Freeze-out Process Research Articles

Relativistic Hydrodynamics in Heavy-Ion Collisions: General Aspects and Recent Developments

Relativistic hydrodynamics has been quite successful in explaining the collective behaviour of the QCD matter produced in high energy heavy-ion collisions at RHIC and LHC. We briefly review the latest developments in the hydrodynamical modeling of relativistic heavy-ion collisions. Essential ingredients of the model such as the hydrodynamic evolution equations, dissipation, initial conditions, equation of state, and freeze-out process are reviewed. We discuss observable quantities such as particle spectra and anisotropic flow and effect of viscosity on these observables. Recent developments such as event-by-event fluctuations, flow in small systems (proton-proton and proton-nucleus collisions), flow in ultracentral collisions, longitudinal fluctuations, and correlations and flow in intense magnetic field are also discussed.

Relic neutrino freeze-out: Dependence on natural constants

Analysis of cosmic microwave background radiation fluctuations favors an effective number of neutrinos, Nν>3. This motivates a reinvestigation of the neutrino freeze-out process. Here we characterize the dependence of Nν on the Standard Model (SM) parameters that govern neutrino freeze-out. We show that Nν depends on a combination η of several natural constants characterizing the relative strength of weak interaction processes in the early Universe and on the Weinberg angle sin2⁡θW. We determine numerically the dependence Nν(η,sin2⁡θW) and discuss these results. The extensive numerical computations are made possible by two novel numerical procedures: a spectral method Boltzmann equation solver adapted to allow for strong reheating and emergent chemical non-equilibrium, and a method to evaluate Boltzmann equation collision integrals that generates a smooth integrand.

Mechanism for thermal relic dark matter of strongly interacting massive particles.

We present a new paradigm for achieving thermal relic dark matter. The mechanism arises when a nearly secluded dark sector is thermalized with the standard model after reheating. The freeze-out process is a number-changing 3→2 annihilation of strongly interacting massive particles (SIMPs) in the dark sector, and points to sub-GeV dark matter. The couplings to the visible sector, necessary for maintaining thermal equilibrium with the standard model, imply measurable signals that will allow coverage of a significant part of the parameter space with future indirect- and direct-detection experiments and via direct production of dark matter at colliders. Moreover, 3→2 annihilations typically predict sizable 2→2 self-interactions which naturally address the "core versus cusp" and "too-big-to-fail" small-scale structure formation problems.

Scaling properties of light (anti)nuclei and (anti)hypertriton production in Au+Au collisions at GeV

We present a mass number scaling property of light (anti)nuclei production in midrapidity Au + Au collisions at GeV, based on the parton and hadron cascade and dynamically constrained phase-space coalescence (PACIAE + DCPC) model. We found that the integrated yields of light (anti)nuclei decrease exponentially with the increase of mass numbers, which can be characterized quantitatively with temperature T at the hadronic freeze-out process. Furthermore, we found that the integrated yields of (anti)nuclei per participant nucleon increase from peripheral to central collisions more rapidly with increasing mass number, and the increasing rates for different mass numbers can be approximately described by a universal line.

Non-extensive statistics and understanding particle production and kinetic freeze-out process from pT-spectra at 2.76 TeV

An approach, based on Tsallis non-extensive statistics, has been employed, here, to analyse, systematically, the p T -spectra of various identified secondary hadrons like pions, kaons, protons and antiprotons, produced in different central Pb + Pb interactions at LHC energy 2.76TeV in terms of multiplicity and temperature fluctuations. The results, thus obtained, have been utilized to understand the various stages of different types of hadron production during evolution of the fireball produced in such collisions.

Measurement of Ionization Energies of Nitrogen in 4H-SiC by Traveling-Wave Method

The impurity bands and corresponding ionization energies of nitrogen atoms in a 4H-SiC crystal with a concentration of 1×1019 cm-3 are measured by a nondestructive and noncontact traveling-wave method. When a SiC sample was placed near the surface of a surface acoustic wave device, its conductivity can be obtained by measuring the attenuation of the piezo-potential traveling-wave grazing along the surface of the sample. Temperature-dependent conductivities corresponding to a freeze-out process of free carriers excited from nitrogen atoms were observed, and the corresponding ionization energies of the nitrogen atoms were estimated by the Arrhenius plot method. The ionization energies in the impurity bands originating from splits of the doping atoms at cubic and hexagonal sites in the carbon sublattice are 72.89 and 47.89 meV, respectively, at room temperature. The ionization energies are in good agreement with the results reported in other theoretical and experimental studies. We also found that the skin depth of the traveling wave in the sample is below 1 mm and that the mobility of the carriers is strongly affected by both ionized dopants and charged surface defects in the depletion region near the surface of the sample. The effects of the sample and traveling wave such as the polarization effects of the crystal and the frequency effects of the traveling wave are discussed.

