Showing papers in "Physical Review C in 2004"

Identified charged particle spectra and yields in Au + Au collisions at √sNN = 200 GeV

Abstract: The centrality dependence of transverse momentum distributions and yields for ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},{K}^{\ifmmode\pm\else\textpm\fi{}},p$, and $\overline{p}$ in $\text{Au}+\text{Au}$ collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{0.3em}{0ex}}\text{GeV}$ at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. We observe a clear particle mass dependence of the shapes of transverse momentum spectra in central collisions below $\ensuremath{\sim}2\phantom{\rule{0.3em}{0ex}}\text{GeV}∕c$ in ${p}_{T}$. Both mean transverse momenta and particle yields per participant pair increase from peripheral to midcentral and saturate at the most central collisions for all particle species. We also measure particle ratios of ${\ensuremath{\pi}}^{\ensuremath{-}}∕{\ensuremath{\pi}}^{+}$, ${K}^{\ensuremath{-}}∕{K}^{+}$, $\overline{p}∕p$, $K∕\ensuremath{\pi}$, $p∕\ensuremath{\pi}$, and $\overline{p}∕\ensuremath{\pi}$ as a function of ${p}_{T}$ and collision centrality. The ratios of equal mass particle yields are independent of ${p}_{T}$ and centrality within the experimental uncertainties. In central collisions at intermediate transverse momenta $\ensuremath{\sim}1.5--4.5\phantom{\rule{0.3em}{0ex}}\text{GeV}∕c$, proton and antiproton yields constitute a significant fraction of the charged hadron production and show a scaling behavior different from that of pions.

Simple parametrization of nucleon form factors

Abstract: This Brief Report provides simple parametrizations of the nucleon electromagnetic form factors using functions of ${Q}^{2}$ that are consistent with dimensional scaling at high ${Q}^{2}$. Good fits require only four parameters each for ${G}_{Ep}$, ${G}_{Mp}$, and ${G}_{Mn}$ and only two for ${G}_{En}$.

Measurements of cross sections and decay properties of the isotopes of elements 112, 114, and 116 produced in the fusion reactions {sup 233,238}U, {sup 242}Pu, and {sup 248}Cm+{sup 48}Ca

Abstract: We have studied the dependence of the production cross sections of the isotopes {sup 282,283}112 and {sup 286,287}114 on the excitation energy of the compound nuclei {sup 286}112 and {sup 290}114. The maximum cross section values of the xn-evaporation channels for the reaction {sup 238}U({sup 48}Ca,xn){sup 286-x}112 were measured to be {sigma}{sub 3n}=2.5{sub -1.1}{sup +1.8} pb and {sigma}{sub 4n}=0.6{sub -0.5}{sup +1.6} pb; for the reaction {sup 242}Pu({sup 48}Ca,xn){sup 290-x}114: {sigma}{sub 2n}{approx}0.5 pb, {sigma}{sub 3n}=3.6{sub -1.7}{sup +3.4} pb, and {sigma}{sub 4n}=4.5{sub -1.9}{sup +3.6} pb. In the reaction {sup 233}U({sup 48}Ca,2-4n){sup 277-279}112 at E*=34.9=2.2 MeV we measured an upper cross section limit of {sigma}{sub xn}{<=}0.6 pb. The observed shift of the excitation energy associated with the maximum sum evaporation residue cross section {sigma}{sub ER}(E*) to values significantly higher than that associated with the calculated Coulomb barrier can be caused by the orientation of the deformed target nucleus in the entrance channel of the reaction. An increase of {sigma}{sub ER} in the reactions of actinide targets with {sup 48}Ca is consistent with the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell N=184. In the present work we detected 33 decay chains arising inmore » the decay of the known nuclei {sup 282}112, {sup 283}112, {sup 286}114, {sup 287}114, and {sup 288}114. In the decay of {sup 287}114({alpha}){yields}{sup 283}112({alpha}){yields}{sup 279}110(SF), in two cases out of 22, we observed decay chains of four and five sequential {alpha} transitions that end in spontaneous fission of {sup 271}Sg (T{sub {alpha}}{sub /SF}=2.4{sub -1.0}{sup +4.3} min) and {sup 267}Rf (T{sub SF}{approx}2.3 h), longer decay chains than reported previously. We observed the new nuclide {sup 292}116 (T{sub {alpha}}=18{sub -6}{sup +16} ms,E{sub {alpha}}=10.66{+-}0.07 MeV) in the irradiation of the {sup 248}Cm target at a higher energy than in previous experiments. The observed nuclear decay properties of the nuclides with Z=104-118 are compared with theoretical nuclear mass calculations and the systematic trends of spontaneous fission properties. As a whole, they give a consistent pattern of decay of the 18 even-Z neutron-rich nuclides with Z=104-118 and N=163-177. The experiments were performed with the heavy-ion beam delivered by the U400 cyclotron of the FLNR (JINR, Dubna) employing the Dubna gas-filled recoil separator.« less

