Measurement of branching fractions for B -˃ J/psi eta K decays and search for a narrow resonance in the J/psi eta final state

Abstract

We report an observation of the B ± →J/ψηK ± and B 0 →J/ψηK 0 S decays using 772×10 6 BB ¯ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider. We obtain the branching fractions B(B ± →J/ψηK ± )=(1.27±0.11(stat.)±0.11(syst.))×10 −4 and B(B 0 →J/ψηK 0 S )=(5.22±0.78(stat.)±0.49(syst.))×10 −5 . We search for a new narrow charmonium(-like) state X in the J/ψη mass spectrum and find no significant excess. We set upper limits on the product of branching fractions, B(B ± →XK ± )B(X→J/ψη) , at 3872 MeV/c 2 where a C -odd partner of X(3872) may exist, at ψ(4040) and ψ(4160) assuming their known mass and width, and over a range from 3.8 to 4.8 GeV/c 2 . % at a 5 MeV/c 2 step. The obtained upper limits at 90% confidence level for X C−odd (3872) , ψ(4040) and ψ(4160) are 3.8×10 −6 , 15.5×10 −6 and 7.4×10 −6 , respectively.