Dark matter asymmetry in supersymmetric Dirac leptogenesis

We discuss asymmetric or symmetric dark matter candidate in the supersymmetric Dirac leptogenesis scenario. By introducing a singlet superfield coupling to right-handed neutrinos, the overabundance problem of dark matter can be evaded and various possibilities for dark matter candidate arise. If the singlino is the lightest supersymmetric particle (LSP), it becomes naturally asymmetric dark matter. On the other hand, the right-handed sneutrino is a symmetric dark matter candidate whose relic density can be determined by the usual thermal freeze-out process. The conventional neutralino or gravitino LSP can be also a dark matter candidate as its non-thermal production from the right-handed sneutrino can be controlled appropriately. In our scenario, the late-decay of heavy supersymmetric particles mainly produces the right-handed sneutrino and neutrino which is harmless to the standard prediction of the Big-Bang Nucleosynthesis.

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.

Optimized production of hyperpolarized 129Xe at 2 bars for in vivo lung magnetic resonance imaging

In this work, the production rate of a spin-exchange optical pumping 129Xe gas polarizer was optimized for routine generation of hyperpolarized 129Xe for in vivo lung MRI. This system uses a narrow (∼ 0.1 nm linewidth), tuneable external cavity laser (operating at ∼25 W) for SEOP of 3% gas mixtures of Xe inside a mid-pressure (2 bars) cell of 491 cm3 volume. Under this regime, theoretical and experimentally measured 129Xe polarizations were calculated to be 24% and 12%, respectively, for a gas flow rate of 300 sccm and a cell temperature of 373 K. The photon efficiency was evaluated, yielding theoretical and experimental values of 0.039 and 0.046, respectively. The theoretical efficiency was calculated from spin-exchange and spin-destruction cross sections and the experimental photon efficiency was measured under flow for a gas-cell residency time equal to an empirically determined spin-exchange time of 45 s. In addition, details of the Xe freeze-out process were analyzed with a model of polarization decay during Xe accumulation in the frozen phase, where a T1 of 87 ± 2 min was observed. To demonstrate the system's application, in vivo lung magnetic resonance images (signal-to-noise ratio ∼ 50 from a voxel of 15 mm× 4 mm× 4 mm) were acquired using modest volumes (<400 ml) of isotopically enriched (86% 129Xe) Xe gas polarized to >10%. Despite the experimental polarization being a factor of 2 lower than the predicted polarization for typical operating parameters, the system is close to the theoretical photon efficiency and the system has so far produced polarized gas for more than 100 in vivo 129Xe lung imaging studies.

Viscous Hydrodynamic Model for Relativistic Heavy Ion Collisions

Viscous hydrodynamical modeling of relativistic heavy ion collisions has been highly successful in explaining bulk of the experimental data in RHIC and LHC energy collisions. We briefly review viscous hydrodynamics modeling of high energy nuclear collisions. Basic ingredients of the modeling, the hydrodynamic equations, relaxation equations for dissipative forces, are discussed. Hydrodynamical modeling being a boundary value problem, we discuss the initial conditions, freeze-out process. We also show representative simulation results in comparison with experimental data. We also discuss the recent developments in event-by-event hydrodynamics.

Sterile neutrino, hidden dark matter and their cosmological signatures

Though thermal dark matter has been the central idea behind the dark matter candidates, it is highly possible that dark matter of the universe is non-thermal in origin or it might be in thermal contact with some hidden or dark sector but not with standard model. Here we explore the cosmological bounds as well as the signatures on two types of non-thermal dark matter candidates. First we discuss a hidden dark matter with almost no interaction (or very feeble) with standard model particles so that it is not in thermal contact with visible sector but we assume it is thermalized with in a hidden sector due to some interaction. While encompassing the standard cold WIMP scenario, we do not require the freeze-out process to be non-relativistic. Rather, freeze-out may also occur when dark matter particles are semi-relativistic or relativistic. Especially we focus on the warm dark matter scenario in this set up and find the constraints on the warm dark matter mass, cross-section and hidden to visible sector temperature ratio which accounts for the observed dark-matter density, satisfies the Tremaine-Gunn bound on dark-matter phase space density and has a free-streaming length consistent with cosmological constraints on the matter power spectrum. Our method can also be applied to keV sterile neutrino dark matter which is not thermalized with standard model but is thermalized with in a dark sector. The second part of this proceeding focuses on an exotic dark matter candidate which arises from the existence of eV mass sterile neutrino through a late phase transition. Due to existence of a strong scalar force the light sterile states get trapped into stable degenerate micro nuggets. We find that its signature in matter power spectra is close to a warm dark matter candidate.