New effective interaction for pf-shell nuclei and its implications for the stability of the N = Z = 28 closed core

Abstract: The effective interaction GXPF1 for shell-model calculations in the full $pf$ shell is tested in detail from various viewpoints such as binding energies, electromagnetic moments and transitions, and excitation spectra. The semimagic structure is successfully described for $N$ or $Z=28$ nuclei, $^{53}\mathrm{Mn}$, $^{54}\mathrm{Fe}$, $^{55}\mathrm{Co}$, and $^{56,57,58,59}\mathrm{Ni}$, suggesting the existence of significant core excitations in low-lying nonyrast states as well as in high spin yrast states. The results of $N=Z$ odd-odd nuclei, $^{54}\mathrm{Co}$ and $^{58}\mathrm{Cu}$, also confirm the reliability of GXPF1 interaction in the isospin dependent properties. Studies of shape coexistence suggest an advantage of Monte Carlo shell model over conventional calculations in cases where full-space calculations still remain too large to be practical.

Parity violation in hot QCD: How to detect it

Abstract: In a recent paper (hep-ph∕0406125) Kharzeev argues for the possibility of $P$- and∕or $CP$-violation effects in heavy-ion collisions, the effects that can manifest themselves via asymmetry in ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ production with respect to the direction of the system angular momentum. Here we present an experimental observable that can be used to detect and measure the effects.

New effective interactions in relativistic mean field theory with nonlinear terms and density-dependent meson-nucleon coupling

Abstract: New parameter sets for the Lagrangian density in the relativistic mean field (RMF) theory, PK1 with nonlinear sigma- and omega-meson self-coupling, PK1R with nonlinear sigma-, omega-, and rho-meson self-coupling, and PKDD with the density-dependent meson-nucleon coupling are proposed. They are able to provide an excellent description not only for the properties of nuclear matter but also for the nuclei in and far from the valley of beta stability. For the first time in the parametrization of the RMF Lagrangian density, the center-of-mass correction is treated by a microscopic way, which is essential to unify the description of nuclei from light to heavy regions with one effective interaction.

Experiments on the synthesis of element 115 in the reaction 243 Am ( 48 Ca ,xn) 291-x 115

Abstract: The results of experiments designed to synthesize element 115 isotopes in the $^{243}\mathrm{Am}+^{48}\mathrm{Ca}$ reaction are presented. With a beam dose of $4.3\ifmmode\times\else\texttimes\fi{}{10}^{18}$ $248\text{\ensuremath{-}}\text{MeV}$ $^{48}\mathrm{Ca}$ projectiles, we observed three similar decay chains consisting of five consecutive $\ensuremath{\alpha}$ decays, all detected in time intervals of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ and terminated at a later time by a spontaneous fission with a high-energy release (total kinetic energy $\ensuremath{\sim}220\phantom{\rule{0.3em}{0ex}}\text{MeV}$). At a higher bombarding energy of $253\phantom{\rule{0.3em}{0ex}}\text{MeV}$, with an equal $^{48}\mathrm{Ca}$ beam dose, we registered a different decay chain of four consecutive $\ensuremath{\alpha}$ decays detected in a time interval of about $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$, also terminated by spontaneous fission. The $\ensuremath{\alpha}$ decay energies and half-lives for nine new $\ensuremath{\alpha}$-decaying nuclei are given. The decay properties of these synthesized nuclei are consistent with consecutive $\ensuremath{\alpha}$ decays originating from the parent isotopes of the new element 115, $^{288}115$ and $^{287}115$, produced in the $3n$- and $4n$-evaporation channels with cross sections of about $3\phantom{\rule{0.3em}{0ex}}\text{pb}$ and $1\phantom{\rule{0.3em}{0ex}}\text{pb}$, respectively. The radioactive properties of the new odd-$Z$ nuclei $(105--115)$ are compared with the predictions of the macroscopic-microscopic theory. The experiments were carried out at the U400 cyclotron with the recoil separator DGFRS at FLNR, JINR.

