Isospin Effects Research Articles

Comparative analysis of nucleonic flows for isospin degree of freedom

We performed a detailed investigation on the role of neutron-proton asymmetry on collective flows viz directed flow (transverse flow), elliptic flow, differential transverse flow, and differential elliptic flow for central collisions of 132Sn+124Sn and 56Ca+132Sn at 100 MeV/nucleon. The role of nuclear symmetry energy (and its density dependence) and isospin dependence of binary collisions cross-sections is examined on various types of nucleonic flows. Our findings reveal that transverse flow in mass symmetric collisions is best suited to explore isospin effects via nuclear symmetry energy at energies close to Fermi energies, however, isospin effects at high energies could be probed via elliptic and differential elliptic flows. A confrontation of our theoretical calculations with available measurements on the energy of disappearance of transverse flow favored the stiff behavior of nuclear symmetry energy at supra-saturation densities.

Hyperon Dynamics in Heavy-Ion Collisions near Threshold EnergySupported by the National Natural Science Foundation of China (Grant Nos. 12175072 and 11722546), and the Talent Program of South China University of Technology.

Within the framework of quantum molecular dynamics transport model, the isospin and in-medium effects on the hyperon production in the reaction of 197Au + 197Au are investigated thoroughly. A repulsive hyperon-nucleon potential from the chiral effective field theory is implemented into the model, which is related to the hyperon momentum and baryon density. The correction on threshold energy of the elementary hyperon cross section is taken into account. It is found that the Σ yields are suppressed in the domain of midrapidity and kinetic energy spectra with the potential. The hyperons are emitted in the reaction plane because of the strangeness exchange reaction and reabsorption process in the nuclear medium. The Σ −/Σ + ratio depends on the stiffness of nuclear symmetry energy, in particular in the high-energy region (above 500 MeV).

Isospin effects of projectile fragmentation in a Boltzmann-Langevin approach * *Supported by the National Natural Science Foundation of China (11635003, 11025524, 11161130520), National Basic Research Program of China (2010CB832903) and the European Commissions 7th Framework Programme (FP7-PEOPLE-2010-IRSES) under Grant Agreement Project (269131)

The isospin effects of projectile fragmentation at intermediate energies are investigated using an isospin-dependent Boltzmann-Langevin model. The collisions of mass-symmetric reactions including 58Fe, 58Ni + 58Fe, and 58Ni at intermediate energies, in the 30 to 100 MeV/A range, are studied for different symmetry energies. Yield ratios of the isotopic, isobaric, and isotonic pairs of fragments from the intermediate-mass region using three symmetry energies are extracted as functions of the N/Z ratio of the composite systems in the entrance channel and the incident energies. It is found that the yield ratios are sensitive to symmetry energies, especially for neutron-rich systems, and the calculations using soft symmetry energy are closer to the experimental data. The isospin effect is stronger for the soft symmetry energy, owing to the competition of the repulsive Coulomb force and the symmetry energy attractive force on the proton. For the first time, the splits are presented, revealing a transition from the isospin equilibrium at lower energies to translucency at intermediate energies. The results show a degree of transparency in that intermediate mass fragments undergo a transition from dependence on the composite systems in the entrance channel to reliance on the projectile and target nuclei.

Isospin effects on the chiral phase transition

We discuss the chiral and pion-superfluidity phase transitions at finite isospin chemical potential within the framework of Dyson–Schwinger equations. With the help of the quark scalar and pseudo-scalar condensates, which are widely accepted as order parameters of chiral and pion-superfluidity phase transitions respectively, we show that there is a second-order phase transition at isospin chemical potential μ I ∼ m π . Thereafter, we investigate the isospin effects on the chiral phase transition and on the critical point as well as the critical end point (CEP). The chiral phase transition line splits into two lines at finite μ I , and the CEP splits into two CEPs as well.

Interplay of Coulomb and symmetry potential in peak fragment production in asymmetric collisions

Role of the nuclear symmetry potential and Coulomb potential is explored on the peak energy of intermediate mass fragments ([Formula: see text]) and on peak multiplicity ([Formula: see text]) and their dependence on mass asymmetry of the reaction is also investigated. The calculations are done using Isospin-dependent Quantum Molecular Dynamics (IQMD) model. We also showed that the momentum-dependent interactions have uniform effects of [Formula: see text] and these effects are independent of mass asymmetry of the reaction. Further, we see that isospin effects that enter through the Coulomb and symmetry potential show much significant role as one increases the mass asymmetry of reaction. Mass asymmetric reactions thus serve a sensitive tool to investigate the nuclear symmetry energy effects.

