Showing papers in "Physical Review C in 2001"

The High precision, charge dependent Bonn nucleon-nucleon potential (CD-Bonn)

Abstract: We present a charge-dependent one-boson-exchange nucleon-nucleon $(\mathrm{NN})$ potential that fits the world proton-proton data below 350 MeV available in the year 2000 with a ${\ensuremath{\chi}}^{2}$ per datum of 1.01 for 2932 data and the corresponding neutron-proton data with ${\ensuremath{\chi}}^{2}/\mathrm{datum}$ $=1.02$ for 3058 data. This reproduction of the $\mathrm{NN}$ data is more accurate than by any phase-shift analysis and any other $\mathrm{NN}$ potential. This is achieved by the introduction of two effective $\ensuremath{\sigma}$ mesons the parameters of which are partial-wave dependent. The charge dependence of the present potential (which we call ``CD-Bonn'') is based upon the predictions by the Bonn full model for charge symmetry and charge-independence breaking in all partial waves with $Jl~4.$ The potential is represented in terms of the covariant Feynman amplitudes for one-boson exchange which are nonlocal. Therefore, the off-shell behavior of the CD-Bonn potential differs in a characteristic way from commonly used local potentials and leads to larger binding energies in nuclear few- and many-body systems, where underbinding is a persistent problem.

Enhanced $J/\psi$ production in deconfined quark matter

Abstract: In high energy heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider at CERN, each central event will contain multiple pairs of heavy quarks. If a region of deconfined quarks and gluons is formed, a mechanism for additional formation of heavy quarkonium bound states will be activated. This is a result of the mobility of heavy quarks in the deconfined region, such that bound states can be formed from a quark and an antiquark that were originally produced in separate incoherent interactions. Model estimates of this effect for $J/\ensuremath{\psi}$ production at RHIC indicate that significant enhancements are to be expected. Experimental observation of such enhanced production would provide evidence for deconfinement unlikely to be compatible with competing scenarios.

Realistic models of pion-exchange three-nucleon interactions

Abstract: We present realistic models of pion-exchange three-nucleon interactions obtained by fitting the energies of all the 17 bound or narrow states of $3l~Al~8$ nucleons, calculated with less than 2% error using the Green's function Monte Carlo method. The models contain two-pion-exchange terms due to $\ensuremath{\pi}N$ scattering in S and P waves, three-pion-exchange terms due to ring diagrams with one $\ensuremath{\Delta}$ in the intermediate states, and a phenomenological repulsive term to take into account relativistic effects, the suppression of the two-pion-exchange two-nucleon interaction by the third nucleon, and other effects. The models have five parameters, consisting of the strength of the four interactions and the short-range cutoff. The 17 fitted energies are insufficient to determine all of them uniquely. We consider five models, each having three adjustable parameters and assumed values for the other two. They reproduce the observed energies with an rms error $l1%$ when used together with the Argonne ${v}_{18}$ two-nucleon interaction. In one of the models the $\ensuremath{\pi}N$ S-wave scattering interaction is set to zero; in all others it is assumed to have the strength suggested by chiral effective-field theory. One of the models also assumes that the $\ensuremath{\pi}N$ P-wave scattering interaction has the strength suggested by effective-field theories, and the cutoff is adjusted to fit the data. In all other models the cutoff is taken to be the same as in the ${v}_{18}$ interaction. The effect of relativistic boost correction to the two-nucleon interaction on the strength of the repulsive three-nucleon interaction is estimated. Many calculated properties of $Al~8$ nuclei, including radii, magnetic dipole, and electric quadrupole moments, isobaric analog energy differences, etc., are tabulated. Results obtained with only Argonne ${v}_{8}^{\ensuremath{'}}$ and ${v}_{18}$ interactions are also reported. In addition, we present results for seven- and eight-body neutron drops in external potential wells.