Causal theories of dissipative relativistic fluid dynamics for nuclear collisions

Abstract: Nonequilibrium fluid dynamics derived from the extended irreversible thermodynamics of the causal M\"uller-Israel-Stewart theory of dissipative processes in relativistic fluids based on Grad's moment method is applied to the study of the dynamics of hot matter produced in ultrarelativistic heavy ion collisions. The temperature, energy density, and entropy evolution are investigated in the framework of the Bjorken boost-invariant scaling limit. The results of these second order theories are compared to those of first order theories due to Eckart and to Landau and Lifshitz and those of zeroth order (perfect fluid) due to Euler. In the presence of dissipation perfect fluid dynamics is no longer valid in describing the evolution of the matter. First order theories fail in the early stages of evolution. Second order theories give a better description in good agreement with transport models. It is shown in which region the Navier-Stokes-Fourier laws (first order theories) are a reasonable limiting case of the more general extended thermodynamics (second order theories).

Hadronic production of thermal photons

Abstract: We study the thermal emission of photons from hot and dense strongly interacting hadronic matter at temperatures close to the expected phase transition to the quark-gluon plasma (QGP). Earlier calculations of photon radiation from ensembles of interacting mesons are reexamined with additional constraints, including new production channels as well as an assessment of hadronic form factor effects. Whereas strangeness-induced photon yields turn out to be moderate, the hitherto not considered $t$-channel exchange of $\ensuremath{\omega}$ mesons is found to contribute appreciably for photon energies above $\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\text{GeV}$. The role of baryonic effects is assessed using existing many-body calculations of lepton pair production. We argue that our combined results constitute a rather realistic emission rate, appropriate for applications in relativistic heavy-ion collisions. Supplemented with recent evaluations of QGP emission, and an estimate for primordial (hard) production, we compute photon spectra at Super Proton Synchrotron (SPS), Relativistic Heavy Ion Collider (RHIC), and Large Hadron Collider (LHC) energies.

Effect of breakup on the fusion of Li 6 , Li 7 , and Be 9 with heavy nuclei

Abstract: Comprehensive high precision complete and incomplete fusion cross sections have been measured for the $^{6}\mathrm{Li}+^{209}\mathrm{Bi}$, $^{7}\mathrm{Li}+^{209}\mathrm{Bi}$, and $^{9}\mathrm{Be}+^{208}\mathrm{Pb}$ reactions, at energies near and below the Coulomb barrier. The experimental details and the analyses procedures for the characteristic $\ensuremath{\alpha}$-decay and fission measurements are described. Three different methods are used to conclusively show the large suppression of complete fusion at energies around and above the average barrier, which is associated with the projectile nuclei having a low energy threshold against breakup. First, theoretical predictions of fusion cross sections above the average barrier are compared with the data, and second the area under the measured barrier distribution is compared with expectations. The sensitivity of the suppression factors to variables which can affect the calculated cross sections is thoroughly investigated. The third method, essentially model independent, compares the complete fusion cross sections for the $^{7}\mathrm{Li}+^{209}\mathrm{Bi}$ and $^{9}\mathrm{Be}+^{208}\mathrm{Pb}$ reactions with those for the fusion of nuclei with a high threshold against breakup, which produce the same compound nucleus. All methods give consistent results, showing that the complete fusion cross sections at energies around and above the barrier are suppressed by $\ensuremath{\sim}30%$ compared with reactions of nuclei having a high energy threshold against breakup. The cross sections for incomplete fusion are found to be similar to the missing complete fusion cross sections. The experimental controversies regarding the effect of breakup on fusion is discussed, and the importance of unambiguously separating complete fusion from incomplete fusion is emphasized. This distinction is also important to achieve theoretically for realistic modelling of fusion of nuclei which break up readily.

Observable implications of geometrical and dynamical aspects of freeze-out in heavy ion collisions

Abstract: Using an analytical parametrization of hadronic freeze-out in relativistic heavy ion collisions, we present a detailed study of the connections between features of the freeze-out configuration and physical observables. We focus especially on anisotropic freeze-out configurations (expected in general for collisions at finite impact parameter), azimuthally sensitive Hanburry-Brown-Twiss interferometry, and final-state interactions between nonidentical particles. Model calculations are compared with data taken in the first year of running at RHIC; while not perfect, good agreement is found, raising the hope that a consistent understanding of the full freeze-out scenario at RHIC is possible, an important first step towards understanding the physics of the system prior to freeze-out.

