arXiv subjects

P. Danielewicz

Publications and source records attributed to P. Danielewicz.

At least 55 records · Page 3Linked to original sources

arXiv2007

Collective Flow of Protons and Negative Pions in Nucleus-Nucleus Collisions at Momentum of $4.2 ÷4.5$ AGeV/c

Collective flow of protons and negative pions has been studied within the momentum region of $4.2 ÷4.5$ AGeV/c ($E =3.4 ÷3.7$ AGeV) for different projectile-target combinations involving carbon and, specifically, He-C, C-C, C-Ne, C-Cu and C-Ta. The data stem from the SKM-200-GIBS streamer chamber and from Propane Bubble Chamber systems utilized at JINR. The directed flow of protons grows dramatically in the carbon region when the counterpart nucleus grows in mass between He and Ta. The elliptic proton flow points out of the reaction plane and also strengthens as system mass increases. Within the reaction plane, the negative pions flow in the same direction as protons for the lighter of the investigated systems, He-C, C-C and C-Ne, and in the opposite direction for the heavier, C-Cu and C-Ta. The Quark-Gluon String Model reproduces observed changes in the flow with system mass.

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arXiv2007

Analysis of Emission Shapes

Shapes of relative emission sources can be accessed by expanding shapes of correlations at low relative velocities in pair center of mass in Cartesian harmonics. Coefficients of expansion for correlations are related to the respective coefficients of expansion for the sources through one dimensional integral transforms involving properties of pair relative wavefunctions. The methodology is illustrated with analyses of NA49 and PHENIX correlation data.

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arXiv2007

Has the QCD Critical Point been Signaled by Observations at RHIC ?

The shear viscosity to entropy ratio ($η/s$) is estimated for the hot and dense QCD matter created in Au+Au collisions at RHIC ($\sqrt{s_{NN}}=200$ GeV). A very low value is found $η/s \sim 0.1$, which is close to the conjectured lower bound ($1/4π$). It is argued that such a low value is indicative of thermodynamic trajectories for the decaying matter which lie close to the QCD critical end point.

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arXiv2006

Symmetry Energy

Examination of symmetry energy is carried out on the basis of an elementary binding-energy formula. Constraints are obtained on the energy value at the normal nuclear density and on the density dependence of the energy at subnormal densities.

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arXiv2006

Comment on "Symmetry energy and the isospin dependent equation of state"

In a recent paper, Shetty et al [Phys. Rev. C 70, 011601R (2004)] reported that the experimental isoscaling parameters aexp from several reactions favor the Gogny-AS effective nucleon-nucleon interaction over Gogny interaction. This conclusion is reached by comparing data to Antisymmetrized Molecular Dynamics (AMD) predictions for collisions of calcium isotopes. The specific simulations produce excited fragments at t=300 fm/c before they decay. Sequential decay calculations of the excited fragments suggest that the isoscaling parameter apri from the AMD calculations could be reduced by as much as 50%. We also explore the linear equation used to relate other reactions to the AMD calculations for Ca+Ca collisions. The uncertainty in the relation is larger than the differences between the predicted apri when using Gogny and Gogny-AS interactions.

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arXiv2005

Nuclear Equation of State: Picture from Medium Energy Heavy Ion Collisions

Characteristics of the nuclear equation of state (EOS) and its importance, in particular for astrophysics, are discussed. Selected observables in nuclear collisions are sensitive to the EOS and can be used to constrain it. For central collisions, these include collective flow asymmetries, subthreshold kaon yields and isospin diffusion. Comparisons between the data and transport theory suggest an energy per nucleon that rises relatively slowly with density for symmetric matter and symmetry energy that rises relatively quickly around the normal nuclear density.

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arXiv2005

Imaging Three Dimensional Two-particle Correlations for Heavy-Ion Reaction Studies

We report an extension of the source imaging method for analyzing three-dimensional sources from three-dimensional correlations. Our technique consists of expanding the correlation data and the underlying source function in spherical harmonics and inverting the resulting system of one-dimensional integral equations. With this strategy, we can image the source function quickly, even with the finely binned data sets common in three-dimensional analyses.

