arXiv subjects
Publications and source records attributed to J. Z. Bai.
We report values of $R = σ(e^+e^-\to {hadrons})/σ(e^+e^-\toμ^+μ^-)$ for 85 center-of-mass energies between 2 and 5 GeV measured with the upgraded Beijing Spectrometer at the Beijing Electron-Positron Collider.
Data are presented on the reaction J/psi--> p pbar eta using 7.8 million J/psi triggers collected by the BEjing Spectrometer (BES). A partial wave analysis is performed. A clear enhancement near the p eta(pbar eta) threshold is observed. It is fitted with a JP=1/2- resonance with mass $M= 1530\pm 10$ MeV and width $Γ= 95\pm 25$ MeV. In addition, there is a peak around 1650 MeV with JP=1/2- preferred also, fitted with $M =1647\pm 20$ MeV and $Γ= 145^{+80}_{-45}$ MeV. These two N* resonances are believed to be the two well established states, S11(1535) and S11(1650), respectively. It is the first partial wave study of the production of these resonances from J/psi decays.
The branching fraction of the psi(2S) decay into tau pair has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is $B_{ττ}=(2.71\pm 0.43 \pm 0.55) \times 10^{-3}$, where the first error is statistical and the second is systematic. This value, along with those for the branching fractions into e+e- and mu+mu of this resonance, satisfy well the relation predicted by the sequential lepton hypothesis. Combining all these values with the leptonic width of the resonance the total width of the psi(2S) is determined to be $(252 \pm 37)$ keV.
A sample of 3.95M $ψ(2S)$ decays registered in the BES detector are used to study final states containing pairs of octet and decuplet baryons. We report branching fractions for $ψ(2S)\to p\bar{p}$, $Λ\barΛ$, $Σ^0\barΣ{}^0$, $Ξ^-\barΞ{}^+$, $Δ^{++}\barΔ{}^{--}$, $Σ^+(1385)\barΣ{}^-(1385)$, $Ξ^0(1530)\barΞ{}^0(1530)$, and $Ω^-\barΩ{}^+$. These results are compared to expectations based on the SU(3)-flavor symmetry, factorization, and perturbative QCD.
BES data on $J/ψ\to γ(K^{\pm} K_S^0 π^{\mp})$ are presented. There is a strong peak due to $η(1440)/ι$, which is fitted with a Breit-Wigner amplitude with $s$-dependent widths for decays to $K^*K$, $κK$, $ηππ$ and $ρρ$; $κ$ refers to the $Kπ$ S-wave. At a $K\bar{K}π$ mass of $\sim 2040$ MeV, there is a second peak with width $\sim 400$ MeV; $J^P = 0^-$ is preferred over $1^+$ and $2^-$ respectively by 5.2 and 6.8 standard deviations. It is a possible candidate for a $0^-$ $s\bar sg$ hybrid partner of $π(1800)$.
BES data on $J/ψ\to γ(π^{+} π^{-} π^{+} π^{-})$ have been analyzed into partial waves. We fit with resonances having $J^{PC}=2^{++}$ at 1275 MeV, $0^{++}$ at 1500 MeV, $2^{++}$ at 1565 MeV, $0^{++}$ at 1740 MeV, $2^{++}$ at 1940 MeV and $0^{++}$ at 2104 MeV, plus a broad $0^-$ component. The $0^{++}$ resonances decay dominantly to $σσ$, while $2^{++}$ resonances in the high mass region decay mainly to $f_2(1270)σ$ and $σσ$; $2^{++}$ resonances from the low mass region decay dominantly to $ρρ$.
BES data on $J/ψ\to γ(K^+K^-π^+π^-)$ are presented. The $K^*\bar K^*$ contribution peaks strongly near threshold. It is fitted with a broad $0^{-+}$ resonance with mass $M = 1800 \pm 100$ MeV, width $Γ= 500 \pm 200$ MeV. A broad $2^{++}$ resonance peaking at 2020 MeV is also required with width $\sim 500$ MeV. There is further evidence for a $2^{-+}$ component peaking at 2.55 GeV. The non-$K^*\bar K^*$ contribution is close to phase space; it peaks at 2.6 GeV and is very different from $K^{*}\bar{K^{*}}$.
In a sample of 7.8 million $J/ψ$ decays collected in the Beijing Spectrometer, the process J/$ψ\toγη_c$ is observed for five different $η_c$ decay channels: $K^+K^-π^+π^-$, $π^+π^-π^+π^-$, $K^\pm K^0_S π^\mp$ (with $K^0_S\toπ^+π^-$), $ϕϕ$ (with $ϕ\to K^+K^-$) and $K^+K^-π^0$. From these signals, we determine the mass of $η_c$ to be $2976.6\pm2.9\pm1.3$ MeV. Combining this result with a previously reported result from a similar study using $ψ(2S)\toγη_c$ detected in the same spectrometer gives $m_{η_c} = 2976.3\pm2.3\pm1.2$ MeV. For the combined samples, we obtain $Γ_{η_c} = 11.0\pm 8.1\pm 4.1$ MeV.
