Isospin Effects Research Articles

Probing shadowed nuclear sea with massive gauge bosons in the future heavy-ion collisions

The production of the massive bosons $Z^0$ and $W^{\pm}$ could provide an excellent tool to study cold nuclear matter effects and the modifications of nuclear parton distribution functions (nPDFs) relative to parton distribution functions (PDFs) of a free proton in high energy nuclear reactions at the LHC as well as in heavy-ion collisions (HIC) with much higher center-of mass energies available in the future colliders. In this paper we calculate the rapidity and transverse momentum distributions of the vector boson and their nuclear modification factors in p+Pb collisions at $\sqrt{s_{NN}}=63$TeV and in Pb+Pb collisions at $\sqrt{s_{NN}}=39$TeV in the framework of perturbative QCD by utilizing three parametrization sets of nPDFs: EPS09, DSSZ and nCTEQ. It is found that in heavy-ion collisions at such high colliding energies, both the rapidity distribution and the transverse momentum spectrum of vector bosons are considerably suppressed in wide kinematic regions with respect to p+p reactions due to large nuclear shadowing effect. We demonstrate that in the massive vector boson productions processes with sea quarks in the initial-state may give more contributions than those with valence quarks in the initial-state, therefore in future heavy-ion collisions the isospin effect is less pronounced and the charge asymmetry of W boson will be reduced significantly as compared to that at the LHC. Large difference between results with nCTEQ and results with EPS09 and DSSZ is observed in nuclear modifications of both rapidity and $p_T$ distributions of $Z^0$ and $W$ in the future HIC.

Rod-shaped Nuclei at Extreme Spin and Isospin.

The anomalous rod shape in carbon isotopes has been investigated in the framework of the cranking covariant density functional theory, and two mechanisms to stabilize such a novel shape with respect to the bending motion, extreme spin, and isospin are simultaneously discussed for the first time in a self-consistent and microscopic way. By adding valence neutrons and rotating the system, we have found the mechanism stabilizing the rod shape; i.e., the σ orbitals (parallel to the symmetry axis) of the valence neutrons, important for the rod shape, are lowered by the rotation due to the Coriolis term. The spin and isospin effects enhance the stability of the rod-shaped configuration. This provides a strong hint that a rod shape could be realized in nuclei towards extreme spin and isospin.

Modified version of the combined model of photonucleon reactions

A refined version of the combined photonucleon-reaction model is described. This version makes it possible to take into account the effect of structural features of the doorway dipole state on photonucleon reactions in the energy range of Eγ ≤ 30 MeV. In relation to the previous version of the model, the treatment of isospin effects at the preequilibrium and evaporation reaction stages is refined; in addition, the description of the semidirect effect caused by nucleon emission from the doorway dipole state is improved. The model in question is used to study photonucleon reactions on the isotopes 35-56Ca and 102-134Sn in the energy range indicated above.

Examining the effect of a binding-energy-dependent clusterization algorithm on isospin-sensitive observables in heavy-ion collisions

The phase space obtained using the isospin quantum molecular dynamical (IQMD) model is analyzed by applying the binding energy cut in the most commonly and widely used secondary cluster recognition algorithm. In addition, for the present study, the energy contribution from momentum-dependent and symmetry potentials is also included during the calculation of total binding energy, which was absent in clusterization algorithms used earlier. The stability of fragments and isospin effects are explored by using the new clusterization algorithm. The findings are summarized as follows: (1) The clusterization algorithm identifies the fragments at quite early time. (2) It is more sensitive for free nucleons and light charged particles compared to intermediate mass fragments, which results in the enhanced (reduced) production of free nucleons (light charged particles, or LCPs). (3) It has affected the yield of isospin-sensitive observables-neutrons (n), protons (p), H-3, He-3, and the single ratio [R(n/p)]-to a greater extent in the mid-rapidity and low kinetic energy region. In conclusion, the inclusion of the binding energy cut in the clusterization algorithm is found to play a crucial role in the study of isospin physics. This study will give another direction for the determination of symmetry energy in heavy-ion collisions at intermediate energies.