Transverse single-spin asymmetries of midrapidity π0 and η mesons in polarized p+p collisions at s=200 GeV

We present a measurement of the transverse single-spin asymmetry for $\pi^0$ and $\eta$ mesons in $p^\uparrow$$+$$p$ collisions in the pseudorapidity range $|\eta|<0.35$ and at a center-of-mass energy of 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider. In comparison with previous measurements in this kinematic region, these results have a factor of 3 smaller uncertainties. As hadrons, $\pi^0$ and $\eta$ mesons are sensitive to both initial- and final-state nonperturbative effects for a mix of parton flavors. Comparisons of the differences in their transverse single-spin asymmetries have the potential to disentangle the possible effects of strangeness, isospin, or mass. These results can constrain the twist-3 trigluon collinear correlation function as well as the gluon Sivers function.

Influence of nuclear thermometers and hot nuclei properties on nuclear temperature isospin effect measurement

The influence of nuclear thermometers and hot nuclei properties on nuclear temperature isospin dependence is studied. A systematic study of the nuclear temperature isospin effect is presented for heavy-ion collisions at moderate energy via the isospin-dependent quantum molecular dynamics model in the company of the statistical decay model (GEMINI). It is found that the isospin dependence of nuclear temperatures which is calculated by a momentum quadrupole fluctuation thermometer is more easily affected by hot nuclei mass range. However, the isospin dependence of nuclear temperatures which is calculated by an isotope yields ratio thermometer is more easily affected by neutron-proton ratio range of the hot nuclei. It is also found that the isospin dependence of nuclear temperatures can be changed by hot nuclei properties.

Influence of secondary decay on chemical freeze-out temperature isospin effect measurement

The influence of sequential decay on isospin dependence of isotope yield ratio temperatures is studied and a systematic study of isotope temperatures is presented for heavy-ion collisions at 600 MeV/nucleon via the isospin-dependent quantum molecular dynamics model in the company of the statistical decay model (GEMINI). Calculations show that the isospin dependence of the isotope yield ratio temperature is not affected by secondary decay.

Role of mass asymmetry on the peak energy of intermediate mass fragments production and its influence towards isospin effects

Using the isospin-dependent quantum molecular dynamics model, we study the peak intermediate mass fragments production and corresponding center-of-mass energy in various mass asymmetric reactions. The study is spanned over different mass asymmetries for total system mass 92, 162 and 240. We propose the peak energy as a powerful probe to constrain the low-density behavior of symmetry energy. This peak energy is found to remove the dual dependence of isospin effects in symmetry energy and cross-section.

Statistical treatment of nuclear clusters in the continuum

The evaluation of the sub-saturation nuclear equation of state at finite temperature requires a proper state counting of the internal partition sum of nuclei which are immersed in the background of their continuum states. This classical statistical problem is addressed within the self-consistent mean-field approximation, which naturally accounts for isospin and effective mass effects in the nuclear density of states. The nuclear free energy is decomposed into bulk and surface terms, allowing a simple analytical prescription for the subtraction of gas states from the nuclear partition sum, that avoids double counting of unbound single particle states. We show that this correction leads to a sizeable effect in the composition of matter at high temperature and low proton fractions, such as it is formed in supernova collapse, early proto-neutron star evolution, as well as laboratory experiments. Specifically, the energy stored in the internal nuclear degrees of freedom is reduced, as well as the mass fraction of heavy clusters in the statistical equilibrium. The gas subtraction prescription is compared to different phenomenological methods proposed in the literature, based on a high energy truncation of the partition sum. We show that none of these methods satisfactorily reproduces the gas subtracted level density, if the temperature overcomes ~4 MeV.

Quark star matter in heavy quark stars

We study the thermodynamic properties of asymmetric quark matter and large mass quark stars within the confined-isospin-density-dependent-quark-mass model. We find that the quark matter symmetry energy should be very large in order to describe the recent discovered heavy compact stars PSR J0348+0432 (text {2.01}pm text {0.04}M_{odot }), MSP J0740+6620 (text {2.14}pm ^text {0.10}_text {0.09}M_{odot } of 68.3% credibility interval and text {2.14}pm ^text {0.20}_text {0.18}M_{odot } of 95.4% credibility interval) and PSR J2215+5135 (2.27pm ^text {0.10}_text {0.09}M_{odot }) as QSs. The tidal deformability Lambda _{1.4} of the QSs is also investigated in this work, and the result indicates that Lambda _{1.4} may depend on the isospin effects and the strength / orientation distribution of the magnetic fields inside the quark stars.

