Freeze-out Dynamics Research Articles

QCD critical point and experimental observables

The presence of a critical point in the QCD phase diagram can deform the trajectories describing the evolution of the expanding fireball in the μB–T phase diagram. If the average emission time of hadrons is a function of transverse velocity, as microscopic simulations of the hadronic freeze-out dynamics suggest, the deformation of the hydrodynamic trajectories will change the transverse velocity (βT) dependence of the proton–antiproton ratio when the fireball passes in the vicinity of the critical point. An unusual βT-dependence of the ratio in a narrow beam energy window would thus signal the presence of the critical point.

Anisotropies in momentum space at finite shear viscosity in ultrarelativistic heavy-ion collisions

Within a parton cascade we investigate the dependence of anisotropies in momentum space, namely the elliptic flow v 2 = 〈 cos ( 2 ϕ ) 〉 and the v 4 = 〈 cos ( 4 ϕ ) 〉 , on both the finite shear viscosity η and the freeze-out (f.o.) dynamics at the RHIC energy of 200 GeV. In particular the impact of the f.o. dynamics is discussed looking at two different procedures: switching-off the collisions when the energy density goes below a fixed value or reducing the cross section according to the increase in η / s from a QGP phase to a hadronic one. We address the relation between the scaling of v 2 ( p T ) with the eccentricity ϵ x and with the integrated elliptic flow. We show that the breaking of the v 2 ( p T ) / ϵ x scaling is not coming mainly from the finite η / s but from the f.o. dynamics and that the v 2 ( p T ) is weakly dependent on the f.o. scheme. On the other hand the v 4 ( p T ) is found to be much more sensitive to both the η / s and the f.o. dynamics and hence is indicated to put better constraints on the properties of the QGP. A first semi-quantitative analysis shows that both v 2 and v 4 (with the smooth f.o.) consistently indicate a plasma with 4 π η / s ∼ 1 − 2 .

Transverse Velocity Dependence of the Proton-Antiproton Ratio as a Signature of the QCD Critical Point

The presence of a critical point in the QCD phase diagram can deform the trajectories describing the evolution of the expanding fireball in the mu_B-T phase diagram. If the average emission time of hadrons is a function of transverse velocity, as microscopic simulations of the hadronic freeze-out dynamics suggest, the deformation of the hydrodynamic trajectories will change the transverse velocity (beta_T) dependence of the proton-antiproton ratio when the fireball passes in the vicinity of the critical point. An unusual beta_T dependence of the [over]p/p ratio in a narrow beam energy window would thus signal the presence of the critical point.

Electroweak resonant leptogenesis in the singlet Majoron model

We study resonant leptogenesis at the electroweak phase transition in the singlet Majoron model with right-handed neutrinos. We consider a scenario where the SM gauge group and the lepton number break down spontaneously during a second-order electroweak phase transition. We calculate the flavor- and temperature-dependent leptonic asymmetries, by including the novel contributions from the transverse polarizations of the ${W}^{\ifmmode\pm\else\textpm\fi{}}$ and $Z$ bosons. The required resummation of the gauge-dependent, off-shell, heavy-neutrino self-energies is consistently treated within the gauge-invariant framework of the pinch technique. Taking into consideration the freeze-out dynamics of sphalerons, we delineate the parameter space of the model that is compatible with successful electroweak resonant leptogenesis. The phenomenological and astrophysical implications of the model are discussed.

Elliptic flow at RHIC: Where and when was it formed?

Evolution of the elliptic flow of hadrons in heavy-ion collisions at RHIC energies is studied within the microscopic quark–gluon string model. The elliptic flow is shown to have a multi-component structure caused by (i) rescattering and (ii) absorption processes in spatially asymmetric medium. Together with different freeze-out dynamics of mesons and baryons, these processes lead to the following trend in the flow formation: the later the mesons are frozen, the weaker their elliptic flow, whereas baryon fraction develops stronger elliptic flow during the late stages of the fireball evolution. The phase-space distributions of the emitted particles are studied as well. The flow is shown to be formed both in the central and in the fragmentation regions of the reaction. Comparison with the PHOBOS data demonstrates the model ability to reproduce the v2ch(η) signal in different centrality bins.

