Measurements of the branching fractions of the semileptonic decays $Xi_{c}^{0} to Xi^{-} ell^{+} u_{ell}$ and the asymmetry parameter of $Xi_{c}^{0} to Xi^{-} pi^{+}$

Using data samples of 89.5 and 711 fb$^{-1}$ recorded at energies of $sqrt{s}=10.52$ and $10.58$ GeV, respectively, with the Belle detector at the KEKB $e^+e^-$ collider, we report measurements of branching fractions of semileptonic decays $Xi_{c}^{0} to Xi^{-} ell^{+} u_{ell}$ ($ell=e$ or $mu$) and the $CP$-asymmetry parameter of $Xi_{c}^{0} to Xi^{-} pi^{+}$ decay. The branching fractions are measured to be ${cal B}(Xi_{c}^{0} to Xi^{-} e^{+} u_{e})=(1.31 pm 0.04 pm 0.07 pm 0.38)%$ and ${cal B}(Xi_{c}^{0} to Xi^{-} mu^{+} u_{mu})=(1.27 pm 0.06 pm 0.10 pm 0.37)%$, and the decay parameter $alpha_{Xipi}$ is measured to be $0.63 pm 0.03 pm 0.01$ with much improved precision compared to the current world average. The corresponding ratio ${cal B}(Xi_{c}^{0} to Xi^{-} e^{+} u_{e})/{cal B}(Xi_{c}^{0} to Xi^{-} mu^{+} u_{mu})$ is $1.03 pm 0.05pm 0.07$, which is consistent with the expectation of lepton flavor universality. The first measured asymmetry parameter ${cal A}_{CP} = (alpha_{Xi^{-}pi^{+}} + alpha_{bar{Xi}^{+}pi^{-}})/(alpha_{Xi^{-}pi^{+}} - alpha_{bar{Xi}^{+}pi^{-}}) = 0.024 pm 0.052 pm 0.014$ is found to be consistent with zero. The first and the second uncertainties above are statistical and systematic, respectively, while the third ones arise due to the uncertainty of the $Xi_{c}^{0} to Xi^{-} pi^+$ branching fraction.

First measurements of absolute branching fractions of $Xi_c^0$ at Belle

We present the first measurements of absolute branching fractions of $Xi_c^0$ decays into $Xi^- pi^+$, $Lambda K^- pi^+$, and $p K^- K^- pi^+$ final states. The measurements are made using a data set comprising $(772pm 11)times 10^{6}$ $Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+e^-$ collider. We first measure the absolute branching fraction for $B^- to bar{Lambda}_c^- Xi_c^0$ using a missing-mass technique; the result is ${cal B}(B^- to bar{Lambda}_c^- Xi_c^0) = (9.51 pm 2.10 pm 0.88) times 10^{-4}$. We subsequently measure the product branching fractions ${cal B}(B^- to bar{Lambda}_c^- Xi_c^0){cal B}(Xi_c^0 to Xi^- pi^+)$, ${cal B}( B^- to bar{Lambda}_c^- Xi_c^0) {cal B}(Xi_c^0 to Lambda K^- pi^+)$, and ${cal B}( B^- to bar{Lambda}_c^- Xi_c^0) {cal B}(Xi_c^0 to p K^- K^- pi^+)$ with improved precision. Dividing these product branching fractions by the result for $B^- to bar{Lambda}_c^- Xi_c^0$ yields the following branching fractions: ${cal B}(Xi_c^0 to Xi^- pi^+)= (1.80 pm 0.50 pm 0.14)%$, ${cal B}(Xi_c^0 to Lambda K^- pi^+)=(1.17 pm 0.37 pm 0.09)%$, and ${cal B}(Xi_c^0 to p K^- K^- pi^+)=(0.58 pm 0.23 pm 0.05)%.$ For the above branching fractions, the first uncertainties are statistical and the second are systematic. Our result for ${cal B}(Xi_c^0 to Xi^- pi^+)$ can be combined with $Xi_c^0$ branching fractions measured relative to $Xi_c^0 to Xi^- pi^+$ to yield other absolute $Xi_c^0$ branching fractions.