Chemical equilibrium study in nucleus-nucleus collisions at relativistic energies

Abstract: We present a detailed study of chemical freeze-out in nucleus-nucleus collisions at beam energies of $116A$, $30A$, $40A$, $80A$, and $158A\phantom{\rule{03em}{0ex}}\text{GeV}$ By analyzing hadronic multiplicities within the statistical hadronization approach, we have studied the strangeness production as a function of center-of-mass energy and of the parameters of the source We have tested and compared different versions of the statistical model, with special emphasis on possible explanations of the observed strangeness hadronic phase space undersaturation We show that, in this energy range, the use of hadron yields at midrapidity instead of in full phase space artificially enhances strangeness production and could lead to incorrect conclusions as far as the occurrence of full chemical equilibrium is concerned In addition to the basic model with an extra strange quark nonequilibrium parameter, we have tested three more schemes: a two-component model superimposing hadrons coming out of single nucleon-nucleon interactions to those emerging from large fireballs at equilibrium, a model with local strangeness neutrality and a model with strange and light quark nonequilibrium parameters The behavior of the source parameters as a function of colliding system and collision energy is studied The description of strangeness production entails a nonmonotonic energy dependence of strangeness saturation parameter ${\ensuremath{\gamma}}_{S}$ with a maximum around $30A\phantom{\rule{03em}{0ex}}\text{GeV}$ We also present predictions of the production rates of still unmeasured hadrons including the newly discovered ${\ensuremath{\Theta}}^{+}(1540)$ pentaquark baryon

Pseudospin symmetry and the relativistic harmonic oscillator

Abstract: A generalized relativistic harmonic oscillator for spin 1∕2 particles is studied. The Dirac Hamiltonian contains a scalar $S$ and a vector $V$ quadratic potentials in the radial coordinate, as well as a tensor potential $U$ linear in $r$. Setting either or both combinations $\ensuremath{\Sigma}=S+V$ and $\ensuremath{\Delta}=V\ensuremath{-}S$ to zero, analytical solutions for bound states of the corresponding Dirac equations are found. The eigenenergies and wave functions are presented and particular cases are discussed, devoting a special attention to the nonrelativistic limit and the case $\ensuremath{\Sigma}=0$, for which pseudospin symmetry is exact. We also show that the case $U=\ensuremath{\Delta}=0$ is the most natural generalization of the nonrelativistic harmonic oscillator. The radial node structure of the Dirac spinor is studied for several combinations of harmonic-oscillator potentials, and that study allows us to explain why nuclear intruder levels cannot be described in the framework of the relativistic harmonic oscillator in the pseudospin limit.

High-pT charged hadron suppression in Au+Au collisions at √sNN=200 GeV

Abstract: The PHENIX experiment at the Relativistic Heavy Ion Collider has measured charged hadron yields at midrapidity over a wide range of transverse momenta $(0.5l{p}_{T}l10\phantom{\rule{0.3em}{0ex}}\text{GeV}∕c)$ in $\text{Au}+\text{Au}$ collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{0.3em}{0ex}}\text{GeV}$. The data are compared to ${\ensuremath{\pi}}^{0}$ measurements from the same experiment. For both charged hadrons and neutral pions, the yields per nucleon-nucleon collision are significantly suppressed in central compared to peripheral and nucleon-nucleon collisions. The suppression sets in gradually and increases with increasing centrality of the collisions. Above $4--5\phantom{\rule{0.3em}{0ex}}\text{GeV}∕c$ in ${p}_{T}$, a constant and almost identical suppression of charged hadrons and ${\ensuremath{\pi}}^{0}$'s is observed. The ${p}_{T}$ spectra are compared to published spectra from $\text{Au}+\text{Au}$ at $\sqrt{{s}_{NN}}=130$ in terms of ${x}_{T}$ scaling. Central and peripheral ${\ensuremath{\pi}}^{0}$ as well as peripheral charged spectra exhibit the same ${x}_{T}$ scaling as observed in $p+p$ data.

