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A. Schiller

Publications and source records attributed to A. Schiller.

At least 163 records · Page 9Linked to original sources
arXiv2001

Level densities and $γ$-strength functions in $^{148,149}$Sm

The level densities and $γ$-strength functions of the weakly deformed $^{148}$Sm and $^{149}$Sm nuclei have been extracted. The temperature versus excitation energy curve, derived within the framework of the micro canonical ensemble, shows structures, which we associate with the break up of Cooper pairs. The nuclear heat capacity is deduced within the framework of both the micro canonical and the canonical ensemble. We observe negative heat capacity in the micro canonical ensemble whereas the canonical heat capacity exhibits an S-shape as function of temperature, both signals of a phase transition. The structures in the $γ$-strength functions are discussed in terms of the pygmy resonance and the scissors mode built on exited states. The samarium results are compared with data for the well deformed $^{161,162}$Dy, $^{166,167}$Er and $^{171,172}$Yb isotopes and with data from (n,$γ$)-experiments and giant dipole resonance studies.

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arXiv2001

Monopoles, confinement and deconfinement in lattice compact QED in (2+1)D with external fields

Finite temperature compact electrodynamics in (2+1) dimensions is studied in the presence of external electromagnetic fields. The deconfinement temperature is found to be insensitive to the external fields. This result corroborates our observation that external fields create additional small--size magnetic dipoles from the vacuum which do not spoil the confining properties of the model at low temperature. However, the Polyakov loop is not an order parameter of confinement. It can vanish in deconfinement in the presence of external field. This does not mean the restoration of confinement for certain external field fluxes. As a next step in the study of (2+1)D QED, the influence of monopoles on the photon propagator is studied. First results are presented showing this connection in the confining phase (without external field).

hep-lat↗
arXiv2001

Monopoles, confinement and deconfinement of (2+1)D compact lattice QED in external fields

The compact Abelian model in three space--time dimensions is studied in the presence of external electromagnetic fields at finite temperatures. We show that the deconfinement phase transition is independent on the strength of the external fields. This result is in agreement with our observation that the external fields create small--size magnetic dipoles from the vacuum which do not influence the confining properties of the model. Contrary to the deconfinement phase, the internal field in the direction of the applied external field is attenuated in the confinement phase, this screening becomes stronger with decreasing temperature.

hep-lat↗
arXiv2001

A lattice study of 3D compact QED at finite temperature

We study the deconfinement phase transition and monopole properties in the finite temperature 3D compact Abelian gauge model on the lattice. We predict the critical coupling as function of the lattice size in a simplified model to describe monopole binding. We demonstrate numerically that the monopoles are sensitive to the transition. In the deconfinement phase the monopoles appear in the form of a dilute gas of magnetic dipoles. In the confinement phase both monopole density and string tension differ from semiclassical estimates if monopole binding is neglected. However, the analysis of the monopole clusters shows that the relation between the string tension and the density of monopoles in charged clusters is in reasonable agreement with those predictions. We study the cluster structure of the vacuum in both phases of the model.

hep-lat↗
arXiv2001

Heat capacity and pairing transition in nuclei

A simple model based on the canonical-ensemble theory is outlined for hot nuclei. The properties of the model are discussed with respect to the Fermi gas model and the breaking of Cooper pairs. The model describes well the experimental level density of deformed nuclei in various mass regions. The origin of the so-called S-shape of the heat capacity curve Cv(T) is discussed.

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arXiv2000

Charge asymmetry of pions in the process $e^-e^+\to e^-e^+π^+π^-$

The study of the charge asymmetry of produced particles allows to investigate the interference of different production mechanisms and to determine new features of the corresponding amplitudes. In the process $e^- e^+ \to e^- e^+ π^+ π^-$ the two-pion system is produced via two mechanisms: two-photon (C-even state) and bremsstrahlung (C-odd state) production. We study the charge asymmetry of pions in a differential in the pion momenta cross section originating from an interference between these two mechanisms. At low effective mass of dipions this asymmetry is directly related to the s- and p-phases of elastic $ππ$ scattering. At higher energies it can give new information about the $f_0$ meson family, $f_2(1270)$ meson, etc. The asymmetry is expressed via the pion form factor $F_π$ and helicity amplitudes $M_{ab}$ for the subprocess $γ^*γ\to π^+π^-$ as $\sum G_{ab}{\rm Re}(F_π^*M_{ab})$ where we have calculated analytically the coefficients $G_{ab}$ for the region giving the main contribution to the effect. Several distributions of pions are presented performing a numerical analysis in a model with point-like pions. In the region near the dipion threshold the asymmetry is of the order of 1%. We show that with suitable cuts the signal to background ratio can be increased up to about 10%.

hep-ph↗
arXiv2000

Observation of the scissors mode in the quasicontinuum

The experimental resonance parameters of the pygmy resonance in rare earth nuclei are compared to global parameters of the scissors-mode states. It is argued that the pygmy resonance in rare earth nuclei can be described in terms of orbital M1 strength observed in (gamma,gamma') experiments. The pygmy resonance is therefore interpreted as the scissors mode in the quasicontinuum.

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arXiv2000

Thermal and electromagnetic properties of 166-Er and 167-Er

The primary gamma-ray spectra of 166-Er and 167-Er are deduced from the (3-He,alpha gamma) and (3-He,3-He' gamma) reaction, respectively, enabling a simultaneous extraction of the level density and the gamma-ray strength function. Entropy, temperature and heat capacity are deduced from the level density within the micro-canonical and the canonical ensemble, displaying signals of a phase-like transition from the pair-correlated ground state to an uncorrelated state at Tc=0.5 MeV. The gamma-ray strength function displays a bump around E-gamma=3 MeV, interpreted as the pygmy resonance.

