Search Results (74 total)

The Courant-Snyder theory gives a complete description of the uncoupled transverse dynamics of charged particles in electromagnetic focusing lattices. In this paper, the Courant-Snyder theory is generalized to the case of coupled transverse dynamics with two degrees of freedom. The generalized theory has the same structure as the original Courant-Snyder theory for one degree of freedom. The four basic components of the original Courant-Snyder theory, i.e., the envelope equation, phase advance, transfer matrix, and the Courant-Snyder invariant, all have their counterparts, with remarkably similar expressions, in the generalized theory presented here. In the generalized theory, the envelope function is generalized into an envelope matrix, and the envelope equation becomes a matrix envelope equation with matrix operations that are noncommutative. The generalized theory gives a new parametrization of the 4D symplectic transfer matrix that has the same structure as the parametrization of the 2D symplectic transfer matrix in the original Courant-Snyder theory. All of the parameters used in the generalized Courant-Snyder theory correspond to physical quantities of importance, and this parametrization can provide a valuable framework for accelerator design and particle simulation studies. A time-dependent canonical transformation is used to develop the generalized Courant-Snyder theory. Applications of the new theory to strongly and weakly coupled dynamics are given. It is shown that the stability of coupled dynamics can be determined by the generalized phase advance developed. Two stability criteria are given, which recover the known results about sum and difference resonances in the weakly coupled limit.

Based on 58×10⁶ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider, the baryon pair processes J/ψ→Σ⁺Σ̅ ^- and J/ψ→Ξ⁰Ξ̅ ⁰ are observed for the first time. The branching fractions are measured to be B(J/ψ→Σ⁺Σ̅ ^-)=(1.50±0.10±0.22)×10^-3 and B(J/ψ→Ξ⁰Ξ̅ ⁰)=(1.20±0.12±0.21)×10^-3, where the first errors are statistical and the second ones are systematic.

We observe an obvious anomalous line shape of the e⁺e^-→ hadrons total cross sections in the energy region between 3.700 and 3.872 GeV. It is inconsistent with the explanation for only one simple ψ(3770) resonance with a statistical significance of 7σ. The anomalous line shape may be explained by two possible enhancements of the inclusive hadron production near the center-of-mass energies of 3.764 and 3.779 GeV, indicating that either there is likely a new structure in addition to the ψ(3770) resonance around 3.773 GeV, or there are some physics effects reflecting the DD̅ production dynamics.

Using ψ(2S)→π⁺π^-J/ψ events in a sample of 14.0×10⁶ ψ(2S) decays collected with the BES-II detector, a search for the decay of the J/ψ to invisible final states is performed. No signal is found, and an upper limit at the 90% confidence level is determined to be 1.2×10^-2 for the ratio B(J/ψ→invisible) / B(J/ψ→μ⁺μ^-). This is the first search for J/ψ decays to invisible final states.

The decays of J/ψ→ηϕf₀(980)[η→γγ,ϕ→K⁺K^-,f₀(980)→π⁺π^-] are analyzed using a sample of 5.8×10⁷ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider. A structure at around 2.18  GeV/c² with about 5σ significance is observed in the ϕf₀(980) invariant mass spectrum. A fit with a Breit-Wigner function gives the peak mass and width of m=2.186±0.010(stat)±0.006(syst)  GeV/c² and Γ=0.065±0.023(stat)±0.017(syst)  GeV/c², respectively, which are consistent with those of Y(2175), observed by the BABAR Collaboration in the initial-state radiation process e⁺e^-→γ_ISRϕf₀(980). The production branching ratio is determined to be Br(J/ψ→ηY(2175))Br(Y(2175)→ϕf₀(980))Br(f₀(980)→π⁺π^-)=[3.23±0.75(stat)±0.73(syst)]×  10^-4, assuming that the Y(2175) is a 1^-- state.

The decays of J/ψ→ωKK̅ π and J/ψ→ϕKK̅ π are studied using 5.8×10⁷ J/ψ events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider (BEPC). The K_S⁰K^±π^∓ and K⁺K^-π⁰ systems, produced in J/ψ→ωKK̅ π, have enhancements in the invariant mass distributions at around 1.44  GeV/c². However, there is no evidence for mass enhancements in the KK̅ π system in J/ψ→ϕKK̅ π. The branching fractions of J/ψ→ωK_S⁰K^±π^∓, ϕK_S⁰K^±π^∓, ωK^*K̅ +c.c., and ϕK^*K̅ +c.c. are obtained, and the J/ψ→ηK_S⁰K^±π^∓ branching fraction is measured for the first time.

