808 research outputs found

    Electromagnetic Form Factors of Nucleons in a Light-cone Diquark Model

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    We investigate the electromagnetic form factors of nucleons within a simple relativistic quark spectator-diquark model using the light-cone formalism. Melosh rotations are applied to both quark and vector diquark. It is shown that the difference between vector and scalar spectator diquarks reproduces the right electric form factor of neutrons, and both the form factors GE(Q2)G_E(Q^2) and GM(Q2)G_M(Q^2) of the proton and neutron agree with experimental data well up to Q2=2 GeV2Q^2=2 ~\rm{GeV}^2 in this simple model.Comment: 16 pages, Revtex4, minor changes, to appear in Phys. Rev.

    Utilizing the Updated Gamma-Ray Bursts and Type Ia Supernovae to Constrain the Cardassian Expansion Model and Dark Energy

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    We update gamma-ray burst (GRB) luminosity relations among certain spectral and light-curve features with 139 GRBs. The distance modulus of 82 GRBs at z>1.4z>1.4 can be calibrated with the sample at z1.4z\leq1.4 by using the cubic spline interpolation method from the Union2.1 Type Ia supernovae (SNe Ia) set. We investigate the joint constraints on the Cardassian expansion model and dark energy with 580 Union2.1 SNe Ia sample (z<1.4z<1.4) and 82 calibrated GRBs data (1.4<z8.21.4<z\leq8.2). In Λ\LambdaCDM, we find that adding 82 high-\emph{z} GRBs to 580 SNe Ia significantly improves the constrain on ΩmΩΛ\Omega_{m}-\Omega_{\Lambda} plane. In the Cardassian expansion model, the best fit is Ωm=0.240.15+0.15\Omega_{m}= 0.24_{-0.15}^{+0.15} and n=0.160.52+0.30n=0.16_{-0.52}^{+0.30} (1σ)(1\sigma), which is consistent with the Λ\LambdaCDM cosmology (n=0)(n=0) in the 1σ1\sigma confidence region. We also discuss two dark energy models in which the equation of state w(z)w(z) is parametrized as w(z)=w0w(z)=w_{0} and w(z)=w0+w1z/(1+z)w(z)=w_{0}+w_{1}z/(1+z), respectively. Based on our analysis, we see that our Universe at higher redshift up to z=8.2z=8.2 is consistent with the concordance model within 1σ1\sigma confidence level.Comment: 17 pages, 6 figures, 2 tables; accepted for publication in Advances in Astronomy, special issue on Gamma-Ray Burst in Swift and Fermi Era. arXiv admin note: text overlap with arXiv:0802.4262, arXiv:0706.0938 by other author

    Fragmentation function of gQQˉ(3S1[8])g\to Q\bar{Q}(^3S_1^{[8]}) in soft gluon factorization and threshold resummation

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    We study the fragmentation function of the gluon to color-octet 3S1^3S_1 heavy quark-antiquark pair using the soft gluon factorization (SGF) approach, which expresses the fragmentation function in a form of perturbative short-distance hard part convoluted with one-dimensional color-octet 3S1^3S_1 soft gluon distribution (SGD). The short distance hard part is calculated to next-to-leading order in αs\alpha_s and a renormalization group equation for the SGD is derived. By solving the renormalization group equation, threshold logarithms are resummed to all orders in perturbation theory. The comparison with gluon fragmentation function calculated in NRQCD factorization approach indicates that the SGF formula resums a series of velocity corrections in NRQCD which are important for phenomenological study.Comment: 38 pages, 8 figure

    A CRASH simulation of the contribution of binary stars to the epoch of reionization

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    We use a set of 3D radiative transfer simulations to study the effect that a large fraction of binary stars in galaxies during the epoch of reionization has on the physical properties of the intergalactic medium (i.e. the gas temperature and the ionization state of hydrogen and helium), on the topology of the ionized bubbles and on the 21 cm power spectra. Consistently to previous literature, we find that the inclusion of binary stars can speed up the reionization process of HI and HeI, while HeII reionization is still dominated by more energetic sources, especially accreting black holes. The earlier ionization attained with binary stars allows for more time for cooling and recombination, so that gas fully ionized by binary stars is typically colder than that ionized by single stars at any given redshift. With the same volume averaged ionization fraction, the inclusion of binary stars results in fewer small ionized bubbles and more large ones, with visible effects also on the large scales of the 21 cm power spectrum.Comment: 14 pages, 11 figures, MNRAS accepte

    A New Method to Calculate Electromagnetic Impedance Matching Degree in One-Layer Microwave Absorbers

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    A delta-function method was proposed to quantitatively evaluate the electromagnetic impedance matching degree. Measured electromagnetic parameters of {\alpha}-Fe/Fe3B/Y2O3 nanocomposites are applied to calculate the matching degree by the method. Compared with reflection loss and quarter-wave principle theory, the method accurately reveals the intrinsic mechanism of microwave transmission and reflection properties. A possible honeycomb structure with promising high-performance microwave absorption according to the method is also proposed.Comment: 13 pages, 3 figure