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Amand Faessler

Publications and source records attributed to Amand Faessler.

At least 163 records · Page 9Linked to original sources
arXiv2005

The exclusive rare decays B -> K l(bar) l and B_c -> D(D*) l(bar) l in a relativistic quark model

We study the exclusive rare decay B -> K l(bar) l. We calculate the relevant form factors within a relativistic constituent quark model, for the first time without employing the impulse approximation. The calculated form factors are used to evaluate differential decay rates and polarization observables. We present results on the q2-dependence of a set of observables with and without long-distance contributions. A similar analysis is done for the exclusive rare decays B_c -> D (D*) l(bar) l with special emphasis on the cascade decay B_c -> D* (-> D pi) l(bar) l. We derive a four-fold angular decay distribution for this process in terms of helicity amplitudes including lepton mass effects. The four-fold angular decay distribution allows to define a number of physical observables which are amenable to measurement. We compare our results with the results of other studies.

hep-ph↗
arXiv2005

Scalar meson mediated nuclear mu-e conversion

We study the nuclear mu-e conversion in the general framework of the effective Lagrangian approach without referring to any specific realization of the physics beyond the standard model (SM) responsible for lepton flavor violation (LFV). We analyze the role of scalar meson exchange between the lepton and nucleon currents and show its relevance for the coherent channel of mu-e conversion. We show that this mechanism introduces modifications in the predicted mu-e conversion rates in comparison with the conventional direct nucleon mechanism, based on the contact type interactions of the nucleon currents with the LFV leptonic current. We derive from the experimental data lower limits on the mass scales of the generic LFV lepton-quark contact terms and demonstrate that they are more stringent than the similar limits existing in the literature.

hep-ph↗
arXiv2005

Scalar nonet quarkonia and the scalar glueball: mixing and decays in an effective chiral approach

We study the strong and electromagnetic decay properties of scalar mesons above 1 GeV within a chiral approach. The scalar-isoscalar states are treated as mixed states of quarkonia and glueball configurations. A fit to the experimental mass and decay rates listed by the Particle Data group is performed to extract phenomenological constraints on the nature of the scalar resonances and to the issue of the glueball decays. A comparison to other experimental results and to other theoretical approaches in the scalar meson sector is discussed.

hep-ph↗
arXiv2005

Scalar meson and glueball decays within a effective chiral approach

We study the strong and electromagnetic decay properties of scalar mesons above 1 GeV within a chiral approach. The scalar-isoscalar states are treated as mixed states of quarkonia and glueball configurations. A fit to the experimental decay rates listed by the Particle Data group is performed to extract phenomenological constraints on the nature of the scalar resonances. A comparison to other theoretical approaches in the scalar meson sector is discussed.

hep-ph↗
arXiv2005

Nucleon polarizabilities in the perturbative chiral quark model

The nucleon polarizabilities alpha(E) and beta(M) are studied in the context of the perturbative chiral quark model. We demonstrate that meson cloud effects are sufficient to explain the electric polarizability of nucleon. Contributions of excite quark states to the paramagnetic polarizability are dominant and cancel the diamagnetic polarizability arising from the chiral field. The obtained results are compared to data and other theoretical predictions.

hep-ph↗
arXiv2005

Anisotropic flow of strange particles at RHIC

Space-time picture of the anisotropic flow evolution in Au+Au collisions at BNL RHIC is studied for strange hadrons within the microscopic quark-gluon string model. The directed flow of both mesons and hyperons demonstrates wiggle structure with the universal antiflow slope at |y| < 2 for minimum bias events. This effect increases as the reaction becomes more peripheral. The development of both components of the anisotropic flow is closely related to particle freeze-out. Hadrons are emitted continuously, and different hadronic species are decoupled from the system at different times. These hadrons contribute differently to the formation and evolution of the elliptic flow, which can be decomposed onto three components: (i) flow created by hadrons emitted from the surface at the onset of the collision; (ii) flow produced by jets; (iii) hydrodynamic flow. Due to these features, the general trend in elliptic flow formation is that the earlier mesons are frozen, the weaker their elliptic flow. In contrast, baryons frozen at the end of the system evolution have stronger v2.

