Search Results (182 total)

We demonstrate experimentally the efficient fusion neutron generation from Coulomb explosion (CE) of laser irradiated large-size heteronuclear deuterated methane clusters. A conversion efficiency of 2.1×10⁶ neutrons/J of incident laser energy is obtained with a 120 mJ, 70 fs laser pulse. It is 50 times higher than that of homonuclear deuterium clusters of similar size. This enhancement is attributed to the significant increase in the deuteron kinetic energies by fourfold due to energetic boosting and overrun effects during CE of heteronuclear clusters. The yield of 5.5×10⁶ neutrons per pulse is obtained with a 100 TW, 50 fs driving laser pulse at an intensity of 1.5×10¹⁹ W/cm². This work may facilitate the development of a high-flux table-top neutron source.

Using a sample of 58 million J/ψ events collected with the BESII detector at the BEPC, more than 100 000 J/ψ→pp̅ π⁰ events are selected, and a detailed partial wave analysis is performed. The branching fraction is determined to be Br(J/ψ→pp̅ π⁰)=(1.33±0.02±0.11)×10^-3. A long-sought missing N^*, first observed in J/ψ→pn̅ π^-, is observed in this decay too, with mass and width of 2040_-4⁺³±25   MeV/c² and 230_-8⁺⁸±52   MeV/c², respectively. Its spin-parity favors 3 / 2⁺. The masses, widths, and spin parities of other N^* states are obtained as well.

The self-field and the associated plasma response of an externally injected charged particle in plasmas are investigated within a self-consistent formalism that extends a formula used in plasma wake-field acceleration. The response of plasmas to such a self-field is also investigated through this self-consistent theory. This self-consistent theory leads to a correct formula of plasma wake field.

We investigate the J/ψ polarization in photoproduction at the Hadron Electron Ring Accelerator (HERA) up to the next-to-leading order of QCD. The results show that the transverse momentum p_t and energy fraction z distributions of J/ψ production do not agree with the observed ones very well. The theoretical uncertainties for the z distributions of the J/ψ polarization parameters with respect to various choices of the renormalization and factorization scales are too large to give a definite predication. The uncertainties for the p_t distributions of these parameters are small when p_t>3  GeV and the obtained p_t distributions cannot describe the experimental data even in this region.

We investigate the pair production of S-wave heavy quarkonium at the LHC in the color-singlet mechanism (CSM) and estimate the contribution from the gluon fragmentation process in the color-octet mechanism (COM) for comparison. With the matrix elements extracted previously in the leading-order calculations, the numerical results show that the production rates are quite large for the pair production processes at the LHC. The p_t distribution of double J/ψ production in the CSM is dominant over that in the COM when p_t is smaller than about 10 GeV. For the production of double Υ, the contribution of the COM is always larger than that in the CSM. The large differences in the theoretical predictions between the CSM and COM for the p_t distributions in the large p_t region are useful in clarifying the effects of COM on the quarkonium production. We also investigate the pair production of S-wave B_c and B_c^* mesons, and the measurement of these processes is useful to test the CSM and extract the long-distance matrix elements for the B_c and B_c^* mesons. Aside from numerical calculations, analytical expressions for the production differential cross sections of all these processes are also given.

We propose a scheme to compensate dynamically the intrinsic chirp of the attosecond harmonic pulses. By adding a weak second harmonic laser field to the driving laser field, the chirp compensation can be varied from the negative to the positive continuously by simply adjusting the relative time delay between the two-color pulses. Using this technique, the compensation of the negative chirp in harmonic emission is demonstrated experimentally for the first time and the nearly transform-limited attosecond pulse trains are obtained.

Based on the color-singlet model, we investigate the photoproduction of J/ψ associated with a cc̅ pair with all subprocesses including the direct, single-resolved, and double-resolved channels. The amplitude squared of these subprocesses are obtained analytically. By choosing corresponding parameters, we give theoretical predictions for the J/ψ transverse momentum and rapidity distributions both at the LEPII and at the future photon colliders for these subprocesses. The numerical results show that at the LEPII these processes cannot give enough contributions to account for the experimental data, and it indicates that the color-octet mechanism may still be needed. At the photon collider with the laser backscattering photons, the resolved photon channel will dominate over the direct one in small and moderate p_t regions with large sqrt[s]. For their rapidity distribution with the laser backscattering photons, the resolved processes will be dominant in the whole region -2<y<2. By measuring the J/ψ production associated with a cc̅ pair, this process can be separated from the inclusive J/ψ production and may provide a new chance to test the color-singlet contributions.

We propose a scheme to identify and separate the contributions from the short and long electron trajectories in the high-order harmonic generation in argon atoms. The modification of the diving laser field by using a diaphragm prior to focusing is demonstrated to be an effective way of controlling the short and long path contributions in harmonic emission since different macroscopic phase-matching conditions are satisfied, respectively. We show that the short and the long path harmonic emission are separated by adjusting the laser focusing geometry and modifying the laser field. This offers a way of selectively probing and controlling the electron dynamics in intense laser fields.

We calculate α²α_s² order QED contribution to J/ψ production in the pp(p̅ )→J/ψ+cc̅ color-singlet process at the Tevatron and LHC in the framework of nonrelativistic QCD. The contribution of the interference between the α²α_s² QED and α_s⁴ QCD is also taken into account. The J/ψ production associated with a charm-quark pair could be a measurable signal at hadron collider experiment. Our calculations show that by including the QED contribution, the p_t distribution is enhanced by a factor of 1.5 (1.9) at the Tevatron (LHC) at p_t=50(100)  GeV. In addition, the polarization of J/ψ turns from unpolarized in all regions to increasingly transverse when p_t becomes larger.

