Relativistic Collisions Research Articles

Electron- and proton-impact excitation of heliumlike uranium in relativistic collisions

We have studied the K-shell excitation of He-like uranium (U90+) in relativistic collisions with hydrogen and argon atoms. Performing measurements with different targets, as well as with different ...

Neutron Star Mergers: Probing the EoS of Hot, Dense Matter by Gravitational Waves

Gravitational waves, electromagnetic radiation, and the emission of high energy particles probe the phase structure of the equation of state of dense matter produced at the crossroad of the closely related relativistic collisions of heavy ions and of binary neutron stars mergers. 3 + 1 dimensional special- and general relativistic hydrodynamic simulation studies reveal a unique window of opportunity to observe phase transitions in compressed baryon matter by laboratory based experiments and by astrophysical multimessenger observations. The astrophysical consequences of a hadron-quark phase transition in the interior of a compact star will be focused within this article. Especially with a future detection of the post-merger gravitational wave emission emanated from a binary neutron star merger event, it would be possible to explore the phase structure of quantum chromodynamics. The astrophysical observables of a hadron-quark phase transition in a single compact star system and binary hybrid star merger scenario will be summarized within this article. The FAIR facility at GSI Helmholtzzentrum allows one to study the universe in the laboratory, and several astrophysical signatures of the quark-gluon plasma have been found in relativistic collisions of heavy ions and will be explored in future experiments.

Dense cold matter

Opportunities to study unexplored domains of nuclear matter phase diagram with BM@N and SPD-NICA is discussed. Since the optimal initial energy range depends on the phase diagram domain, the subject and experimental methods for its study have a lot of opportunities both for the fixed target and collider experiments. Fixed target experiments provide an access to the laboratory study of nuclear matter with the unprecedented density as high as the one for the neutron star core and even larger. A specialised kinematic trigger for relativistic ion-ion collisions can be used for this study. On the other hand, the NICA collider energy range will provide unique opportunities to study interactions between multi-quark states. The article discusses the importance of experimental study of the diquark component of nuclear structure function and diquark-diquark interactions. It is important to develop neutron identification methods both for the fixed target and collider experiments.

Production of $ \Lambda$Λ-hypernuclei and evaluation of their binding energies via the double yield ratio

Relativistic collisions of ions, hadrons and leptons with nuclei can produce various hypernuclei by the capture of hyperons in nuclear residues. In many cases the disintegration of such hypernuclear systems can be described with statistical approaches suggesting that the fragment production is related to the binding energies of hypernuclei. We demonstrate how the hyperon binding energies can be effectively evaluated from the yields of different hyper-isotopes using the double ratio method. Its universality and the possibility to involve many different isotopes are the advantages of this method. The same procedure can also be applied for multi-strange nuclei, for which binding energies were very difficult to measure in previous hypernuclear experiments. Modifications caused by secondary de-excitation processes in hot hypernuclei are considered additionally.

Holographic Collisions across a Phase Transition.

We use holography to analyze relativistic collisions in a one-parameter family of strongly coupled gauge theories with thermal phase transitions. For a critical value of the parameter, the transition is second order, for subcritical values it is first order, and for supercritical values it a smooth crossover. We extract the gauge theory stress tensor from collisions of gravitational shock waves on the dual geometries. Regardless of the nature of the transition, for values of the parameter close to the critical value, almost all the energy of the projectiles is deposited into a long-lived, quasistatic blob of energy at midrapidity. This configuration is well described by the constitutive relations of second-order hydrodynamics that include all second-order gradients that are purely spatial in the local rest frame. In contrast, a Müller-Israel-Stewart-type formulation of hydrodynamics fails to provide a good description. We discuss possible implications for searches of the QCD critical point.

Evaluation of relativistic transport coefficients and the generalized Spitzer function in devices with 3D geometry and finite collisionality

The relativistic generalization of the linearized drift kinetic equation solver NEO-2 is presented which is used for computation of neoclassical transport coefficients and the generalized Spitzer function in 3D toroidal fusion devices (tokamaks and stellarators). This upgrade allows computations of the Spitzer function playing the role of current drive efficiency in the whole experimentally relevant temperature range, from mild temperatures where finite plasma collisionality effects are important to high temperatures where relativistic effects should be taken into account. Within the Galerkin method used for problem discretization over energy, relativistic effects are included into a set of matrices constant on a flux surface. Those matrices determine coefficients of a coupled set of integro-differential equations with a reduced dimension which is of the same form as in the non-relativistic case. For energy discretization of the linearized relativistic Coulomb collision operator, it is presented in spherical momentum space variables in the symmetric integral form derived directly from Beliaev-Budker expressions. The cancellation problem pertinent to the fully analytical representation of Braams and Karney [Phys. Fluids B 1, 1355 (1989)] does not appear in this form. Examples of evaluation of relativistic transport coefficients and the Spitzer function are presented.

Gain of regularity for the relativistic collision operator

We study a regularity property for the gain part of the relativistic Boltzmann collision operator. Our assumptions on the collisional cross-section cover the full range of generic hard and soft potentials with angular cut-off. We first take the Fourier transform of the gain operator and extract the modulus of Fourier symbol of the first order by exploiting the positive-definiteness of the relativistic correction in the post-collisional momentum. We then provide two different proofs based on Young’s convolution inequality and the Hardy–Littlewood–Sobolev inequality.

