Measurement of branching fractions for $B \to J/\psi \eta K$ decays and search for a narrow resonance in the $J/\psi \eta$ final stateIwashita, T. (Avtor) Bračko, Marko (Avtor) Golob, Boštjan (Avtor) Klučar, Jure (Avtor) Korpar, Samo (Avtor) Križan, Peter (Avtor) Pestotnik, Rok (Avtor) Petrič, Marko (Avtor) Stanič, Samo (Drugo) Starič, Marko (Avtor) Šantelj, Luka (Avtor) Zupanc, Anže (Avtor) C07C21particle propertieslepton collider experimentsexperimental particle physicsWe report an observation of the $B^{\pm } \to J/\psi \eta K^{\pm }$ and $B^0 \to J/\psi \eta K^0_S$ decays using $772\times 10^{6}B\overline {B}$ pairs collected at the $\Upsilon (4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We obtain the branching fractions ${\mathcal B}(B^{\pm }\rightarrow J/\psi \eta K^{\pm })=(1.27\pm 0.11(\hbox {stat.})\pm 0.11(\hbox {syst.}))\times 10^{-4}$ and ${\mathcal B}(B^0\to J/\psi \eta K^0_S)=(5.22 \pm 0.78 (\hbox {stat.}) \pm 0.49 (\hbox {syst.}))\times 10^{-5}$. We search for a new narrow charmonium(-like) state $X$ in the $ J/\psi \eta$ mass spectrum and find no significant excess. We set upper limits on the product of branching fractions, ${\mathcal B}(B^\pm \to XK^\pm){\mathcal B}(X \to J/\psi \eta)$, at $3872\,\hbox {MeV}\,c^{-2}$ where a $C$-odd partner of $X(3872)$ may exist, at $\psi (4040)$ and $\psi (4160)$ assuming their known mass and width, and over a range from 3.8 to $4.8\,\hbox {GeV}\,c^{-2}$. The obtained upper limits at 90% confidence level for $X^{C\hbox {-}{\rm odd}}(3872)$, $\psi (4040)$, and $\psi (4160)$ are $3.8\times 10^{-6}$, $15.5\times 10^{-6}$, and $7.4\times 10^{-6}$, respectively.20142017-07-21 12:45:37Znanstveno delo66977ISSN: 2050-3911UDK: 539.1OceCobissID: 2704635COBISS_ID: 28162343DOI: 10.1093/ptep/ptu043ISSN pri članku: 2050-3911NUK URN: URN:SI:UM:DK:GX0ZKAEFsl