Processes In Heavy-ion Collisions Research Articles

Radial and Tangential Friction Coefficients: Derivation of Formulas, Calculation and Application in Modeling of the Heavy Ion Collision Process

Radial and Tangential Friction Coefficients: Derivation of Formulas, Calculation and Application in Modeling of the Heavy Ion Collision Process

Transient Joule- and (ac) Josephson-like photon emission in one- and two- nucleon tunneling processes between superfluid nuclei: Blackbody and coherent spectral functions

Effective charged neutrons involved in one- and two- nucleon tunneling processes in heavy ion collisions between superfluid nuclei are expected to emit photons. Although the centroid, width and integrated energy area characterizing the associated gamma-strength functions are rather similar, the corresponding line shapes reflect the thermal equilibrated-like character of the quasiparticle transfer (1n-channel, blackbody spectral functional dependence), and the quantal coherent character of the Cooper pair transfer (2n-channel, Gaussian functional dependence) respectively. The predicted angular distributions, polarizations and analyzing power provide further insight into the profoundly different physics to be found at the basis of what can be considered a transient Joule-like and a (ac) Josephson-like nuclear processes

Photoproductions of dileptons, photons, and light vector mesons in p−p , p−Pb , and Pb−Pb collisions at Large Hadron Collider energies

The photoproduction of large $p_{T}$ dileptons, photons and light vector mesons in p-p, p-Pb and Pb-Pb collisions at LHC energies is studied, where the fragmentation processes and the ultra-incoherent photon channel are included. An exact treatment is developed for photoproduction processes in heavy ion collisions, which recovers the equivalent photon approximation (EPA) in the limit $Q^{2}\rightarrow0$ and can avoid double counting effectively. The full kinematical relations are also achieved. We present the results as the distributions in $Q^{2}$, $p_{T}$ and $y_{r}$, the total cross sections are also estimated. The numerical results indicate that: the contribution of photoproduction processes is evident in the large $p_{T}$ and $y_{r}$ regions, and starts to play a fundamental role in p-Pb collisions. The ultra-incoherent photon emission is a important channel of photoproduction processes, which can provide the meaningful contributions. EPA is only applicable in small $y$ and $Q^{2}$ domains, and is very sensitive to the values of $y_{\mathrm{max}}$ and $Q^{2}_{\mathrm{max}}$. The EPA errors appear when $y>0.29$ and $Q^{2}>0.1~\mathrm{GeV}^{2}$, and are rather serious in p-Pb and Pb-Pb collisions. When dealing with widely kinematical regions, the exact treatment needs to be adopted.

Monte Carlo Based Glauber Model in Pb-Pb Collisions at LHC

Glauber model is a model used to describe heavy ion collision process. In this paper, we describe an implementation of a Monte Carlo based Glauber Model calculation used for the PHOBOS experiment, which simulates the collision of two heavy nuclei. The nucleon distribution in two initial Pb nuclei is parameterized with Woods-Saxon. The energy profile is √(S_NN )=5.5TeV and inelastic cross-section area, i.eσ_NN= 72mb. Geometric quantities such as impact parameter(b), the participant elliptical and triangular eccentricities (ε₂, ε₃) are studied and presented graphically and quantitatively in this paper. The results for collisions of Pb-Pb are compared with other Glauber model calculations which agrees with the results from other studies to a great extent. In this simulation, a decline trend of ε₂ and ε₃ is observed with increasing Npart. The centralities of peripheral events (40 to 50 percent centrality) are ε₂ ranging from 0.5 to 0.4 and ε₃ being roughly 0.3. The ratio of ε₃ to ε₂ is roughly 0.5 at Npart=0 and 1 at Npart=416 (a hundred percent centrality), showing the significance of triangular flow of the area of intersection. The model can also be used to predict geometric quantities in the LHC experiments and other collision processes.

