arXiv subjects

Amand Faessler

Publications and source records attributed to Amand Faessler.

At least 217 records · Page 12Linked to original sources

arXiv2002

Perspectives of Nuclear Physics

The organizers of this meeting have asked me to present perspectives of nuclear physics. This means to identify the areas where nuclear physics will be expanding in the next future. In six chapters a short overview of these areas will be given, where I expect that nuclear physics willdevelop quite fast: A. Quantum Chromodynamics and effective field theories in the confinement region; B. Nuclear structure at the limits; C. High energy heavy ion collisions; D. Nuclear astrophysics; E. Neutrino physics; F. Test of physics beyond the standard model by rare processes. After a survey over these six points I will pick out a few topics where I will go more in details. There is no time to give for all six points detailed examples. I shall discuss the following examples of the six topics mentionned above: 1. The perturbative chiral quark model and the nucleon $Σ$-term, 2. VAMPIR (Variation After Mean field Projection In Realistic model spaces and with realistic forces) as an example of the nuclear structure renaissance, 3. Measurement of important astrophysical nuclear reactions in the Gamow peak, 4. The solar neutrino problem. As examples for testing new physics beyond the standard model by rare processes I had prepared to speak about the measurement of the electric neutron dipole moment and of the neutrinoless double beta decay. But the time is limited and so I have to skip these points, although they are extremely interesting.

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arXiv2002

Transition to meson-dominated matter at RHIC. Consequences for kaon flow

Anisotropic flow of kaons and antikaons is studied in heavy-ion collisions at CERN SPS and BNL RHIC energies within the microscopic quark-gluon string model. In the midrapidity range the directed flow of kaons v_1 differs considerably from that of antikaons at SPS energy (E_{lab} = 160 AGeV), while at RHIC energy (\sqrt{s} = 130 AGeV) the excitation functions of both, kaon and antikaon, flows coincide within the statistical error bars. The change is attributed to formation of dense meson-dominated matter at RHIC, where the differences in interaction cross-sections of kaons and antikaons become unimportant. The time evolution of the kaon anisotropic flow is also investigated. The elliptic flow of these hadrons is found to develop at midrapidity at times 3 < t < 10 fm/c, which is much larger than the nuclear passing time t^{pass} = 0.12 fm/c. As a function of transverse momentum the elliptic flow increases almost linearly with rising p_t. It stops to rise at p_t > 1.5 GeV/c reaching the saturation value $v_2^K (p_t) \approx 10%$.

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arXiv2002

Fully-Renormalized QRPA fulfills Ikeda sum rule exactly

The renormalized quasiparticle-RPA is reformulated for even-even nuclei using restrictions imposed by the commutativity of the phonon creation operator with the total particle number operator. This new version, Fully-Renormalized QRPA (FR-QRPA), is free from the spurious low-energy solutions. Analytical proof is given that the Ikeda sum rule is fullfiled within the FR-QRPA.

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arXiv2002

Phi meson production at RHIC

The production of phi mesons in Au+Au collisions at RHIC and their propagation in a hot and dense nuclear medium is studied within the microscopic quark-gluon string model. The inverse slope parameter of the transverse mass distribution agrees well with that extracted from the STAR data, while the absolute yield of phi is underestimated by a factor 2. It appears that the fusion of strings alone cannot increase the phi yield either. Less than 30% of detectable phi's experience elastic scattering, this rate is insufficient for the full thermalization of phi. The directed flow of phi at |y|<2 demonstrates strong antiflow behavior, whereas its elliptic flow rises up to about 3.5% in the same rapidity interval. As a function of transverse momentum it rises linearly with increasing p_t, in agreement with the STAR data, and saturates at p_t > 2 GeV/c.

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arXiv2002

Strange nucleon form factors in the perturbative chiral quark model

We apply the perturbative chiral quark model at one loop to calculate the strange form factors of the nucleon. A detailed numerical analysis of the strange magnetic moments and radii of the nucleon, and also the momentum dependence of the form factors is presented.

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arXiv2002

Anisotropic flow of strange particles in heavy ion collisions at RHIC energies

Anisotropic flow of K's, anti-K's, and lambdas is studied in heavy ion collisions at SPS and RHIC energies within the microscopic quark-gluon string model. At SPS energy the directed flow of kaons differs considerably at midrapidity from that of antikaons, while at RHIC energy kaon and antikaon flows coincide. The change is attributed to formation of dense meson-dominated matter at RHIC, where the differences in interaction cross-sections of hadrons become unimportant. The directed flows of strange particles, $v_1^{K,\bar{K}, Λ}(y)$, have universal negative slope at $|y| \leq 2$ at RHIC. The elliptic flow of strange hadrons is developed at midrapidity at times 3<t<10 fm/c. It increases almost linearly with rising p_t and stops to rise at p_t < 1.5 GeV/c reaching the same saturation value $v_2 ^{K,Λ}(p_t) \approx 10%$ in accord with experimental results.