We reinvestigate the modulation inversion of angular distribution of high-order harmonic generation (HHG) from impulsively aligned CO₂ molecules. The angular distribution is found to be sensitive to both the harmonic order and the intensity of the driving laser pulse for HHG. The roles of intramolecular quantum interference and the laser intensity are clarified. A laser-field-related recombination interference model is proposed to explain the modulation inversion of harmonic yield with respect to the molecular alignment. We conclude that the two-center interference in the recombination process can be manipulated by changing the laser intensity.

We investigate theoretically the high-order harmonic generation in the argon gas cell driven with a 7-fs 800-nm intense laser pulse, by taking into account the phase-matching effect during the propagation process in the gas cell. We demonstrate a successful selection of single quantum path of the returning electrons and therefore the generation of an isolated xuv attosecond pulse in the plateau region of high-order harmonic spectrum, instead of the emission in the form of pulse trains if without optimizing the phase-matching process.

A modified model based on the Coulomb explosion model [H. Li , Phys. Rev. A 74, 023201 (2006)] is proposed to roughly estimate the nuclear fusion yields produced in the Coulomb explosion of deuterium cluster jet with irradiation of the intense laser pulses by taking the attenuation of laser energy absorbed by the clusters with a logarithmic-normal size distribution into account. The neutron yield generated inside the heated plasma filament, as the sum of the intercluster fusion yield and beam-target fusion yield, is calculated as a function of laser-cluster parameters such as the cluster size, the laser energy, and the focus spot radius and position. Only if these parameters match with each other can the neutron yield or the neutron conversion efficiency be maximized. The rough qualitative comparison about the variation tendency of fusion yield with laser-cluster parameters is made between our simulations and the reported measurements.

The Debye series expansion expresses the Mie scattering coefficients into a series of Fresnel coefficients and gives physical interpretation of different scattering modes, but when an infinite multilayered cylinder is obliquely illuminated by electromagnetic plane waves, the scattering process becomes very complicated because of cross polarization. Based on the relation of boundary conditions between global scattering process and local scattering processes, the generalized Debye series expansion of plane wave scattering by an infinite multilayered cylinder at oblique incidence is derived in this paper. The formula and the code are verified by the comparison of the results with that of Lorenz-Mie theory in special cases and those presented in the literatures.

The spatiotemporal evolutions of ultrashort pulses in two dimensions are investigated numerically by solving the coupled Maxwell-Bloch equations without invoking the slowly varying envelope approximation and rotating-wave approximation. For an on-axis 2nπ sech pulse, local delay makes the temporal split 2π sech pulses crescent-shaped in the transverse distribution. Due to the transverse effect, the temporal split 2π sech pulses become unstable and experience reshaping during the propagation process. Then, interference occurs between the successive crescent-shaped pulses and multiple self-focusing can form.

The electronic structures of undoped and fluorine-doped LaFeAsO superconductors and fluorine-doped SmFeAsO superconductors are studied using soft x-ray absorption and emission spectroscopy combined with full potential linearized augmented planes waves (FP LAPW) calculations. The comparison between the numerical simulations and the experimental Fe L-emission spectra shows that the Fe states are concentrated near E_F, suggesting that the materials are not highly correlated systems. The comparison of the O K-edge and F K-edge spectra with the calculated density of states shows that the F dopants do not directly participate in the electronic structure near E_F, and so they serve the same purpose as an oxygen vacancy leading to an increase in the number of available charge carriers. Increasing the amount of fluorine doped into the structure of SmFeAsO is shown to cause a narrowing of the bandwidth of occupied O valence states.

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 experimentally investigate the high-order harmonic generation in argon gas using a driving laser pulse at a center wavelength of 1240  nm. High-contrast fine interference fringes could be observed in the harmonic spectra near the propagation axis, which is attributed to the interference between long and short quantum paths. We also systematically examine the variation of the interference fringe pattern with increasing energy of the driving pulse and with different phase-matching conditions.

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.

A conceptual molecular phase-coherent transistor is proposed based on first-principles quantum transport calculations. The device is formed by two molecules connected via a one-dimensional wire and it is operated by gating the interconnect between the molecules and exploiting quantum-mechanical interference. The transistor thus works by controlling the electron phase instead of the position of the molecular energy levels, and it paves the way for phase-controllable electronic circuits.

Confinement of electromagnetic energy into a single well-controlled oscillation of light is very important for generation of intense supercontinuum radiation. We find that the pulse breakup of few-cycle ultrashort laser pulses via resonant propagation effects can achieve this aim. By extracting such pulses and then focusing them to drive the He atoms, about 200  eV intense supercontinuum radiation can be generated, which is capable of supporting ∼20  attosecond isolated pulse generation.

We investigate experimentally the high-order harmonic generation from aligned CO₂ molecules and demonstrate that the modulation inversion of the harmonic yield with respect to molecular alignment can be altered dramatically by fine-tuning the intensity of the driving laser pulse for harmonic generation. The results can be modeled by employing the strong field approximation including a ground state depletion factor. The laser intensity is thus proved to be a parameter that can control the high-harmonic emission from aligned molecules.

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.

Stimulated Raman transition is generally used for a coherent population transfer in a cold atom interferometer. We have experimentally investigated the magnetic field dependence of coherent population transfer by the stimulated Raman process. Experimental data show that both Rabi frequency and population greatly depend on the amplitude and sense of the magnetic field. Theoretical analysis is also carried out, and the experimental results are in good agreement with the theoretical calculations. The study is helpful for optimizing the interferometer fringes and for improving the rotation measurement precision of a Sagnac interferometer.

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.