Nucleons In Heavy-ion Collisions Research Articles

Estimation of collision centrality in terms of the number of participating nucleons in heavy-ion collisions using deep learning

Estimation of collision centrality in terms of the number of participating nucleons in heavy-ion collisions using deep learning

In-medium nucleon-nucleon cross section in nuclear matter

In-medium nucleon-nucleon ($NN$) cross sections at various densities and isospin asymmetries are investigated systematically in the framework of the Brueckner-Hartree-Fock approach. The calculations are base on the exact treatment of the center-of-mass momentum instead of the total momentum approximation employed in previous works. The neutron-proton in-medium cross section is shown to exhibit nearly isospin asymmetry independence. For convenience in application, analytical formulas embodying the medium correction to free $NN$ cross section with parameters calibrated to the calculated results have been provided. Using these formulas, the transverse and elliptic flows of emitted nucleons in heavy-ion collisions are studied within the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model. We find the medium effect of the cross section from the $G$ matrix contributes to the transverse and elliptic flows oppositely to and obviously smaller than that from the effective mass. In addition, the present microscopic in-medium cross sections can significantly revise the predictions of the transverse and elliptic flows compared to the in-medium cross sections previously adopted in transport models for heavy-ion collisions.

Scaling behaviors of heavy flavor meson suppression and flow in different nuclear collision systems at the LHC

We explore the system size dependence of heavy-quark-QGP interaction by studying the heavy flavor meson suppression and elliptic flow in Pb–Pb, Xe–Xe, Ar–Ar and O–O collisions at the LHC. The space-time evolution of the QGP is simulated using a (3+1)-dimensional viscous hydrodynamic model, while the heavy-quark-QGP interaction is described by an improved Langevin approach that includes both collisional and radiative energy loss inside a thermal medium. Within this framework, we provides a reasonable description of the D meson suppression and flow coefficients in Pb–Pb collisions, as well as predictions for both D and B meson observables in other collision systems yet to be measured. We find a clear hierarchy for the heavy meson suppression with respect to the size of the colliding nuclei, while their elliptic flow coefficient relies on both the system size and the geometric anisotropy of the QGP. Sizable suppression and flow are predicted for both D and B mesons in O–O collisions, which serve as a crucial bridge of jet quenching between large and small collision systems. Scaling behaviors between different collision systems are shown for heavy meson suppression factor and the bulk-eccentricity-rescaled heavy meson elliptic flow as functions of the number of participant nucleons in heavy-ion collisions.

Stopped Nucleons in Configuration Space

In this note, using the colour string model, we study the configuration space distribution of stopped nucleons in heavy-ion collisions. We find that the stopped nucleons from the target and the projectile end up separated from each other by the distance increasing with the collision energy. In consequence, for the center of mass energies larger than 6 or 10 GeV (depending on the details of the model) it appears that the system created is not in thermal and chemical equilibrium, and the net baryon density reached is likely not much higher than that already present in the colliding nuclei.

Collective flow and correlation with higher harmonic event planes

Abstract Azimuthal event anisotropy and particle correlation have been used to analyze the collectivity of the system created in the high-energy heavy-ion collisions in order to study the properties of Quark Gluon Plasma (QGP). Higher harmonic event anisotropy is recently recognized to carry the information of initial participant geometrical fluctuation because of the finite number of participating nucleons in heavy-ion collisions. The system response after the collective expansion can be observed as higher harmonic event anisotropy, the n-th harmonic order dependence can be used to further constrain the hydro-dynamical properties of the system. The multi-particle azimuthal correlation with respect to the higher harmonic event plane can be used as a tool to understand the origin of the higher harmonic event anisotropy and its relation to the medium response from the jet-quenching as soft-hard interplay. Recent results on the higher harmonic event anisotropy measurements and an attempt of two-particle correlation analysis with respect to the higher harmonic event planes are discussed.

Comissioning and calibration of the Zero Degree Calorimeters for the ALICE experiment

The Zero Degree Calorimeters (ZDCs) for the ALICE experiment will measure the energy of the spectator nucleons in heavy ion collisions at the CERN LHC. Since all the spectator nucleons have the same energy, the calorimeter reponse is proportional to their umber providing a direct information on the centrality of the collision. Two sets of ZDCs are located at opposite sides with respect to the interaction point (I), 116 m away from it Each set consists of a neutron (ZN) calorimeter, placed between the two beam pipes, an a proton (ZP) calorimeter, posioned externally to the outgoing beam pipe. The ZDCs spaghetti calorimeters, which detect the Cherenkov light produced by the shower particle in silica optical fibers embedded in a dense absorber. In summer 2007 the ZN and ZP calorimeters have been placed on a movable platform and then installed in the LHC tunnel. The results of the commissioning studies and in particular the solutions adopted to control the stability of the PMTs response will be shown: light injection with a larser diode and cosmic rays. The foreseen calibration with e.m. dissociation event in Pb-Pb collisions will also be disussed. Finally the it meaurements carried out during the commissining in the LHC tunnel will be presentend.

