit on half-life of 7.5 × 10²² yr at the 90% confidence level. This is the first experimental limit on such a rare decay in a natural xenon-based detector.