Search Results (178 total)

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.

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 the idea of a transformation medium, a cloak can be designed to make a domain invisible to one target frequency. We examine the possibility of extending the bandwidth of such a cloak. We find that causality requirements impose severe constraints on the system parameters of the transformation medium, and we show that a specific form of “reduction” can help us to create a cloak that offers a reduced cross section in a finite frequency range. We also give a simple inequality that limits the bandwidth of operation.

The present experiment exploits the interference between the deeply virtual Compton scattering (DVCS) and the Bethe-Heitler processes to extract the imaginary part of DVCS amplitudes on the neutron and on the deuteron from the helicity-dependent D(e→,e^′γ)X cross section measured at Q²=1.9  GeV² and x_B=0.36. We extract a linear combination of generalized parton distributions (GPDs) particularly sensitive to E_q, the least constrained GPD. A model dependent constraint on the contribution of the up and down quarks to the nucleon spin is deduced.

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.

Tunneling anisotropic magnetoresistance (TAMR) is observed in tunnel junctions with transition metal electrodes as the moments are rotated from in-plane to out-of-plane in sufficiently large magnetic fields that the moments are nearly parallel to one another. A complex angular dependence of the tunneling resistance is found with twofold and fourfold components that vary strongly with bias voltage. Distinctly different TAMR behaviors are obtained for devices formed with highly textured crystalline MgO(001) and amorphous Al₂O₃ tunnel barriers. A tight-binding model shows that a fourfold angular dependence can be explained by the presence of an interface resonant state that affects the transmission of the contributing tunneling states through a spin-orbit interaction.

High-precision measurements of the proton elastic form-factor ratio, μ_pG_E^p/G_M^p, have been made at four-momentum transfer, Q², values between 0.2 and 0.5  GeV². The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q² range the deviation from unity is primarily due to G_E^p being smaller than expected.

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.

We investigated simultaneously the ¹²C(e,e^′p) and ¹²C(e,e^′pp) reactions at Q²=2  (GeV/c)², x_B=1.2, and in an (e, e^′p) missing-momentum range from 300 to 600  MeV/c. At these kinematics, with a missing momentum greater than the Fermi momentum of nucleons in a nucleus and far from the delta excitation, short-range nucleon-nucleon correlations are predicted to dominate the reaction. For (9.5±2)% of the ¹²C(e,e^′p) events, a recoiling partner proton was observed back-to-back to the ¹²C(e,e^′p) missing-momentum vector, an experimental signature of correlations.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

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^-.

A high-resolution (σ_instr.=1.5 MeV) search for narrow states (Γ<10 MeV) with masses of M_x≈1500–1850 MeV in ep→e^′K⁺X,e^′K^-X, and e^′π⁺X electroproduction at small angles and low Q² was performed. These states would be candidate partner states of the reported Θ⁺(1540) pentaquark. No statistically significant signal was observed in any of the channels at 90% C.L. Upper limits on forward production were determined to be between 0.8% and 4.9% of the Λ(1520) production cross section, depending on the channel and the assumed mass and width of the state.

Under Anderson localization, two types of correlation of spontaneous emission in one-dimensional random media are investigated: i.e., time-domain field correlation C_t^E(r₁,r₂,τ) and energy-spectrum correlation C_ω^P(r₁,r₂,Δω). The results show that the spatial correlation length of C_t^E(r₁,r₂,τ=0) is unrelated to the localization length; however, the increase of the correlation length of max_τ∣C_t^E(r₁,r₂,τ)∣ with the localization length is sensitive and monotonous. In particular, we find that the fields at the different locations keep on exchanging with each other in a certain fixed speed by investigation of time-domain field correlation. The speed is almost not affected by the random strength and almost equal to the group velocity of the corresponding periodic structure. Therefore the localized mode is really a dynamic equilibrium state though the energy of the localized mode is localized. In addition, because it is not convenient to characterize the localization length by C_t^E(r₁,r₂,τ), another correlation—energy-spectrum correlation C_ω^P(r₁,r₂,Δω)—is proposed. By investigation of the energy-spectrum correlation for Δω=0, we obtain that there is an approximately linear relation between the spatial correlation length of energy-spectrum and the localization length. Obviously, in the aspect of characterizing the localization length, the energy-spectrum correlation is more convenient than the time-domain field correlation.