Molecular states of heavy quark mesons

Abstract: We explore molecular states of two open heavy-quark mesons $(Q\overline{q})\ensuremath{-}(q\overline{Q})$ in a quark-based model in terms of a four-body nonrelativistic Hamiltonian with pairwise effective interactions. Molecular states are found in the combinations of ${D,{D}^{*},B,{B}^{*}}$ with ${\overline{D},{\overline{D}}^{*},\overline{B},{\overline{B}}^{*}}$, including a weakly bound $D{\overline{D}}^{*}$ state near the threshold which may be qualitatively identified as the 3872 state observed recently by the Belle Collaboration.

Inclusive quasielastic charged-current neutrino-nucleus reactions

Abstract: The quasielastic (QE) contribution of the nuclear inclusive electron scattering model developed by Gil, Nieves, and Oset [Nucl. Phys. A627, 543 (1997); A627, 599 (1997)] is extended to the study of electroweak charged current (CC) induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for, among other nuclear effects, long range nuclear [random phase approximation (RPA)] correlations, final state interaction (FSI), and Coulomb corrections. Predictions for the inclusive muon capture in $^{12}\mathrm{C}$ and the reaction $^{12}\mathrm{C}$ $({\ensuremath{ u}}_{\ensuremath{\mu}},{\ensuremath{\mu}}^{\ensuremath{-}})X$ near threshold are also given. RPA correlations are shown to play a crucial role and their inclusion leads to one of the best existing simultaneous description of both processes, with accuracies of the order of 10\char21{}15 % for the muon capture rate and even better for the Liquid Scintillating Neutrino Detector (LSND) measurement.

Measurements of cross sections for the fusion-evaporation reactions pu-244(ca-48, xn) 114-(292-x) and cm-245(ca-48, xn) 116-(293-x)

Abstract: We have studied the excitation functions of the reactions $^{244}\mathrm{Pu}$($^{48}\mathrm{Ca}$,$xn$). Maximum cross sections for the evaporation of 3--5 neutrons in the complete-fusion reaction $^{244}\mathrm{Pu+}^{48}\mathrm{Ca}$ were measured to be ${\ensuremath{\sigma}}_{3n}=2\phantom{\rule{0.3em}{0ex}}\text{pb}$, ${\ensuremath{\sigma}}_{4n}=5\phantom{\rule{0.3em}{0ex}}\text{pb}$, and ${\ensuremath{\sigma}}_{5n}=1\phantom{\rule{0.3em}{0ex}}\text{pb}$. The decay properties of $3n$-evaporation product $^{289}114$, in the decay chains observed at low $^{48}\mathrm{Ca}$ energy coincide well with those previously observed in the $^{244}\mathrm{Pu+}^{48}\mathrm{Ca}$ and $^{248}\mathrm{Cm+}^{48}\mathrm{Ca}$ reactions and assigned to $^{288}114$. Two isotopes of element 114 and their descendant nuclei were identified for the first time at higher bombarding energies: $^{288}114$ (${E}_{\ensuremath{\alpha}}=9.95\phantom{\rule{0.3em}{0ex}}\text{MeV}$, ${T}_{1∕2}=0.6\phantom{\rule{0.3em}{0ex}}\mathrm{s}$) and $^{287}114$ (${E}_{\ensuremath{\alpha}}=10.04\phantom{\rule{0.3em}{0ex}}\text{MeV}$, ${T}_{1∕2}=1\phantom{\rule{0.3em}{0ex}}\mathrm{s}$). We also report on the observation of new isotopes of element 116, $^{290,291}116$, produced in the $^{245}\mathrm{Cm}+^{48}\mathrm{Ca}$ reaction with cross sections of about $1\phantom{\rule{0.3em}{0ex}}\text{pb}$. A discussion of self-consistent interpretations of all observed decay chains originating at $Z=118$, 116, and 114 is presented.

Local density approximation for proton-neutron pairing correlations: Formalism

Abstract: In the present study we generalize the self-consistent Hartree-Fock-Bogoliubov (HFB) theory formulated in the coordinate space to the case which incorporates an arbitrary mixing between protons and neutrons in the particle-hole $(p\text{\ensuremath{-}}h)$ and particle-particle ($p\text{\ensuremath{-}}p$ or pairing) channels. We define the HFB density matrices, discuss their spin-isospin structure, and construct the most general energy-density functional that is quadratic in local densities. The consequences of the local gauge invariance are discussed and the particular case of the Skyrme energy-density functional is studied. By varying the total energy with respect to the density matrices the self-consistent one-body HFB Hamiltonian is obtained and the structure of the resulting mean fields is shown. The consequences of the time-reversal symmetry, charge invariance, and proton-neutron symmetry are summarized. The complete list of expressions required to calculate total energy is presented.