Features of particle multiplicities and strangeness production in central heavy ion collisions between 1.7A and 158A GeV/c

Abstract: A systematic study is performed of fully integrated particle multiplicities in central Au-Au and Pb-Pb collisions at beam momenta of $1.7A \mathrm{GeV}/c, 11.6A \mathrm{GeV}/c$ (Au-Au), and $158A \mathrm{GeV}/c$ (Pb-Pb) by using a statistical-thermal model. The close similarity of the colliding systems makes it possible to study heavy ion collisions under definite initial conditions over a range of center-of-mass energies covering more than 1 order of magnitude. In order to further study the behavior of strangeness production, an updated study of Si-Au collisions at $14.6A \mathrm{GeV}$ is also presented. The data analysis has been performed with two completely independent numerical algorithms giving closely consistent results. We conclude that a thermal model description of particle multiplicities, with additional strangeness suppression, is possible for each energy. The degree of chemical equilibrium of strange particles and the relative production of strange quarks with respect to u and d quarks are higher than in ${e}^{+}{e}^{\ensuremath{-}},$ $pp,$ and $p\overline{p}$ collisions at comparable and even at lower energies. The behavior of strangeness production as a function of center-of-mass energy and colliding system is presented and discussed. The average energy per hadron in the comoving frame is close to 1 GeV per hadron despite the fact that the energy increases more than tenfold.

Flow analysis from multiparticle azimuthal correlations

Abstract: We present a new method for analyzing directed and elliptic flow in heavy-ion collisions. Unlike standard methods, it separates the contribution of flow to azimuthal correlations from contributions due to other effects. The separation relies on a cumulant expansion of multiparticle azimuthal correlations, and includes corrections for detector inefficiencies. This new method allows the measurement of the flow of identified particles in narrow phase-space regions, and can be used in every regime, from intermediate to ultrarelativistic energies.

Benchmark Test Calculation of a Four-Nucleon Bound State

Abstract: In the past, several efficient methods have been developed to solve the Schr\"odinger equation for four-nucleon bound states accurately. These are the Faddeev-Yakubovsky, the coupled-rearrangement-channel Gaussian-basis variational, the stochastic variational, the hyperspherical variational, the Green's function Monte Carlo, the no-core shell model, and the effective interaction hyperspherical harmonic methods. In this article we compare the energy eigenvalue results and some wave function properties using the realistic $\mathrm{AV}{8}^{\ensuremath{'}}$ NN interaction. The results of all schemes agree very well showing the high accuracy of our present ability to calculate the four-nucleon bound state.

Parity violating measurements of neutron densities

Abstract: Parity violating electron nucleus scattering is a clean and powerful tool for measuring the spatial distributions of neutrons in nuclei with unprecedented accuracy. Parity violation arises from the interference of electromagnetic and weak neutral amplitudes, and the ${Z}^{0}$ of the standard model couples primarily to neutrons at low ${Q}^{2}.$ The data can be interpreted with as much confidence as electromagnetic scattering. After briefly reviewing the present theoretical and experimental knowledge of neutron densities, we discuss possible parity violation measurements, their theoretical interpretation, and applications. The experiments are feasible at existing facilities. We show that theoretical corrections are either small or well understood, which makes the interpretation clean. The quantitative relationship to atomic parity nonconservation observables is examined, and we show that the electron scattering asymmetries can be directly applied to atomic parity nonconservation because the observables have approximately the same dependence on nuclear shape.

Chiral phase transition within effective models with constituent quarks

Abstract: We study the chiral phase transition at nonzero temperature T and baryon chemical potential ${\ensuremath{\mu}}_{B}$ within the framework of the linear sigma model and the Nambu--Jona-Lasinio (NJL) model. For small bare quark masses we find in both models a smooth crossover transition for nonzero T and ${\ensuremath{\mu}}_{B}=0$ and a first order transition for $T=0$ and nonzero ${\ensuremath{\mu}}_{B}.$ We calculate explicitly the first order phase transition line and spinodal lines in the $(T,{\ensuremath{\mu}}_{B})$ plane. As expected they all end at a critical point. We find that, in the linear sigma model, the sigma mass goes to zero at the critical point. This is in contrast to the NJL model, where the sigma mass, as defined in the random phase approximation, does not vanish. We also compute the adiabatic lines in the $(T,{\ensuremath{\mu}}_{B})$ plane. Within the models studied here, the critical point does not serve as a ``focusing'' point in the adiabatic expansion.

