Peripheral Heavy-ion Collisions Research Articles

Particle and light fragment emission in peripheral heavy ion collisions at Fermi energies

A systematic investigation of the average multiplicities of light charged particles and intermediate mass fragments emitted in peripheral and semiperipheral collisions is presented as a function of the beam energy, violence of the collision, and mass of the system. The data have been collected with the FIASCO setup in the reactions $^{93}\mathrm{Nb}+^{93}\mathrm{Nb}$ at $(17,23,30,38)A$ MeV and $^{116}\mathrm{Sn}+^{116}\mathrm{Sn}$ at $(30,38)A$ MeV. The midvelocity emission has been separated from the emission of the projectile-like fragment. This last component appears to be compatible with an evaporation from an equilibrated source at normal density, as described by the statistical code GEMINI at the appropriate excitation energy. On the contrary, the midvelocity emission presents remarkable differences in both the dependence of the multiplicities on the energy deposited in the midvelocity region and the isotopic composition of the emitted light charged particles.

Lepton-pair production from deep inelastic scatteringin peripheral relativistic heavy ion collisions

We calculate the inelastic electron- and muon-pair production in peripheral relativistic heavy ion collisions in the region of large Q 2 of one of the photons. This offers a possibility to study the quark distribution functions in ions in "ultraperipheral heavy ion collisions". The calculations are compared with those making use of the equivalent photon and the equivalent lepton approximation. We compare the results for Pb-Pb and Pb-p collisions at RHIC (γ ≈ 100) and LHC (γ ≈ 3000) energies. Furthermore we include nuclear modifications to the parton distribution functions in our calculations to study their effect on the cross sections.

Neutron tagging of quasielastic [formula omitted] photoproduction off nucleus in ultraperipheral heavy ion collisions at RHIC energies

We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisions in kinematics of Relativistic Heavy Ion Collider (RHIC) at sNN=200 GeV. Our improved estimate of the total coherent cross section is a factor of two smaller than the earlier ones. We find that the counting rate of quasielastic J/ψ photoproduction tagged by neutrons emitted due to cascading of the recoiled nucleon within the residual nucleus exceeds the rate of the coherent events. We argue that measurements of this process can be used to learn about the dynamics of color dipole–nucleon interactions in nuclei in the wide effective range of energies of γA interactions.

Nuclear modification factors of strange and multi-strange hadrons

Comparing data from central heavy-ion collisions to data from p+p or peripheral heavy-ion collisions can give us insight into the particle production mechanisms. The pT differential ratios we obtain, i.e., the ‘nuclear modification factors’, are the basis of many recent theoretical models which try to reproduce the shape and magnitude observed. We will show the first measurements of RAA and RdA for K±, K0S, Ω, Λ and Ξ STAR data from dAu and AuAu collisions at in order to review the applicability of some of these models and learn about the details of hadronic particle production.

Resolving multiple particles in a highly segmented silicon array

The design, construction, and performance of a new highly segmented charged particle detector array, FIRST, are described. This forward angle annular array ( 2 ∘ ⩽ θ lab ⩽ 28 ∘ ) has been developed to study peripheral and mid-central heavy-ion collisions at intermediate energies ( E / A ≈ 50 MeV ). FIRST consists of three individual telescopes that each utilize ion-passivated silicon detectors in either a Si(IP)–Si(IP)–CsI(Tl) stack or a Si(IP)–CsI(Tl) stack. This array provides elemental identification for 1 ⩽ Z ⩽ 50 with isotopic identification of lighter elements, Z ⩽ 13 , over a wide dynamic range in energy. The high segmentation of each silicon detector provides good angular resolution in a compact geometry and allows deconvolution of multiple particles incident on a single telescope. The performance of the array in a commissioning experiment Zn 64 + Zn 64 , Bi 209 at E / A = 45 MeV is shown.

Conditions for the manifestation of transient effects in fission

The conditions for the manifestation of transient effects in fission are carefully examined by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Experimental total nuclear fission cross sections of 238U at 1 A GeV on gold and uranium targets are compared with the predictions of a nuclear-reaction code, where transient effects in fission are modeled using different approximations to the numerical time-dependent fission-decay width: a new analytical description based on the solution of the Fokker–Planck equation and two widely used but less realistic descriptions, a step function and an exponential-like function. The experimental data are only reproduced when dissipation is considered. The influence of transient effects on the fission process, as well as the deduced value of the dissipation strength β, depends strongly on the approximation applied for the time-dependent fission-decay width. In particular, a meticulous analysis sheds severe doubts on the use of the exponential-like in-growth function. Finally, we investigate which should be the characteristics of experimental observables to be most sensitive to transient effects in fission. The pertinence of observables related to the excitation energy at saddle is discussed.