A variational symplectic integrator for the guiding-center motion of charged particles in general magnetic fields is developed for long-time simulation studies of magnetized plasmas. Instead of discretizing the differential equations of the guiding-center motion, the action of the guiding-center motion is discretized and minimized to obtain the iteration rules for advancing the dynamics. The variational symplectic integrator conserves exactly a discrete Lagrangian symplectic structure, and has better numerical properties over long integration time, compared with standard integrators, such as the standard and variable time-step fourth order Runge-Kutta methods.

Using 58×10⁶ J/ψ events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider, the decays J/ψ→γϕρ and J/ψ→γωρ are searched for, and upper limits on their branching fractions are reported at the 90% C.L. No clear structures are observed in the γρ, γϕ, or ρϕ mass spectra for J/ψ→γϕρ nor in the γρ, γω, or ρω mass spectra for J/ψ→γωρ.

By analyzing the data collected at the center-of-mass energy E_cm=3.773  GeV and below the DD̅ meson pair production threshold with the BES-II detector at the BEPC Collider, we directly measured the observed non-DD̅ cross section of ψ(3770) decay to be σ_{ψ(3770)→non-DD̅} ^obs=(0.95±0.35±0.29)  nb at E_cm=3.773  GeV, and the branching fraction BF[ψ(3770)→non-DD̅ ]=(13.4±5.0±3.6)% for inclusive non-DD̅ decay of ψ(3770). We also determined the cross section for DD̅ meson pair production to be σ_DD̅ ^obs=(6.12±0.37±0.23)  nb at E_cm=3.773  GeV.

Using 14.0×10⁶ψ(2S) events collected with the BES-II detector, the C-parity violating process J/ψ→γγ via ψ(2S)→π⁺π^-J/ψ is studied. We determine a new upper limit for the J/ψ→γγ branching ratio of B(J/ψ→γγ)<2.2×10^-5 at the 90% C.L., which is about 20 times lower than the previous measurement.

Using 58×10⁶ J/ψ and 14×10⁶ ψ(2S) events collected by the BESII detector at the BEPC, branching fractions or upper limits for the decays J/ψ and ψ(2S)→ΛΛ̅ π⁰ and ΛΛ̅ η are measured. For the isospin violating decays, the upper limits are determined to be B(J/ψ→ΛΛ̅ π⁰)<6.4×10^-5 and B[ψ(2S)→ΛΛ̅ π⁰]<4.9×10^-5 at the 90% confidence level. The isospin conserving process J/ψ→ΛΛ̅ η is observed for the first time, and its branching fraction is measured to be B(J/ψ→ΛΛ̅ η)=(2.62±0.60±0.44)×10^-4, where the first error is statistical and the second one is systematic. No ΛΛ̅ η signal is observed in ψ(2S) decays, and B[ψ(2S)→ΛΛ̅ η]<1.2×10^-4 is set at the 90% confidence level. Branching fractions of J/ψ decays into Σ⁺π^-Λ̅ and Σ̅ ^-π⁺Λ are also reported, and the sum of these branching fractions is determined to be B(J/ψ→Σ⁺π^-Λ̅ +c.c.)=(1.52±0.08±0.16)×10^-3.

Using 14×10⁶ ψ(2S) events accumulated at the BESII detector, we report first measurements of branching fractions or upper limits for ψ(2S) decays into γpp̅ , γ2(π⁺π^-), γK_S⁰K⁺π^-+c.c., γK⁺K^-π⁺π^-, γK^*0K^-π⁺+c.c., γK^*0K̅ ^*0, γπ⁺π^-pp̅ , γ2(K⁺K^-), γ3(π⁺π^-), and γ2(π⁺π^-)K⁺K^- with the invariant mass of hadrons below 2.9  GeV/c². We also report branching fractions of ψ(2S) decays into 2(π⁺π^-)π⁰, ωπ⁺π^-, ωf₂(1270), b₁^±π^∓, and π⁰2(π⁺π^-)K⁺K^-.