New method for measuring azimuthal distributions in nucleus-nucleus collisions

Abstract: The methods currently used to measure azimuthal distributions of particles in heavy-ion collisions assume that all azimuthal correlations between particles result from their correlation with the reaction plane. However, other correlations exist, and it is safe to neglect them only if azimuthal anisotropies are much larger than $1/\sqrt{N},$ with N the total number of particles emitted in the collision. This condition is not satisfied at ultrarelativistic energies. We propose a new method, based on a cumulant expansion of multiparticle azimuthal correlations, which allows measurements of much smaller values of azimuthal anisotropies, down to $1/N.$ It is simple to implement and can be used to measure both integrated and differential flow. Furthermore, this method automatically eliminates the major systematic errors, which are due to azimuthal asymmetries in the detector acceptance.

Density dependent hadron field theory for asymmetric nuclear matter and exotic nuclei

Abstract: Published in: Phys. Rev. C 64 (2001) , pp.034314 citations recorded in [Science Citation Index] Abstract: The density dependent relativistic hadron field (DDRH) theory is applied to strongly asymmetric nuclear matter and finite nuclei far off stability. A new set of in-medium meson-nucleon vertices is derived from Dirac-Brueckner Hartree-Fock (DBHF) calculations in asymmetric matter, now accounting also for the density dependence of isovector coupling constants. The scalar-isovector $delta$ meson is included. Nuclear matter calculations show that it is necessary to introduce a momentum correction in the extraction of coupling constants from the DBHF self-energies in order to reproduce the DBHF equation of state by DDRH mean-field calculations. The properties of DDRH vertices derived from the Groningen and the Bonn A nucleon-nucleon (NN) potentials are compared in nuclear matter calculations and for finite nuclei. Relativistic Hartree results for binding energies, charge radii, separation energies and shell gaps for the Ni and Sn isotopic chains are presented. Using the momentum corrected vertices an overall agreement to data on a level of a few percent is obtained. In the accessible range of asymmetries the $delta$ meson contributions to the self-energies are found to be of minor importance but asymmetry dependent fluctuations may occur.

Neutron radii and the neutron equation of state in relativistic models

Abstract: The root-mean-square radii for neutrons in nuclei and their relationship to the neutron equation of state are investigated in the relativistic Hartree model. A correlation between the neutron skin in heavy nuclei and the derivative of the neutron equation of state is found which provides a linear continuation of results obtained from nonrelativistic Hartree-Fock models. The relativistic models tend to give larger neutron radii and an associated stiffer neutron equation of state compared with the nonrelativistic models.

Measurements of the Elastic Electromagnetic Form Factor Ratio Mu(P)G(Ep)/G(Mp) Via Polarization Transfer

Abstract: We present measurements of the ratio of the proton elastic electromagnetic form factors, μ_pG_(Ep)/G_(Mp). The Jefferson Lab Hall A Focal Plane Polarimeter was used to determine the longitudinal and transverse components of the recoil proton polarization in ep elastic scattering; the ratio of these polarization components is proportional to the ratio of the two form factors. These data reproduce the observation of Jones et al. [Phys. Rev. Lett. 84, 1398 (2000)], that the form factor ratio decreases significantly from unity above Q^2=1 GeV^2.

Lane-consistent, semimicroscopic nucleon-nucleus optical model

Abstract: A semimicroscopic, Lane-consistent optical model is established up to 200 MeV for nucleons incident on spherical and near-spherical nuclei with masses $40l~Al~209.$ This model, based on the earlier approach of Jeukenne, Lejeune, and Mahaux in nuclear matter, is an extension of our previous work [E. Bauge, J. P. Delaroche, and M. Girod, Phys. Rev. C 58, 1118 (1998)]. The modulus of the isovector potential is extracted and compared with measurements and fully microscopic predictions. Good overall descriptions of nucleon scattering, of transitions to isobaric analog states, and of reaction observables are obtained down to 1 keV. Those results are discussed in detail.

Is early thermalization achieved only near midrapidity in Au + Au collisions at (s NN )=130 GeV?

