Search

The first observation of ${\chi }_{\mathrm{cJ}}\mathrm{}(J=0,1,2\mathrm{})\mathrm{}$ decays to $\Lambda \overline{\Lambda }$ is reported using $\psi \mathrm{}\left(2S\right)\mathrm{}$ data collected with the Beijing Spectrometer detector at the Beijing Electron Positron Collider. The branching ratios are determined to be $\mathcal{B}({\chi }_{c0\mathrm{}}\to \Lambda \overline{\Lambda }{)=(4.7\mathrm{}}_{-1.2}^{+1.3\mathrm{}}\pm 1.0)\times {10}^{-4},$ $\mathcal{B}({\chi }_{c1\mathrm{}}\to \Lambda \overline{\Lambda }{)=(2.6\mathrm{}}_{-0.9}^{+1.0\mathrm{}}\pm 0.6)\times {10}^{-4}$ and $\mathcal{B}({\chi }_{c2\mathrm{}}\to \Lambda \overline{\Lambda }{)=(3.3\mathrm{}}_{-1.3}^{+1.5\mathrm{}}\pm 0.7)\times {10}^{-4}.$ The results are compared with model predictions.

The processes ${\psi }^{\prime }\to p\overline{p}{\pi }^0$ and ${\psi }^{\prime }\to p\overline{p}\eta$ are studied using a sample of $14\times {10}^6$ ${\psi }^{\prime }$ decays collected with the Beijing Spectrometer at the Beijing Electron-Positron Collider. The branching fraction of ${\psi }^{\prime }\to p\overline{p}{\pi }^0$ is measured with improved precision as $(13.2\pm 1.0\pm 1.5)\times {10}^{-5}$, and ${\psi }^{\prime }\to p\overline{p}\eta$ is observed for the first time with a branching fraction of $(5.8\pm 1.1\pm 0.7)\times {10}^{-5}$, where the first errors are statistical and the second ones are systematic.

A shell model effective interaction for nuclei beyond the double magic nucleus ${}^{78}\mathrm{Ni}$ is constructed. First, the single-particle evolutions for valence neutrons above the double magic ${}^{78}\mathrm{Ni}$ are systematically explored in the $N=51$ isotones using the large scale shell model (LSSM) calculations based on the constructed effective interaction. Subsequently, we calculate the excitation energies of $2_1^{+}$ states and reduced electric quadrupole transition probabilities $B(E2;2^{+}\to 0^{+})$ for $N=52$ isotones. Notably, our calculation gives the result most consistent with the trend of the $B\left(E2\right)$ values observed in the $N=52$ isotones, especially for ${}^{84}\mathrm{Ge}$, a result that poses a serious challenge to the theoretical model. Furthermore, the collectivity in $N=52$ isotones, as well as the roles of pseudo-SU(3) symmetry, are investigated via the calculated primary configurations of their ground states and the first excited states. Additionally, the low-lying structures and band characteristics of neutron-rich Ge and Se isotopes are investigated. The ground state and the $\gamma$-soft band are constructed in our LSSM calculations, aligning well with available experimental evidence. Finally, we present the calculated evolutions of low-lying states in neutron-rich Ge and Se isotopes. The predictions for the as-yet unobserved low-lying states in these nuclei provide a comprehensive dataset to guide and inform future experimental efforts to decipher the evolution of shell structures and collectivity.

Using photon conversions to $e^{+}{e}^{-}$ pairs, the energy spectrum of inclusive photons from $\psi (2S)$ radiative decays is measured with photon energy resolution (${\sigma }_{E_{\gamma }}$) in the range from 2.3 to 3.8 MeV by BESII at the Beijing Electron-Positron Collider. The ${\chi }_{cJ}(1P)$ states ($J=0,1,2$) are clearly observed, and their masses and the spin-averaged ${\chi }_{cJ}$ mass are determined to be $M_{{\chi }_{c0}}=3414.21\pm 0.39\pm 0.27$, $M_{{\chi }_{c1}}=3510.30\pm 0.14\pm 0.16$, $M_{{\chi }_{c2}}=3555.70\pm 0.59\pm 0.39$, and $M({}^{3}P_{\mathrm{cog}})=3524.85\pm 0.32\pm 0.30\mathrm{MeV}/c^2$, respectively.