Isotopic tungsten targets

In order to explore the isospin dependence of the quasifission process, a set of reactions with a wide range of N/Z was required. To maximize the sensitivity of the measurement to the isospin effects it was required that the Z of the projectile and target be fixed. Therefore, a set of isotopic targets spanning a relatively large N/Z range was desired. Tungsten, having five stable isotopes, provided a 6 neutron difference from 180W to 186W and can be obtained with high enrichment. Therefore, the production of enriched 180,182,184,186W targets was needed. Additionally, the targets were required to be relatively thin (<100 μg/cm2) in order to minimize the energy loss and scattering of the fission or quasifission fragments resulting from the reactions. Details of the W target preparation as well as target performance will be presented.

S= −1 meson-baryon interaction in hot and dense nuclear matter: chiral symmetry, many-body and unitarization for a road to GSI/FAIR

The dynamics of strange pseudoscalar and vector mesons in hot and dense nuclear matter is studied within a chiral unitary framework in coupled channels. Our results set up the starting point for implementations in microscopic transport approaches of heavy-ion collisions, particularly at the conditions of the forthcoming experiments at GSI/FAIR and NICA-Dubna. In the K̄ N sector we focus on the calculation of (off-shell) transition rates for the most relevant binary reactions involved in strangeness production close to threshold energies, with special attention to the excitation of sub-threshold hyperon resonances and isospin effects (e.g. K̄ p vs K̄ n). We also give an overview of recent theoretical developments regarding the dynamics of strange vector mesons (K*, K̄* and ϕ) in the nuclear medium, in connection with experimental activity from heavy-ion collisions and nuclear production reactions. We emphasize the role of hadronic decay modes and the excitation of hyperon resonances as the driving mechanisms modifying the properties of vector mesons.

Isospin effects in fragmentation reactions

Fragmentation reactions induced by relativistic heavy ions are characterized by large fluctuations in excitation energy and isospin. These features justifies the interest of this reaction mechanism for the production of nuclei far from stability. A particularly interesting reaction channel proving those fluctuations is the nucleon removal. Moreover, those processes are of interest to characterize the structural properties of the nuclei involved in the reactions.

The InKiIsSY experiment at LNS: A study of size vs. isospin effects with124Xe+64Zn and124,112Sn+64,58Ni reactions

The systems 124Sn+64Ni and 112Sn+58Ni at 35 AMeV were analyzed in previous experiments, by using the 4π multi-detector CHIMERA in order to investigate the time-scale and mechanisms of fragments formation. By comparing the two systems, effects that have been correlated to Isospin transport in reaction dynamics and symmetry energy term of nuclear equation of state at sub-saturation density were found. Recently, in order to disentangle the Isospin effects from size effects the 124Xe+64Ni reaction was investigated at the same beam energy of 35 AMeV.

Isospin effects in heavy-ion reactions: Results from transport theories

We discuss recent studies, within the framework of transport theories, on heavy ion reactions between charge asymmetric systems, from low up to Fermi energies. We focus on isospin sensitive observables, aiming at extracting information on the density dependence of the isovector part of the nuclear effective interaction and of the nuclear symmetry energy. Results are critically reviewed, also trying to establish a link, when possible, between the outcome of different transport models.

Centrality, rapidity and pT dependence of isolated prompt photon production in lead–lead collisions at sNN=2.76 TeV with the ATLAS detector at the LHC

Abstract The ATLAS experiment at the LHC has measured prompt photon production in s NN = 2.76 TeV Pb + Pb collisions using data collected in 2011 with an integrated luminosity of 0.14 nb − 1 . The measurement is performed with a hermetic, longitudinally segmented calorimeter, which gives excellent spatial and energy resolution, and detailed information about the shower shape of each measured photon. Using a selection based on shower shapes and an isolation criterion gives measured purities between approximately 50–90%, depending on p T , η and centrality. Photon yields, scaled by the mean nuclear thickness function, are determined as a function of collision centrality, pseudorapidity (in two intervals | η | 1.37 and 1.52 | η | 2.37 ) and transverse momentum ( 22 p T 280 GeV ). The scaled yields are shown compared to expectations from JETPHOX (perturbative QCD calculations at next to leading order), as are the ratios of the forward yields to those near mid-rapidity (for 22 p T 140 GeV ). The observed photon yields agree well with the predictions for proton–proton within statistical and systematic uncertainties, but are unable to distinguish between these calculations and others including isospin and nuclear PDF effects.