In this paper, we investigate the physical mechanism and properties of the two classes of photonic gaps in the aperiodic Thue-Morse (TM) lattice: the traditional gaps and the fractal gaps. Based on the analysis of the field distributions of the midgap and gap-edge states, we demonstrate that the two classes of gaps originate from the different spatial correlations. For the case of the traditional gaps the correlation mechanism is confirmed by the approximated transfer matrix method, in which only the relevant interfaces are considered. Then we find that the edges of the traditional gaps can be obtained from the condition x₂=2, where x₂ is the second order trace map. Around the basic traditional gap, there are two finite spectral regions in which the transmission spectra look like the bands of a periodic system and the eigenstates behave like the Bloch states, so that they are defined as the traditional bands. Simultaneously, it is numerically illuminated that the traditional bands are well defined physically. Finally, the localization properties of the fractal gap-edge state are studied by the quality factor Q. The result indicates the fractal gap-edge states become hyperexponentially localized as the system size increases.

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5–11 and -t=2–7  GeV² with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0±0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

We measured angular distributions of differential cross section, beam analyzing power, and recoil polarization for neutral pion electroproduction at Q²=1.0 (GeV/c)² in 10 bins of 1.17≤W≤1.35 GeV across the Δ resonance. A total of 16 independent response functions were extracted, of which 12 were observed for the first time. Comparisons with recent model calculations show that response functions governed by real parts of interference products are determined relatively well near the physical mass, W=M_Δ≈1.232 GeV, but the variation among models is large for response functions governed by imaginary parts, and for both types of response functions, the variation increases rapidly with W>M_Δ. We performed a multipole analysis that adjusts suitable subsets of ℓ_π≤2 amplitudes with higher partial waves constrained by baseline models. This analysis provides both real and imaginary parts. The fitted multipole amplitudes are nearly model independent—there is very little sensitivity to the choice of baseline model or truncation scheme. By contrast, truncation errors in the traditional Legendre analysis of N→Δ quadrupole ratios are not negligible. Parabolic fits to the W dependence around M_Δ for the multiple analysis gives values for Re(S₁₊/M₁₊)=(-6.61±0.18)% and Re(E₁₊/M₁₊)=(-2.87±0.19)% for the pπ⁰ channel at W=1.232 GeV and Q²=1.0 (GeV/c)² that are distinctly larger than those from the Legendre analysis of the same data. Similarly, the multipole analysis gives Re(S₀₊/M₁₊)=(+7.1±0.8)% at W=1.232 GeV, consistent with recent models, while the traditional Legendre analysis gives the opposite sign because its truncation errors are quite severe.

We report new measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets with ⟨θ_lab⟩≈6.0°. The ⁴He result is A_PV=(+6.40±0.23(stat)±0.12(syst))×10^-6. The hydrogen result is A_PV=(-1.58±0.12(stat)±0.04(syst))×10^-6. These results significantly improve constraints on the electric and magnetic strange form factors G_E^s and G_M^s. We extract G_E^s=0.002±0.014±0.007 at ⟨Q²⟩=0.077  GeV², and G_E^s+0.09G_M^s=0.007±0.011±0.006 at ⟨Q²⟩=0.109  GeV², providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.

A large data set of charged-pion (π^±) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms.

We present the first measurements of the e→p→epγ cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q² dependence (from 1.5 to 2.3  GeV²) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q². The helicity-independent cross section is also measured at Q²=2.3  GeV². We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.

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.