Semi-peripheral Collisions Research Articles

Characteristic time scale of cluster production at the Fermi energy

The study of heavy-ion collisions in the Fermi energy domain (20 MeV/nucleon < E/A < 100 MeV/nucleon) is a fundamental research topic in modern nuclear physics. In the case of semi-peripheral collisions, the Fermi energy regime is characterized by the formation of a transient, neck-like structure that connects a projectile-like fragment (PLF) with a target-like fragment (TLF). The neck structure represents a precursor of the fireball which is a typical overlap of participating nucleons at relativistic energies (E/A> 200 MeV/nucleon). It undergoes an expansion phase in a short time scale of the order of 100 fm/c with the formation of a low-density region of nuclear matter, therefore favoring the clusterization of intermediate mass fragments (IMFs) of atomic number Z (typically) less than 20. Particular emphasis is given to some relevant results obtained by the CHIMERA collaboration in the last decade, regarding the time scale of the production mechanisms of the intermediate mass fragments in neck fragmentation and their neutron enrichment.

Nuclear neck-density determination at Fermi energy with CHIMERA detector

Binary collisions of either stable or radioactive heavy ions at Fermi energies allow to study the nuclear reaction mechanisms under dynamical conditions of non-equilibrium and the formation of transient pieces of nuclear matter of very short mean life times at sub-normal density. An important role in the evolutionary phase of the collision is played by the gradient of the nuclear density affecting the isospin asymmetry of the reaction products by typical transport phenomena such as the isospin diffusion and drift. Experimental determination of the value of the nuclear matter density in the early phase of the collision is a crucial step towards understanding the underlying mechanism responsible for the production of clusters. In this paper a method for evaluating the nuclear density in semi-peripheral collisions in the reaction $$^{124}Sn + ^{64}Ni$$ at 35 MeV/nucleon as studied with the CHIMERA multi-particle detector is described.

Dynamical fission of the quasiprojectile and isospin equilibration for the system Kr80+Ca48 at 35 MeV/nucleon

Experimental results concerning the dynamical fission of quasiprojectiles in semiperipheral collisions for the system 80 Kr+ 48 Ca at 35 MeV/nucleon are presented. Data have been collected with four blocks of the FAZIA setup in the first physics experiment of the FAZIA Collaboration. The degree of isospin equilibration between the two fission fragments and its dependence on their charge asymmetry is investigated. The data are compared with the results of the AMD model coupled to GEMINI as an afterburner, in order to get hints about the timescale of the process.

Dynamic description of ternary and quaternary splits of heavy nuclear systems in the deep-inelastic regime

Colliding heavy nuclear systems in the deep-inelastic regime may undergo partitioning into multiple fragments when fusion can not be achieved. While multiple breakups are common at Fermi energy, they are rather exotic in the deep-inelastic regime, where density, excitation and, in general, transport conditions, are expected to be different. Abundant ternary and quaternary splits have been observed in recent experiments, for instance in symmetric semi-central and semi-peripheral collisions with heavy systems, like $^{197}\mathrm{Au}$ + $^{197}\mathrm{Au}$ at 15 MeV per nucleon. In these conditions, we undertook a microscopic description of the reaction dynamics. Relying on the full solution of the Boltzmann-Langevin equation implemented in the BLOB approach, we could follow in time the development of instabilities along deformation.

Lights and (some) shadows in the comparison among experimental data of heavy ion collisionat Fermi energies and the dynamical model AMD

The simulation of heavy ion collisions in the Fermi energy region is a challenge for the theoretical models; in particular it is difficult to obtain a coherent description in all the impact parameter range and to reproduce all the experimental observables. In this contribution we will show the very good job done by the dynamical model AMD [1] followed by the statistical code GEMINI [2, 3] as an afterburner. The model is able to reproduce the main characteristics of peripheral and semiperipheral collisions, although some discrepancies still persist.

