Search Results (136 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.

This work focuses on organic-organic heterojunctions (OOHs) formed between two small-molecule, low band-gap, semiconductors, tris{2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12 -hexaaza-triphenylene (THAP), and copper phthalocyanine (CuPc). The organic layers are deposited on various substrates, and the energy level alignment between them is investigated by ultraviolet photoemission spectroscopy. The electronic structure of the OOH is found to depend on the work function of the organic underlayer predeposited on the different substrates. The vacuum level offset between THAP and CuPc, which consists of the sum of the interface dipole and the molecular level shift, varies from 0.26  to  1.37  eV. The interface dipole between the two organic films linearly changes with the work function of the organic underlayer.

Fast electron emission from the interaction of femtosecond laser pulses with shaped solid targets has been studied. It is found that the angular distributions of the forward fast electrons are highly dependent upon the target shapes. The important roles played by the electrostatic fields built up at the non-laser-irradiated target surfaces and the collisions in the target are identified. Our two-dimensional particle-in-cell simulations with binary collisions included reproduce the main experimental observations.

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.

A protocol for steering Rydberg electrons towards targeted final states is realized with the aid of a chirped train of half-cycle pulses (HCPs). Its novel capabilities are demonstrated experimentally by transporting potassium atoms excited to the lowest-lying quasi-one-dimensional states in the n_i=350 Stark manifold to a narrow range of much higher-n states. We demonstrate that this coherent state transfer is, to a high degree, reversible. The protocol allows for remarkable selectivity and is highly efficient, with typically over 80% of the parent atoms surviving the HCP sequence.

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^-.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

We report the first observation of echoes in the electric dipole moment of an ensemble of Rydberg atoms precessing in an external electric field F. Rapid reversal of the field direction is shown to play a role similar to that of a π pulse in NMR in rephasing a dephased ensemble of electric dipoles resulting in the buildup of an echo. The mechanisms responsible for this are discussed with the aid of classical trajectory Monte Carlo simulations.

The time evolution of Stark wave packets in Rydberg atoms under the influence of colored noise is analyzed. The Stark wave packets are generated by exposing K(350p) Rydberg atoms to an electric field step. Their time evolution is monitored using a half-cycle probe pulse that is applied after a variable time delay. The noise is generated by randomly modulating the amplitude of the static field. We show that noise results in an enhanced and irreversible dephasing of Stark quantum beats, as monitored by the damping of the beat amplitude. This source of decoherence is most effective when the characteristic frequency of the noise power spectrum matches the orbital frequency of the Rydberg electron. Noise driven resonant Δn=1 transitions broaden the energy distribution thereby accelerating the dephasing.

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.

The ability to localize and to steer Rydberg wave packets in phase space using tailored sequences of half-cycle pulses is demonstrated. Classical phase-space portraits are used to explain the method and to illustrate the level of control that can be achieved. This is confirmed experimentally by positioning a phase-space-localized wave packet at the center of a stable island or navigating it around its periphery. This work provides a valuable starting point for further engineering of electronic wave functions.

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.

By Polnarev’s method we analytically calculate the polarization spectra of the cosmic microwave background radiation (CMB) generated by cosmic relic gravitational waves (RGW). In this analytic approach the physics involved in this generating process is more transparent. Consequently, the effects due to various elements of physics can be isolated easily. In solving the equation for evolution of RGW in the expanding universe, both the sudden transition and the WKB approximation for the scale factor during the radiation-matter transition have been taken. To describe more precisely the decoupling process, we have introduced an analytic expression for the visibility function, consisting of two pieces of half-Gaussian curves. We also include the damping on polarizations due to the photon diffusion up to the second order of the tight coupling. Analytic polarization spectra C_l^XX have been obtained with the following several improvements over the previous results. 1. The approximate analytic result is quite close to the numerical one evaluated from the cmbfast code, especially, for the first three peaks of the spectrum that are observable. By using the analytic exact solution of RGW in the sudden transition approximation, we have demonstrated the dependence of C_l^XX on the dark energy and the baryons. 2. Our analytic half-Gaussian approximation of the visibility function fits better than the usual Gaussian model, and its time integration yields an analytic damping factor, which is parameter-dependent. This improves the spectra by ∼30% around the second and third peaks. 3. The second order of tight coupling reduces the overall amplitude of C_l^XX by 58%, comparing with the tight-coupling limit. 4. The influences of inflation on RGW and on CMB polarization are explicitly demonstrated.

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.

As strong evidence for inflation, relic gravitational waves (RGW) have been extensively studied. Although they have not been detected yet, some constraints have been achieved by observations. Future experiments for RGW detection are mainly of two kinds: CMB experiments and laser interferometers. In this paper, we study these current constraints and the detective abilities of future experiments. We calculate the strength of RGW Ω_g(k) using two methods: the analytic method and the numerical method, by solving the inflationary flow equations. By the first method, we obtain a bound Ω_g<3.89×10^-16 at ν=0.1  Hz, where we have used the current constraints on the scalar spectral index and the tensor-scalar ratio; furthermore, we have taken into account the redshift-suppression effect, the accelerating expansion effect, and the neutrino damping effect on RGW. But the analytic expression of Ω_g(k) depends on specific inflationary models and does not apply well for the waves with very high frequencies. The numerical method is more precise for the waves with high frequencies. It gives a bound Ω_g<8.62×10^-14, which is independent of the inflationary parameters, and applies to any single-field slow-roll inflationary model. After considering the current constraints on the inflationary parameters, this bound becomes Ω_g<2×10^-17. These two methods give consistent conclusions: the current constraints on RGW from LIGO, big bang nucleosynthesis, and pulsar timing are too loose to give any constraint for the single-field inflationary models, and the constraints from WMAP are relatively tighter. Future laser interferometers are more effective for detecting RGW with the smaller tensor-scalar ratio, but the CMB experiments are more effective for detecting the waves with the larger ratio. These detection methods are complementary to each other for the detection of RGW.

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.

We investigate the interband optical absorption spectra near the band edge of a cylindrical semiconductor quantum wire in the presence of a static electric field and a terahertz electric field polarized along the axis. Optical absorption spectra are nonperturbatively calculated by solving the low-density semiconductor Bloch equations in real space and real time. The influence of the Franz-Keldysh (FK) effect and dynamical FK effect on the absorption spectrum is investigated. To highlight the physics behind the FK effect and dynamical FK effect, the spatiotemporal dynamics of the polarization wave packet are also presented. Under a reasonable static electric field, substantial and tunable absorption oscillations appear above the band gap. A terahertz field, however, will cause the Autler-Townes splitting of the main exciton peak and the emergence of multiphoton replicas. The presented results suggest that semiconductor quantum wires have potential applications in electro-optical devices.

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.