Probing the density dependence of the symmetry potential with peripheral heavy-ion collisions

The peripheral heavy-ion collisions of $^{112, 124}Sn+ ^{86}Kr$ at $E_{b}= 25AMeV$ are studied by means of the Improved Quantum Molecular Dynamics Model(ImQMD). It is shown that the slope of the average N/Z ratio of emitted nucleons vs impact parameters for these reactions is very sensitive to the density dependence of the symmetry energy. Our study also shows that the yields of $^{3}H$ and $^{3}He$ decrease with impact parameters and slope of the yield of $^{3}H$ vs impact parameters as well as the ratio of Y($^{3}H$)/Y($^{3}He$) depend on the symmetry potential strongly for peripheral heavy-ion collisions.

Energetics of Midvelocity Emissions in Peripheral Heavy Ion Collisions at Fermi Energies

Peripheral and semiperipheral collisions have been studied in the system 93Nb+93Nb at 38A MeV. The evaporative and midvelocity components of the light charged particle and intermediate mass fragment emissions have been carefully disentangled. In this way it was possible to obtain the average amount not only of charge and mass, but also of energy, pertaining to the midvelocity emission, as a function of an impact parameter estimator. This emission has a very important role in the overall balance of the reaction, as it accounts for a large fraction of the emitted mass and for more than half of the dissipated energy. As such, it may give precious clues on the microscopic mechanism of energy transport from the interaction zone toward the target and projectile remnants.

Transient effects in fission from new experimental signatures.

A new experimental approach is introduced to investigate the relaxation of the nuclear deformation degrees of freedom. Highly excited fissioning systems with compact shapes and low angular momenta are produced in peripheral relativistic heavy-ion collisions. Both fission fragments are identified in atomic number. Fission cross sections and fission-fragment element distributions are determined as a function of the fissioning element. From the comparison of these new observables with a nuclear-reaction code a value for the transient time is deduced.

Can we observe the radion and Higgs signals in peripheral heavy ion collisions?

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 <FONT FACE=Symbol>gg</FONT> -> <FONT FACE=Symbol>gg</FONT>, <FONT FACE=Symbol>gg</FONT> -> <img id="_x0000_i1026" src="../../img/revistas/bjp/v34n1a/a08img09.gif" align=absmiddle>, and <FONT FACE=Symbol>gg ® gg</FONT>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.

MODELLING OF DISSIPATION IN NUCLEAR FISSION

Peripheral heavy-ion collisions at relativistic energy are proposed as a new experimental approach dedicated to nuclear dissipation studies and, in particular, to investigate transient effects which are responsible for the inhibition of fission at the beginning of the process. To extract reliable information from the data, an analytical approximation of the time-dependent fission decay width is used in connection with new experimental signatures of relaxation effects.

Ultra peripheral heavy ion collisions and the energy dependence of the nuclear radius

To estimate realistic cross sections in ultra peripheral heavy ion collisions we must remove effects of strong absorption. One method to eliminate these effects make use of a Glauber model calculation, where the nucleon–nucleon energy dependent cross sections at small impact parameter are suppressed. In another method we impose a geometrical cut on the minimal impact parameter of the nuclear collision (bmin>R1+R2, where Ri is the radius of ion ‘i’). In this last case the effect of a possible nuclear radius dependence with the energy has not been considered in detail up to now. Here we introduce this effect showing that for final states with small invariant mass the effect is negligible. However when the final state has a relatively large invariant mass, e.g., an intermediate mass Higgs boson, the cross section can decrease up to 50%.

The Cronin effect, quantum evolution and the color glass condensate

We show that the numerical solution of the classical SU(3) Yang–Mills equations of motion in the McLerran–Venugopalan model for gluon production in central heavy ion collisions leads to a suppression at low pt and an enhancement at the intermediate pt region as compared to peripheral heavy ion and pp collisions at the same energy. Our results are compared to previous, color glass condensate inspired calculations of gluon production in heavy ion collisions. We revisit the predictions of the color glass condensate model for pA (dA) collisions in leading order and show that quantum evolution—in particular, the phenomenon of geometric scaling and change of anomalous dimensions—preserves the Cronin enhancement of pA cross section (when normalized to the leading twist term) in the leading order approximation even though the pt spectrum can change. We comment on the case when gluon radiation is included.