Coulomb and nuclear breakup of a halo nucleus11Be

Abstract: Breakup reactions of the one-neutron halo nucleus {sup 11}Be on lead and carbon targets at about 70 MeV/nucleon have been investigated at RIKEN by measuring the momentum vectors of the incident {sup 11}Be, outgoing {sup 10}Be, and neutron in coincidence. The relative energy spectra as well as the angular distributions of the {sup 10}Be+n center of mass system (inelastic angular distributions) have been extracted both for Pb and C targets. For the breakup of {sup 11}Be on Pb, the selection of forward-scattering angles, corresponding to large impact parameters, is found to be effective to extract almost purely the first-order E1 Coulomb breakup component and to exclude the nuclear contribution and higher-order Coulomb breakup components. This angle-selected energy spectrum is thus used to deduce the spectroscopic factor for the {sup 10}Be(0{sup +})x{nu}2s{sub 1/2} configuration in {sup 11}Be which is found to be 0.72{+-}0.04 with a B(E1) strength up to E{sub x}=4 MeV of 1.05{+-}0.06 e{sup 2} fm{sup 2}. The energy weighted E1 strength up to E{sub x}=4 MeV explains 70%{+-}10% of the cluster sum rule, consistent with the obtained spectroscopic factor. The non-energy-weighted sum rule within the same energy range is used to extract the root-mean-square distance of the halo neutronmore » to be 5.77(16) fm, consistent with previously known values. In the breakup with the carbon target, we have observed the excitations to the known unbound states in {sup 11}Be at E{sub x}=1.78 MeV and E{sub x}=3.41 MeV. Angular distributions for these states show the diffraction pattern characteristic of L=2 transitions, resulting in a J{sup {pi}}=(3/2,5/2){sup +} assignment for these states. We finally find that even for the C target the E1 Coulomb direct breakup mechanism becomes dominant at very forward angles.« less

Quantum Monte Carlo calculations of excited states in A =6-8 nuclei

Abstract: A variational Monte Carlo method is used to generate sets of orthogonal trial functions, ${\ensuremath{\Psi}}_{T}({J}^{\ensuremath{\pi}};T)$, for given quantum numbers in various light $p$-shell nuclei. These ${\ensuremath{\Psi}}_{T}$ are then used as input to Green's function Monte Carlo (GFMC) calculations of first, second, and higher excited $({J}^{\ensuremath{\pi}};T)$ states. Realistic two- and three-nucleon interactions are used. We find that if the physical excited state is reasonably narrow, the GFMC energy converges to a stable result. With the combined Argonne ${\mathrm{v}}_{18}$ two-nucleon and Illinois-2 three-nucleon interactions, the results for many second and higher states in $A=6\char21{}8$ nuclei are close to the experimental values.

Reactions induced by the halo nucleus He-6 at energies around the coulomb barrier

Abstract: The reaction $^{6}\mathrm{He}+^{64}\mathrm{Zn}$ was studied in order to investigate the effects of the projectile neutron-halo structure on the reaction mechanism at energies around the Coulomb barrier. Elastic scattering angular distributions, transfer∕breakup angular distributions, and fusion excitation functions have been measured. Due to the low-recoil energy of the evaporation residues and the low intensity of the $^{6}\mathrm{He}$ beam, the fusion cross section was measured by detecting off-line the atomic x-ray emission which follows the electron capture decay of the evaporation residues. For comparison the reaction $^{4}\mathrm{He}+^{64}\mathrm{Zn}$ was studied using the same technique. The data for the reaction $^{6}\mathrm{He}+^{64}\mathrm{Zn}$ show that the transfer and breakup mechanisms account for almost $80%$ of the total reaction cross section, moreover we do not observe an enhancement of the fusion cross section when compared with the $^{4}\mathrm{He}+^{64}\mathrm{Zn}$ reaction.

Relativistic harmonic oscillator with spin symmetry

Abstract: The eigenfunctions and eigenenergies for a Dirac Hamiltonian with equal scalar and vector harmonic oscillator potentials are derived. Equal scalar and vector potentials may be applicable to the spectrum of an antinucleon embedded in a nucleus. Triaxial, axially deformed, and spherical oscillator potentials are considered. The spectrum has a spin symmetry for all cases and, for the spherical harmonic oscillator potential, a higher symmetry analogous to the SU(3) symmetry of the nonrelativistic harmonic oscillator is discussed.