Measurement of the production of high-pT electrons from heavy-flavour hadron decays in Pb–Pb collisions at [formula omitted

Electrons from heavy-flavour hadron decays (charm and beauty) were measured with the ALICE detector in Pb–Pb collisions at a centre-of-mass of energy sNN=2.76 TeV. The transverse momentum (pT) differential production yields at mid-rapidity were used to calculate the nuclear modification factor RAA in the interval 3<pT<18 GeV/c. The RAA shows a strong suppression compared to binary scaling of pp collisions at the same energy (up to a factor of 4) in the 10% most central Pb–Pb collisions. There is a centrality trend of suppression, and a weaker suppression (down to a factor of 2) in semi-peripheral (50–80%) collisions is observed. The suppression of electrons in this broad pT interval indicates that both charm and beauty quarks lose energy when they traverse the hot medium formed in Pb–Pb collisions at LHC.

Microscopic dynamics simulations of multinucleon transfer inKr86+Ni64at 25 MeV/nucleon

Multi-nucleon transfer in $^{86}$Kr+$^{64}$Ni at an incident energy of 25 MeV/nucleon is for the first time investigated with a microscopic dynamics model: improved quantum molecular dynamics (ImQMD) model. The measured isotope distributions are reasonably well reproduced by using the ImQMD model together with a statistical code (GEMINI) for describing the secondary decay of fragments. The reaction mechanism is explored with the microscopic dynamics simulations from central to peripheral collisions. In central collisions there exists a strong competition among fusion, deep-inelastic scattering and multi-fragmentation at such an incident energy. In semi-peripheral collisions, binary scattering together with nucleon transfer is dominant, and the probability of elastic+inelastic scattering events increases rapidly with impact parameter in peripheral collisions and approaches to one when $b>14$ fm. The mass-TKE distribution in central collisions due to the competition is quite different from those in peripheral collisions and the distribution of total kinetic energy loss (TKEL) for binary events with nucleon transfer is much more broader than those without transfer.

Dynamical fission following peripheral heavy-ion collisions

A closed-form theoretical approach describing in a single picture both the evaporation component and the fast nonequilibrium component of the sequential fission of projectile-like fragments in a peripheral heavy-ion collision is derived and then applied to the dynamical fission observed in the [Formula: see text]+[Formula: see text] semiperipheral collision at 35A[Formula: see text]MeV. Information on the reaction mechanism is obtained such as the opposite polarization effects and the estimate of the “formation-to-fast fission lifetimes” of the fissioning fragment.

Semiclassical approach to sequential fission in peripheral heavy-ion collisions

A closed-form theoretical approach describing in a single picture both the evaporation component and the fast nonequilibrium component of the sequential fission of projectilelike fragments in a semiperipheral heavy-ion collision is derived and then applied to the dynamical fission observed in the 124 Sn + 64 Ni semiperipheral collision at 35A MeV. Information on opposite polarization effects of the fissioning projectilelike fragments and on their “formation-to-fast fission lifetimes” are obtained.

Chaotic vortical flows and their manifestations

We study vorticity and hydrodynamic helicity in semi-peripheral heavy-ion collisions using the kinetic model of Quark-Gluon Strings. The angular momentum, which is a source of P-odd observables, is preserved with a good accuracy. We observe formation of the specific toroidal structures of the vorticity field. Their existence, accompanied by the strange chemical potential, is mirrored in the polarization of hyperons of the percent order.

Correlation between the fragmentation modes and light charged particles emission in heavy ion collisions

The correlation between the shape of rapidity distribution of the yield of light charged particles and the fragmentation modes in semi-peripheral collisions for 70Zn+70Zn, 64Zn+64Zn and 64Ni+64Ni at the beam energy of 35 MeV/nucleon is investigated based on ImQMD05 code. Our studies show there is an interplay between the binary, ternary and multi-fragmentation break-up modes. The binary and ternary break-up modes more prefer to emit light charged particles at middle rapidity and give larger values of R yield mid compared with the multi-fragmentation break-up mode does. The reduced rapidity distribution for the normalized yields of p, d, t, 3He, 4He and 6He and the corresponding values of R yield mid can be used to estimate the probability of multi-fragmentation break-up modes. By comparing to experimental data, our results illustrate that ≥40% of the collisions events belong to the multifragmentation break-up mode for the reactions we studied.