Using a sample of 3.8 M psi(2S) events accumulated with the BES detector, the process psi(2S) -> pi^+ pi^- J/psi is studied. The angular distributions are compared with the general decay amplitude analysis of Cahn. We find that the dipion system requires some D-wave, as well as S-wave. On the other hand, the J/psi-(pi pi) relative angular momentum is consistent with being pure S-wave. The decay distributions have been fit to heavy quarkonium models, including the Novikov-Shifman model. This model, which is written in terms of the parameter kappa, predicts that D-wave should be present. We determine kappa = 0.183 +/- 0.002 +/- 0.003 based on the joint dipion mass - cos theta distribution. The fraction of D-wave as a function of the dipion mass is found to decrease with increasing dipion mass, in agreement with the model. We have also fit the Mannel-Yan model, another model that allows D-wave.
Using the upgraded Beijing Spectrometer (BESII), we have measured the total cross section for $e^+e^-$ annihilation into hadronic final states at center-of-mass energies of 2.6, 3.2, 3.4, 3.55, 4.6 and 5.0 GeV. Values of $R$, $σ(e^+e^-\to {hadrons})/σ(e^+e^-\toμ^+μ^-)$, are determined.
The first measurement of B(0->phi X0) and an upper limit for B(D+->phi X+) are determined from 22.3 pb-1 of e+e- annihilation data at a C. M. energy of 4.03 GeV. The data were recorded by the Beijing Spectrometer (BES) at BEPC. A recoil charge method is applied for the first time to charm threshold data to determine the charge of the D meson in the recoil from 9054+-309+-416 reconstructed D0, D+ mesons. The branching fractions B(D->phiX0) =(1.71+0.76-0.71+-0.17)%, and B(D+->phiX+) <1.8% are determined from 10 events with a reconstructed D and a recoiling phi. In addition, a 90% C.L. upper limit of B(D+->phi e+X0)<1.6% is determined from a search for semileptonic decays of the D+.
A sample of 3.79 million psi(2S) decays observed in the BES detector is used to study the decays of charmonium to axialvector plus pseudoscalar mesons. The branching fraction for psi(2S) to b1pi agrees with expectations. A contrasting behavior between the psi(2S) and the J/psi decays to K1K cannot be accommodated by an adjustment of the singlet-triplet axialvector mixing angle.
Hadronic decays of the P-wave spin-triplet charmonium states χ_cJ (J=0,1,2) are studied using a sample of ψ(2S) decays collected by the BES detector operating at the BEPC storage ring. Branching fractions for the decays χ_c1 going to K_s^0 K^+ π^- + c.c., χ_c0 going to K_s^0 K_s^0, χ_c2 going to K_s^0 K_s^0, χ_c0 going to ϕϕ, χ_c2 going to ϕϕand χ_cJ going to K^+ K^- K^+ K^- are measured for the first time, and those for χ_cJ going to π^+ π^- π^+ π^-, $ χ_cJ going to π^+ π^- K^+ K^-, χ_cJ going to π^+ π^- p \bar{p} and χ_cJ going to 3(π^+ π^-) are measured with improved precision. In addition, we determine the masses of the χ_c0 and η_c to be M_{χ_{c0}}=3414.1 \pm 0.6(stat) \pm 0.8 (sys) MeV and M_{η_c}=2975.8 \pm 3.9(stat) \pm 1.2 (sys) MeV.
The processes $ψ(2S)\to γπ^+ π^-$, $γK^+ K^-$ and $γp \bar{p}$ have been studied using a sample of $3.7 \times 10^6$ produced $ψ(2S)$ decays. We determine the total width of the $χ_{c0}$ to be $Γ^{tot}_{χ_{c0}} = 14.3\pm 2.0\pm 3.0$ MeV. We present the first measurement of the branching fraction $B(χ_{c0} \to p \bar{p}) = (16.3 \pm 4.4 \pm 5.4)\times 10^{-5}$, where the first error is statistical and the second one systematic. Branching fractions of $χ_{c0,2} \to π^+ π^-$ and $K^+ K^-$ are also reported.
A comparison of the rates for psi(2S) -> pi^+ pi^- J/psi, J/psi -> l^+l^- and J/psi -> anything is used to determine the J/psi leptonic branching fractions. The results are B(J/psi -> e^+e^-) = (5.90 +/- 0.05 +/- 0.10)% and B(J/psi -> mu^+ mu^-) = (5.84 +/- 0.06 +/- 0.10)%, where the first error is statistical and the second is systematic. Assuming lepton universality, the leptonic branching fraction of the J/psi is B(J/psi -> l^+ l^-) = (5.87 +/- 0.04 +/- 0.09)% per species. This result is used to estimate the QCD scale factor Lambda_MSbar and the strong coupling constant alpha_s.
We report the first measurements of the branching fractions: $B(ψ(2S) -> γη') = (1.54 \pm 0.31 \pm 0.23) x 10^{-4}$ and $B(ψ(2S) -> γη) = (0.53 \pm 0.31 \pm 0.08) x 10^{-4}$. The $ψ(2S)$ to $γη'$ result is consistent with expectations of a model that considers the possibility of $η' - η$ mixing. The ratio of the $ψ(2S)$ to $γη'$ and $ψ(2S)$ to $γη$ rates is used to determine the pseudoscalar octet-singlet mixing angle.