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arXiv2002

Strangeness production in heavy ion collisions at SPS and RHIC within two-source statistical model

The experimental data on hadron yields and ratios in central Pb+Pb and Au+Au collisions at SPS and RHIC energies, respectively, are analysed within a two-source statistical model of an ideal hadron gas. These two sources represent the expanding system of colliding heavy ions, where the hot central fireball is embedded in a larger but cooler fireball. The volume of the central source increases with rising bombarding energy. Results of the two-source model fit to RHIC experimental data at midrapidity coincide with the results of the one-source thermal model fit, indicating the formation of an extended fireball, which is three times larger than the corresponding core at SPS.

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arXiv2002

Violation of energy-per-hadron scaling in a resonance matter

Yields of hadrons, their average masses and energies per hadron at the stage of chemical freeze-out in (ultra)relativistic heavy-ion collisions are analyzed within the statistical model. The violation of the scaling / = 1 GeV observed in Au+Au collisions at $\sqrt{s}$ = 130 AGeV is linked to the formation of resonance-rich matter with a considerable fraction of baryons and antibaryons. The rise of the energy-per-hadron ratio in baryon-dominated matter is discussed. A violation of the scaling condition is predicted for a very central zone of heavy-ion collisions at energies around 40 AGeV.

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arXiv2002

Strange quark matter in a chiral SU(3) quark mean field model

We apply the chiral SU(3) quark mean field model to investigate strange quark matter. The stability of strange quark matter with different strangeness fraction is studied. The interaction between quarks and vector mesons destabilizes the strange quark matter. If the strength of the vector coupling is the same as in hadronic matter, strangelets can not be formed. For the case of beta equilibrium, there is no strange quark matter which can be stable against hadron emission even without vector meson interactions.

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arXiv2002

Neutrinoless Double Beta Decay of 134Xe

In view of recent great progress achieved in the experimental study of the neutrinoless double beta decay (0nbb-decay) of 134Xe we discuss theoretical aspects of this process. The light and heavy Majorana neutrino exchange as well as the trilinear R-parity breaking contributions to 0nbb-decay are considered. We show that the sensitivity of the studied process to the signal of lepton number violation is only by factor 2-3 weaker in comparison with the 0nbb-decay of 136Xe. The current limits on effective neutrino mass (light and heavy) and trilinear R-parity violating parameter lambda'_111 deduced from lower limits on the 0nbb-decay half-lifes of various nuclei are reviewed and perspectives of the experimental verification of recently announced evidence of the 0nbb-decay of 76Ge are discussed.

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arXiv2002

Neutron-Proton High-Energy Charge Exchange Scattering

The high energy proton-neutron charge exchange scattering reaction is studied in an effective hadron model for the energy range of s from 45.9 to 414.61 GeV*GeV. The main features of the observed differential cross section, the forward peak and the scaling behavior over a large energy region, are well reproduced.

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arXiv2002

Field-theoretical description of the multichannel gamma-p scattering reaction in the Delta resonance region and determination of the magnetic moment of the Delta+ resonance

The cross-sections of the $γp-γ' N'$, $γp-π' N'$ and $γp-γ'π' N'$ reactions are calculated in the framework of the field-theoretical one-particle ($π,ω,ρ$-mesons, nucleon and $Δ$-resonance) exchange model. Unlike the other relativistic approaches, our resulting amplitudes of the $γp$ multichannel reactions require one-variable covariant vertex functions as input ingredient and every diagram of these amplitudes satisfies the current conservation condition in the Coulomb gauge. The complete set of the model independent skeleton diagrams for the $γ p\toγ'π'N'$ reaction is presented. The separable model of the $πN$ interaction is generalized to construct the spin 3/2 particle propagator of the $Δ$-resonance. This procedure allows to obtain the $πN-Δ$ form factor and $Δ$ propagator directly from the $πN$ $P_{33}$ phase shifts. The numerical calculation of the differential cross section of the $γp-γ' N'$, $γp-{π^o}' N'$ and $γp-γ'{π^o}'N'$ reactions are performed with two different separable models of the $Δ$ propagator and with the propagator of Breit-Wigner shape. It is demonstrated that the numerical description of these reactions in the $Δ$-resonance region are very sensitive to the form of the $Δ$-propagator. The sensitivity of the cross-sections of the $γp\toγ'{π^o}'p'$ reaction to the magnitude of the $Δ^+$ magnetic moment is examined and the most convenient kinematical region for the determination of the magnetic moment of the $Δ^+$-resonance from the forthcoming data is indicated.

