Search results for "PERTURBATION"
showing 10 items of 811 documents
A dispersion theoretical approach to the threshold amplitudes of pion photoproduction
1996
We give predictions for the partial wave amplitudes of pion photoproduction near threshold by means of dispersion relations at fixed t. The free parameters of this approach are determined by a fit to experimental data in the energy range 160 MeV $\le E_{\gamma} \le$ 420 MeV. The observables near threshold are found to be rather sensitive to the amplitudes in the resonance region, in particular to the $\Delta$ (1232) and $N^*$ (1440). We obtain a good agreement with the existing threshold data for both charged and neutral pion production. Our predictions also agree well with the results of chiral perturbation theory, except for neutral pion production off the neutron.
Partial-wave analysis of proton Compton scattering data below the pion-production threshold
2018
Low-energy Compton scattering off the proton is used for determination of the proton polarizabilities. However, the present empirical determinations rely heavily on the theoretical description(s) of the experimental cross sections in terms of polarizabilities. The most recent determinations are based on either the fixed-$t$ dispersion relations (DR) or chiral perturbation theory in the single-baryon sector ($\chi$PT). The two approaches obtain rather different results for proton polarizabilities, most notably for $\beta_{M1}$ (magnetic dipole polarizability). We attempt to resolve this discrepancy by performing a partial-wave analysis of the world data on proton Compton scattering below thr…
Generalized polarizabilities and electroexcitation of the nucleon
1999
Generalized nucleon polarizabilities for virtual photons can be defined in terms of electroproduction cross sections as function of the 4-momentum transfer $Q^2$. In particular, the sum of the generalized electric and magnetic polarizabilities $\Sigma=\alpha+\beta$ and the spin polarizability $\gamma$ can be expressed by virtual photon absorption cross sections integrated over the excitation energy. These quantities have been calculated within the framework of the recently developed unitary isobar model for pion photo- and electroproduction on the proton, which describes the available experimental data up to an excitation energy of about 1 GeV. Our results have been compared to the predicti…
Reply to “Comment on ‘Polarizability of the pion: No conflict between dispersion theory and chiral perturbation theory’”
2009
We show that the alleged discrepancies between chiral perturbation theory (ChPT) and dispersion theory, reported for the polarizability of the pion by Fil'kov and Kashevarov [Phys. Rev. C 72, 035211 (2005)], result from applying dispersion theory to nonanalytic functions.
Chiral approach to the rho meson in nuclear matter
2000
In this work, the properties of the $\rho$ meson at rest in cold symmetric nuclear matter are studied. We make use of a chiral unitary approach to pion-pion scattering in the vector-isovector channel, calculated from the lowest order Chiral Perturbation Theory ($\chi PT$) lagrangian including explicit resonance fields. Low energy chiral constraints are considered by matching our expressions to those of one loop $\chi PT$. To account for the medium corrections, the $\rho$ couples to $\pi\pi$ pairs which are properly renormalized in the nuclear medium, accounting for both $p-h$ and $\Delta -h$ excitations. The terms where the $\rho$ couples directly to the hadrons in the $p-h$ or $\Delta -h$ …
The polarizability of the pion: no conflict between dispersion theory and chiral perturbation theory
2008
Recent attempts to determine the pion polarizability by dispersion relations yield values that disagree with the predictions of chiral perturbation theory. These dispersion relations are based on specific forms for the absorptive part of the Compton amplitudes. The analytic properties of these forms are examined, and the strong enhancement of intermediate-meson contributions is shown to be connected with spurious singularities. If the basic requirements of dispersion relations are taken into account, the results of dispersion theory and effective field theory are not inconsistent.
Rare decay η→ππγγ in chiral perturbation theory
1996
We investigate the rare radiative {eta} decay modes {eta}{r_arrow}{pi}{sup +}{pi}{sup {minus}}{gamma}{gamma} and {eta}{r_arrow}{pi}{sup 0}{pi}{sup 0}{gamma}{gamma} within the framework of chiral perturbation theory at {ital O}({ital p}{sup 4}). We present photon spectra and partial decay rates for both processes as well as a Dalitz contour plot for the charged decay. {copyright} {ital 1996 The American Physical Society.}
Resonances, chiral symmetry, coupled channel unitarity and effective Lagrangians
1999
By means of a coupled channel non-perturbative unitary approach, it is possible to extend the strong constrains of Chiral Perturbation Theory to higher energies. In particular, it is possible to reproduce the lowest lying resonances in meson-meson scattering up to 1.2 GeV using the parameters of the O(p^2) and O(p^4) Chiral Lagrangian. We report on an update of these results examining their possible relevance for meson spectroscopy.
CHIRAL SYMMETRY CONSTRAINTS ON THE $K^+$ INTERACTION WITH THE NUCLEAR PION CLOUD
1995
7 páginas, 4 figuras.-- arXiv:nucl-th/9503011v1
Pion generalized dipole polarizabilities by virtual Compton scatteringπe→πeγ
2001
We present a calculation of the cross section and the event generator of the reaction $\ensuremath{\pi}e\ensuremath{\rightarrow}\ensuremath{\pi}e\ensuremath{\gamma}.$ This reaction is sensitive to the pion generalized dipole polarizabilities, namely, the longitudinal electric ${\ensuremath{\alpha}}_{L}{(q}^{2}),$ the transverse electric ${\ensuremath{\alpha}}_{T}{(q}^{2}),$ and the magnetic $\ensuremath{\beta}{(q}^{2})$ which, in the real-photon limit, reduce to the ordinary electric and magnetic polarizabilities $\overline{\ensuremath{\alpha}}$ and $\overline{\ensuremath{\beta}},$ respectively. The calculation of the cross section is done in the framework of chiral perturbation theory at $…