We report the investigations of specific heat, thermal conductivity, as well as thermoelectric power on the charge-density-wave (CDW) compound ${\mathrm{Lu}}_5{\mathrm{Ir}}_4{\mathrm{Si}}_{10}$ as a function of temperature. All thermal measurements consistently exhibit anomalous features around the CDW transition temperature $T_o\sim 80\mathrm{K}.$ Although the observations can be associated with the CDW formation, the measured anomalies are significantly large, in contrast to those in weak-coupled CDW materials. A quantitative analysis for the specific-heat data near the fluctuation region yields a critical exponent $\alpha \sim 2,$ much larger than the predicted value $\alpha =0.5\mathrm{}$ in the extended mean-field theory assuming three-dimensional fluctuations. We also obtained a ratio ${\gamma }^{*}/\gamma =8.4,$ a factor of 6 larger than the BCS value 1.43 in the weak-coupling limit, indicating a strong coupling of this phase transition. Besides, the observed giant excess specific heat $\Delta C_p{/C}_p\sim 26\%$ and thermal conductivity $\Delta \kappa /\kappa \sim 15\%$ at $T_o$ further support this strong-coupling scenario. These large enhancements in $C_p$ and κ are attributed to the results of substantially thermal excitation and heat carried by the soft phonons at the transition. In addition, a rapid change in the sign of thermoelectric power at $T_o$ was observed, which provides a better understanding of the evolution of electronic band structure of the system below and above the CDW formation.

$K_S^0{K}_L^0$ from ${\psi }^{\prime \prime }$ decays is searched for using the ${\psi }^{\prime \prime }$ data collected by BESII at BEPC, and the upper limit of the branching fraction is determined to be $\mathcal{B}({\psi }^{\prime \prime }\to K_S^0{K}_L^0)<2.1\times {10}^{-4}$ at 90% C.L. The measurement is compared with the prediction of the $S$- and $D$-wave mixing model of the charmonia, based on the measurements of the branching fractions of $J/\psi \to K_S^0{K}_L^0$ and ${\psi }^{\prime }\to K_S^0{K}_L^0$.

Inclusive momentum spectra and multiplicity distributions of charged particles measured with the BESII detector at center of mass energies of 2.2, 2.6, 3.0, 3.2, 4.6 and 4.8 GeV are presented. Values of the second binomial moment $R_2$ obtained from the multiplicity distributions are reported. These results are compared with both experimental data from high energy $e^{+}{e}^{-},$ ep, and $p\overline{p}$ experiments and QCD calculations.

The polarization of the ${\chi }_{c2}$ produced in $\psi (2S)$ decays into $\gamma {\chi }_{c2}$ is measured using a sample of $14\times {10}^6\psi (2S)$ events collected by the BESII detector at the BEPC storage ring. A fit to the ${\chi }_{c2}$ production and decay angular distributions in $\psi (2S)\to \gamma {\chi }_{c2}$, ${\chi }_{c2}\to {\pi }^{+}{\pi }^{-}$ and $K^{+}{K}^{-}$ yields values $x=A_1/A_0=2.08\pm 0.44$ and $y=A_2/A_0=3.03\pm 0.66$, with a correlation $\rho =0.92$ between them, where $A_{0,1,2}$ are the ${\chi }_{c2}$ helicity amplitudes. The measurement agrees with a pure $E1$ transition, and $M2$ and $E3$ contributions do not differ significantly from zero.

Isospin-symmetry breaking is responsible for the energy difference of excited states in mirror nuclei. It also influences the coefficient of the isobaric multiplet mass equation. In the present work, we extensively investigate isospin-symmetry breaking in medium-mass nuclei within ab initio frameworks. For this, we employ the ab initio valence-space in-medium similarity renormalization group approach, in which charge-symmetry and charge-independence breakings are included in the adopted nuclear force. The mirror energies of $sd$- and $pf$-shell nuclei are computed for that matter. The effects of single-particle states on weakly bound and unbound nuclear states, especially those of the $s$ wave, are discussed. Predictions are also made concerning proton drip-line nuclei bearing large mirror energy difference. Finally, the coefficient of the isobaric multiplet mass equation in $T=1/2$ and $T=1$ isospin multiplets for $A=18$ to $A=76$ is calculated.

Based on $5.8\times {10}^{7}J/\psi$ events detected in BESII, the branching fractions of $J/\psi \to \varphi \eta$ and $\varphi {\eta }^{\prime }$ are measured for different $\eta$ and ${\eta }^{\prime }$ decay modes. The results are significantly higher than previous measurements. An upper limit on $B(J/\psi \to \varphi {\pi }^0)$ is also obtained.

