Dissipative processes in peripheral heavy ion collisions at Fermi energies

Peripheral heavy ion reactions are of interest for the production of new isotopes. In this contribution we present an investigation of reactions of light projectiles O and Ne on Be and Ta targets at Fermi energies in the framework of transport theory. Transport theory describes dissipative (deep-inelastic) processes, where considerable amounts of mass and energy are exchanged. The data, on the other hand, also seem to include a more direct component with small energy loss. We separate the two components on the basis of the velocity distribution and compare the dissipative component to the transport calculations. The primary fragments of the transport calculation still have considerable excitation energies. For the comparison with experiment we take into account the secondary evaporation in a statistical model. This improves the qualitative agreement with the data.     

Isospin effect in peripheral heavy-ion collisions at Fermi energies

The isospin effect in peripheral heavy-ion collisions was thoroughly investigated within the framework of the Lanzhou quantum molecular dynamics (LQMD) transport model. A coalescence approach was used to recognize the primary fragments formed in nucleus-nucleus collisions. The secondary decay process of these fragments was described using the statistical code GEMINI. The production mechanism and isospin effect of the projectile-like and target-like fragments were analyzed using the combined approach. It was found that the isospin migration from the high-isospin density to the low-density matter occurred in the neutron-rich nuclear reactions, i.e., ⁴⁸Ca+²⁰⁸Pb, ⁸⁶Kr+⁴⁸Ca/²⁰⁸Pb/¹²⁴Sn, ¹³⁶Xe+²⁰⁸Pb, ¹²⁴Sn+¹²⁴Sn, and ¹³⁶Xe+¹³⁶Xe. A hard symmetry energy was available for creating the neutron-rich fragments, particularly in the medium-mass region. The isospin effect of the neutron-to-proton (n/p) ratio of the complex fragments was reduced when the secondary decay process was included. However, a soft symmetry energy enhanced the n/p ratio of the light particles, particularly at kinetic energies greater than 15 MeV/nucleon.     

Enhanced production of neutron-rich rare isotopes in peripheral collisions at Fermi energies

A large enhancement in the production of neutron-rich projectile residues is observed in the reactions of a 25 MeV/nucleon 86Kr beam with the neutron-rich 124Sn and 64Ni targets relative to the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to the reaction with the less neutron-rich 112Sn target. A hybrid model based on a deep-inelastic transfer (DIT) code followed by a statistical deexcitation code accounts for part of the observed large cross sections. The DIT simulation indicates that the production of neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange in which the neutron skins of the neutron-rich 124Sn and 64Ni target nuclei may play an important role. From a practical viewpoint, such reactions offer a novel synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.     

Intermediate mass fragment emission pattern in peripheral heavy-ion collisions at Fermi energies

The emission pattern in the v(perp)-v(par) plane of intermediate mass fragments with Z = 3--7 (IMF) has been studied in the collision 116Sn+ 93Nb at 29.5A MeV as a function of the total kinetic energy loss of the reaction. This pattern shows that for peripheral reactions most IMF's are emitted at velocities intermediate between those of the projectile- and target-like products. Coulomb trajectory calculations show that these IMF's are produced in the interaction zone in a short time interval at the end of the target-projectile interaction.     

Photon-photon luminosities in relativistic heavy ion collisions at LHC energies

Effective – luminosities are calculated for various realistic hadron collider scenarios. The main characteristics of photon-photon processes at relativistic heavy-ion colliders are established and compared to the corresponding –-luminosities ate ⁺-e ^– — and future photon linear colliders (PLC). Higher order corrections as well as inelastic processes are discussed. It is concluded that feasible high luminosity Ca–Ca collisions at the Large Hadron Collider (LHC) are an interesting option for – physics up to about 100 GeV – CM energy.     

Isotope analysis in central heavy ion collisions at intermediate energies

Symmetry energy is a key quantity in the study of the equation of state of asymmetric nuclear matter. Heavy ion collisions at low and intermediate energies, performed at Laboratori Nazionali di Legnaro and Laboratori Nazionali del Sud, can be used to extract information on the symmetry energy coefficient C_sym, which is currently poorly known but relevant both for astrophysics and for deeper knowledge of the structure of exotic nuclei.     

