arXiv subjects

A. Abada

Publications and source records attributed to A. Abada.

66 records · Page 4Linked to original sources

arXiv1998

Direct computation of inelastic photon-neutrino processes in the Standard Model

In this paper, we compute the Standard Model polarized amplitudes and cross sections of the processes $γν\toγγν$, $γγ\to γν\barν$ and $ν\barν\to γγγ$, for centre-of-mass energies $ω$ within the range of validity of the Fermi theory. By using a large electron-mass expansion of the exact result, we also derive the first correction term to the effective, low-energy ($ω< m_e$) formula for $γν\toγγν$. Finally, we discuss possible astrophysical implications of our results and provide simple fits to the exact expressions.

hep-ph↗

arXiv1998

Exact four-spinon dynamical correlation function in isotropic Heisenberg model

We discuss some properties of the exact four-spinon dynamical correlation function $S_4$ in the antiferromagnetic spin 1/2 $XXX$-model, the expression of which we derived recently. We show that the region in which it is not identically zero is different from and larger than the spin-wave continuum. We describe its behavior as a function of the neutron momentum transfer $k$ for fixed values of the neutron energy $ω$ and compare it to the one corresponding to the exact two-spinon dynamical correlation function $S_2$. We show that the overall shapes are quite similar, even though the expression of $S_4$ is much more involved than that of $S_2$. We finish with concluding remarks.

cond-mat↗

arXiv1997

A potential infrared problem with the damping rates for gluons with soft momentum in hot QCD

We calculate the damping rate $γ_l$ for longitudinal gluons with zero momentum in finite high temperature QCD and show that some of its contributing terms are infrared divergent. This is in contrast with the expectation that this damping rate is to be equal to the corresponding one $γ_t$ for transverse gluons which is known to be finite. Our calculation was motivated by the fact that similar divergent terms occur when we calculated in a previous work $γ_t$ to order $ p^2$, p being the momentum of the gluon. After we present our results, we briefly discuss them.

hep-ph↗

arXiv1997

Damping rate for transverse gluons with finite soft momentum in hot QCD

We calculate the damping rate for transverse gluons with {\nineti finite} soft momentum to leading order in perturbative hot QCD. The internal momenta of the one-loop contributing diagrams are soft. This means we have to use effective vertices and propagators which incorporate the so-called hard thermal loops. We expand the damping rate in powers of the incoming momentum and argue that the series ought to converge within a finite radius of convergence. We contrast such a behavior with the one obtained from a previous calculation that produced a logarithmic behavior, a calculation based on letting the gluon momentum come from the hard limit down towards the interior of the soft region. This difference in behavior may point to interesting physics around some `critical' region.

hep-ph↗

arXiv1997

Exact four-spinon dynamical correlation function of the Heisenberg model

In this paper we derive the exact expression of the four-spinon contribution to the dynamical correlation function of the spin S= 1/2 anisotropic (XXZ) Heisenberg model in the antiferromagnetic regime. We extensively study its isotropic (XXX) limit and derive perturbatively the Ising one. Our method relies on the quantum affine symmetry of the model, which allows for a systematic diagonalization of the Hamiltonian in the thermodynamic limit and for an exact calculation of matrix elements of local spin operators. In fact, we argue that the familiar criticism of this method related to the complication of these matrix elements is not justified. First, we give, in the form of contour integrals, an exact expression for the n-spinon contribution. After we compile recently found results concerning the two-spinon contribution, we specialize the n-spinon formula to the new case n=4. Then we give an explicit series representation of this contribution in the isotropic limit. Finally, after we show that this representation is free of divergences, we discuss the Ising limit in which a simple expression is found up to first order in the anisotropy parameter.

hep-th↗

arXiv1995

Radiative Decays of Hyperons in the Skyrme Model: E2/M1 Transitions

We study the radiative decays of $J^π=\frac{3}{2}^+$ baryons in the framework of the SU(3) collective approach to the Skyrme model. We present the predictions for the decay widths and the corresponding $E2/M1$ ratios. We find that all considered ratios are negative and of the order of a few percent only. We discuss the effects of flavor symmetry breaking and compare our results to those obtained in related models.

hep-ph↗

arXiv1995

Strange Nucleons and Hyperons as Chiral Solitons in the NJL Model

In the chirally symmetric Nambu--Jona-Lasinio model baryons are described as chiral solitons of mesonic quark--antiquark bound states. Hyperons are investigated in two complementary pictures: the bound state approach of Callan and Klebanov and the collective approach of Yabu and Ando. The latter is used to compute the strange vector form factors of the nucleon providing estimates for the strange electric mean square radius and the strange magnetic moment of the nucleon.

hep-ph↗

arXiv1995

Monopole Excitations of Baryons in the Nambu--Jona--Lasinio Soliton Model

We investigate the monopole excitations of the soliton in the Nambu--Jona--Lasinio model. By studying the solutions to the corresponding Bethe--Salpeter equation in the background of the soliton we exclude the existence of real large amplitude fluctuations. This allows us to treat the collective coordinate for the monopole excitations, which parametrizes the extension of the soliton, in the harmonic approximation. The canonical quantization of this coordinate yields a spectrum which agrees reasonably well with the empirical data for the Roper resonance, N(1440), and the corresponding one for the Delta, $Δ$(1600). We also comment on going beyond the harmonic approximation.

hep-ph↗

arXiv1995

Isovector response function of hot nuclear matter with Skyrme interactions

We investigate the role of the effective nucleon-nucleon interaction in the description of giant dipole resonances in hot nuclei. For this purpose we calculate the response function of hot nuclear matter to a small isovector external perturbation using various effective Skyrme interactions. We find that for Skyrme forces with an effective mass close to unity an undamped zero sound mode occurs at zero temperature. This mode gives rise in finite nuclei (calculated via the Steinwedel-Jenssen model) to a resonance whose energy agrees with the observed value. We find that zero sound disappears at a temperature of a few MeV, leaving only a broad peak in the dipole strength. For Skyrme forces with a small value of the effective mass (0.4- 0.5), there is no zero sound at zero temperature but only a weak peak located too high in energy. The strength distribution in this case is nearly independent of temperature and shows small collective effects. The relevance of these results for the saturation of photon multiplicities observed in recent experiments is pointed out.

hep-ph↗

arXiv1995

On the strange vector form factors of the nucleon in the NJL soliton model

Within the Nambu--Jona--Lasinio model strange degrees of freedom are incorporated into the soliton picture using the collective approach of Yabu and Ando. The form factors of the nucleon associated with the nonet vector current are extracted. The numerical results provide limits for the strange magnetic moment: $-0.05\leμ_s\le0.25$. For the strange magnetic form factor of the nucleon the valence quark and vacuum contributions add coherently while there are significant cancellations for the strange electric form factor.

hep-ph↗

arXiv1992

On the role of vector mesons in topological soliton stability

Isospin one vector mesons (in particular the $ρ$) are usually described as massive Yang-Mills particles in the chiral Lagrangian. We investigate some aspects of an alternative approach in the soliton sector. It is found that the soliton is stable in very much the same way as with the $ω$-meson and that spontaneous parity violating classical solutions do not exist. The formulation in terms of antisymmetric tensors is shown to be canonically related to a vector field description provided the Skyrme term is added to the latter.

hep-ph↗

arXiv1992

Deformation of the Wakimoto construction

We present the extension of the Wakimoto construction to the $su(2)_k$ quantum current algebra and its associated $Z_k$ quantum parafermion algebra. This construction is achieved in terms of various deformations of three classical free boson fields. We also give the vertex operators corresponding to the quantum spin-$j$ representation.

hep-th↗