Isospin effects on pt-differential flow in heavy ion collisions at intermediate energies

This paper aims to study the role of isospin degree of freedom in heavy-ion collisions through the transverse momentum (pt), neutron to proton ratio and system mass dependence of pt-differential transverse flow. Our study shows that (pt)-differential transverse flow dependence can act as sensitive probe to study symmetry energy and its density dependence compared to the energy of vanishing flow. Symmetry energy and its density dependence play a dominant role over the isospin-dependence of nucleon–nucleon cross-section at Fermi energy.

The cosmic ray antiproton background for AMS-02

The AMS-02 experiment is measuring the cosmic ray antiproton flux with high precision. The interpretation of the upcoming data requires a thorough understanding of the secondary antiproton background. In this work, we employ newly available data of the NA49 experiment at CERN, in order to recalculate the antiproton source term arising from cosmic ray spallations on the interstellar matter. We systematically account for the production of antiprotons via hyperon decay and discuss the possible impact of isospin effects on antineutron production. A detailed comparison of our calculation with the existing literature as well as with Monte Carlo based evaluations of the antiproton source term is provided. Our most important result is an updated prediction for the secondary antiproton flux which includes a realistic assessment of the particle physics uncertainties at all energies.

System-size and energy dependence of particle momentum spectra: The UrQMD analysis ofp+pand Pb + Pb collisions

The UrQMD transport model is used to study a system-size and energy dependence of the pion production in high energy collisions. New data of the NA61/SHINE Collaboration on spectra of negatively charged pions in proton-proton interactions at SPS energies are considered. These results are compared with the corresponding data of the NA49 Collaboration in central Pb+Pb collisions at the same collision energies per nucleon. Mean pion multiplicity per participant nucleon, inverse slope parameter of the transverse momentum spectra, and width of rapidity distribution are investigated. A role of isospin effects is discussed. We find that the UrQMD model predicts a non-monotonous behavior of mean transverse mass with collision energy for positively charged pions at the mid-rapidity in inelastic proton-proton interactions. This will be checked soon experimentally by NA61/SHINE Collaboration.

An analysis of β-delayed neutron emission of even–even neutron-rich nuclei with proton-neutron QRPA

β-delayed neutron emission probabilities of even–even nuclei are examined with a proton-neutron quasi-particle random-phase-approximation (pnQRPA). To calculate a light particle evaporation from excited states of daughter nucleus populated by β-decay, the Hauser-Feshbach statistical model is used. We discuss effects of isospin T = 0 pairing, tensor force and nuclear deformation on the β-delayed neutron emission probability. We find that the effect of nuclear deformation is essential rather than the that of isospin T = 0 pairing and tensor force to give an appropriate β-feedings to energy window of neutron emission. The obtained β-delayed neutron emission probabilities are in reasonable agreement with the experimental data of several typical delayed neutron emitters. The results are also compared with the FRDM + QRPA calculation. We conclude that the present approach is promising to calculate β-delayed neutron.

Isospin effects on the system mass dependence of nuclear stopping around the energy of vanishing flow

We study the effect of isospin degree of freedom on nuclear stopping throughout the mass range 50 and 350 for two sets of isotopic systems with N/Z ≈ 1.5 and 1.8, as well as isobaric systems with N/Z = 1.0 and 1.4. Analysis is carried out at incident energies below, at, and above the energy of vanishing flow (EVF) using the isospin-dependent quantum molecular dynamics model. Our findings reveal that nuclear stopping does not show any particular behavior at the EVF. Moreover, system size effects dominate the isospin effects throughout the range of colliding geometry. The Coulomb effects, however, become important at peripheral geometry. The comparative study of the counterbalancing of Coulomb and mean field by removing the nucleon–nucleon collisions and symmetry potential clearly indicates the dominance of nucleon–nucleon cross-section over the Coulomb repulsions. Moreover, the theoretical results presented in this manuscript for the set of reactions can be experimentally verified.