We have measured the low-temperature specific heat $C\left(T\right)$ for polycrystalline ${\mathrm{MgB}}_2$ prepared by high pressure synthesis. $C\left(T\right)$ below 10 K vanishes exponentially, which unambiguously indicates a fully opened superconducting energy gap. However, this gap is found to be too small to account for $T_c$ of ${\mathrm{MgB}}_2$. Together with the small specific heat jump $\Delta C/{\gamma }_n{T}_c=1.09\mathrm{}$, scenarios such as anisotropic $s$-wave or multicomponent order parameter are called for. The magnetic field dependence of $\gamma \left(H\right)$ is neither linear for a fully gapped $s$-wave superconductor nor $H^{1/2}$ for nodal order parameter. It seems that this intriguing behavior of $\gamma \left(H\right)$ is associated with the intrinsic electronic properties other than flux pinning.

We observe a narrow enhancement near $2m_p$ in the invariant mass spectrum of $p\overline{p}$ pairs from radiative $J/\psi \to \gamma p\overline{p}$ decays. No similar structure is seen in $J/\psi \to {\pi }^{0}p\overline{p}$ decays. The results are based on an analysis of a $58\times {10}^6$ event sample of $J/\psi$ decays accumulated with the BESII detector at the Beijing electron-positron collider. The enhancement can be fit with either an $S$- or $P$-wave Breit-Wigner resonance function. In the case of the $S$-wave fit, the peak mass is below $2m_p$ at $M={1859}_{-10}^{+3}{\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}}_{-25}^{+5}\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ and the total width is $\Gamma <30\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ at the 90% confidence level. These mass and width values are not consistent with the properties of any known particle.

$K^{*}(892{)}^0{\overline{K}}^{*}(892{)}^0$ signals from ${\chi }_{cJ}(J=0,1,2)$ decays are observed for the first time using a data sample of 14 million $\psi (2S)$ events accumulated in the BES II detector. The branching fractions $\mathcal{B}[{\chi }_{cJ}\to K^{*}(892{)}^0{\overline{K}}^{*}(892{)}^0]$ $(J=0,1,2)$ are determined to be $(1.78\pm 0.34\pm 0.34)\times {10}^{-3}$, $(1.67\pm 0.32\pm 0.31)\times {10}^{-3}$, and $(4.86\pm 0.56\pm 0.88)\times {10}^{-3}$ for the ${\chi }_{c0}$, ${\chi }_{c1}$, and ${\chi }_{c2}$ decays, respectively, where the first errors are statistical and the second are systematic. The significances of these signals are about $4.7\sigma$, $4.5\sigma$, and $7.6\sigma$, respectively.

Using 58 million $J/\psi$ events taken with the BES II detector at the Beijing Electron Positron Collider, the decay $J/\psi \to \gamma f_2(1270)f_2(1270)\to \gamma {\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ is observed, and the branching fraction $Br(J/\psi \to \gamma f_2{f}_2)=(9.5\pm 0.7\pm 1.6)\times {10}^{-4}$ is measured for the first time, where the quoted errors are statistical and systematic, respectively. The branching fraction of $J/\psi \to \gamma {\eta }_c$, ${\eta }_c\to f_2{f}_2$ is also measured to be $Br(J/\psi \to \gamma {\eta }_c)·Br({\eta }_c\to f_2{f}_2)=(1.3\pm {0.3}_{-0.4}^{+0.3})\times {10}^{-4}$. The nonresonant branching fraction of $J/\psi \to \gamma f_2{f}_2$ is $Br(J/\psi \to \gamma f_2{f}_2)=(8.2\pm 0.8\pm 1.7)\times {10}^{-4}.$

Using 58 million $J/\psi$ and 14 million $\psi \mathrm{}\left(2S\right)\mathrm{}$ decays obtained by the BES II experiment, the branching fraction of $J/\overrightarrow{\psi }{\pi }^{+}{\pi }^{-}{\pi }^0$ is determined. The result is $(2.10\mathrm{}\pm 0.12)\times {10}^{-2},$ which is significantly higher than previous measurements.