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arXiv2002

The nuclear equation of state probed by K+ production in heavy ion collisions

The dependence of K+ production on the nuclear equation of state is investigated in heavy ion collisions. An increase of the excitation function of K+ multiplicities obtained in heavy (Au+Au) over light (C+C) systems when going far below threshold which has been observed by the KaoS Collaboration strongly favours a soft equation of state. This observation holds despite of the influence of an in-medium kaon potential predicted by effective chiral models which is necessary to reproduce the experimental K+ yields. Phase space effects are discussed with respect to the K+ excitation function.

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arXiv2001

Distinguishing the neutrinoless double beta decay mechanisms

Many new neutrinoless double beta decay (0nbb) experiments are planned or in preparation. If the 0nbb-decay will be detected, the key issue will be what is the dominant mechanism of this process. By measuring only transitions to the ground state one can not distinguish among many of the 0nbb-decay mechanisms (the light and heavy Majorana neutrino exchange mechanisms, the trilinear R-parity breaking mechanisms etc.). We show that if the ratio of the 0nbb-decay half-lifes for transitions to the 0^+ first excited and ground states is determined both theoretically and experimentally, it might be possible to determine, which 0nbb-decay mechanisms is dominant. For that purpose the corresponding nuclear matrix elements have to be evaluated with high reliability. The present work is giving strong motivations for experimental studies of the 0nbb-decay transitions to the first excited 0^+ states of the final nuclei.

hep-ph↗

arXiv2001

The (muon^-,muon^+) conversion in nuclei as a probe of new physics

A detailed study of the muonic analogue of neutrinoless double beta decay, (muon^-,muon^+) conversion, has been carried out for the A=44 nuclear system. We studied several lepton number violating (LNV) mechanisms potentially triggering this process: exchange by light and heavy Majorana neutrinos as well as exchange by supersymmetric particles participating in R-parity violating interactions. The nuclear structure has been taken into account within the renormalized Quasiparticle Random Phase Approximation method. To our knowledge, this is the first realistic treatment of nuclear structure aspects of the (muon^-,muon^+) conversion. We estimated the rate of this process utilizing the existing experimental constraints on the parameters of the underlying LNV interactions and conclude that the (muon^-,muon^+) conversion is hardly detectable in the near future experiments.

hep-ph↗

arXiv2001

Testing Dirac-Brueckner models in collective flow of heavy-ion collisions

We investigate differential in-plane and out-of-plane flow observables in heavy ion reactions at intermediate energies from $0.2÷2$ AGeV within the framework of relativistic BUU transport calculations. The mean field is based on microscopic Dirac-Brueckner-Hartree-Fock (DBHF) calculations. We apply two different sets of DBHF predictions, those of ter Haar and Malfliet and more recent ones from the Tübingen group, which are similar in general but differ in details. The latter DBHF calculations exclude spurious contributions from the negative energy sector to the mean field which results in a slightly softer equation of state and a less repulsive momentum dependence of the nucleon-nucleus potential at high densities and high momenta. For the application to heavy ion collisions in both cases non-equilibrium features of the phase space are taken into account on the level of the effective interaction. The systematic comparison to experimental data favours the less repulsive and softer model. Relative to non-relativistic approaches one obtains larger values of the effective nucleon mass. This produces a sufficient amount of repulsion to describe the differential flow data reasonably well.

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arXiv2001

Electromagnetic transition form factors and dilepton decay rates of nucleon resonances

Relativistic, kinematically complete phenomenological expressions for the dilepton decay rates of nucleon resonances with arbitrary spin and parity are derived in terms of the magnetic, electric, and Coulomb transition form factors. The dilepton decay rates of the nucleon resonances with masses below 2 GeV are estimated using the extended vector meson dominance model for the transition form factors. The model provides a unified description of the photo- and electroproduction data, the vector meson decays, and the dilepton decays of the nucleon resonances. The constraints on the transition form factors from the quark counting rules are taken into account. The parameters of the model are fixed by fitting the available photo- and electroproduction data and using results of the multichannel partial-wave analysis of the $πN$ scattering. Where experimental data are not available, predictions of the non-relativistic quark models are used as an input. The vector meson coupling constants of the magnetic, electric, and Coulomb types are determined. The dilepton widths and the dilepton spectra from decays of nucleon resonances with masses below 2 GeV are calculated.

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arXiv2001

Electromagnetic couplings of the ChPT Lagrangian from the perturbative chiral quark model

We apply the perturbative chiral quark model to the study of the low-energy pi-N interaction. Using an effective chiral Lagrangian we reproduce the Weinberg-Tomozawa result for the S-wave pi-N scattering lengths. After inclusion of the photon field we give predictions for the electromagnetic O(p^2) low-energy couplings of the chiral perturbation theory effective Lagrangian that define the electromagnetic mass shifts of nucleons and first-order (e^2) radiative corrections to the pi-N scattering amplitude. Finally, we estimate the leading isospin-breaking correction to the strong energy shift of the pi(-)p atom in the 1s state, which is relevant for the experiment "Pionic Hydrogen" at PSI.

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