Observation of a Prethermal U(1) Discrete Time Crystal

We report the first evidence for the

h_{b} (2 P) \to ϒ (1 S) η

transition with a significance of 3.5 standard deviations. The decay branching fraction is measured to be

B [h_{b} (2 P) \to ϒ (1 S) η] = ({7.1}_{- 3.2}^{+ 3.7} \pm 0.8) \times 10^{- 3}

, which is noticeably smaller than expected. We also set upper limits on

π^{0}

transitions of

B [h_{b} (2 P) \to ϒ (1 S) π^{0}] < 1.8 \times 10^{- 3}

, and

B [h_{b} (1 P) \to ϒ (1 S) π^{0}] < 1.8 \times 10^{- 3}

, at the 90% confidence level. These results are obtained with a

131.4 {fb}^{- 1}

data sample collected near the

ϒ (5 S)

resonance with the Belle detector at the KEKB asymmetric-energy

e^{+} e^{-}

collider. Published by the American Physical Society2024