39,905 research outputs found
Spin Polarisability of the Nucleon in the Heavy Baryon Effective Field Theory
We have constructed a heavy baryon effective field theory with photon as an
external field in accordance with the symmetry requirements similar to the
heavy quark effective field theory. By treating the heavy baryon and
anti-baryon equally on the same footing in the effective field theory, we have
calculated the spin polarisabilities of the nucleon at
third order and at fourth-order of the spin-dependent Compton scattering. At
leading order (LO), our results agree with the corresponding results of the
heavy baryon chiral perturbation theory, at the next-to-leading order(NLO) the
results show a large correction to the ones in the heavy baryon chiral
perturbation theory due to baryon-antibaryon coupling terms. The low energy
theorem is satisfied both at LO and at NLO. The contributions arising from the
heavy baryon-antibaryon vertex were found to be significant and the results of
the polarisabilities obtained from our theory is much closer to the
experimental data.Comment: 21pages, title changed, minimal correction
Rayleigh-Brillouin light scattering spectroscopy of nitrous oxide (NO)
High signal-to-noise and high-resolution light scattering spectra are
measured for nitrous oxide (NO) gas at an incident wavelength of 403.00 nm,
at 90 scattering, at room temperature and at gas pressures in the range
bar. The resulting Rayleigh-Brillouin light scattering spectra are
compared to a number of models describing in an approximate manner the
collisional dynamics and energy transfer in this gaseous medium of this
polyatomic molecular species. The Tenti-S6 model, based on macroscopic gas
transport coefficients, reproduces the scattering profiles in the entire
pressure range at less than 2\% deviation at a similar level as does the
alternative kinetic Grad's 6-moment model, which is based on the internal
collisional relaxation as a decisive parameter. A hydrodynamic model fails to
reproduce experimental spectra for the low pressures of 0.5-1 bar, but yields
very good agreement (\%) in the pressure range bar. While these
three models have a different physical basis the internal molecular relaxation
derived can for all three be described in terms of a bulk viscosity of Pas. A 'rough-sphere' model, previously
shown to be effective to describe light scattering in SF gas, is not found
to be suitable, likely in view of the non-sphericity and asymmetry of the N-N-O
structured linear polyatomic molecule
Large Component QCD and Theoretical Framework of Heavy Quark Effective Field Theory
Based on a large component QCD derived directly from full QCD by integrating
over the small components of quark fields with , an
alternative quantization procedure is adopted to establish a basic theoretical
framework of heavy quark effective field theory (HQEFT) in the sense of
effective quantum field theory. The procedure concerns quantum generators of
Poincare group, Hilbert and Fock space, anticommutations and velocity
super-selection rule, propagator and Feynman rules, finite mass corrections,
trivialization of gluon couplings and renormalization of Wilson loop. The
Lorentz invariance and discrete symmetries in HQEFT are explicitly illustrated.
Some new symmetries in the infinite mass limit are discussed. Weak transition
matrix elements and masses of hadrons in HQEFT are well defined to display a
manifest spin-flavor symmetry and corrections. A simple trace
formulation approach is explicitly demonstrated by using LSZ reduction formula
in HQEFT, and shown to be very useful for parameterizing the transition form
factors via expansion. As the heavy quark and antiquark fields in HQEFT
are treated on the same footing in a fully symmetric way, the quark-antiquark
coupling terms naturally appear and play important roles for simplifying the
structure of transition matrix elements, and for understanding the concept of
`dressed heavy quark' - hadron duality. In the case that the `longitudinal' and
`transverse' residual momenta of heavy quark are at the same order of power
counting, HQEFT provides a consistent approach for systematically analyzing
heavy quark expansion in terms of . Some interesting features in
applications of HQEFT to heavy hadron systems are briefly outlined.Comment: 59 pages, RevTex, no figures, published versio
Symmetry-preserving Loop Regularization and Renormalization of QFTs
A new symmetry-preserving loop regularization method proposed in \cite{ylw}
is further investigated. It is found that its prescription can be understood by
introducing a regulating distribution function to the proper-time formalism of
irreducible loop integrals. The method simulates in many interesting features
to the momentum cutoff, Pauli-Villars and dimensional regularization. The loop
regularization method is also simple and general for the practical calculations
to higher loop graphs and can be applied to both underlying and effective
quantum field theories including gauge, chiral, supersymmetric and
gravitational ones as the new method does not modify either the lagrangian
formalism or the space-time dimension of original theory. The appearance of
characteristic energy scale and sliding energy scale offers a
systematic way for studying the renormalization-group evolution of gauge
theories in the spirit of Wilson-Kadanoff and for exploring important effects
of higher dimensional interaction terms in the infrared regime.Comment: 13 pages, Revtex, extended modified version, more references adde
Electronic States and Magnetism of Mn Impurities and Dimers in Narrow-Gap and Wide-Gap III-V Semiconductors
Electronic states and magnetic properties of single impurity and dimer
doped in narrow-gap and wide-gap - semiconductors have been studied
systematically. It has been found that in the ground state for single
impurity, - complex is antiferromagnetic (AFM) coupling when -
hybridization is large and both the hole level and the
impurity level are close to the midgap; or very weak ferromagnetic (FM)
when is small and both and are deep in the valence band.
In dimer situation, the spins are AFM coupling for half-filled or
full-filled orbits; on the contrast, the Mn spins are double-exchange-like
FM coupling for any -orbits away from half-filling. We propose the strong
{\it p-d} hybridized double exchange mechanism is responsible for the FM order
in diluted - semiconductors
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