Lepton pair production through two photon process in heavy ion collisions

This paper investigates the electromagnetic production of lepton pairs with low transverse momentum in relativistic heavy ion collisions. We estimate the initial photons' transverse momentum contributions by employing models where the average transverse momentum squared of the incoming photon can be calculated in the equivalent photon approximation. We further derive an all order QED resummation for the soft photon radiation, which gives an excellent description of the ATLAS data in ultra-peripheral collisions at the LHC. For peripheral and central collisions, additional $p_T$-broadening effects from multiple interaction with the medium and the magnetic field contributions from the quark-gluon plasma are also discussed.

Simulating V+jet processes in heavy ion collisions with JEWEL

Processes in which a jet recoils against an electroweak boson complement studies of jet quenching in heavy ion collisions at the LHC. As the boson does not interact strongly it escapes the dense medium unmodified and thus provides a more direct access to the hard scattering kinematics than can be obtained in di-jet events. First measurements of jet modification in these processes are now available from the LHC experiments and will improve greatly with better statistics in the future. We present an extension of JEWEL to boson-jet processes. JEWEL is a dynamical framework for jet evolution in a dense background based on perturbative QCD, that is in agreement with a large variety of jet observables. We also obtain a good description of the CMS and ATLAS data for y+jet and Z+jet processes at 2.76 TeV and 5.02 TeV.

Dipolar degrees of freedom and isospin equilibration processes in heavy ion collisions

Background: In heavy ion collision at the Fermi energies Isospin equilibration processes occur- ring when nuclei with different charge/mass asymmetries interacts have been investigated to get information on the nucleon-nucleon Iso-vectorial effective interaction. Purpose: In this paper, for the system 48Ca +27 Al at 40 MeV/nucleon, we investigate on this process by means of an observable tightly linked to isospin equilibration processes and sensitive in exclusive way to the dynamical stage of the collision. From the comparison with dynamical model calculations we want also to obtain information on the Iso-vectorial effective microscopic interaction. Method: The average time derivative of the total dipole associated to the relative motion of all emitted charged particles and fragments has been determined from the measured charges and velocities by using the 4? multi-detector CHIMERA. The average has been determined for semi- peripheral collisions and for different charges Zb of the biggest produced fragment. Experimental evidences collected for the systems 27Al+48Ca and 27Al+40Ca at 40 MeV/nucleon used to support this novel method of investigation are also discussed.

Study of the freeze-out process in heavy ion collisions at relativistic energies

In this work, we investigate the freeze-out process in heavy ion collisions at different relativistic energies. We present a study of standard blast-wave fits and Tsallis blast-wave fits performed to the transverse momentum spectra obtained in Au+Au collisions at RHIC energies. In addition, comparisons with simulated heavy ion collisions data using the UrQMD code will be presented to provide more detailed insight into the properties of the space-time evolution such as collective dynamics of the dense matter.

The evolution from quasi-elastic to deep inelastic processes and its connection to fusion

Substantial experimental and theoretical efforts have been made in the last decade to study into detail the transition from quasi-elastic to deep inelastic processes in heavy ion collisions. The understanding of this transition is relevant not only to probe which degrees of freedom play a role in the multinucleon transfer process but also for its connection with near barrier fusion reactions. A boost in the field has been given with the implementation of large solid angle magnetic spectrometers coupled to γ arrays with which extensive investigations have been carried out for nuclear structure and reaction dynamics. In the present paper aspects of these studies will be presented, with emphasis on processes undergoing significant energy loss in binary collisions.

Strange hadron ratios from quark coalescence at RHIC and LHC energies

Quark coalescence models have been applied successfully to reproduce measured hadron production data in relativistic heavy ion collisions at SPS and RHIC energies, which strongly supports the formation of deconfined quark matter in these collisions. The investigation of meson and baryon production is an ideal tool to understand dynamical details of hadronization, especially strange hadron numbers and ratios. We display latest results on the production of strange particles in quark coalescence processes in heavy ion collisions at different collider energies.