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arXiv2005

Decomposition of Harmonic and Jet Contributions to Particle-pair Correlations at Ultra-relativistic Energies

Methodology is presented for analysis of two-particle azimuthal angle correlation functions obtained in collisions at ultra-relativistic energies. We show that harmonic and di-jet contributions to these correlation functions can be reliably decomposed by two techniques to give an accurate measurement of the jet-pair distribution. Results from detailed Monte Carlo simulations are used to demonstrate the efficacy of these techniques in the study of possible modifications to jet topologies in heavy ion reactions.

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arXiv2005

Analysis of Low-Momentum Correlations with Cartesian Harmonics

Exploiting final-state interactions and/or identity interference, analysis of anisotropic correlations of particles at low-relative velocities yields information on the anisotropy of emission sources in heavy-ion reactions. We show that the use of cartesian surface-spherical harmonics in such analysis allows for a systematic expansion of the correlations in terms of real angular-moment coefficients dependent on relative momentum. The coefficients are directly related to the analogous coefficients for emission sources. We illustrate the analysis with an example of correlations generated by classical Coulomb interaction.

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arXiv2005

Angular Moment Analysis of Correlations

For analyzing anisotropic low relative-velocity correlation-functions and the associated emission sources, we propose an expansion in terms of cartesian spherical harmonics. The expansion coefficients represent angular moments of the investigated functions. The respective coefficients for the correlation and source are directly related to each other via one-dimensional integral transforms. The shape features of the source may be partly read off from the respective features of the correlation function and can be, otherwise, imaged.

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arXiv2005

Transport Theories for Heavy Ion Collisions in the 1 AGeV Regime

We compare multiplicities as well as rapidity and transverse momentum distributions of protons, pions and kaons calculated within presently available transport approaches for heavy ion collisions around 1 AGeV. For this purpose, three reactions have been selected: Au+Au at 1 and 1.48 AGeV and Ni+Ni at 1.93 AGeV.

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arXiv2004

Symmetry energy for fragmentation in dynamical nuclear collisions

We extract values for the free symmetry energy as a function of the fragment size (the proton number Z) from antisymmetrized molecular dynamics (AMD) calculations of calcium collisions. Simple statistical physics describe well the distribution of hot nuclei at breakup, provided the surface symmetry term in the free energy is much smaller at high excitation than in ground state nuclei. This result may reflect the condition of low density and finite temperature when these systems disassemble.

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arXiv2004

Towards the 3D-Imaging of Sources

Geometric details of a nuclear reaction zone, at the time of particle emission, can be restored from low relative-velocity particle-correlations, following imaging. Some of the source details get erased and are a potential cause of problems in the imaging, in the form of instabilities. These can be coped with by following the method of discretized optimization for the restored sources. So far it has been possible to produce 1-dimensional emission source images, corresponding to the reactions averaged over all possible spatial directions. Currently, efforts are in progress to restore angular details.

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arXiv2004

Effects of in-medium cross-sections and optical potential on thermal-source formation in p+197Au reactions at 6.2-14.6 GeV/c

Effects of in-medium cross-sections and of optical potential on pre-equilibrium emission and on formation of a thermal source are investigated by comparing the results of transport simulations with experimental results from the p+{197}Au reaction at 6.2-14.6 GeV/c. The employed transport model includes light composite-particle production and allows for inclusion of in-medium particle-particle cross-section reduction and of momentum dependence in the particle optical-potentials. Compared to the past, the model incorporates improved parameterizations of elementary high-energy processes. The simulations indicate that the majority of energy deposition occurs during the first ~25 fm/c of a reaction. This is followed by a pre-equilibrium emission and readjustment of system density and momentum distribution toward an equilibrated system. Good agreement with data, on the d/p and t/p yield ratios and on the residue mass and charge numbers, is obtained at the time of ~ 65 fm/c from the start of a reaction, provided reduced in-medium cross-sections and momentum-dependent optical potentials are employed in the simulations. By then, the pre-equilibrium nucleon and cluster emission, as well as mean-field readjustments, drive the system to a state of depleted average density, rho/rho_{0} ~ 1/4-1/3 for central collisions, and low-to-moderate excitation, i.e. the region of nuclear liquid-gas phase transition.