Recombination of shower partons at high p T in heavy-ion collisions

Abstract: A formalism for hadron production at high ${p}_{T}$ in heavy-ion collisions has been developed such that all partons hadronize by recombination The fragmentation of a hard parton is accounted for by the recombination of shower partons that it creates Such shower partons can also recombine with the thermal partons to form particles that dominate over all other possible modes of hadronization in the $3l{p}_{T}l8\phantom{\rule{03em}{0ex}}\mathrm{GeV}$ range The results for the high ${p}_{T}$ spectra of pion, kaon, and proton agree well with experiments Energy loss of partons in the dense medium is taken into account on the average by an effective parameter by fitting data, and is found to be universal independent of the type of particles produced, as it should Due to the recombination of thermal and shower partons, the structure of jets produced in nuclear collisions is different from that in $pp$ collisions The consequence on same-side correlations is discussed

Rethinking the properties of the quark-gluon plasma at T c T 4 T c

Abstract: We argue that although at asymptotically high temperatures the quark-gluon plasma (QGP) in bulk behaves as a gas of weakly interacting quasiparticles (modulo long-range magnetism), it is different at temperatures up to few times the critical temperature ${T}_{c}$. In particular, we emphasize the role of several zero binding lines on the phase diagrams, below which $\overline{c}c$, light quarks, $gg$ as well as exotic $gq,qq$ bound states exist. Near these lines quasiparticle rescattering is enhanced dramatically, possibly explaining why heavy-ion collisions at the relativistic heavy-ion collider exhibit robust collective phenomena. Although in QCD the coupling constant in the QGP phase reaches only values $g\ensuremath{\sim}1$, using Maldacena duality one may study these phenomena in the strong coupling limit ${g}^{2}{N}_{c}⪡1$ in conformal gauge theories at finite temperature [e.g., $\mathcal{N}=4$ supersymmetric Yang-Mills (SUSY YM)]. Trapped atoms is another system which allows us to study strongly interacting matter, in the limit of infinite scattering length.

Hyperon photoproduction in the nucleon resonance region

Abstract: High-statistics cross sections and recoil polarizations for the reactions gamma+p-->K++Lambda and gamma+p-->K++Sigma(0) have been measured at CLAS for center-of-mass energies between 1.6 and 2.3 GeV. In the K(+)Lambda channel we confirm a resonance-like structure near W=1.9 GeV at backward kaon angles. Our data show more complex s- and u- channel behavior than previously seen, since structure is also present at forward angles, but not at central angles. The position and width change with angle, indicating that more than one resonance is playing a role. Large positive Lambda polarization at backward angles, which is also energy dependent, is consistent with sizable s- or u-channel contributions. Presently available model calculations cannot explain these aspects of the data.

Microscopic determination of the nuclear incompressibility within the nonrelativistic framework

Abstract: The nuclear incompressibility ${K}_{\ensuremath{\infty}}$ is deduced from measurements of the isoscalar giant monopole resonance (ISGMR) in medium-heavy nuclei, and the resulting value turns out to be model dependent. Since the considered nuclei have neutron excess, it has been suggested that the model dependence is due to the different behavior of the symmetry energy in different models. To clarify this issue, we make a systematic and careful analysis based on new Skyrme forces, which span a wide range of values for ${K}_{\ensuremath{\infty}}$, for the value of the symmetry energy at saturation and for its density dependence. By calculating, in a fully self-consistent fashion, the ISGMR centroid energy in $^{208}\mathrm{Pb}$, we reach three important conclusions: (i) the monopole energy, and consequently the deduced value of ${K}_{\ensuremath{\infty}}$, depend on a well-defined parameter related to the shape of the symmetry energy curve and called ${K}_{\mathit{sym}}$; (ii) Skyrme forces of the type of SLy4 predict ${K}_{\ensuremath{\infty}}$ around $230\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, in agreement with the Gogny force (previous estimates using Skyrme interactions having been plagued by a lack of full self-consistency); (iii) it is possible to build forces which predict ${K}_{\ensuremath{\infty}}$ around $250\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, although part of this increase is due to our poor knowledge of the density dependence and effective mass.