Study of bulk properties of QCD matter by the relativistic heavy ion collisions

We review the present understandings of the bulk properties of strong interacting matter obtained from Relativistic Heavy Ion collision processes and the origin of collective flow. We discuss some open questions in the hydrodynamical approach to these processes.

CMS Physics Technical Design Report, Volume II: Physics Performance

CMS is a general purpose experiment, designed to study the physics of pp collisions at 14 TeV at the Large Hadron Collider (LHC). It currently involves more than 2000 physicists from more than 150 institutes and 37 countries. The LHC will provide extraordinary opportunities for particle physics based on its unprecedented collision energy and luminosity when it begins operation in 2007.The principal aim of this report is to present the strategy of CMS to explore the rich physics programme offered by the LHC. This volume demonstrates the physics capability of the CMS experiment. The prime goals of CMS are to explore physics at theTeV scale and to study the mechanism of electroweak symmetry breaking—through the discovery of the Higgs particle or otherwise. To carry out this task, CMS must be prepared to search for new particles, such as the Higgs boson or supersymmetric partners of the Standard Model particles, from the start-up of the LHC since new physics at theTeV scale may manifest itself with modest data samples of the order of a few fb−1 or less.The analysis tools that have been developed are applied to study in great detail and with all the methodology of performing an analysis on CMS data specific benchmark processes upon which to gauge the performance of CMS. These processes cover several Higgs boson decay channels, the production and decay of new particles such as Z′ and supersymmetric particles, Bs production and processes in heavy ion collisions. The simulation of these benchmark processes includes subtle effects such as possible detector miscalibration and misalignment. Besides these benchmark processes, the physics reach of CMS is studied for a large number of signatures arising in the Standard Model and also in theories beyond the Standard Model for integrated luminosities ranging from 1 fb−1 to 30 fb−1. The Standard Model processes include QCD, B-physics, diffraction, detailed studies of the top quark properties, and electroweak physics topics such as the W and Z0 boson properties. The production and decay of the Higgs particle is studied for many observable decays, and the precision with which the Higgs boson properties can be derived is determined. About ten different supersymmetry benchmark points are analysed using full simulation. The CMS discovery reach is evaluated in the SUSY parameter space covering a large variety of decay signatures. Furthermore, the discovery reach for a plethora of alternative models for new physics is explored, notably extra dimensions, new vector boson high mass states, little Higgs models, technicolour and others. Methods to discriminate between models have been investigated.This report is organized as follows. Chapter 1, the Introduction, describes the context of this document. Chapters 2–6 describe examples of full analyses, with photons, electrons, muons, jets, missing ET, B-mesons and τ's, and for quarkonia in heavy ion collisions. Chapters 7–15 describe the physics reach for Standard Model processes, Higgs discovery and searches for new physics beyond the Standard Model.

Resonance Multiplicities in a Hadron Transport Approach

In this article we focus on the multiplicities of resonances, ratios of resonant over non-resonant states and rescattering processes in heavy ion collisions. Therefore we utilize a hadron transport model (UrQMD vl.3). We find that rescattering of decay particles is of great importance when studying resonances in a hadronic medium.

Studying charm quark elliptic flow via single electron measurements

The observed scaling of elliptic flow (v2) with the number of constituent quarks for mesons and baryons in Au+Au collisions at RHIC energies has theoretically been described through hadron production via quark coalescence from a thermalized parton system. Within the framework of quark coalescence, the elliptic flow of individual quark flavours can be deduced from v2 measurements of identified hadrons. While the light quarks are predominantly created in soft collisions, heavy quarks are believed to be created in hard collisions and are not expected to carry flow at the time of their production. An analysis of the elliptic flow of charmed hadrons, e.g. D-mesons, might therefore not only reveal information about charm quark flow, but also about hadronization and thermalization processes in heavy ion collisions. As direct v2 measurements of charmed hadrons are not available yet, single electron v2 at sufficiently high transverse momenta can serve as a substitute. The production of single electrons from non-photonic sources is expected to be dominated by the decay of D-mesons at transverse momenta between 2 GeV/c and 5 GeV/c. Simulations show a strong correlation between the flow of the D-meson and its decay electron in this momentum range. We will present preliminary STAR results from our single electron v2 measurements from Au+Au collisions at RHIC energies and compare with predictions from quark coalescence models.