Freeze-out Dynamics at RHIC

Investigation of the final hadronic state properties of ultra-relativistics pp and Au+Au collisions supplies information on freeze-out conditions at RHIC and possible insights into early stages of these collisions. A variety of particle spectra measured by STAR are studied within the framework of chemical and local kinetic equlibrium models. Here we present the extracted chemical and final kinetic freeze-out temperatures, strangeness saturation factor, final collective flow velocity, and the inferred flow velocity at chemical freeze-out. In light of those measurements we disscuss dynamical evoution of the collission system.

Modified Boltzmann Transport Equation and Freeze Out

We study the Freeze-Out process in high-energy heavy-ion reaction. The description of the process is based on the Boltzmann Transport Equation (BTE). We point out the basic limitations of the BTE approach and introduce the Modified BTE. The Freeze-Out dynamics is presented in the 4-dimensional space-time in a layer of finite thickness, and we employ the Modified BTE for the realistic Freeze-Out description.

Bulk properties at RHIC: systematic study

A variety of particle species measured by the STAR experiment in 200 GeV Au+Au collisions yielded rich information about freeze-out dynamics at RHIC. The results on chemical and kinetic freeze-out properties and their centrality dependences appeared to be consistent with QGP formation in the early stage of the collision. To further advance our understanding of the collision evolution, lower energy 62.4 GeV Au+Au collisions were measured. Here, we present a review of freeze-out dynamics in 200 GeV collisions at RHIC in comparison with new results from the 62.4 GeV data.

Erratum: Strange hadrons and their resonances: A diagnostic tool of quark-gluon plasma freeze-out dynamics [Phys. Rev. C64, 054907 (2001)

We update our chemical analysis of (strange) hadrons produced at the SPS in Pb--Pb collisions at 158A GeV and and present a first chemical analysis of RHIC results. We report that the shape of (anti)hyperon m_T-spectra in a thermal freeze-out analysis leads to freeze-out conditions found in chemical analysis, implying sudden strange hyperon production. We discuss how a combined analysis of several strange hadron resonances of differing lifespan can be used to understand the dynamical process present during chemical and thermal freeze-outs. In medium resonance quenching is considered.

Strange hadrons and their resonances: A diagnostic tool of quark-gluon plasma freeze-out dynamics

We update our chemical analysis of (strange) hadrons produced at the SPS in Pb-Pb collisions at 158A GeV and discuss chemical analysis of RHIC results. We report that the shape of (anti)hyperon ${m}_{\ensuremath{\perp}}$ spectra in a thermal freeze-out analysis leads to freeze-out conditions found in chemical analysis, implying sudden strange-hyperon production. We discuss how a combined analysis of several strange-hadron resonances of differing lifespan can be used to understand the dynamical process present during chemical and thermal freeze-outs. In medium resonance quenching is considered.

Strange hadron resonances as a signature of freeze-out dynamics

We study the production and the observability of Lambda*(1520), K*0(892), and Sigma*(1385), strange hadron resonances as function of the freeze-out conditions within the statistical model of hadron production. We obtain an estimate of how many decay products are rescattered in evolution towards thermal freeze-out following chemical freeze-out, and find that the resonance decay signal is strong enough to be detected. We show how a combined analysis of at least two resonances can be used to understand the chemical freeze-out temperature, and the time between chemical and thermal freeze-outs.

DX-like behavior of oxygen in GaN

The role of oxygen as a shallow donor and a DX-state in GaN is elucidated by recent Raman experiments under hydrostatic pressure and the findings of first principles calculations. A pressure induced transfer of electrons from a shallow donor state to a deep DX-like state of the same donor can be correlated with vibrational gap modes by monitoring the freeze-out dynamics. Both features are unique to oxygen doped GaN and cannot be observed in Si doped material. The gap modes can be well explained by a linear chain model of impurity vibrations of substitutional O on the N site. A mode variation, and switching steps in its pressure behavior, which occurs in parallel to the carrier freeze-out are proposed to reflect three different charge states of the strongly localized states of O. This DX-type behavior as well as the experimental threshold pressure values are in excellent agreement with the theory results.

Sensitivity of pion interferometry to the microscopic dynamics of freeze-out

We study the HBT interferometry of ultrarelativistic nuclear collisions using a freeze-out model in which free pions emerge in the course of the last binary collisions in the hadron gas. We show that the HBT correlators of both identical and nonidentical pions change with respect to the case of independent pion production. Practical consequences for the design of the event generator with the built in Bose-Einstein correlations are discussed. We argue that the scheme of inclusive measurement of the HBT correlation function does not require the symmetrization of the multipion transition amplitudes (wave functions).