Entrance-Channel Dependence of Isospin Effects on Double n/p Ratio in HIC

Within the hadronic transport model IBUU04, we investigate the effect of density-dependent symmetry energy on double neutron/proton (n/p) ratio of free nucleons in heavy ion collisions by taking four isotopic Sn+Sn reaction systems. Especially the entrance-channel asymmetry and impact-parameter dependence of the effect of symmetry energy are discussed. It is found that in both central and semi-central collisions the sensitivity of the double n/p ratio to the density-dependent symmetry energy is more pronounced in neutron-richer systems. Our results also indicate clearly that the effect of symmetry energy is stronger in central collisions than that in semi-central collisions.

The zero degree calorimeters for the ALICE experiment

The Zero Degree Calorimeters (ZDC) for the ALICE experiment will measure the energy of the spectator nucleons in heavy ion collisions at the CERN LHC, providing a direct measure of the centrality of the collisions. ZDC are spaghetti calorimeters, which detect the Cherenkov light produced by the shower particles in silica optical fibers embedded in a dense absorber. The main characteristics of the ZP and ZN detectors are described in this article. The calorimeters were tested at the CERN SPS using pion and electron beams with momenta ranging from 50 to 200 GeV / c . Test beam results such as the calorimeter response, the energy resolution, the signal uniformity and the localizing capability are presented.

Performance of the Zero Degree Calorimeters for the ALICE Experiment

The zero degree calorimeters (ZDCs) for the ALICE experiment will measure the energy of the spectator nucleons in heavy ion collisions at the CERN LHC. The dipole magnets of the LHC beam optics will separate the ion beams from the spectator protons; the spectator protons and neutrons will be respectively detected by the proton (ZP) and the neutron (ZN) calorimeters. Since all the spectator nucleons have the same energy, the calorimeter response is proportional to their number, providing a direct information on the centrality of the collision. ZDCs are spaghetti calorimeters, which detect the Cherenkov light produced by the shower particles in silica optical fibers embedded in a dense absorber. The technical characteristics of ZP and ZN detectors are described. The calorimeters have been tested at the CERN SPS using pion and electron beams with momenta ranging from 50 to 200 GeV/c; the ZN detector behaviour has also been studied with an indium beam of 158 GeV/c per nucleon. The beam test results are presented: the calorimeters response, the energy resolution and the localizing capability. Also the signal uniformity and a comparison between the transverse profile of the hadronic and electromagnetic shower are discussed. Moreover the differences between the ZP detector responses to protons and pions of the same energy have been investigated, exploiting the proton contamination in the positive pion beams.

Status of the construction and performances of the neutron Zero Degree Calorimeters of the ALICE experiment

The details of the construction of the neutron Zero Degree Calorimeters (ZN) of the ALICE Experiment, as well as their performances, will be presented. These spaghetti calorimeters will measure the energy lost by spectator nucleons in heavy-ion collisions. They are made of an absorber (tungsten alloy) filled with silica fibers, in which the charged particles of the shower produce Cherenkov light. The final neutron calorimeters have been built and their performances studied at the CERN SPS using pion and positron beams with momentum ranging from 50 to 150 GeV/ c . The main features like linearity of the response and resolution as a function of energy will be presented.

Thermodynamics from three-dimensional many-body fragmentation simulations on a cellular automaton model

The thermal equilibrium of many-body systems subject to finite range interactions is investigated numerically, by means of a multipurpose 3D cellular automaton dynamic model developed by the authors. The numerical experiments, carried out at fixed number of bodies, volume and energy, demonstrate the formation of an equilibrium among 3D aggregates of bodies. The distribution of the aggregates against size obeys a power law of (negative) exponent tau approximately 2.2 (against 1.3 in 2D). Our experiments, indicating that the exponent is insensitive to the precise parameter values and the precise parametrization of the interactions, are consistent with the idea of the existence of a universality class corresponding to the thermal equilibrium. The numerical value for the exponent tau is in agreement with the theoretical thermal equilibrium analyses based on various other approaches, numerical and semianalytical, indicating that the cellular automaton approach provides an adequate methodology to investigate thermal equilibria. In this paper, as an illustration of this method, we refer to the problem of formation of clusters of nucleons in heavy ion collisions of nuclei leading on to fragmentation. The theoretical tau value, however, corresponding to the thermal equilibrium among the aggregation clusters, is 15 percent lower than the empirical value ( approximately 2.6 ) , as measured in laboratory nuclear fragmentation experiments induced by collision. There is then only a very approximate correspondence between the experimental and the thermal equilibrium value. On the basis of the results of this paper and of a previous paper of this series, we conjecture that the approximate agreement is due to a partial establishment of a thermodynamic equilibrium during the collision of the nuclei. The thermal equilibrium gives the main contribution to the observed tau value; the deviation from this possibly universal value is largely the consequence of the lack of full thermal equilibrium in actual laboratory experiments. This conjecture is extended to interpret the observed ubiquity of power laws of exponents exceeding 2.2, which refer to the distribution of various types of matter in 3D space.