Cosmological limits on hidden sector dark matter

We explore the model-independent constraints from cosmology on a dark-matter particle with no prominent standard model interactions that interacts and thermalizes with other particles in a hidden sector. Without specifying detailed hidden-sector particle physics, we characterize the relevant physics by the annihilation cross section, mass, and temperature ratio of the hidden to visible sectors. While encompassing the standard cold WIMP scenario, we do not require the freeze-out process to be nonrelativistic. Rather, freeze-out may also occur when dark matter particles are semirelativistic or relativistic. We solve the Boltzmann equation to find the conditions that hidden-sector dark matter accounts for the observed dark-matter density, satisfies the Tremaine-Gunn bound on dark-matter phase space density, and has a free-streaming length consistent with cosmological constraints on the matter power spectrum. We show that for masses <1.5 keV no region of parameter space satisfies all these constraints. This is a gravitationally-mediated lower bound on the dark-matter mass for any model in which the primary component of dark matter once had efficient interactions -- even if it has never been in equilibrium with the standard model.

THERMINATOR 2: THERMal heavy Io N gener ATOR 2

We present an extended version of THERMINATOR, a Monte Carlo event generator dedicated to studies of the statistical production of particles in relativistic heavy-ion collisions. The increased functionality of the code contains the following features: The input of any shape of the freeze-out hypersurface and the expansion velocity field, including the 3+1 dimensional profiles, in particular those generated externally with various hydrodynamic codes. The hypersufraces may have variable thermal parameters, which allows for studies departing significantly from the mid-rapidity region, where the baryon chemical potential becomes large. We include a library of standard sets of hypersurfaces and velocity profiles describing the RHIC Au+Au data at sqrt(s_(NN)) = 200 GeV for various centralities, as well as those anticipated for the LHC Pb+Pb collisions at sqrt(s_(NN)) = 5.5 TeV. A separate code, FEMTO-THERMINATOR, is provided to carry out the analysis of femtoscopic correlations which are an important source of information concerning the size and expansion of the system. We also include several useful scripts that carry out auxiliary tasks, such as obtaining an estimate of the number of elastic collisions after the freeze-out, counting of particles flowing back into the fireball and violating causality (typically very few), or visualizing various results: the particle p_T-spectra, the elliptic flow coefficients, and the HBT correlation radii. We also investigate the problem of the back-flow of particles into the hydrodynamic region, as well as estimate the elastic rescattering in terms of trajectory crossings. The package is written in C++ and uses the CERN ROOT environment.

Phenomenology of the little bang

I review recent selected developments in the theory and modeling of ultrarelativistic heavy-ion collisions. I explain why relativistic viscous hydrodynamics is now used to model the expansion of the matter formed in these collisions. I give examples of first quantitative predictions, and I discuss remaining open questions associated with the description of the freeze-out process. I argue that while the expansion process is now well understood, our knowledge of initial conditions is still poor. Recent analyses of two-particle correlations have revealed fine structures known as ridge and shoulder, which extend over a long range in rapidity. These correlations are thought to originate from initial state fluctuations, whose modeling is still crude. I discuss triangular flow, a simple mechanism recently put forward, through which fluctuations generate the observed correlation pattern.

DENSE IRON EJECTA AND CORE-COLLAPSE SUPERNOVA EXPLOSION IN THE YOUNG SUPERNOVA REMNANT G11.2-0.3

We present the results of near-infrared spectroscopic observations of dense (≳103 cm−3) iron ejecta in the young core-collapse supernova remnant G11.2–0.3. Five ejecta knots projected to be close to its center show a large dispersion in their Doppler shifts: two knots in the east are blueshifted by more than 1000 km s-1, while three western knots have relatively small blueshifts of 20–60 km s-1. This velocity discrepancy may indicate that the western knots have been significantly decelerated or that there exists a systematic velocity difference among the knots. One ejecta filament in the northwestern boundary, on the other hand, is redshifted by ≳200 km s-1, while opposite filament in the southeastern boundary shows a negligible radial motion. Some of the knots and filaments have secondary velocity components, and one knot shows a bow shock-like feature in the velocity structure. The iron ejecta appear to be devoid of strong emission from other heavy elements, such as S, which may attest to the α-rich freezeout process in the explosive nucleosynthesis of the core-collapse supernova explosion close to its center. The prominent bipolar distribution of the Fe ejecta in the northwestern and southeastern direction, along with the elongation of the central pulsar wind nebula in the perpendicular direction, is consistent with the interpretation that the supernova exploded primarily along the northwestern and southeastern direction.