Comparing spin observables in the reaction $^{12} C(\vec p,\vec p')^{12} C$ at 100–500 MeV with various distorted-wave impulse approximations

Spin observables in the process \((\vec p,\vec p')\) are analyzed at intermediate energies in the range 100–500 MeV. Experimental and theoretical data are compared for the reaction \(^{12} C(\vec p,\vec p')^{12} C\) involving excitation of states 1+, T = 0 (12.71 MeV) and 1+, T = 1 (15.11 MeV). Two varieties of DWIA-type calculations are mainly used, allowing the role of isospin effects to be distinguished.

Isospin Against Size Effects In Projectile Dynamical Fission For 112,124Sn+58,64Ni and 124Xe+64Zn Reactions At 35 A.MeV

In past experiments, mass asymmetric projectile-target combinations124Sn+64Ni and 112Sn+58Ni were investigated at ELab(112'124Sn)=35 A.MeVbeam energybyusing the 4n multi-detector CHIMERA. From a quantitative comparison of cross sections associated to Statistical and Dynamical Fission of the Projectile-Like Fragments, it resulted that Dynamical Fission process is about two times more probable in the neutron rich 124Sn+64Ni system than in the 112 Sn +58 Ni neutron poor one. In contrast, no sizable difference was found for Statistical Fission mechanism. The observed difference in the strength of the Dynamical effects could arise from the difference in entrance channel Isospin (N/Z) content. In order to disentangle Isospin effects from effects due to the different masses of the two systems, a new experiment 124Xe+64Zn at 35 A.MeV beam energy has been recently carried out.

Isospin effect of the in-medium nucleon-nucleon cross section in excited nuclear reactions

Using relativistic mean field theory, the neutron and the proton density distribution of 56Ni nuclei could be obtained in the ground state and the excited state. Based on the framework of the quantum molecular dynamics model, the 56Ni nuclei have been simulated in ground state and in the neutron or proton excited state. We then used the three different states of 56Ni to collide with the 56Ni in the ground state. To discuss the evolution of the nuclear stopping in different reactions, two kinds of different excited nuclear reactions were studied at different reaction energies and at different impact parameters. Studies have shown that the nuclear stopping of an excited nuclear reaction is sensitive to the isospin-dependent in-medium nucleon-nucleon cross section, compared with the response value of the ground state nuclear reaction. So, it is better for the excited nuclei to extract the isospin dependence of nucleon-nucleon cross section information.

Isospin effects and symmetry energy studies with INDRA

The equation of state of asymmetric nuclear matter is still controversial, as predictions at subsaturation as well as above normal density widely diverge. We discuss several experimental results measured in heavy-ion collisions with the INDRA array in the incident energy range 5-80 MeV/nucleon. In particular an estimate of the density dependence of the symmetry energy is derived from isospin diffusion results compared with a transport code: the potential part of the symmetry energy linearly increases with the density. We demonstrate that isospin equilibrium is reached in mid-central collisions for the two reactions Ni+Au at 52 MeV/nucleon and Xe+Sn at 32 MeV/nucleon. New possible variables and an improved modelization to investigate symmetry energy are discussed.

Theoretical predictions of experimental observables sensitive to the symmetry energy

In the framework of mean-field based transport approaches, we discuss recent results concerning heavy ion reactions between charge asymmetric systems, from low up to intermediate energies. We focus on isospin sensitive observables, aiming at extracting information on the density dependence of the isovector part of the nuclear effective interaction and of the nuclear symmetry energy. For reactions close to the Coulomb barrier, we explore the structure of collective dipole oscillations, rather sensitive to the low-density behavior of the symmetry energy. In the Fermi energy regime, we investigate the interplay between dissipation mechanisms, fragmentation and isospin effects. At intermediate energies, where regions with higher density and momentum are reached, we discuss collective flows and their sensitivity to the momentum dependence of the isovector interaction channel, which determines the splitting of neutron and proton effective masses. Finally, we also discuss the isospin effect on the possible phase transition from nucleonic matter to quark matter. Results are critically reviewed, also trying to establish a link, when possible, with the outcome of other transport models.