Abstract: The pseudorapidity dependence of elliptic flow in Au+Au collisions at $130A \mathrm{GeV}$ is studied within a full three-dimensional hydrodynamic model in the light-cone coordinate. First, we prepare two initial conditions in the hydrodynamic model for analyzing elliptic flow. Both initial conditions lead to reasonable agreement with single particle spectra in central and semicentral collisions. Second, by using these hydrodynamic simulations, we compare elliptic flow as a function of pseudorapidity with experimental data recently measured by the PHOBOS Collaboration. Our results are in agreement with experimental data only near midrapidity. This suggests that thermalization in the early stage of collisions is not achieved in forward and backward rapidity regions.

Constrained molecular dynamics approach to fermionic systems

Abstract: We propose a constrained molecular dynamics model for a fermionic system In this approach the equations of motion of the centroids related to the single-particle phase-space distributions are solved by imposing that the one-body occupation probability ${f}_{i},$ evaluated for each particle, can assume only values less than or equal to 1 This condition reflects the fermionic nature of the studied systems, and it is implemented with a fast algorithm which allows also the study of the heaviest colliding system The parameters of the model have been chosen to reproduce the average binding energy and radii of nuclei in the mass region $A=30--208$ Some comparison to the data is given

Synthesis of superheavy nuclei: Nucleon collectivization as a mechanism for compound nucleus formation

Abstract: A consistent systematic analysis of the synthesis of very heavy nuclei is performed within a ‘‘standard’’ theoretical approach without any adjustable parameters and additional simplification. Good agreement with experimental data was obtained in all the cases up to synthesis of the 102 element. It was confirmed that a process of the compound nucleus formation, starting from the instant when two heavy nuclei touch and proceeding in strong competition with the fission and quasifission processes, plays an important role in the asymmetric synthesis of superheavy elements with ZCN>104 as well as in the symmetric fusion at ZCN>90. A new mechanism of the fusion-fission process for a heavy nuclear system is proposed, which takes place in the (A1 , A2) space, where A1 and A2 are two nuclei, surrounded by a certain number of common nucleons DA . These nuclei gradually lose ~or acquire! their individualities with increasing ~or decreasing! the number of collectivized nucleons DA . The driving potential in the (A1 , A2) space is derived, which allows the calculation of both the probability of the compound nucleus formation and the mass distribution of fission fragments in heavy ion fusion reactions.

Synthesis of superheavy nuclei: How accurately can we describe it and calculate the cross-sections?

Abstract: A thorough analysis of all stages of heavy ion fusion reaction leading to the formation of a heavy evapora- tion residue has been performed. The main goal of the analysis was to gain better understanding of the whole process and to find out what factors and quantities, in particular, bring major uncertainty into the calculated cross sections, how reliable the calculation of the cross sections of superheavy element formation may be and what additional theoretical and experimental studies should be made in this field. The interest in the synthesis of superheavy nuclei has lately grown due to the new experimental results @1-4 # dem- onstrating a real possibility of producing and investigating the nuclei in the region of the so-called ''island of stability.'' The new reality demands a more substantial theoretical sup- port of these expensive experiments which will allow a more reasonable choice of fusing nuclei and collision energies as well as a better estimation of the cross sections and unam- biguous identification of evaporation residues ~ER!. Unfortu- nately, at present it is quite difficult ~and hardly possible! to make an accurate qualitative analysis of the complex dynam- ics of the heavy ion fusion reaction leading to the formation in the exit channel of ER of easily fissile superheavy nucleus. However, lately a number of speculative papers have ap- peared in which predictions are made in terms of rather sim- plified models concerning the values of formation cross sec- tions of new superheavy elements in reactions with different colliding nuclei. A thorough analysis of all reaction stages is made in the present study. It is aimed at better understanding of how well we can describe them in the framework of ex- isting theoretical models, what quantities the cross sections are sensitive to and how accurate the values of these quanti- ties have been determined, and finally, how accurate are the predictions of the cross sections of the heavy ER produced in reactions for which no experimental data are available. The production cross section of a cold residual nucleus B, which is the product of light particle evaporation and g emis- sion from an excited compound nucleus C, formed in the fusion process of two heavy nuclei A11A2!C!B 1n, p,a,g at a center-of mass energy close to the Coulomb barrier in the entrance channel, is usually decomposed over partial waves and given by s ER11A2!B ~ E!' p\ 2