The dust condensation sequence in red supergiant stars

Context. Red supergiant (RSG) stars exhibit significant mass loss by means of a slow, dense wind. They are often considered to be the more massive counterparts of Asymptotic Giant Branch (AGB) stars. While AGB mass loss is related to their strong pulsations, the RSG are often only weakly variable. This raises the question of whether their wind-driving mechanism and the dust composition of the wind are the same. Aims. We study the conditions at the base of the wind by determining the dust composition of a sample of RSG. The dust composition is assumed to be sensitive to the density, temperature, and acceleration at the base of the wind. We compare the derived dust composition with the composition measured in AGB star winds. Methods. We compile a sample of 27 RSG infrared spectra (ISO-SWS) and supplement these with photometric measurements to derive the full spectral energy distribution (SED). These data are modelled using a dust radiative-transfer code, taking into account the optical properties of the relevant candidate materials to search for correlations between mass-loss rate, density at the inner edge of the dust shell, and stellar parameters. Results. We find strong correlations between the dust composition, mass-loss rate, and the stellar luminosity, roughly in agreement with the theoretical dust condensation sequence. We identify the need for a continuous (near-)IR dust opacity and tentatively propose amorphous carbon, and we note significant differences with AGB star winds in terms of the presence of PAHs, absence of “the” 13 μm band, and a lack of strong water bands. Conclusions. Dust condensation in RSG is found to experience a freeze-out process that is similar to that in AGB stars. Together with the positive effect of the stellar luminosity on the mass-loss rate, this suggests that radiation pressure on dust grains is an important ingredient in the driving mechanism. Still, differences with AGB stars are manifold and thus the winds of RSG should be studied individually in further detail.

Pion mass shift and the kinetic freeze-out process

The kinetic freeze-out process of a pion gas through a finite layer with time-like normal is considered. The pion gas is described by a Boltzmann gas with elastic collisions among the pions. Within this model, the impact of the in-medium pion mass modification on the freeze-out process is studied. A marginal change of the freeze-out variables temperature and flow velocity and an insignificant modification of the frozen-out particle distribution function has been found.

Cryogenic Heat-Exchanger Design for Freeze-out Removal of Carbon Dioxide from Landfill Gas

A cryogenic heat exchanger to remove carbon dioxide from landfill gas (LFG) is proposed and designed for applications to LNG production in distributed-scale. Since the major components of LFG are methane and carbon dioxide, CO2 removal is a significant pre-process in the liquefaction systems. A new and simple approach is proposed to directly remove carbon dioxide as frost on the surface wall along the cooling passage in a liquefying heat exchanger and to install two identical heat exchangers in parallel for alternative switching. As a first step of feasibility study, combined heat and mass transfer analysis is performed on the freeze-out process of CO2 in a counterflow heat exchanger, where CH4-CO2 mixture is cooled below its frost temperature in thermal contact with cold refrigerant. Engineering correlations for the analogy of heat and mass transfer are incorporated into numerical heat exchanger analysis with detailed fluid properties. The developed analytical model is used to estimate the distribution of CO2 accumulation and the required heat exchanger size with latent thermal load for the cryogenic CO2 removal in various operating conditions.

Jozsó's Legacy: Chemical and kinetic freeze-out in heavy-ion collisions

We review J. Zimányi's key contributions to the theoretical understanding of dynamical freeze-out in nuclear collisions and their subsequent applications to ultra-relativistic heavy-ion collisions, leading to the discovery of a freeze-out hierarchy where chemical freeze-out of hadron yields precedes the thermal decoupling of their momentum spectra. Following Zimányi's lines of reasoning we show that kinetic freeze-out necessarily leads to a dependence of the corresponding freeze-out temperature on collision centrality. This centrality dependence can be predicted within hydrodynamic models, and for Au+Au collisions at RHIC this prediction is shown to reproduce the experimentally observed centrality dependence of the thermal decoupling temperature, extracted from hadron momentum spectra. The fact that no such centrality dependence is observed for the chemical decoupling temperature, extracted from the hadron yields measured in these collisions, excludes a similar kinetic interpretation of the chemical decoupling process. We argue that the chemical decoupling data from Au+Au collisions at RHIC can only be consistently understood if the chemical freeze-out process is driven by a phase transition, and that the measured chemical decoupling temperature therefore measures the critical temperature of the quark-hadron phase transition. We propose additional experiments to further test this interpretation.

Hydrodynamic description of heavy ion collisions

We give a short review of hydrodynamic models at heavy ion collisions from the point of view of initial conditions, an equation of states (EoS) and a freezeout process. Then we show our latest results of a combined fully three-dimensional macroscopic/microscopic transport approach. In this model for the early, dense, deconfined stage relativistic 3D hydrodynamics of the reaction and a microscopic non-equilibrium model for the later hadronic stage where the equilibrium assumptions are no longer valid are employed. Within this approach we study the dynamics of hot, bulk QCD matter, which is being created in ultra-relativistic heavy ion collisions at RHIC.