Measurement of branching fractions for B → J/ψηK decays and search for a narrow resonance in the J/ψη final state

Abstract

We report an observation of the B±→J/ψηK± and B0→J/ψηK0S decays using 772×106BB¯¯¯¯ 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(B0→J/ψηK0S)=(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 3872MeVc−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 4.8GeVc−2. The obtained upper limits at 90% confidence level for XC-odd(3872), ψ(4040), and ψ(4160) are 3.8×10−6, 15.5×10−6, and 7.4×10−6, respectively.