Neutron radii of 208 Pb and neutron stars

Abstract: A new relation between the neutron skin of a heavy nucleus and the radius of a neutron star is proposed: the larger the neutron skin of the nucleus the larger the radius of the star. Relativistic models that reproduce a variety of ground-state observables cannot determine uniquely the neutron skin of a heavy nucleus. Thus, a large range of neutron skins is generated by supplementing the models with nonlinear couplings between isoscalar and isovector mesons. We illustrate how the correlation between the neutron skin and the radius of the star can be used to place important constraints on the equation of state and how it may help elucidate the existence of a phase transition in the interior of the neutron star.

Measurement of k-p → ηΛ near threshold

Abstract: We present measurements of the differential and total cross sections and the {Lambda} polarization for the reaction K{sup -}p{yields}{eta}{Lambda} from threshold to p{sub K}-=770 MeV/c, with much better precision than previous measurements. Our cross-section data show a remarkable similarity to the SU(3) flavor-related {pi}{sup -}p{yields}{eta}n cross-section results. The reaction K{sup -}p{yields}{eta}{Lambda} at threshold is dominated by formation of the intermediate {Lambda}(1670)1/2- state.

J/ψ absorption by π and ρ mesons in a meson exchange model with anomalous parity interactions

Abstract: We reanalyze the dissociation process of the $J/\ensuremath{\psi}$ by $\ensuremath{\pi}$ and $\ensuremath{\rho}$ mesons into $D+\overline{D},$ ${D}^{*}+\overline{D},$ $D+{D}^{*},$ and ${D}^{*}+{D}^{*}$ within a meson exchange model. In addition to the dissociation mechanisms considered in the literature, we consider anomalous parity interactions, whose couplings are constrained by heavy quark spin symmetry and phenomenology. This opens new dissociation channels and adds new diagrams in the previously considered processes. Compared to the previous results, we find that these new additions have only a minor effect on the $\ensuremath{\rho}+J/\ensuremath{\psi}$ total inelastic cross section, but reduce the one for $\ensuremath{\pi}+J/\ensuremath{\psi}$ by about 50% near the threshold.

Measurement of neutron total cross sections up to 560 MeV

Abstract: We have completed a new set of total cross section measurements of 31 elements and isotopes spanning the periodic table from A=1 to 238. We employed the same technique as in Finley [Phys. Rev. C 47, 237 (1993)] with refinements intended to allow measurements on separated isotopes and improved systematic error control. The goal of the new measurement was 1% statistical accuracy in 1% energy bins with systematic errors less than 1%. This was achieved for all but the thinnest samples. Stringent checks of systematic errors in this measurement resulted in a reassignment of systematic uncertainties to the neutron total cross sections reported in Finley Microscopic optical model calculations were carried out to interpret the results of the experiment. Two specific types of optical models were employed. The Jeukenne-Lejeune-Mahaux model was used in the range of 5--160 MeV, and a model based on the empirical effective interaction of Kelly was used from 135 to 650 MeV. These models are shown to be useful for predicting both neutron total cross sections and proton reaction cross sections. They are particularly important for light nuclei, for which standard global phenomenological parametrizations of the optical potential are insufficiently accurate.

Dynamical study of the {Delta} excitation in N(e,e{prime}{pi}) reactions

Abstract: The dynamical model developed by us [Phys. Rev. C 54, 2660 (1996)] has been applied to investigate the pion electroproduction reactions on the nucleon. It is found that the model can describe to a very large extent the recent data of ${p(e,e}^{\ensuremath{'}}{\ensuremath{\pi}}^{0})$ reaction from Jefferson Laboratory and MIT-Bates. The extracted magnetic dipole $(M1),$ electric dipole $(E2),$ and Coulomb $(C2)$ strengths of the $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\Delta}$ transition are presented. It is found that the $C2/M1$ ratio drops significantly with ${Q}^{2}$ and reaches about $\ensuremath{-}14%$ at ${Q}^{2}=4 (\mathrm{GeV}{/c)}^{2},$ while the $E2/M1$ ratio remains close to the value $\ensuremath{\sim}\ensuremath{-}3%$ at the ${Q}^{2}=0$ photon point. The determined $M1$ transition form factor drops faster than the usual dipole form factor of the proton. We also find that the nonresonant interactions can dress the $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\Delta}$ vertex to enhance strongly its strength at low ${Q}^{2},$ but much less at high ${Q}^{2}.$ Predictions are presented for future experimental tests. Possible developments of the model are discussed.