Collective effects with strong coupling between the longitudinal and transverse dynamics are of fundamental importance for applications of high-intensity bunched beams. The self-consistent Vlasov-Maxwell equations are applied to high-intensity finite-length charge bunches, and a generalized δf particle simulation algorithm is developed for bunched beams with or without energy anisotropy. The nonlinear δf method exhibits minimal noise and accuracy problems in comparison with standard particle-in-cell simulations. Systematic studies are carried out under conditions corresponding to strong 3D nonlinear space-charge forces in the beam frame. For charge bunches with isotropic energy, finite bunch-length effects are clearly evident by the fact that the spectra for an infinitely long coasting beam and a nearly spherical charge bunch have strong similarities, whereas the spectra have distinctly different features when the bunch length is varied between these two limiting cases. For bunched beams with anisotropic energy, there exists no exact kinetic equilibrium because the particle dynamics do not conserve transverse energy and longitudinal energy separately. A reference state in approximate dynamic equilibrium has been constructed theoretically, and a quasi-steady state has been established in the simulations for the anisotropic case. Collective excitations relative to the reference state have been simulated using the generalized δf algorithm. In particular, the electrostatic Harris instability driven by strong energy anisotropy is investigated for a finite-length charge bunch. The observed growth rates are larger than those obtained for infinitely long coasting beams. However, the growth rate decreases for increasing bunch length to a value similar to the case of a long coasting beam. For long bunches, the instability is axially localized symmetrically relative to the beam center, and the characteristic wavelength in the longitudinal direction is comparable to the transverse dimension of the beam.

We report measurements of the continuum R_uds near the center-of-mass energy of 3.70 GeV, the R_{uds(c)+ψ(3770)}(s) and the R_had(s) values in e⁺e⁻ annihilation at 68 energy points in the energy region between 3.650 and 3.872 GeV with the BES-II detector at the BEPC Collider. We obtain the R_uds for the continuum light hadron (containing u, d, and s quarks) production near the DD̅ threshold to be R_uds=2.141±0.025±0.085.

Using 14M ψ(2S) events collected at BESII, the branching fraction of ψ(2S)→τ⁺τ^- is measured to be Br_ττ=(3.08±0.21±0.38)×10^-3, where the first error is statistical and the second is systematic.

Using a data sample of 58×10⁶ J/ψ decays collected with the Beijing Spectrometer II detector at the Beijing Electron Positron Collider, searches for invisible decays of η and η^′ in J/ψ to ϕη and ϕη^′ are performed. The ϕ signals, which are reconstructed in K⁺K^- final states, are used to tag the η and η^′ decays. No signals are found for the invisible decays of either η or η^′, and upper limits at the 90% confidence level are determined to be 1.65×10^-3 for the ratio B(η→invisible) / B(η→γγ) and 6.69×10^-2 for B(η^′→invisible) / B(η^′→γγ). These are the first searches for η and η^′ decays into invisible final states.

A broad peak is observed at low K⁺K^- invariant mass in J/ψ→K⁺K^-π⁰ decays found in a sample of 5.8×10⁷ J/ψ events collected with the BESII detector. The statistical significance of the broad resonance is much larger than 5σ. A partial wave analysis shows that the J^PC of this structure is 1^--. Its pole position is determined to be [1576_-55⁺⁴⁹(stat)_-91⁺⁹⁸(syst)]  MeV/c²-i / 2[818_-23⁺²²(stat)_-133⁺⁶⁴(syst)]  MeV/c². These parameters are not compatible with any known meson resonances.

Radiative decays of the ψ(2S) into γKK̅ π and γηπ⁺π^- final states are studied using 14×10⁶ ψ(2S) events collected with the BESII detector. Branching fractions or upper limits on the branching fractions of ψ(2S) and χ_cJ decays are reported. No significant signal for η(1405)/η(1475) is observed in the KK̅ π or ηπ⁺π^- mass spectra, and upper limits on the branching fractions of ψ(2S)→γη(1405)/η(1475), η(1405)/η(1475)→KK̅ π, and ηπ⁺π^- are determined.

We measure the branching fractions for ψ(3770)→D⁰D̅ ⁰, D⁺D^-, DD̅ , and non-DD̅ to be (46.7±4.7±2.3)%, (36.9±3.7±2.8)%, (83.6±7.3±4.2)%, and (16.4±7.3±4.2)%, respectively. The resonance parameters of ψ(3770) and ψ(2S) are measured to be M_ψ(3770)=3772.2±0.7±0.3  MeV, Γ_ψ(3770)^tot=26.9±2.4±0.3  MeV, and Γ_ψ(3770)^ee=251±26±11  eV; M_ψ(2S)=3685.5±0.0±0.3  MeV, Γ_ψ(2S)^tot=331±58±2  keV, and Γ_ψ(2S)^ee=2.330±0.036±0.110  keV. We also measure the light hadron R value to be R_uds=2.262±0.054±0.109 in the energy region from 3.660 to 3.872 GeV.

