Matter In Heavy-ion Collisions Research Articles

Investigation of surface homogeneity of mirrors for the CBM-RICH detector and low-mass di-electron feasibility studies

The Compressed Baryonic Matter (CBM) experiment at the future FAIR facility will investigate high net-baryon density matter at moderate temperatures in A+A collisions from 4-45 AGeV. One of the key observables of the CBM physics program is electromagnetic radiation as a probe of strongly interacting matter in heavy-ion collisions, carrying undistorted information on its conditions to the detector. This includes detailed investigations of low-mass vector mesons in their di-electron channel. A clean and efficient identification of electrons is required for such measurements. In CBM the electron identification will be performed by a Ring Imaging Cherenkov detector and several layers of Transition Radiation Detectors. The RICH detector will be operated with CO2 radiator gas, MAPMTs as photodetector and spherical glass mirrors as focusing elements. A high quality of the mirrors in terms of reflectivity and surface homogeneity is required. In the first part of the contribution results on measurements of the mirror surface homogeneity are presented. Results on the feasibility studies of low-mass di-electron measurements with realistic detector response are discussed in the second part of the contribution.

NICA Complex and JINR - status and plans

One of the main directions of the scientific research at the Joint Institute for Nuclear Research (JINR) in Dubna is the relativistic nuclear and spin physics. The new JINR flagship program in this direction is now realized within the project NICA (Nuclotron-based Ion Collider fAcility). The main goal of the NICA scientific program is an experimental study of hot and dense strongly interacting matter in heavy ion collisions at nucleon-nucleon centre-of-mass energies of 4-11 GeV and at average luminosity of 1027 cm−2 s −1 for Au (79+) in the collider mode. In parallel, fixed target experiments at the upgraded JINR superconducting synchrotron Nuclotron are carried out with the extracted beams of various nuclei species up to gold with the momenta up to 13 GeV/c for protons. The program also foresees a study of spin physics with extracted and colliding beams of polarized deuterons and protons at the center-of-mass energies up to 26 GeV for proton collisions. The proposed program allows to search for possible signs of the mixed phase and critical endpoint, and to shed more light on the problem of nucleon spin structure. The survey of the main directions of the JINR scientific research program and general design and construction status of the NICA complex are presented.

Detecting Strangeness -4 Dibaryon States

Recent experiments at Jefferson Laboratory, potential new facilities at the Japan Proton Accelerator Research Complex (J-PARC), the energy, beam intensity, and detection systems available at RHIC, and recent lattice calculations make it evident that the discovery of a 1S0 di-cascade bound state of two Ξ particles is feasible. We state the simple arguments, based on SU(3) flavor symmetry, for the existence of this bound state, discuss the previous predictions, and comment on the experimental conditions necessary for detection. We conclude that observing this new state of matter in heavy ion collisions at RHIC is not unlikely.

Project NICA at JINR

Scientific program of NICA (Nuclotron-based Ion Collider fAcility) is now under realization phase at JINR (Dubna). The main goal of the program is an experimental study of hot and dense strongly interacting matter in heavy ion collisions at nucleon–nucleon centre-of-mass energies of 4–11 GeV and at average luminosity of 1027 cm−2s−1 for Au (79+) in the collider mode. In parallel, fixed target experiments at the upgraded JINR superconducting synchrotron Nuclotron are carried out with the extracted beams of various nuclei species up to gold with the momenta up to 13 GeV/c for protons. The program also foresees a study of spin physics with extracted and colliding beams of polarized deuterons and protons at the centre-of-mass energies up to 26 GeV for proton collisions. The proposed program allows to search for possible signs of the mixed phase and critical endpoint, and to shed more light on the problem of nucleon spin structure. General design and construction status of the complex is presented.

Separating jets from bulk matter in heavy ion collisions at the LHC

We discuss a theoretical scheme that accounts for bulk matter, jets, and the interaction between the two. The physical picture of our approach is the following: Initial hard scatterings result in mainly longitudinal flux tubes, with transversely moving pieces carrying the pt of the partons from hard scatterings. These flux tubes constitute eventually both bulk matter (which thermalizes, flows, and finally hadronizes) and jets, according to some criteria based on partonic energy loss. High energy flux tube segments will leave the fluid, providing jet hadrons via the usual Schwinger mechanism of flux-tube breaking caused by quark-antiquark production. But the jets may also be produced at the freeze-out surface. Here we assume that the quark-antiquark needed for the flux tube breaking is provided by the fluid, with properties (momentum, flavor) determined by the fluid rather than the Schwinger mechanism. Considering transverse fluid velocities up to 0.7c, and thermal parton momentum distributions, one may get a "push" of a couple of GeV to be added to the transverse momentum of the string segment. This will be a crucial effect for intermediate pt jet hadrons.