Valence quark distributions in the pion

Abstract: We calculate the pion's valence-quark momentum-fraction probability distribution using a Dyson-Schwinger equation model. Valence quarks with an active mass of 0.30 GeV carry 71% of the pion's momentum at a resolving scale q{sub 0}=0.54 GeV=1/(0.37 fm). The shape of the calculated distribution is characteristic of a strongly bound system and, evolved from q{sub 0} to q=2 GeV, it yields first, second, and third moments in agreement with lattice and phenomenological estimates, and valence-quarks carrying 49% of the pion's momentum. However, pointwise there is a discrepancy between our calculated distribution and that hitherto inferred from parametrizations of extant pion-nucleon Drell-Yan data.

Isoscaling in statistical models

Abstract: Different statistical multifragmentation models have been used to study isoscaling, i.e., the factorization of the isotope ratios from two reactions, into fugacity terms of proton and neutron number, ${R}_{21}{(N,Z)=Y}_{2}{(N,Z)/Y}_{1}(N,Z)=C\mathrm{exp}(\ensuremath{\alpha}N+\ensuremath{\beta}Z).$ Even though the primary isotope distributions are quite different from the final distributions due to evaporation from the excited fragments, the values of $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ are not much affected by sequential decays. $\ensuremath{\alpha}$ is shown to be mainly sensitive to the proton-to-neutron composition of the emitting source and may be used to study isospin-dependent properties in nuclear collisions, such as, the symmetry energy in the equation of state of asymmetric nuclear matter.

Single-neutron knockout reactions: Application to the spectroscopy of 16,17,19 C

Abstract: The structure of the neutron-rich carbon isotopes ${}^{16,17,19}\mathrm{C}$ has been investigated using one-neutron knockout reactions on a ${}^{9}\mathrm{Be}$ target at approximately 60 MeV/nucleon. Partial cross sections and associated momentum distributions corresponding to final states of the ${}^{15,16,18}\mathrm{C}$ residues were measured and compared with predictions based on a shell-model theory and an eikonal model of the reaction mechanism. Spectroscopic factors and l-value assignments are given. The ground-state spins of ${}^{17,19}\mathrm{C}$ are ${\frac{3}{2}}^{+}$ and ${\frac{1}{2}}^{+},$ respectively. It is suggested that the accepted one-neutron separation energy for the ground state of ${}^{19}\mathrm{C}$ needs to be revised upwards.

Spectroscopy of medium-heavy Λ hypernuclei via the (π + , K + ) reaction

Abstract: Excitation energy spectra of ${}_{\mathrm{\ensuremath{\Lambda}}}^{89}\mathrm{Y},$ ${}_{\mathrm{\ensuremath{\Lambda}}}^{51}\mathrm{V},$ and ${}_{\mathrm{\ensuremath{\Lambda}}}^{12}\mathrm{C}$ have been measured via the $({\ensuremath{\pi}}^{+}{,K}^{+})$ reaction by using the SKS spectrometer at the K6 beam line in the KEK 12-GeV Proton Synchrotron. In the ${}_{\mathrm{\ensuremath{\Lambda}}}^{89}\mathrm{Y}$ spectrum, obtained with an energy resolution of 1.65 MeV (FWHM) and in the highest statistics so far, we have succeeded in clearly observing a characteristic fine structure in heavy \ensuremath{\Lambda} hypernuclear systems and precisely obtaining a series of \ensuremath{\Lambda} single-particle energies in the very wide excitation energy range of more than 20 MeV, for the first time. Also in the ${}_{\mathrm{\ensuremath{\Lambda}}}^{51}\mathrm{V}$ spectrum, a similar structure to that of ${}_{\mathrm{\ensuremath{\Lambda}}}^{89}\mathrm{Y}$ was observed. In the ${}_{\mathrm{\ensuremath{\Lambda}}}^{12}\mathrm{C}$ spectrum, new core-excited states were clearly resolved thanks to the best energy resolution of 1.45 MeV so far achieved by using the SKS spectrometer.