hep-ph↗
arXiv2005

Effective Nucleon Masses in Symmetric and Asymmetric Nuclear Matter

The momentum and isospin dependence of the in-medium nucleon mass are studied. Two definitions of the effective mass, i.e. the Dirac mass $m^*_D$ and the nonrelativistic mass $m^*_{NR}$ which parameterizes the energy spectrum, are compared. Both masses are determined from relativistic Dirac-Brueckner-Hartree-Fock calculations. The nonrelativistic mass shows a distinct peak around the Fermi momentum. The proton-neutron mass splitting in isospin asymmetric matter is $m^*_{D,n} m^*_{NR,p}$, which is consistent with nonrelativistic approaches.

nucl-th↗
arXiv2005

Description of intermediate- and short-range NN nuclear force within a covariant effective field theory

The present paper is aimed to developing an effective field theory in the GeV region to describe consistently both elastic and inelastic NN-scattering in a fully covariant way. In this development we employ our novel s-channel mechanism of NN-interaction at intermediate and short ranges assuming a formation of the six-quark bag dressed with basic meson fields such as $π$, $σ$, $ρ$ and $ω$ in the intermediate state. The peripheral part of the interaction is governed by t-channel one- and two-pion exchanges described fairly well by chiral perturbation theory. The $σ$-meson dressing should be especially important for the stabilization of the bag due to a high coupling constant of the $σ$-meson with the bag and also a strong interaction between the $σ$-field and space symmetric quark core $|s^6[6]>$. This stabilization of the intermediate dressed bag (which shifts the mass of the bag downward to NN threshold) will be crucially important for the description of $π$, $2π$, $ρ$ and $ω$-meson production in NN-collisions, and, in particular, for the correct description of $π^0$ production near threshold. So that, we expect that the results of the present work will lead to a deeper understanding the short- and intermediate-range nuclear forces and internucleon correlations in nuclei.

nucl-th↗
arXiv2005

Magnetic moments of S-shell pentaquarks in the constituent quark model

We discuss the magnetic moments of the recently discovered exotic baryons Theta(+) and Xi(--) and their multiplet partners in the framework of a naive additive quark model. These baryons are set up as pentaquark states where four quarks and a single antiquark are located in their ground state orbit. The pentaquark states are classified as members of the flavor SU(3) octet and antidecuplet with spin-parity J^P = (1/2)^- and (3/2)^-, where we determine the magnetic moments of these pentaquark states.

hep-ph↗
arXiv2005

Anisotropic flow of charged and identified hadrons in the quark-gluon string model for Au+Au collisions at sqrt(s) = 200 GeV

The pseudorapidity behaviour of the azimuthal anisotropy parameters v_1 and v_2 of inclusive charged hadrons and their dependence on the centrality has been studied in Au+Au collisions at full RHIC energy of sqrt(s) = 200 GeV within the microscopic quark-gluon string model. The QGSM simulation results for the directed flow v_1 show antiflow alignment within the pseudorapidity range |eta| < 2 in a fair agreement with the experimental v_1(eta) data, but cannot reproduce the further development of the antiflow up to |eta| around 3.5. The eta dependence of the elliptic flow v_2 extracted from the simulations agrees well with the experimental data in the whole pseudorapidity range for different centrality classes. The centrality dependence of the integrated elliptic flow of charged hadrons in the QGSM almost coincides with the PHOBOS experimental distribution. The transverse momentum dependence of the elliptic flow of identified and inclusive charged hadrons is studied also. The model reproduces quantitatively the low p_T part of the distributions rather good, but underestimates the measured elliptic flow for transverse momenta p_T > 1 GeV/c. Qualitatively, however, the model is able to reproduce the saturation of the v_2(p_T) spectra with rising p_T as well as the crossing of the elliptic flow for mesons and baryons.