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arXiv2000

Gamma-ray strength function and pygmy resonance in rare earth nuclei

The gamma-ray strength function for gamma energies in the 1-7 MeV region has been measured for 161,162-Dy and 171,172-Yb using the (3-He,alpha gamma) reaction. Various models are tested against the observed gamma-ray strength functions. The best description is based on the Kadmenskii, Markushev and Furman E1 model with constant temperature and the Lorentzian M1 model. A gamma-ray bump observed at E_gamma=3 MeV is interpreted as the so-called pygmy resonance, which has also been observed previously in (n,gamma) experiments. The parameters for this resonance have been determined and compared to the available systematics.

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arXiv2000

Entropy of thermal quasiparticles in nuclei

Information on level density for nuclei with mass numbers A = 20 - 250 is deduced from discrete low-lying levels and neutron resonance data. The odd-mass nuclei exhibit in general 4 - 7 times the level density found for its neighboring even-even nuclei at the same excitation energy. This excess corresponds to an entropy of approx. 1.7 k for the odd particle. The value is approximately constant for all mid-shell nuclei and for all ground state spins. For these nuclei it is argued that the entropy scales with the number of quasiparticles. A simple model based on the canonical ensemble theory accounts qualitatively for the observed properties.

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arXiv2000

Level density and thermal properties in rare earth nuclei

A convergent method to extract the nuclear level density and the gamma-ray strength function from primary gamma-ray spectra has been established. Thermodynamical quantities have been obtained within the microcanonical and canonical ensemble theory. Structures in the caloric curve and in the heat capacity curve are interpreted as fingerprints of breaking of Cooper pairs and quenching of pairing correlations. The strength function can be described using models and common parameterizations for the E1, M1 and pygmy resonance strength. However, a significant decrease of the pygmy resonance strength at finite temperatures has been observed.

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arXiv2000

Renormalisation and off-shell improvement in lattice perturbation theory

We discuss the improvement of flavour non-singlet point and one-link lattice quark operators, which describe the quark currents and the first moment of the DIS structure functions respectively. Suitable bases of improved operators are given, and the corresponding renormalisation factors and improvement coefficients are calculated in one-loop lattice perturbation theory, using the Sheikholeslami-Wohlert (clover) action. To this order we achieve off-shell improvement by eliminating the effect of contact terms. We use massive fermions, and our calculations are done keeping all terms up to first order in the lattice spacing, for arbitrary m^2/p^2, in a general covariant gauge. We also compare clover fermions with fermions satisfying the Ginsparg-Wilson relation, and show how to remove O(a) effects off-shell in this case too, and how this is in many aspects simpler than for clover fermions. Finally, tadpole improvement is also considered.

hep-lat↗
arXiv2000

Critical temperature for quenching of pair correlations

The level density at low spin in the 161,162-Dy and 171,172-Yb nuclei has been extracted from primary gamma rays. The nuclear heat capacity is deduced within the framework of the canonical ensemble. The heat capacity exhibits an S-formed shape as a function of temperature, which is interpreted as a fingerprint of the phase transition from a strongly correlated to an uncorrelated phase. The critical temperature for the quenching of pair correlations is found at Tc=0.50(4) MeV.

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arXiv2000

Energy shifted level densities in the rare earth region

The density of accessible levels at low spin in the (3-He,alpha gamma) reaction has been extracted for the 161,162-Dy and 171,172-Yb nuclei. The energy shift between the level densities of the even-odd and even-even isotopes is measured as a function of excitation energy. The results are compared with predictions from various semi-empirical models. The energy shift procedure works well for excitation energies between 3.5 and 7 MeV in the even-even nucleus, provided that a proper level density function is used. The experimental energy shift is close to the pairing gap parameter Delta.

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arXiv1999

Embedded vortices and their interactions at electroweak crossover

We study properties of Z-vortices in the crossover region of the 3D SU(2) Higgs model. Correlators of the vortex currents with gauge field energy and Higgs field squared ("quantum vortex profile") reveal a structure that can be compared with a classical vortex. We define a core size and a penetration depth from the vortex profile. Z-vortices are found to interact with each other analogously to Abrikosov vortices in a type-I superconductor.

hep-lat↗
arXiv1999

Level density and gamma strength function in 162-Dy from inelastic 3-He scattering

Complementary measurements have been performed for the level density and gamma strength function in 162-Dy using inelastic 3-He scattering. Comparing these results to previous measurements using the 163-Dy(3-He,alpha) reaction, reveals that the measured quantities above 1.5 MeV do not depend significantly on the nuclear reaction chosen.

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arXiv1999

Entropy in hot ^{161,162}Dy and ^{171,172}Yb nuclei

The density of accessible levels at low spin in the (^3He,αγ) reaction has been extracted for the ^{161,162}Dy and ^{171,172}Yb nuclei. The entropy of the even-odd and even-even nuclei has been deduced as a function of excitation energy, and found to reach a maximum of 15 k_B before neutron evaporation. The entropy of one quasi-particle outside an even-even core is found to be 1.70(15) k_B. This quasi-particle picture of hot nuclei is well accounted for within a simple pairing model. The onset of two, four and six quasi-particle excitations in the ^{162}Dy and ^{172}Yb nuclei is discussed and compared to theory. The number of quasi-particles excited per excitation energy is a measure for the ratio of the level energy spacing and the pairing strength.

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