α-nucleus optical potential in the double-folding model

Abstract: The double-folding formalism for the $\ensuremath{\alpha}$-nucleus optical potential is revised to study the exchange effects and density dependence of the effective nucleon-nucleon $(\mathrm{NN})$ interaction in detail. A realistic density dependent M3Y interaction, based on the G-matrix elements of the Paris NN potential, has been used in the folding calculation. The local approximation for the nonlocal one-body density matrix in the calculation of the exchange potential was tested by using the harmonic oscillator representation of the nonlocal density matrices of the $\ensuremath{\alpha}$-particle and target nucleus. The inclusion of a realistic density dependence into the effective NN interaction was shown to be vital for a correct description of the refractive $\ensuremath{\alpha}$-nucleus scattering data. A high sensitivity of the density distributions of the $\ensuremath{\alpha}$-particle and target nucleus to the shape of the $\ensuremath{\alpha}$-nucleus potential was found, which can be used in the folding analysis to test various density models for the $\ensuremath{\alpha}$-particle and target as well as to choose the most realistic approximation for the overlap density in the dinuclear system. Our results also stress the importance of $\ensuremath{\alpha}$-nucleus scattering experiment in the nuclear structure study.

Jet quenching and azimuthal anisotropy of large p(T) spectra in noncentral high-energy heavy ion collisions

Abstract: Parton energy loss inside a dense medium leads to the suppression of large pT hadrons and can also cause azimuthal anisotropy of hadron spectra at large transverse momentum in non-central high-energy heavy-ion collisions. Such azimuthal anisotropy is studied qualitatively in a parton model for heavy-ion collisions at the RHIC energies. The coefficientv2(pT) of the elliptic anisotropy at large pT is found to be very sensitive to parton energy loss. It decreases slowly with pT contrary to its low pT behavior where v2 increases very rapidly with pT. The turning point signals the onset of contributions of hard processes and the magnitude of parton energy loss. The centrality dependence of v2(pT) is shown to be sensitive to both size and density dependence of the parton energy loss and the later can also be studied via variation of the colliding energy. The anisotropy coefficient v2/" normalized by the spatial ellipticity " is found to decrease significantly toward semi-peripheral collisions, differing from the hydrodynamic results for low pT hadrons. Constrained by the existing WA98 experimental data at the SPS energy on parton energy loss, both hadron spectra suppression and azimuthal anisotropy at high pT are predicted to vanish for b >7-8 fm in Au + Au collisions at p s =130-200 GeV when the hadron rapidity density per unit area of the initial overlapped region is less than what is achieved in the central Pb + Pb collisions at the SPS energy.

Signatures of thermal dilepton radiation at ultrarelativistic energies

Abstract: The properties of thermal dilepton production from heavy-ion collisions in the RHIC energy regime are evaluated for invariant masses ranging from 0.5 to 3 GeV. Using an expanding thermal fireball to model the evolution through both quark-gluon and hadronic phases various features of the spectra are addressed. In the low-mass region, due to an expected large background, the focus is on possible medium modifications of the narrow resonance structures from $\ensuremath{\omega}$ and $\ensuremath{\varphi}$ mesons, whereas in the intermediate-mass region the old idea of identifying QGP radiation is reiterated including effects of chemical undersaturation in the early stages of central Au+Au collisions.

Calculations of three-body observables in 8 B breakup

Abstract: We discuss calculations of three-body observables for the breakup of ${}^{8}\mathrm{B}$ on a ${}^{58}\mathrm{Ni}$ target at low energy using the coupled discretized continuum channels approach. Calculations of both the angular distribution of the ${}^{7}\mathrm{Be}$ fragments and their energy distributions are compared with those measured at several laboratory angles. In these observables there is interference between the breakup amplitudes from different spin-parity excitations of the projectile. The resulting angle and the energy distributions reveal the importance of the higher-order continuum state couplings for an understanding of the measurements.