Isospin Effect on Baryon and Charge Fluctuations from the pNJL Model

We have studied the possible isospin corrections on the skewness and kurtosis of net-baryon and net-charge fluctuations in the isospin asymmetric matter formed in relativistic heavy-ion collisions at RHIC-BES energies, based on a 3-flavor Polyakov-looped Nambu–Jona–Lasinio model. With typical scalar–isovector and vector–isovector couplings leading to the splitting of u and d quark chiral phase transition boundaries and critical points, we have observed considerable isospin effects on the susceptibilities, especially those of net-charge fluctuations. Reliable experimental measurements at even lower collision energies are encouraged to confirm the observed isospin effects.

Isospin Effect on fragment productions and reaction mechanisms for Ni+Ca systems at 25 AMeV

Main features of fragments produced in 58Ni+40Ca and 58Ni+48Ca systems at 25 AMeV, collected by Chimera multidetector at INFN Laboratori Nazionali del Sud (Italy), are analysed in order to study properties of moderately excited systems (Ex=2-5 AMeV) formed in central collisions. By means of correlations between specific global variables, sensitive to the centrality of the collision, a stringent selection of Fusion-Evaporation residues, Multifragmentation sources and their decay products has been performed. The influence of the isospin contents of the systems on the reaction mechanism is carefully investigated and relevant observables such as isospin asymmetry N/Z or charge and mass of fragments, are discussed in order to probe the fragmentation path, characterised by short living ( ∼ 100fm/c) low density states of nuclear matter out of equilibrium. Preliminary comparisons with predictions of the BLOB model, in a semi-classical mean field framework, are presented, investigating cluster production emerging from the occurrence of low-density instabilities in central collisions.

Temperature and isospin dependence of the level-density parameter in the A≈110 mass region

$\ensuremath{\alpha}$-Particle evaporation spectra were measured at backward angles in $^{16}\mathrm{O}+\phantom{\rule{0.16em}{0ex}}^{94,100}\mathrm{Mo}$ reactions at multiple beam energies. Inverse level-density parameter, $K$, was determined for Cd nuclei of different isospins by simulating high-energy tail of the measured $\ensuremath{\alpha}$-particle evaporation spectra with statistical model code pace2. An overall increasing behavior of the $K$ value is observed with increasing temperature in the range of 1 to 2.5 MeV. It is observed that in the temperature region below 1.8 MeV, the parameter $K$ is higher for the neutron-rich nuclei by around 1 MeV. Semiclassical calculations were performed which treat single-particle level density of neutron and proton on different footing and this can account for the isospin effects. These calculations reproduce the $K$ values as determined from the statistical model analysis of $\ensuremath{\alpha}$-particle evaporation spectra. Present results clearly demonstrate the isospin dependence of the level-density parameter as conjectured by theoretical works.

Symmetry energy effect on emissions of light particles in coincidence with fast fission

The emission of light particles in coincidence with fast fission in near peripheral reactions of 40Ar+197Au at 30 MeV/u beam energy has been studied by using the Improved Quantum Molecular Dynamics Model (ImQMD). It is demonstrated that in the symmetrical fast fission of the heavy target-like fragment (TLF), the emission of the coalescence-invariant neutrons (CIN) is enhanced particularly at large angles in comparison to that in the events without fission. Due to the enhanced CIN emission and the accumulation of the isospin effect in the long-time process, the yield ratio of the CIN to the coalescence-invariant protons (CIP) in the fission events exhibits significant dependence on the symmetry energy Esym(ρ) varying with density. Considering that the large angle neutrons are experimentally advantageous to measure for the lower flying velocity and lower multiplicity, we emphasize that the yield ratio RCINP at large angles in the fast fission events can be used as a novel probe of the nuclear symmetry energy at sub-saturation densities.

Spectator-induced electromagnetic effects in heavy-ion collisions and space-time-momentum conditions for pion emission

We present our calculation of electromagnetic effects, induced by the spectator charge on Feynman-$x_F$ distributions of charged pions in peripheral $Pb+Pb$ collisions at CERN SPS energies, including realistic initial space-time-momentum conditions for pion emission. The calculation is performed in the framework of our simplified implementation of the fire-streak model, adopted to the production of both $\pi^-$ and $\pi^+$ mesons. Isospin effects are included to take into account the asymmetry in production of $\pi^+$ and $\pi^-$ at high rapidity. A comparison to a simpler model from the literature is made. We obtain a good description of the NA49 data on the $x_F$- and $p_T$-dependence of the ratio of cross sections $\pi^+/\pi^-$. The experimental data favors short times ($0.5<\tau<2$~fm/$c$) for fast pion creation in the local fire-streak rest frame. The possibility of the expansion of the spectators is considered in our calculation, and its influence on the electromagnetic effect observed for the $\pi^+/\pi^-$ ratio is discussed. In addition we discuss the relation between anisotropic flow and the electromagnetic distortion of $\pi^+/\pi^-$ ratios, and study the influence of transverse expansion of fire streaks as well as their vorticity on this distortion. In this latter study we find that inclusion of rotation of fire streaks in our model gives a satisfactory description of the rapidity dependence of pion directed flow. We conclude that {our implementation of the} fire-streak model, which properly describes the centrality dependence of $\pi^-$ rapidity spectra at CERN SPS energies, also provides a quantitative description of the electromagnetic effect on the $\pi^+/\pi^-$ ratio as a function of $x_F$.