Evidence of a structure in $bar{K}^{0} Lambda_{c}^{+}$ consistent with a charged $Xi_c(2930)^{+}$, and updated measurement of $bar{B}^{0} to bar{K}^{0} Lambda_{c}^{+} bar{Lambda}_{c}^{-}$ at Belle

We report evidence for the charged charmed-strange baryon $Xi_{c}(2930)^+$ with a signal significance of 3.9$sigma$ with systematic errors included. The charged $Xi_{c}(2930)^+$ is found in its decay to $K_{S}^{0} Lambda_{c}^+$ in the substructure of $bar{B}^{0} to K^{0}_{S} Lambda_{c}^{+} bar{Lambda}_{c}^{-}$ decays. The measured mass and width are $[2942.3 pm 4.4 (rm stat.) pm 1.5(rm syst.)]$~MeV/$c^{2}$ and $[14.8 pm 8.8(rm stat.) pm 2.5(rm syst.)]$~MeV, respectively, and the product branching fraction is $cal{B}(bar{B}^{0} to Xi_c(2930)^{+} bar{Lambda}_{c}^{-}) cal{B}(Xi_c(2930)^{+}to bar{K}^{0} Lambda_{c}^{+})=[2.37 pm 0.51 (rm stat.)pm 0.31(rm syst.)]times 10^{-4}$. We also measure $cal{B}(bar{B}^{0} to bar{K}^{0} Lambda_{c}^{+} bar{Lambda}_{c}^{-}) = [3.99 pm 0.76(rm stat.) pm 0.51(rm syst.)] times 10^{-4}$ with greater precision than previous experiments, and present the results of a search for the charmonium-like state $Y(4660)$ and its spin partner, $Y_{eta}$, in the $Lambda_{c}^{+}bar{Lambda}_{c}^{-}$ invariant mass spectrum. No clear signals of the $Y(4660)$ or $Y_{eta}$ are observed and the 90% credibility level (C.L.) upper limits on their production rates are determined. These measurements are obtained from a sample of $(772pm11)times 10^{6} Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.

Observation of $Xi_{c}(2930)^0$ and updated measurement of $B^{-} to K^{-} Lambda_{c}^{+} bar{Lambda}_{c}^{-}$ at Belle

We report the first observation of the $Xi_{c}(2930)^0$ charmed-strange baryon with a significance greater than 5$sigma$. The $Xi_{c}(2930)^0$ is found in its decay to $K^- Lambda_{c}^+$ in $B^{-} to K^{-} Lambda_{c}^{+} bar{Lambda}_{c}^{-}$ decays. The measured mass and width are $[2928.9 pm 3.0(rm stat.)^{+0.9}_{-12.0}(rm syst.)]$ MeV/$c^{2}$ and $[19.5 pm 8.4(rm stat.) ^{+5.9}_{-7.9}(rm syst.)]$ MeV, respectively, and the product branching fraction is ${cal B}(B^{-} to Xi_{c}(2930)^0 bar{Lambda}_{c}^{-}) {cal B}(Xi_{c}(2930)^0 to K^- Lambda_{c}^{+})=[1.73 pm 0.45(rm stat.) pm 0.21(rm syst.)]times 10^{-4}$. We also measure ${cal B}(B^{-} to K^{-} Lambda_{c}^{+} bar{Lambda}_{c}^{-}) = [4.80 pm 0.43(rm stat.) pm 0.60(rm syst.)] times 10^{-4}$ with improved precision, and search for the charmonium-like state $Y(4660)$ and its spin partner, $Y_{eta}$, in the $Lambda_{c}^{+}bar{Lambda}_{c}^{-}$ invariant mass spectrum. No clear signals of the $Y(4660)$ nor its spin partner are observed and the 90% credibility level (C.L.) upper limits on their production rates are determined. These measurements are obtained from a sample of $(772pm11)times 10^{6} Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance by the Belle detector at the KEKB asymmetric energy electron-positron collider.