Theoretical analysis of dilepton spectra in heavy ion collisions at GSI-FAIR energies

This paper addresses the theoretical analysis of dilepton spectra in C+C collisions at GSI-SIS energies. Theoretical predictions for the recent data of the HADES Collaboration at SIS energies are made with the help of a hadron-string transport model, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. A mass shift of the ρ meson due to kinematical effects is discussed.     

Asymmetry of velocity distributions in peripheral reactions with heavy ions at Fermi energies

The asymmetry of velocity distributions of projectile-like fragments produced in heavy ion collisions is considered. The calculations performed in the transport model approach (Vlasov kinetic equation with the collision term) are compared with the experimental data for the ²²Ne (40MeV/nucleon) + ⁹Be and ¹⁸O (35 MeV/nucleon) + ⁹Be(¹⁸¹Ta) reactions. It is found that the velocity distributions contain two components: a direct component centered at the beam velocity and a dissipative component at lower energies, leading to asymmetry of velocity distributions. The direct component is interpreted empirically within the Goldhaber model, and the centroids and widths σ₀ of the distributions for each fragment are extracted. It is found that value of σ₀ derived from experimental data is smaller by a factor of 2 than the theoretical one. The dissipative (also called deep inelastic) component is described well by the transport calculations. It is shown that the ratio of yields of direct and dissipative components, which determines the asymmetry of velocity distributions, depends on shape of the deflection function. Original Russian Text ? T.I. Mikhailova, B. Erdemchimeg, G. Kaminski, A.G. Artyukh, M. Colonna, M. Di Toro, I.N. Mikhailov, Yu.M. Sereda, H.H. Wolter, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 6, pp. 898–903.     

Radion and Higgs signals in peripheral heavy ion collisions at the LHC

We investigate the sensitivity of the heavy ion mode of the LHC to Higgs boson and Radion production via photon–photon fusion through the analysis of the processes γγ→γγ, , and γγ→gg in peripheral heavy ion collisions. We suggest cuts to improve the Higgs and Radion signal over standard model background ratio and determine the capability of LHC to detect these particles production.     

Photoproduction of top quarks in peripheral heavy ion collisions

In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs?cker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the Large Hadron Collider (LHC) will be the first accessible non-hadronic top production channel. We calculate the photoproduction cross sections, pair mass and top quark rapidity distributions in peripheral lead-lead and oxygen-oxygen collisions. The cross sections are sensitive to the top quark charge and the large- gluon distribution in the nucleus. We find a cross section of 15 pb in oxygen-oxygen collisions, leading to 210 pairs in a one month ( s) LHC run. In pA collisions, the rate is higher, 1100 pairs per month for pO. A comparison of the AA and pA data might allow for a study of gluon shadowing at high Q . Received: 18 May 2001 / Revised version: 13 June 2001 / Published online: 24 August 2001     

One- and two-body dissipation in peripheral heavy ion collisions

For peripheral collisions of heavy ions we solve the many-body Schrödinger equation in second order time-dependent perturbation theory. The two nuclei interact via a two-body interaction of finite range. With controllable approximations we get to a sensible comparison between 1p?1h excitations caused by the coherent Hartree part and direct 2p?2h excitations both created by the same two-body interaction. The results of the calculation show that for peripheral collisions almost all excitation energy originates from one-body dissipation. Furthermore we encounter large virtual excitations during the collision indicating a non Markovian process.     

z-scaling in heavy ion collisions at the RHIC

Experimental data on transverse particle spectra obtained by the STAR, PHENIX, PHOBOS, and BRAHMS collaborations at the RHIC are analyzed in the framework of the generalized concept of z-scaling. It was developed for analysis of inclusive particle production in proton-(anti)proton collisions at high p _T and high multiplicities. The general scheme of the approach based on the physical principles of self-similarity, locality, and fractality is reviewed. Independence of the scaling function ψ(z) from energy, multiplicity, and atomic weight for h ^±, π ^±,0, K _S ⁰ , and Λ hadrons produced in Au-Au and Cu-Cu collisions at √s = 130 and 200 GeV is discussed. Based on z-scaling, the multiplicity dependence of pion transverse spectra up to p _T = 25 GeV/c in Au-Au collisions at √s = 200 GeV for experiments at the RHIC is predicted. The text was submitted by the author in English.     

Partonic effects in heavy ion collisions at RHIC

Effects of partonic interactions in heavy ion collisions at RHIC are studied in a multiphase transport model (AMPT) that includes both initial partonic and final hadronic interactions. It is found that a large parton scattering cross section is needed to understand the measured elliptic flow of pions and two-pion correlation function.     