Topological String in Quantum-Chromodynamical Chiral PhaseTransitions

It is pointed out that if in heavy ion collision processes, thequark–gluon plasma SU(2) chiral phase transition really takes placeand the phase transition is a second order. Then the topological string,i.e., the π string, will be formed. The main effect of thisphenomenon is that there will be a number of pions produced by decay ofthe π string in the final state. The pions from the decay of theπ string lead to the same effect of decreasing theHanbury–Brown–Twiss peak in two-pion spectra which is just as that ofthe long-lived hadronic resonances. At relativistic heavy-ion collisionand large hadron collision energies, it is expected that thefactors are about α~0.7−0.9 and α~0.6−0.85,respectively.

Suggesting a di-omega dibaryon search in heavy ion collision experiments

The structure of a new dibaryon $(\ensuremath{\Omega}\ensuremath{\Omega}{)}_{{0}^{+}}$ is studied theoretically in the framework of the chiral SU(3) quark model by solving a resonating group method equation. The binding energy of this dibaryon is predicted to be around 100 MeV, the mean-square root of the distance between two \ensuremath{\Omega}'s is 0.84 fm, and the preliminary estimated mean lifetime is about two times that of the free $\ensuremath{\Omega}$'s. All these interesting properties, as well as the two negative charge units it carries, could make it easily identifiable experimentally in the heavy ion collision process. The production probability of this new dibaryon in a 158 GeV $\mathrm{Pb}+\mathrm{Pb}$ collision is estimated using the thermal model. The rate is of the order of ${10}^{\ensuremath{-}6}$ to ${10}^{\ensuremath{-}5}$ per event. It is expected that, with the increase of the temperature, the production rate will also be increased.

Two-body correlation transport theory for heavy ion collision

Abstract A two-body correlation transport theory (TBCTT) for describing the dynamical process in heavy ion collisions has been proposed by means of coupling the equation of motion for coherent single particle (representation basis) states with two-body correlation dynamics. This model contains time-dependent mean field effects, two-body correlations to all orders of interactions as well as fermionic (antisymmetric) effects. It is capable of describing the time-evolution of nonuniform nuclear matter including dynamical fluctuations and dynamical formation of fragments in heavy ion collisions. The preliminary numerical results show that this model is promising for the description of heavy ion collision dynamics.

Dissipative processes in an expanding massive gluon gas

The temperature dependence of the kinetic coefficients is obtained in the nonperturbative region with the help of Green-Kubo-type formulae in the model of massive gluon gas motivated by numerical results from simulations of lattice QCD. The entropy production rate is estimated using scaling hydrodynamics. It is shown that the increase in the viscosity coefficients leads to entropy generation in heavy ion collision processes which could be big, especially for temperatures close to the critical one.

Collective kinetic energy in the heavy ion collision process

Collective kinetic energy in the heavy ion collision process

Quantum molecular dynamics — A model for nucleus-nucleus collisions from medium to high energies

The thermalisation process in heavy ion collisions is investigated employing the Quantum Molecular Dynamics approach. The validity of thermodynamical quantities, e.g. density, temperature and pressure are discussed. The investigation of the local degree of equilibrium leads to a three component picture, where in addition to the projectile and target a “fireball” component is defined, which includes all those nucleons which have suffered at least one hard collision. Local equilibration is observed in all of those components at all times. The dynamical evolution of the reaction is dominated by the interaction of both the projectile and the target with this fireball component and not, as was assumed in the two fluid model, by the interaction of projectile and target with each other. The structure of the stress tensor can be cast in a Newtonian form only for low energies. Then the values of the shear viscosity are in agreement with those values used in present viscous hydrodynamical calculations.