38,011 research outputs found

    Electromagnetic fields in a 3D cavity and in a waveguide with oscillating walls

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    We consider classical and quantum electromagnetic fields in a three-dimensional (3D) cavity and in a waveguide with oscillating boundaries of the frequency Ω\Omega . The photons created by the parametric resonance are distributed in the wave number space around Ω/2\Omega/2 along the axis of the oscillation. When classical waves propagate along the waveguide in the one direction, we observe the amplification of the original waves and another wave generation in the opposite direction by the oscillation of side walls. This can be understood as the classical counterpart of the photon production. In the case of two opposite walls oscillating with the same frequency but with a phase difference, the interferences are shown to occur due to the phase difference in the photon numbers and in the intensity of the generated waves.Comment: 8 pages revTeX including 1 eps fi

    Off-Forward Parton Distributions in 1+1 Dimensional QCD

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    We use two-dimensional QCD as a toy laboratory to study off-forward parton distributions (OFPDs) in a covariant field theory. Exact expressions (to leading order in 1/NC1/N_C) are presented for OFPDs in this model and are evaluated for some specific numerical examples. Special emphasis is put on comparing the x>ζx>\zeta and x<ζx<\zeta regimes as well as on analyzing the implications for the light-cone description of form factors.Comment: Revtex, 6 pages, 4 figure

    Exciton Valley Dynamics probed by Kerr Rotation in WSe2 Monolayers

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    We have experimentally studied the pump-probe Kerr rotation dynamics in WSe2_2 monolayers. This yields a direct measurement of the exciton valley depolarization time τv\tau_v. At T=4K, we find τv≈6\tau_v\approx 6ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases with τv\tau_v being as short as 1.5ps at 125K. The temperature dependence is well explained by the developed theory taking into account the exchange interaction and a fast exciton scattering time on short-range potentials.Comment: 5 pages, 3 figure

    Leading Chiral Contributions to the Spin Structure of the Proton

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    The leading chiral contributions to the quark and gluon components of the proton spin are calculated using heavy-baryon chiral perturbation theory. Similar calculations are done for the moments of the generalized parton distributions relevant to the quark and gluon angular momentum densities. These results provide useful insight about the role of pions in the spin structure of the nucleon, and can serve as a guidance for extrapolating lattice QCD calculations at large quark masses to the chiral limit.Comment: 8 pages, 2 figures; a typo in Ref. 7 correcte

    Quark Orbital-Angular-Momentum Distribution in the Nucleon

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    We introduce gauge-invariant quark and gluon angular momentum distributions after making a generalization of the angular momentum density operators. From the quark angular momentum distribution, we define the gauge-invariant and leading-twist quark {\it orbital} angular momentum distribution Lq(x)L_q(x). The latter can be extracted from data on the polarized and unpolarized quark distributions and the off-forward distribution E(x)E(x) in the forward limit. We comment upon the evolution equations obeyed by this as well as other orbital distributions considered in the literature.Comment: 8 pages, latex, no figures, minor corrections mad

    SU(2) gluon propagator on a coarse anisotropic lattice

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    We calculated the SU(2) gluon propagator in Landau gauge on an anisotropic coarse lattice with the improved action. The standard and the improved scheme are used to fix the gauge in this work. Even on the coarse lattice the lattice gluon propagator can be well described by a function of the continuous momentum. The effect of the improved gauge fixing scheme is found not to be apparent. Based on the Marenzoni's model, the mass scale and the anomalous dimension are extracted and can be reasonably extrapolated to the continuum limit with the values α∌0.3\alpha\sim 0.3 and M∌600MeVM\sim 600MeV. We also extract the physical anisotropy Ο\xi from the gluon propagator due to the explicit Ο\xi dependence of the gluon propagator.Comment: LaTeX, 14 pages including 4 ps figure
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