Jet charge modification in finite QCD matter

Jet production and jet substructure modification in heavy-ion collisions have played an essential role in revealing the in-medium evolution of parton showers and the determination of the properties of strongly-interacting matter under extreme conditions. It is imperative to extend these studies to include flavor tagging and to devise observables that are sensitive to the partonic origin of jets. The average jet charge, defined as the momentum-weighted sum of the electric charges of particles inside the jet, is a proxy of the electric charge of the quark or gluon that initiates the jet. We demonstrate how the factorization framework of soft-collinear effective theory can be generalized to evaluate the jet charge in a dense strongly-interacting matter environment, such as the one produced in nuclear reactions at collider energies. Observables that can separate the contribution of in-medium branching from the trivial isospin effects are identified and their connection to established jet quenching effects is elucidated. We present predictions for the transverse momentum dependence of the jet charge distribution in nucleus-nucleus collisions and its modification relative to the proton case.

New probe to study the symmetry energy at low nuclear density with the deuteron breakup reaction

The reactions of nucleon and polarized deuteron scattered off a heavy target at large impact parameter with intermediate energies have been investigated by using the improved quantum molecular dynamics model. It is found that, due to the difference effect of isovector potential on proton and neutron, there is a significant difference between the angle distribution of elastic scattering protons and neutrons. To overcome the lack of monochromatic neutron beam, the reaction of polarized deuteron peripherally scattered off the heavy target is used to replace the reaction of individual proton and neutron scattered off heavy target to study the isospin effect. It is found that the distributions of elastic scattering angle of proton and neutron originating from the breakup of deuteron are very similar to the results of the individual proton- and neutron-induced reaction. A new probe more effective and more clean, namely the difference between elastic scattering angle of proton and neutron originating from the breakup of polarized deuteron, is promoted to constrain the symmetry energy at subsaturation density.

Effects of isospin and dissipation coefficient of the K coordinate in simulation of fission dynamics of excited compound nuclei

A stochastic approach based on four-dimensional (4D) dynamical model has been used to simulate the fission process of the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr produced in fusion reactions. Effects of isospin and dissipation coefficient of the [Formula: see text] coordinate, [Formula: see text], on estimation of the evaporation residue (ER) cross-section, the prescission neutron multiplicity, the variance of the mass and energy distributions of fission fragments and the anisotropy of fission fragments angular distribution have been investigated for the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr. Three collective shape coordinates [Formula: see text] plus the projection of total spin of the compound nucleus to the symmetry axis, [Formula: see text], were considered in the 4D dynamical model. In the 4D dynamical model, the magnitude of the dissipation coefficient of [Formula: see text], [Formula: see text], was considered as a free parameter and its magnitude inferred by fitting measured data on the ER cross-section. Results of the extracted dissipation coefficients of [Formula: see text] for different isotopes of Fr were shown that the magnitude of the dissipation coefficient of [Formula: see text] increases with decreasing isospin of fissioning compound nucleus. It was also shown that the prescission neutron multiplicity and the anisotropy of fission fragments angular distribution increase with increasing isospin whereas the variance of the mass and energy distributions of fission fragments decrease with increasing isospin of fissioning compound nucleus. Furthermore, it was shown that the calculated values of prescission neutron multiplicity and the variance of the mass distribution of fission fragments for the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr decrease with the dissipation strength of [Formula: see text], whereas the variance of the energy distribution of fission fragments and the anisotropy of fission fragments angular distribution increase with the dissipation strength of [Formula: see text].

Z boson production in Pb+Pb collisions at [formula omitted] TeV measured by the ATLAS experiment

The production yield of Z bosons is measured in the electron and muon decay channels in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 nb−1 are used for the analysis. The Z boson yield, normalised by the total number of minimum-bias events and the mean nuclear thickness function, is measured as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in proton–proton collisions at the same centre-of-mass energy. The nuclear modification factor is found to be consistent with unity for all centrality intervals. The results are compared with theoretical predictions obtained at next-to-leading order using nucleon and nuclear parton distribution functions. The normalised Z boson yields in Pb+Pb collisions lie 1–3σ above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity and is consistent with a next-to-leading-order QCD calculation including the isospin effect.