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arXiv2004

Azimuthal dependence of collective expansion for symmetric heavy ion collisions

Detailed studies of the azimuthal dependence of the mean fragment and flow energies in the Au+Au and Xe+CsI systems are reported as a function of incident energy and centrality. Comparisons between data and model calculations show that the flow energy values along different azimuthal directions could be viewed as snapshots of the fireball expansion with different exposure times. For the same number of participating nucleons more transversally elongated participant shapes from the heavier system produce less collective transverse energy. Good agreement with BUU calculations is obtained for a soft nuclear equation of state.

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arXiv2003

Isospin Diffusion in Heavy Ion Reactions

Using symmetric 112Sn+112Sn, 124Sn+124Sn collisions as references, we probe isospin diffusion in peripheral asymmetric 112Sn+124Sn, 124Sn+112Sn systems at incident energy of E/A=50 MeV. Isoscaling analyses imply that the quasi-projectile and quasi-target in these collisions do not achieve isospin equilibrium, permitting an assessment of the isospin transport rates. We find that comparisons between isospin sensitive experimental and theoretical observables, using suitably chosen scaled ratios, permit investigation of the density dependence of the asymmetry term of the nuclear equation of state.

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arXiv2003

Nuclear Isospin Diffusivity

The isospin diffusion and other irreversible phenomena are discussed for a two-component nuclear Fermi system. The set of Boltzmann transport equations, such as employed for reactions, are linearized, for weak deviations of a system from uniformity, in order to arrive at nonreversible fluxes linear in the nonuniformities. Besides the diffusion driven by a concentration gradient, also the diffusion driven by temperature and pressure gradients is considered. Diffusivity, conductivity, heat conduction and shear viscosity coefficients are formally expressed in terms of the responses of distribution functions to the nonuniformities. The linearized Boltzmann-equation set is solved, under the approximation of constant form-factors in the distribution-function responses, to find concrete expressions for the transport coefficients in terms of weighted collision integrals. The coefficients are calculated numerically for nuclear matter, using experimental nucleon-nucleon cross sections. The isospin diffusivity is inversely proportional to the neutron-proton cross section and is also sensitive to the symmetry energy. At low temperatures in symmetric matter, the diffusivity is directly proportional to the symmetry energy.

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arXiv2003

Surface Symmetry Energy

Binding energy of symmetric nuclear matter can be accessed straightforwardly with the textbook mass-formula and a sample of nuclear masses. We show that, with a minimally modified formula (along the lines of the droplet model), the symmetry energy of nuclear matter can be accessed nearly as easily. Elementary considerations for a macroscopic nucleus show that the surface tension needs to depend on asymmetry. That dependence modifies the surface energy and implies the emergence of asymmetry skin. In the mass formula, the volume and surface and (a)symmetry energies combine as energies of two connected capacitors, with the volume and surface capacitances proportional to the volume and area, respectively. The net asymmetry partitions itself into volume and surface contributions in proportion to the capacitances. A combination of data on skin sizes and masses constrains the volume symmetry parameter to 27 MeV < alpha < 31 MeV and the volume-to-surface symmetry-parameter ratio to 2.0 < alpha/beta < 2.8. In Thomas-Fermi theory, the surface asymmetry-capacitance stems from a drop of the symmetry energy per nucleon S with density. We establish limits on the drop at half of normal density, to 0.57 < S(rho_0/2)/S(rho_0) < 0.83. In considering the feeding of surface by an asymmetry flux from interior, we obtain a universal condition for the collective asymmetry oscillations, in terms of the asymmetry-capacitance ratio.

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