Angular dihadron correlations as an interplay between elliptic and triangular flows

The hybrid model HYDJET++, which combines soft and hard components, is employed for the analysis of dihadron angular correlations measured in PbPb collisions at center-of-mass energy sqrt(s_NN) = 2.76 TeV. The model allows for study both individual contributions and mutual influence of lower flow harmonics, v_2 and v_3, on higher harmonics and dihadron angular correlations. It is shown that the typical structure called ridge in dihadron angular correlations in a broad pseudorapidity range could appear just as interplay of v_2 and v_3. Central, semi-central and semi-peripheral collisions were investigated. Comparison of model results with the experimental data on dihadron angular correlations is presented for different centralities and transverse momenta p_T.

Hexagonal flowv6as a superposition of ellipticv2and triangularv3flows

Partial contributions of elliptic ${v}_{2}$ and triangular ${v}_{3}$ flows to the hexagonal ${v}_{6}$ flow are studied within the hydjet$++$ model for Pb+Pb collisions at $\sqrt{s}=2.76$ TeV. Scaling of the ratio ${v}_{6}^{1/6}{{\ensuremath{\Psi}}_{2}}/{v}_{2}^{1/2}{{\ensuremath{\Psi}}_{2}}$ in the elliptic flow plane, ${\ensuremath{\Psi}}_{2}$, is predicted in the range $1\ensuremath{\le}{p}_{t}\ensuremath{\le}4$ GeV/$c$ for semicentral and semiperipheral collisions. Jets increase this ratio by about 10% and also cause its rise at ${p}_{t}\ensuremath{\ge}3.5$ GeV/$c$. The part of ${v}_{6}$ coming from ${v}_{2}$ is instantly increasing as the reaction becomes more peripheral, whereas the contribution of ${v}_{3}$ to ${v}_{6}$ drops. This behavior explains the experimentally observed increase of correlations between second and sixth harmonics and the decrease of correlations between third and sixth harmonics with rising impact parameter $b$. Our study favors the idea that basic features of the hexagonal flow can be understood in terms of the interplay of elliptic and triangular flows.

Triton/3He ratio as an observable for neutron-skin thickness

Based on the framework of the Isospin-Dependent Quantum Molecular Dynamics (IQMD) model in which the initial neutron and proton densities are sampled according to the droplet model, the correlation between triton-to-$^{3}$He yield ratio (R(t/$^{3}$He)$=$Yield(t)/Yield($^{3}$He)) and neutron skin thickness (${\delta}_{np}$) in neutron-rich projectile induced reactions is investigated. By changing the diffuseness parameter of neutron density distribution in the droplet model for the projectile to obtain different ${\delta}_{np}$, the relationship between ${\delta}_{np}$ and the corresponding R(t/$^{3}$He) in semi-peripheral collisions is obtained. The calculated results show that R(t/$^{3}$He) has a strong linear correlation with ${\delta}_{np}$ for neutron-rich $^{50}$Ca and $^{68}$Ni nuclei. It is suggested that R(t/$^{3}$He) could be regarded as a good experimental observable to extract ${\delta}_{np}$ for neutron-rich nuclei because the yields of charged particles triton and $^{3}$He can be measured quite precisely.

Flow and Correlations in PbPb and pPb Collisions from CMS Experiment

Measurements of two- and four-particle angular correlations for inclusive charged particles in pPb collisions at p sNN = 5:02 TeV are presented over a wide pseu- dorapidity range, | | < 2.5 and full azimuth from the CMS experiment. Results from 31 nb 1 of pPb data, collected during the 2013 LHC running are compared to 2.76 TeV semi peripheral PbPb collision data, from 2011 PbPb run, having similar range of particle multiplicities. The second-order (v2) and third-order (v3) anisotropy harmonics are ex- tracted using the two-particle azimuthal correlation technique. v2 is also extracted using a four-particle cumulant method to further explore the multi-particle nature of correlations in pPb collisions. We observe a remarkable similarity in the v3 signal as a function of multiplicity between pPb and PbPb systems.