nucl-th↗
arXiv2005

Gauge invariant counterparts and quantization of systems under holonomic constraints

Systems under holonomic constraints are classified within the generalized Hamiltonian framework as second-class constraints systems. We show that each system of point particles with holonomic constraints has a hidden gauge symmetry which allows to quantize it in the original phase space as a first-class constraints system. The proposed method is illustrated with quantization of a point particle moving on an $n-1$-dimensional sphere $S^{n-1}$ as well as its field theory analog the O(n) nonlinear sigma model.

hep-th↗
arXiv2005

Dibaryon model for nuclear force and the properties of the $3N$ system

The dibaryon model for $NN$ interaction, which implies the formation of an intermediate six-quark bag dressed by a $σ$-field, is applied to the $3N$ system, where it results in a new three-body force of scalar nature between the six-quark bag and a third nucleon. A new multicomponent formalism is developed to describe three-body systems with nonstatic pairwise interactions and non-nucleonic degrees of freedom. Precise variational calculations of $3N$ bound states are carried out in the dressed-bag model including the new scalar three-body force. The unified coupling constants and form factors for $2N$ and $3N$ force operators are used in the present approach, in a sharp contrast to conventional meson-exchange models. It is shown that this three-body force gives at least half the $3N$ total binding energy, while the weight of non-nucleonic components in the $^3$H and $^3$He wavefunctions can exceed 10%. The new force model provides a very good description of $3N$ bound states with a reasonable magnitude of the $σNN$ coupling constant. A new Coulomb $3N$ force between the third nucleon and dibaryon is found to be very important for a correct description of the Coulomb energy and r.m.s. charge radius in $^3$He. In view of the new results for Coulomb displacement energy obtained here for A=3 nuclei, an explanation for the long-term Nolen--Schiffer paradox in nuclear physics is suggested. The role of the charge-symmetry-breaking effects in the nuclear force is discussed.

nucl-th↗
arXiv2005

Sterile neutrinos in neutrinoless double beta decay

We study possible contribution of the Majorana neutrino mass eigenstate $ν_h$ dominated by a sterile neutrino component to neutrinoless double beta ($0νββ$) decay. From the current experimental lower bound on the $0νββ$-decay half-life of $^{76}$Ge we derive stringent constraints on the $ν_h-ν_e$ mixing in a wide region of the values of $ν_h$ mass. We discuss cosmological and astrophysical status of $ν_h$ in this mass region.

hep-ph↗
arXiv2005

The pentaquark $Θ^+(1540)$ in the string model

We consider the $Θ^+(1540)$ pentaquark in the string model that correctly reproduces the linear Regge trajectories for the case of orbital excitations of light $q\bar q$ mesons and $qqq$ baryons. Assuming (and arguing in favour of) the diquark-antiquark-diquark ($[ud]\bar s [ud]$) clustering of this orbitally excited object we found its mass about $290 \rm {MeV}$ above the experimental value $1540 \rm {MeV}$. In the model considered this discrepancy could be attributed to the change of the constituent mass of $\bar s$ antiquark as compared to that of $s$ quark localized at the string end.

hep-ph↗
arXiv2004

Electron-Positron Annihilation into Hadron-Antihadron Pairs

The reactions of electron-positron to nucleon-antinucleon pairs are studied in a non-perturbative quark model. The work suggests that the two-step process, in which the primary quark-antiquark pair forms first a vector meson which in turn decays into a hadron pair, is dominant over the one-step process in which the primary quark-antiquark pair is directly dressed by additional quark-antiquark pairs to form a hadron pair. To reproduce the experimental data of the reactions of electron-positron to proton-antiproton and electron-positron to neutron-antineutron a D-wave omega-like vector meson with a mass of around 2 GeV has to be introduced.

hep-ph↗