The decay J/ψ→N̅ Nπ provides an effective isospin 1/2 filter for the πN system due to isospin conservation. Using 58×10⁶ J/ψ decays collected with the Beijing Electromagnetic Spectrometer at the Beijing Electron Positron Collider, more than 100 thousand J/ψ→pπ^-n̅ +c.c. events are obtained. Besides the two well-known N^* peaks at around 1500  MeV/c² and 1670  MeV/c², there are two new, clear N^* peaks in the pπ invariant mass spectrum around 1360  MeV/c² and 2030  MeV/c² with statistical significance of 11σ and 13σ, respectively. We identify these as the first direct observation of the N^*(1440) peak and a long-sought missing N^* peak above 2  GeV/c² in the πN invariant mass spectrum.

Using 14×10⁶ ψ(2S) events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider, the branching fractions of ψ(2S) decays to pn̅ π^- and p̅ nπ⁺ and the branching fractions of the main background channels ψ(2S)→pn̅ π^-π⁰, ψ(2S)→γχ_c0→γpn̅ π^-, ψ(2S)→γχ_c2→γpn̅ π^-, and ψ(2S)→γχ_cJ→γpn̅ π^-π⁰ are determined. The contributions of the N^* resonances in ψ(2S)→pn̅ π^-+c.c. are also discussed.

The decay channel J/ψ→γωω, ω→π⁺π^-π⁰ is analyzed using a sample of 5.8×10⁷ J/ψ events collected with the BESII detector. The ωω invariant mass distribution peaks at 1.76  GeV/c², just above the ωω threshold. Analysis of angular correlations indicates that the ωω system below 2  GeV/c² is predominantly pseudoscalar. A partial wave analysis confirms the predominant pseudoscalar structure, together with small 0⁺⁺ and 2⁺⁺ contributions, and yields a pseudoscalar mass M=1744±10 (stat)±15 (syst)  MeV/c², a width Γ=244_-21⁺²⁴ (stat)±25 (syst)  MeV/c², and a product branching fraction Br(J/ψ→γη(1760))·Br(η(1760)→ωω)=(1.98±0.08 (stat)±0.32 (syst))×10^-3.

The single-particle dynamics in a time-dependent focusing field is examined. The existence of the Courant-Snyder invariant, a fundamental concept in accelerator physics, is fundamentally a result of the corresponding symmetry admitted by the harmonic oscillator equation with linear time-dependent frequency. It is demonstrated that the Lie algebra of the symmetry group for the oscillator equation with time-dependent frequency is eight dimensional, and is composed of four independent subalgebras. A detailed analysis of the admitted symmetries reveals a deeper connection between the nonlinear envelope equation and the oscillator equation. A general theorem regarding the symmetries and invariants of the envelope equation, which includes the existence of the Courant-Snyder invariant as a special case, is demonstrated. As an application to accelerator physics, the symmetries of the envelope equation enable a fast numerical algorithm for finding matched solutions without using the conventional iterative Newton’s method, where the envelope equation needs to be numerically integrated once for every iteration, and the Jacobi matrix needs to be calculated for the envelope perturbation.

An enhancement near threshold is observed in the ωϕ invariant mass spectrum from the doubly Okubo-Zweig-Iizuka–suppressed decays of J/ψ→γωϕ, based on a sample of 5.8×10⁷ J/ψ events collected with the BESII detector. A partial wave analysis shows that this enhancement favors J^P=0⁺, and its mass and width are M=1812_-26⁺¹⁹(stat)±18(syst)  MeV/c² and Γ=105±20(stat)±28(syst)  MeV/c². The product branching fraction is determined to be B(J/ψ→γX)B(X→ωϕ)=[2.61±0.27(stat)±0.65(syst)]×10^-4.

The decay modes J/ψ→γπ⁰, γη and γη^′ are analyzed using a data sample of 58×10⁶ J/ψ decays collected with the BESII detector at the BEPC. The branching fractions are determined to be: Br(J/ψ→γπ⁰)=(3.13_-0.47^+0.65)×10^-5, Br(J/ψ→γη)=(11.23±0.89)×10^-4, and Br(J/ψ→γη^′)=(5.55±0.44)×10^-3, where the errors are combined statistical and systematic errors. The ratio of partial widths Γ(J/ψ→γη^′)/Γ(J/ψ→γη) is measured to be 4.94±0.40, and the singlet-octet pseudoscalar mixing angle of η-η^′ system is determined.