New perspective on complex cluster radioactivity of heavy nuclei

Abstract: Experimental data of complex cluster radioactivity $(^{14}\mathrm{C}\text{--}^{34}\mathrm{Si})$ are systematically analyzed and investigated with different models. The half-lives of cluster radioactivity are well reproduced by a new formula between half-lives and decay energies and by a microscopic density-dependent cluster model with the renormalized M3Y nucleon-nucleon interaction. The formula can be considered as a natural extension of both the Geiger-Nuttall law and the Viola-Seaborg formula from simple $\ensuremath{\alpha}$ decay to complex cluster radioactivity where different kinds of clusters are emitted. It is useful for experimentalists to analyze the data of cluster radioactivity. A new linear relationship between the decay energy of cluster radioactivity and the number of $\ensuremath{\alpha}$ particles in the cluster is found where the increase of decay energy for an extra $\ensuremath{\alpha}$ particle is between 15 and 17 MeV. The possible physics behind this new linear relationship is discussed.

Neutrino-“pasta” scattering: The opacity of nonuniform neutron-rich matter

Abstract: Neutron-rich matter at subnuclear densities may involve complex structures displaying a variety of shapes, such as spherical, slablike, and∕or rodlike shapes. These phases of the nuclear pasta are expected to exist in the crust of neutron stars and in core-collapse supernovae. The dynamics of core-collapse supernovae is very sensitive to the interactions between neutrinos and nucleons∕nuclei. Indeed, neutrino excitation of the low-energy modes of the pasta may allow for a significant energy transfer to the nuclear medium, thereby reviving the stalled supernovae shock. The linear response of the nuclear pasta to neutrinos is modeled via a simple semiclassical simulation. The transport mean free path for $\ensuremath{\mu}$ and $\ensuremath{\tau}$ neutrinos (and antineutrinos) is expressed in terms of the static structure factor of the pasta, which is evaluated using Metropolis Monte Carlo simulations.

Systematic Failure of the Woods-Saxon Nuclear Potential to Describe Both Fusion and Elastic Scattering: Possible Need for a New Dynamical Approach to Fusion

Abstract: A large number of precision fusion excitation functions, at energies above the average fusion barriers, have been fitted using the Woods-Saxon form for the nuclear potential in a barrier passing model of fusion. They give values for the empirical diffuseness parameter $a$ ranging between 0.75 and $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, compared with values of about $0.65\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ which generally reproduce elastic scattering data. There is a clear tendency for the deduced $a$ to increase strongly with the reaction charge product ${Z}_{1}{Z}_{2}$, and some evidence for the effect of nuclear structure on the value of $a$, particularly with regard to the degree of neutron richness of the fusing nuclei, and possibly with regard to deformation. The measured fusion-barrier energies are always lower than those of the bare potentials used, which is expected as a result of adiabatic coupling to high energy collective states. This difference increases with increasing ${Z}_{1}{Z}_{2}$ and calculations show that about $1∕3$ of it may be attributed to coupling to the isoscalar giant-quadrupole resonances in the target and projectile. Coupling to all giant resonances may account for a significant part. Fluctuations about the trend line may be due to systematic errors in the data and/or structure effects such as coupling to collective octupole states. Previously suggested reasons for the large values of $a$ have been related to departures from the Woods-Saxon potential and to dissipative effects. This work suggests that the apparently large values of $a$ may be an artifact of trying to describe the dynamical fusion process by use of a static potential. Another partial explaination might reside in fusion inhibition, due for example to deep-inelastic scattering, again a process requiring dynamical calculations.

Nuclear parton distribution functions and their uncertainties

Abstract: We analyze experimental data of nuclear structure-function ratios ${F}_{2}^{A}∕{F}_{2}^{{A}^{\ensuremath{'}}}$ and Drell-Yan cross section ratios for obtaining optimum parton distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian method. Valence-quark distributions are determined by the ${F}_{2}$ data at large $x$; however, the small-$x$ part is not obvious from the data. On the other hand, the antiquark distributions are determined well at $x\ensuremath{\sim}0.01$ from the ${F}_{2}$ data and at $x\ensuremath{\sim}0.1$ by the Drell-Yan data; however, the large-$x$ behavior is not clear. Gluon distributions cannot be fixed by the present data and they have large uncertainties in the whole $x$ region. Parametrization results are shown in comparison with the data. We provide a useful code for calculating nuclear PDFs at given $x$ and ${Q}^{2}$.