Fragmentation in peripheral heavy-ion collisions: from neck emission to spectator decays

Invariant cross sections of intermediate mass fragments in peripheral collisions of 197Au on 197Au at incident energies between 40 and 150 MeV per nucleon have been measured with the 4π multi-detector INDRA. The maximum of the fragment production is located near mid-rapidity at the lower energies and moves gradually towards the projectile and target rapidities as the energy is increased. Schematic calculations within an extended Goldhaber model suggest that the observed cross section distributions and their evolution with energy are predominantly the result of the clustering requirement for the emerging fragments and of their Coulomb repulsion from the projectile and target residues. The quantitative comparison with transverse energy spectra and fragment charge distributions emphasizes the role of hard scattered nucleons in the fragmentation process.

PHOTON EXCHANGE IN NUCLEUS–NUCLEUS COLLISIONS

The strong electromagnetic fields in peripheral heavy ion collisions give rise to photon–photon and photon–nucleus interactions. I present a general survey of the photon–photon and photon-hadron physics accessible in these collisions. Among these processes I discuss the nuclear fragmentation through the excitation of giant resonances, the Coulomb dissociation method for application in nuclear astrophysics, and the production of particles.

Hadronic physics in peripheral heavy Ion collisions

We discuss the production of hadronic resonances in very peripheral heavy ion collisions, where the ions collide with impact parameter larger than twice the nuclear radius and remain intact after the collision. We compare the resonance production through two-photon and double Pomeron exchange, showing that when we impose the condition for a peripheral interaction the $\gamma \gamma$ process dominates over the Pomeron interaction, due to the short range propagation of this last one. We also discuss the observation of light resonances through the subprocess $\gamma \gamma \to R \to \gamma \gamma $, which is a clean signal for glueball candidates as well as one way to check the existence of a possible scalar $\sigma$ meson.

Proton and α Sequential Emission in the 16 O + 58 Ni Deep Inelastic Reaction: A Semi-Classical Approach

The 132 MeV 16O + 58Ni reaction has been experimentally investigated by using coincident charged particle techniques. A closed-form theoretical approach, describing in a simple picture the non-equilibrium component and the evaporative one of the angular correlation between light particles and reaction residues emitted in a peripheral heavy-ion collision, is applied — in the hypotesis of a sequential process — to the (C,N,O)-α and (C,N,O)-p differential multiplicities for the 16O + 58Ni at 8.25 MeV/A deep inelastic collision. From this analysis some reaction mechanism information is deduced.

Electroproduction of charmonia off nuclei

In a recent publication we calculated elastic charmonium production in $\mathrm{ep}$ collisions, employing realistic charmonia wave functions and dipole cross sections, and found good agreement with the data in a wide range of s and ${Q}^{2}$. Using the ingredients from those calculations we calculate exclusive electroproduction of charmonia off nuclei. Here new effects become important, (i) color filtering of the $c\overline{c}$ pair on its trajectory through nuclear matter, (ii) dependence on the finite lifetime of the $c\overline{c}$ fluctuation (coherence length), and (iii) gluon shadowing in a nucleus compared to the one in a nucleon. Total coherent and incoherent cross sections for C, Cu, and Pb as functions of s and ${Q}^{2}$ are presented together with some differential cross sections. The results can be tested with future electron-nucleus colliders or in peripheral collisions of ultrarelativistic heavy ions.

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, γγ→b b ̄ , 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.

Fading out of J/ ψ color transparency in high energy heavy ion peripheral collisions

We provide predictions for the J/ ψ coherent production in the peripheral heavy ion collisions at LHC and RHIC using the leading twist model of nuclear shadowing based on the QCD factorization theorem for diffraction and the HERA hard diffraction data. We demonstrate that for LHC kinematics this model leads to a bump-shape distribution in rapidity which is suppressed overall as compared to the expectations of the color transparency regime by a factor ∼6. This is a significantly larger suppression than that expected within the impact parameter eikonal model. Thus we show that the interaction of spatially small wave package for which the total cross section of interaction with nucleons is small is still strongly shadowed by nuclear medium in high energy processes.