Probing the symmetry energy at low density using observable from neck fragmentation

Semi-peripheral collisions of 64 Ni + 124 Sn in direct and inverse kinematics, at 35 A MeV incident energy, are studied to characterize the dynamical origin and align- ments properties of fragments emission at midrapidity. This emission is partly due to the fragmentation of a transient neck joining the projectile and target in the early stages of the reaction. Recently we exploited observable from neck fragmentation to constraint the density dependence of the symmetry energy. In this contribution we focus on the den- sity evolution of the mid-rapidity source. An experimental survey is suggested to check if data are consistent with the formation of neck fragments in a dilute density region in contact with the projectile-like and target-like spectators at normal saturation density.

Emission of fragments in Ca+Ca reactions at 25 MeV/nucleon

We discuss experimental data concerning 40,48Ca+40,48Ca reactions at 25 MeV/nucleon; the 4π multi-detector Chimera has been used as detection device. Effects that can be attributed to the neutron to proton ratios (N/Z) degree of freedom have been investigated. From the analysis of experimental data it seems that the neutron richness of the interacting system plays an important role on the evolution of fusion-like sources formed in semi-central collisions. In particular, it is observed that the larger is the neutron content and the larger is the emission of heavy residues. Experimental data have been compared with CoMD-II model calculations; a moderately stiff symmetry energy should be used to reproduce satisfactorily the data. A thermodynamical analysis on fusion-like sources has been also performed. In semi-peripheral collisions, isospin diffusion signals have been found. They have been investigated by analyzing isobaric emission (7Li/7Be) of quasi-projectile sources. Experimental data indicate that an incomplete N/Z mixing is reached during the interaction phase.

System-size dependence of particle-ratio fluctuations in Pb + Pb collisions at 158AGeV

New measurements by the NA49 experiment of the centrality dependence of event-by-event fluctuations of the particle yield ratios (K++K−)/(π++π−), (p+p¯)/(π++π−), and (K++K−)/(p+p¯) are presented for Pb+Pb collisions at 158A GeV. The absolute values of the dynamical fluctuations of these ratios, quantified by the measure σdyn, increase by about a factor of two from central to semiperipheral collisions. Multiplicity scaling scenarios are tested and found to apply for both the centrality and the previously published energy dependence of the (K++K−)/(π++π−) and (p+p¯)/(π++π−) ratio fluctuations. A description of the centrality and energy dependence of (K++K−)/(p+p¯) ratio fluctuations by a common scaling prescription is not possible since there is a sign change in the energy dependence.3 MoreReceived 11 April 2012DOI:https://doi.org/10.1103/PhysRevC.87.024902©2013 American Physical Society

N/Z effects on nuclear reactions near the multi-fragmentation threshold

We investigate effects that can be attributed to the neutron-to-proton ratio (N/Z) degree of freedom by studying nuclear reactions 40, 48Ca + 40, 48Ca and 40Ca + 46Ti at 25 MeV/nucleon. We used the Chimera 4π detector as detection device. Even-odd staggering in Z and N distributions of light fragments have been seen. They can be attributed, in a qualitative way, to the de-excitation phase of pre-fragments. By selecting central events of reaction, where fusion-like sources can be formed, we observe that the emission of heavy residues is more favored in the case of neutron rich entrance channels; this effect could be attributed to the interplay between Coulomb and symmetry terms in the nuclear equation of state, as obtained by comparing experimental data with CoMD-II model calculations. We characterized also thermodynamically hot-fusion-like sources formed in central collisions; in this case, the studied systems show similar apparent temperatures. In semi-peripheral collisions, isospin diffusion effects have been found. From the data analysis it seems that, at this bombarding energy, the neutron-to-proton ratio degree of freedom does not have time to distribute uniformly in the interacting di-nuclear system.

Correlations between emission timescale of fragments and isospin dynamics in124Sn+64Ni and112Sn+58Ni reactions at 35AMeV

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of $<$N/Z$>$ isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio $<$N/Z$>$ distributions than those produced in sequential statistical emission. All these effects support the concept of isospin "migration", that is sensitive to the density gradient between participant and quasi-spectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a Stochastic Mean Field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.