We report the systematic investigations of specific heat $C,$ thermal conductivity $\kappa ,$ and thermoelectric power on the rare-earth–transition-metal ternary compounds $R_5{\mathrm{Ir}}_4{\mathrm{Si}}_{10}(R$ = Dy, Ho, Er, Tm, Yb, and Lu) as a function of temperature. For all measured thermal properties, pronounced anomalous responses were observed near the charge-density-wave (CDW) transition temperature $T_P$ in these compounds (except $R=\mathrm{Yb}).\mathrm{}$ Particularly, the complex temperature-dependent thermal transport properties observed in these materials indicate very complicated heat transport processes during the phase transitions. Although these phase transitions can be associated with the CDW formation, the measured anomalies are considerably large, in contrast to those found in other weak-coupled CDW materials. In addition, it is found that $T_P$ decreases linearly with the decreasing ionic radius of rare-earth elements from ${\mathrm{Dy}}_5{\mathrm{Ir}}_4{\mathrm{Si}}_{10}$ to ${\mathrm{Lu}}_5{\mathrm{Ir}}_4{\mathrm{Si}}_{10},$ but it increases nonlinearly with x in $({\mathrm{Lu}}_{1-x}{\mathrm{Er}}_x{)}_5{\mathrm{Ir}}_4{\mathrm{Si}}_{10}.$ These observations indicate that both ionic-size and atomic-disorder effects play important roles for the CDW transitions in this class of materials.

An enhancement near the $m_p+M_{\Lambda }$ mass threshold is observed in the combined $p\overline{\Lambda }$ and $\overline{p}\Lambda$ invariant-mass spectrum from $J/\psi \to p{K}^{-}\overline{\Lambda }+\mathrm{c}.\mathrm{c}.$ decays. It can be fit with an $S$-wave Breit-Wigner resonance with a mass $m=2075\pm 12(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 5(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{M}\mathrm{e}\mathrm{V}$ and a width of $\Gamma =90\pm 35(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 9(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{M}\mathrm{e}\mathrm{V}$; it can also be fit with a $P$-wave Breit-Wigner resonance. Evidence for a similar enhancement is also observed in ${\psi }^{\prime }\to p{K}^{-}\overline{\Lambda }+\mathrm{c}.\mathrm{c}.$ decays. The analysis is based on samples of $5.8\times {10}^{7}J/\psi$ and $1.4\times {10}^7{\psi }^{\prime }$ decays accumulated in the BES II detector at the Beijing Electron-Positron Collider.

The processes $\psi \mathrm{}\left(2S\right)\mathrm{}\to \gamma {\chi }_{\mathrm{cJ}},$ ${\chi }_{\mathrm{cJ}}\to p\overline{p}$ $\mathrm{}(J=0,1,2\mathrm{})\mathrm{}$ are studied using a sample of $14\times {10}^6$ $\psi \mathrm{}\left(2S\right)\mathrm{}$ decays collected with the Beijing Spectrometer at the Beijing Electron-Positron Collider. Very clear ${\chi }_{c0\mathrm{}},$ ${\chi }_{c1\mathrm{}}$ and ${\chi }_{c2\mathrm{}}$ signals are observed, and the branching fractions $\mathcal{B}({\chi }_{\mathrm{cJ}}\to p\overline{p})$ $\mathrm{}(J=0,1,2\mathrm{})\mathrm{}$ are determined to be ${(27.1\mathrm{}}_{-3.9}^{+4.3\mathrm{}}\pm 4.7)\times {10}^{-5},$ ${(5.7\mathrm{}}_{-1.5}^{+1.7\mathrm{}}\pm 0.9)\times {10}^{-5},$ and ${(6.5\mathrm{}}_{-2.1}^{+2.4\mathrm{}}\pm 1.0)\times {10}^{-5},$ respectively, where the first errors are statistical and the second are systematic.

Decays of the $\psi (2S)$ into vector plus pseudoscalar meson final states have been studied with $14\times {10}^6$ $\psi (2S)$ events collected with the BESII detector. Branching fractions of $\psi (2S)\to \varphi \eta$, $\varphi {\eta }^{\prime }$, and $\omega {\eta }^{\prime }$, and upper limits of $\psi (2S)\to \varphi {\pi }^0$ and $\omega \eta$ are obtained: $B\mathbf{(}\psi (2S)\to \varphi \eta \mathbf{)}=(3.3\pm 1.1\pm 0.5)\times {10}^{-5}$, $B\mathbf{(}\psi (2S)\to \varphi {\eta }^{\prime }\mathbf{)}=(3.1\pm 1.4\pm 0.7)\times {10}^{-5}$, and $B\mathbf{(}\psi (2S)\to \omega {\eta }^{\prime }\mathbf{)}=({3.2}_{-2.0}^{+2.4}\pm 0.7)\times {10}^{-5}$; and $B\mathbf{(}\psi (2S)\to \varphi {\pi }^0\mathbf{)}<0.40\times {10}^{-5}$, and $B\mathbf{(}\psi (2S)\to \omega \eta \mathbf{)}<3.1\times {10}^{-5}$ at the 90% C.L. These results are used to test the perturbative QCD “12% rule.”