Bulk viscosity and energy-momentum correlations in high energy hadron collisions

We show how the measurement of appropriately constructed particle-energy/momentum correlations allows access to the bulk viscosity of strongly interacting hadron matter in heavy ion collisions. This measurement can be performed by the LHC and RHIC experiments in events with high-particle multiplicity, following up on existing estimates of the shear viscosity based on elliptic flow.

Strong Interactions, (De)coherence and Quarkonia

Quarkonia are the central objects to explore the non-perturbative nature of non-abelian gauge theories. We describe the confinement-deconnnement phases for heavy quarkonia in a hot QCD medium and thereby the statistical nature of the inter-quark forces. In the sense of one-loop quantum effects, we propose that the "quantum" nature of quark matters follows directly from the thermody-namic consideration of Richardson potential. Thereby we gain an understanding of the formation of hot and dense states of quark gluon plasma matter in heavy ion collisions and the early universe. In the case of the non-abelian theory, the consideration of the Sudhakov form factor turns out to be an efficient tool for soft gluons. In the limit of the Block-Nordsieck resummation, the strong coupling obtained from the Sudhakov form factor yields the statistical nature of hadronic bound states, e.g. kaons and Ds particles.

New simple explicit solutions of perfect-fluid hydrodynamics and phase-space evolution

New exact solutions of relativistic perfect-fluid hydrodynamics are presented, including the first family of exact rotating solutions. The method used to search for them is a simultaneous investigation of the relativistic hydrodynamical equations and the collisionless Boltzmann equation. Possible connections to the evolution of hot and dense partonic matter in heavy-ion collisions is discussed.

Rapidity distribution as a probe for elliptical flow at intermediate energies

Abstract The interplay between spectator and participant matter in heavy-ion collisions is investigated within the isospin-dependent quantum molecular dynamics (IQMD) model in terms of the rapidity distribution of light charged particles. The effect of different types and sizes of rapidity distributions is studied in elliptical flow. The elliptical-flow patterns show the important role of nearby spectator matter on the participant zone. This role is further explained on the basis of the passing time of the spectator and the expansion time of the participant zone. The transition from in-plane to out-of-plane emission is observed only when the mid-rapidity region is included into the rapidity bin. Otherwise no transition occurs. The transition energy is found to be highly sensitive to the size of the rapidity bin, while it is only weakly dependent on the type of the rapidity distribution. These theoretical findings are found to be in agreement with experimental results.

Microscopic models and effective equation of state in nuclear collisions in the vicinity ofElab=30AGeV at the GSI Facility for Antiproton and Ion Research (FAIR) and beyond

Two microscopic models, ultrarelativistic quantum molecular dynamics and quark-gluon string model, were employed to study the formation of locally equilibrated hot and dense nuclear matter in heavy-ion collisions at energies from $11.6A$ to $160A$ GeV. Analysis was performed for the fixed central cubic cell of volume $V=125$ fm${}^{3}$ and for the expanding cell that followed the growth of the central area with uniformly distributed energy. To decide whether the equilibrium was reached, results of the microscopic calculations were compared to that of the statistical thermal model. Both dynamical models indicate that the state of kinetic, thermal and chemical equilibrium is nearly approached at any bombarding energy after a certain relaxation period. The higher the energy, the shorter the relaxation time. Equation of state has a simple linear dependence $P=a(\sqrt{s})\ensuremath{\varepsilon}$, where $a\ensuremath{\equiv}{c}_{s}^{2}$ is the sound velocity squared. It varies from $0.12\ifmmode\pm\else\textpm\fi{}0.01$ at ${E}_{\mathrm{lab}}=11.6A$ GeV to $0.145\ifmmode\pm\else\textpm\fi{}0.005$ at ${E}_{\mathrm{lab}}=160A$ GeV. Change of the slope in $a(\sqrt{s})$ behavior occurs at ${E}_{\mathrm{lab}}=40A$ GeV and can be assigned to the transition from baryon-rich to meson-dominated matter. The phase diagrams in the $T\text{\ensuremath{-}}{\ensuremath{\mu}}_{\mathrm{B}}$ plane show the presence of kinks along the lines of constant entropy per baryon. These kinks are linked to the inelastic (i.e., chemical) freeze-out in the system.

Proton–proton physics in ALICE

The ALICE experiment has several unique features which makes it an important contributor to proton–proton physics at the large hadron collider, in addition to its specific design goal of studying the physics of strongly interacting matter in heavy-ion collisions. The unique capabilities include its low transverse momentum (pt) acceptance, excellent vertexing, particle identification over a broad pt range and jet reconstruction. In this paper, a brief review of ALICE capabilities is given for studying bulk properties of produced particles which characterize the underlying events, and the physics of heavy-flavour, quarkonia, photons, di-leptons and jets.

Probing the strange quark condensate by di-electrons from φ-meson decays in heavy-ion collisions at SIS energies

QCD sum rules predict that the change of the strange quark condensate 〈¯ss〉 in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from φ-meson decays. Due to the expansion of hadron matter in heavy-ion collisions, the φ peak suffers a smearing governed by the interval of density in the expanding fireball, which appears as an effective broadening of the di-electron spectrum in the φ region. The emerging broadening is sensitive to the in-medium change of 〈¯ss〉. This allows to probe directly in-medium modifications of 〈¯ss〉 via di-electron spectra in heavy-ion collisions at SIS energies with HADES.