Status of the construction and performances of the neutron Zero Degree Calorimeters of the ALICE experiment

The details of the construction of the neutron Zero Degree Calorimeters (ZN) of the ALICE Experiment, as well as their performances, will be presented. These spaghetti calorimeters will measure the energy lost by spectator nucleons in heavy-ion collisions. They are made of an absorber (tungsten alloy) filled with silica fibers, in which the charged particles of the shower produce Cherenkov light. The final neutron calorimeters have been built and their performances studied at the CERN SPS using pion and positron beams with momentum ranging from 50 to 150 GeV/ c . The main features like linearity of the response and resolution as a function of energy will be presented.

Off-shell effects on particle production

We investigate the observable effects of off-shell propagation of nucleons in heavy-ion collisions at SIS energies. Within a semiclassical Boltzmann-Uehling-Uhlenback transport model we find a strong enhancement of subthreshold particle production when off-shell nucleons are propagated.

Contribution of pi N--> Lambda K to subthreshold kaon production in heavy-ion collisions.

We have studied kaon production from the interaction of pions with nucleons in heavy-ion collisions at energies below the threshold for kaon production in the nucleon-nucleon interaction in free space. It is found that this contribution is less than 25% of the total kaon yield. The main contribution to kaon production is from baryon-baryon interactions and its magnitude depends sensitively on the nuclear equation of state at high densities.

Emission of prompt nucleons in heavy ion collisions

We describe a simple dynamical model for the production of prompt nucleons at the very beginning of a heavy ion collision. Approximations often used in previous calculations (neglect of Pauli blocking, of window velocity and of its extension) have been checked. We apply the model to the calculation of prompt emitted neutrons in incomplete fusion reactions observed on the20Ne+165Ho system at 220, 402 and 700 MeV. Results of the model are extensively discussed.

Shortening of the nucleon's mean free path in heavy ion collisions

The mean free path of nucleons in heavy ion collisions is most essential for the development of nuclear collective phenomena. We discuss the effect of the nucleon Fermi motion in nuclei for shortening the mean free path. This Fermi motion together with the prior Pauli effect makes nuclei nontransparent for heavy ions in the medium energy domain.

Shortening of the nucleon's mean free path in heavy ion collisions

The mean free path of nucleons in heavy ion collisions is most essential for the development of nuclear collective phenomena. We discuss the effect of the nucleon Fermi motion in nuclei for shortening the mean free path. This Fermi motion together with the prior Pauli effect makes nuclei nontransparent for heavy ions in the medium energy domain.

Three-body model for nucleon transfer and emission in heavy-ion collisions

Transfer and emission of nucleons in heavy-ion collisions are investigated by modelling the reaction dynamics as a particle in the field of two potentials moving along given classical trajectories. In a Faddeev-like formulation the probabilities for various reaction channels are calculated in dependence on bombarding energy, impact parameter, and initial nucleon binding energy, using single-term separable potentials and straight-line trajectories.

Coupled-reaction-channel approach for molecular orbital phenomena of valence nucleons in heavy-ion collisions: The 12c + 13c system

A complete coupled-reaction-channel method is applied to molecular orbital phenomena of the valence nucleon in the scattering of 12 C on 13 C, i.e. in elastic and inelastic transitions to the single-particle states of 13 C ∗ (gr., 1 2 − ; 3.086 MeV, 1 2 + ; 3.854 MeV, 5 2 + ) with consi 12 C core-exchange process. The system is analyzed with respect to the occurrence of higher-order transitions of the direct (inelastic) and transfer (core-exchange) processes of the valence nucleon in 13 C. It is found in the states of positive total parity that the transfer and direct processes act additively and lead to extreme higher-order steps of the multi-interactions, if the ground state (1p 1 2 ) and the excited states (2s 1 2 and 1d 5 2 ) are taken into account. Transitions between the excited subchannels show strong-coupling effects, which are due to the reorientations of the d 5 2 orbitals. The reoriented system couples to the ground channel ( l -parity mixing) so favourably that the multi-interaction series hardly converges. The consequence of these observations are discussed in terms of the formation of molecular nucleon orbitals based on the polarization and the hybridization of the original orbitals.

Influence of inertia effects on the transfer of nucleons in heavy ion collisions

The effects induced by the presence of an inertia term in the collective description of mass transfer are analysed in the framework of a transport model. The model is applied to the study of the collision of S on Ge. The transfer regime is shown to be quasi-oscillatory. The main differences with the overdamped regime are described. The implication of the presence of inertia terms on physical observables like differential cross sections is discussed.