Freeze-out in relativistic heavy ion collisions at SPS

The freeze-out conditions in the light (S+S) and heavy (Pb+Pb) colliding systems at 160 AGeV/c are analyzed within the microscopic Quark Gluon String Model (QGSM). Even for the most heavy systems particle emission takes place from the whole space-time domain available for the system evolution, but not from the thin “freeze-out hypersurface”, adopted in fluid dynamical models. Pions are continuously emitted from the whole volume of the reaction and reflect the main trends of the system evolution. Nucleons in Pb+Pb collisions initially come from the surface region. For both reactions there is a separation of the elastic and inelastic freeze-out.     

Coherent photon-photon interactions in very peripheral relativistic heavy ion collisions

Heavy ions at high velocities provide very strong electromagnetic fields for a very short time. The main characteristics of ultraperipheral relativistic heavy ion collisions are reviewed, characteristic parameters are identified. The main interest in ultraperipheral heavy ion collisions at relativistic ion colliders like the LHC is the interactions of very high energy (equivalent) photons with the countermoving (equivalent) photons and hadrons (protons/ions). The physics of these interactions is quite different from and complementary to the physics of the strong fields achieved with current and future lasers.     

Influence of In-medium properties of vector mesons on dilepton production in heavy ion collisions at SIS energies

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy ion collisions. They allow the study of meson properties, like mass and decay width, at various density and temperature regimes. Up to now, in the Tübingen model for dilepton production, modification of meson properties in nuclear medium has been accounted for by allowing a density dependence of the mass (Brown–Rho scaling) together with an ad hoc dependence of the meson decay widths on the same variable. We use the extended vector meson dominance (eVMD) model to extract meson properties in nuclear matter by computing the in-medium meson spectral functions. Dilepton spectra for C+C at 1.0 and 2.0 AGeV are calculated and compared with previous results.     

Hadronic rapidity spectra in heavy ion collisions at SPS and AGS energies in a quark combination model

The quark combination mechanism of hadron production is applied to nucleus-nucleus collisions at the CERN Super Proton Synchrotron (SPS) and the BNL Alternating Gradient Synchrotron (AGS). The rapidity spectra of identified hadrons and their spectrum widths are studied. The data of π^-, K^±, φ, Λ, Λ, Ξ^- and Ξ⁺ at 80 and 40 A GeV, and in particular at 30 and 20 A GeV where the onset of deconfinement is suggested to happen, are consistently described by the quark combination model. However, at AGS 11.6 A GeV below the onset, the π^±, K^± and Λ spectra cannot be simultaneously explained, indicating the disappearance of the intrinsic correlation of their production in the constituent quark level. The collision-energy dependence of the rapidity spectrum widths of the constituent quarks, and the strangeness of the hot and dense quark matter produced in heavy ion collisions, are obtained and discussed.     

γ-γ Physics with peripheral relativistic heavy ion collisions

The high flux of equivalent photons present in relativistic heavy ion collisions of two chargesZ ₁ andZ ₂ gives rise to the collision of two equivalent photons. The cross-sections for various processes are directly related to the correspondingγ- γ cross-sections. As compared toγ- γ physics being studied at e⁺ e^? colliders, we find that high energy states will not be so easily accessible at the existing facilities, however, the enhancement factor (Z ₁ Z ₂)² in the expression for the cross section will provide very large photon fluxes for lower energies.     

Two-pion interferometry in heavy ion collisions at HIRFL-CSR energy

We examine the two-pion Hanbury-Brown-Twiss (HBT) interferometry for the particle-emitting source produced in heavy ion collisions at HIRFL-CSR energy. The source evolution is described by relativistic hydrodynamics with three kinds of equations of state for chemical equilibrium (CE), chemical freeze-out (CFO),and partial chemical equilibrium (PCE) models, respectively. We investigate the effects of particle decay,multiple scattering, and source collective expansion on the two-pion interferometry results. We find that the HBT radii of the evolution source for the CFO and PCE models are smaller than that for the CE model.The HBT lifetime for the CFO model is smaller than those for the PCE and CE models. The particle decay increases the HBT radius and lifetime while the source expansion decreases the HBT radius. The multiple scattering effect on the HBT results can be neglected based on our model calculations.