New Results on Theory of AA Collisions at RHIC Energy

I discuss recent theoretical work about AA collisions at RHIC energies, largely from our group at BNL and the group at the RIKEN-BNL Center. Most of the discussion is about the impact of the Color Glass Condensate on the production and evolution of matter in heavy ion collisions.

Spectator response to the participant blast

The interplay between spectator and participant matter in heavy-ion collisions is investigated in the context of a microscopic transport model. Transport simulations show that flow patterns for the participant matter are strongly influenced by the presence of the nearby spectator matter. However, the influence is mutual. During the explosion of the participant zone, the spectator matter acquires a transverse momentum $<P^x /A>$ that shows sensitivity to the nuclear incompressibility and to the momentum dependence of the nuclear mean field (MF). An observed change in the net average momentum per nucleon, $\Delta |< {\bf P}/A >|$, can be associated with the momentum dependence of the MF. For a momentum-dependent MF and a low impact parameter in a heavy system, the spectators may emerge faster than in the initial state, accelerated by the violent participant explosion. The average excitation energy and the mass of the spectators, in contrast to the momentum, show little sensitivity to the nuclear equation of state.

Transition toresonance-rich matter in heavy-ion collisions at RHIC energies

The equilibration of hot and dense nuclear matter produced in the centralregion in central Au + Au collisions at (s)1/2 = 200 A GeV is studied within the microscopic transport model UrQMD. Thepressure here becomes isotropic at t≈5 fm c-1. Within the next15 fm c-1 the expansion of the matter proceeds almost isentropicallywith the entropy per baryon ratio S/A≈150. During this periodthe equation of state in the (P,ε)-plane has a very simpleform, P = 0.15ε. Comparison with the statistical model(SM) of an ideal hadron gas reveals that the time of≈ 20 fm c-1 may be too short to attain the fully equilibratedstate. In particular, the fractions of resonances are overpopulated incontrast with the SM values. The creation of such a long-livedresonance-rich state slows down the relaxation to chemical equilibriumand can be detected experimentally.

Electromagnetic probes of dense matter in heavy-ion collisions

Dilepton and photon production in heavy-ion collisions at SPS energies are studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. It is found that the dilepton spectra in proton-nucleus reactions can be well described by the conventional mechanism of Dalitz decay and direct vector meson decay. However, to provide a quantitative explanation of the observed dilepton spectra in central heavy-ion collisions requires contributions other than these direct decays and also various medium effects. Introducing a decrease of vector meson masses in hot dense medium, we find that these heavy-ion data can be satisfactorily explained. Furthermore, the single photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper bound deduced from the experiments by the WA80 Collaboration.

Shock-like freeze-out in relativistic hydrodynamics

We have formulated a self-consistent model of freeze-out on an arbitrary hypersurface. It conserves energy and momentum across the discontinuity between ideal fluid and the gas of free particles. Energy and momentum of those free particles have non-equilibrium values that could be a signal for the formation of hot and dense matter in heavy ion collisions.

AZIMUTHAL DEPENDENCE OF ENERGY FLOW IN CENTRAL COLLISIONS OF HEAVY NUCLEI

Microscopic VUU and QMD calculations, which include the momentum dependence of the nuclear interactions, both predict the observed collective off-plane squeeze-out of nuclear matter in heavy ion collisions. A strong projectile mass dependence is found, in agreement with the data. A fragment mass dependence of the emission pattern is predicted. The off-plane squeeze-out is sensitive to the bulk properties of hot and dense nuclear matter, namely the nuclear viscosity and the equation of state.

Strangelets in heavy ion physics

The properties of strangelets and their implication for astrophysics and heavy ion physics is summarized. The Relativistic Quantum Molecular Dynamics is used to investigate the formation of baryon and strangeness rich matter in heavy ion collisions.

VUU and (R)QMD model of high energy heavy ion collisions

The Vlasov Uehling Uhlenbeck (VUU)-model, the Quantum Molecular Dynamics (QMD) and its covariant extension, the Relativistic Quantum Molecular Dynamics (RQMD) are used to investigate the formation of baryon rich nuclear matter in heavy ion collisions over a wide range of bombarding energy from several hundred MeV/nucleon to several hundred GeV/nucleon. Stopping and bounce off are predicted at all energies if heavy systems are investigated. The formation of Δ-matter is predicted at E > 1GeV/nucleon in the baryon rich region, this is supported by the yields and spectra of pions and is manifested by the enhancement of low p T pions. (Anti)-strangeness is created copiously leading to larger kaon and lambda yields. This together with the large baryon densities facilitates the formation of multi hypernuclei and droplets of strange quark matter, so-called strangelets.