Cluster formation in relativistic nucleus-nucleus collisions

An event generator for exclusive simulation of nucleus-nucleus collisions based on quantum molecular dynamics is developed. A comparison of the results obtained using this event generator with earlier measurements show that the yields of pions, kaons, antiprotons, deuterons, and nuclear fragments in the collisions of various nuclei pair such as Si-Al and Pb-Pb in the energy range of a few to $100A$ GeV are predicted successfully. The Lorentz-covariant description of nucleon-nucleon interactions plays a critical role in reproducing fragment yields in peripheral collisions. Secondary meson yields from nucleus-nucleus collisions are well reproduced by incorporating the production and decay of resonances of up to 2 GeV/${c}^{2}$ and strings. This model can therefore be used for analysis and planning of high-energy particle physics experiments, prediction of cosmic ray transport, and shielding design of high-energy heavy-ion accelerators.

Charmed hadrons from coalescence plus fragmentation in relativistic nucleus-nucleus collisions at RHIC and LHC

In a coalescence plus fragmentation approach we calculate the heavy baryon/meson ratio and the p_T spectra of charmed hadrons D^{0}, D_{s} and varLambda _{c}^{+} in a wide range of transverse momentum from low p_T up to about 10 GeV and discuss their ratios from RHIC to LHC energies without any change of the coalescence parameters. We have included the contribution from decays of heavy hadron resonances and also the one due to fragmentation of heavy quarks which do not undergo the coalescence process. The coalescence process is tuned to have all charm quarks hadronizing in the p_Trightarrow 0 limit and at finite p_T charm quarks not undergoing coalescence are hadronized by independent fragmentation. The p_T dependence of the baryon/meson ratios are found to be sensitive to the masses of coalescing quarks, in particular the varLambda _{c}/D^{0} can reach values of about mathrm 1div 1.5 at p_T approx , 3 GeV, or larger, similarly to the light baryon/meson ratio like p/pi and varLambda /K, however a marked difference is a quite weak p_T dependence with respect to the light case, such that a larger value at intermediate p_T implies a relatively large value also for the integrated yields. A comparison with other coalescence model and with the prediction of thermal model is discussed.

Universal freezeout condition for charged hadrons in a hybrid approach

Hadronic freezeout during the evolution of the medium created in heavy-ion collisions is an important phenomena. It is quite useful to find a universal freezeout condition for each and every nuclear collisions. In this article, we have constructed a hybrid model to calculate the ratio of transverse energy to total mean multiplicity $E_{T} /N_{ch}$, since this ratio can possibly act as a freezeout condition in heavy-ion collision experiments. Present hybrid model blends two approaches : Tsallis statistics and wounded quark approach. Recently, Tsallis statistics has been reliably used to obtain the transverse momentum distribution of charged hadrons produced in relativistic ion collisions. On the other side it has been shown that the pseudorapidity distribution of charged hadrons can be calculated satisfactorily using the wounded quark model (WQM). We have used this hybrid model to calculate the transverse energy density distributions, $dE_{T}/d\eta$ at midrapidity using charged particle pseudorapidity distributions, $dN_{ch}/d\eta$ and mean transverse momentum $\langle p_{T} \rangle$ in various type of nuclear collisions. We found that present hybrid model satisfactorily explains the experimental data whether other models fail to reproduce the data at central and at peripheral collisions simultaneously. Finally, ratio of transverse energy to total mean multiplicity, $E_{T} /N_{ch}$ has been computed within hybrid model and compared with the available experimental data at RHIC and LHC energies. We observed no explicit dependence of $E_{T} /N_{ch}$ on energy as well as centrality and thus it can definitely act as a freezeout criteria.

Electron loss from hydrogen-like highly charged ions in collisions with electrons, protons and light atoms

We consider electron loss from a hydrogen-like highly charged ion (HCI) in relativistic collisions with hydrogen and helium in the range of impact velocities vmin ≤ v ≤ vmax (vmin and vmax correspond to the threshold energy εth for electron loss in collisions with a free electron and to ≈5 εth, respectively) where any reliable data for loss cross sections are absent. In this range, where the loss process is characterized by large momentum transfers, we express it in terms of electron loss in collisions with equivelocity protons and electrons and explore by performing a detailed comparative study of these subprocesses. Our results, in particular, show that: (i) compared to equivelocity electrons protons are more effective in inducing electron loss, (ii) the relative effectiveness of electron projectiles grows with increase in the atomic number of a HCI, (iii) collisions with protons and electrons lead to a qualitatively different population of the final-state-electron momentum space and even when the total loss cross sections in these collisions become already equal the spectra of the outgoing electrons still remain quite different in almost the entire volume of the final-state-electron momentum space, (iv) in collisions with hydrogen and helium the contributions to the loss process from the interactions with the nucleus and the electron(s) of the atom could be rather well separated in a substantial part of the final-state-electron momentum space.

On the relativistic large-angle electron collision operator for runaway avalanches in plasmas

Large-angle Coulomb collisions lead to an avalanching generation of runaway electrons in a plasma. We present the first fully conservative large-angle collision operator, derived from the relativistic Boltzmann operator. The relation to previous models for large-angle collisions is investigated, and their validity assessed. We present a form of the generalized collision operator which is suitable for implementation in a numerical kinetic equation solver, and demonstrate the effect on the runaway-electron growth rate. Finally we consider the reverse avalanche effect, where runaways are slowed down by large-angle collisions, and show that the choice of operator is important if the electric field is close to the avalanche threshold.

Processes of hypernuclei formation in relativistic ion collisions

The study of hypernuclei in relativistic ion collisions open new opportunities for nuclear and particle physics. The main processes leading to the production of hypernuclei in these reactions are the disintegration of large excited hyper-residues (target- and projectile-like), and the coalescence of hyperons with other baryons into light clusters. We use the transport, coalescence and statistical models to describe the whole reaction, and demonstrate the effectiveness of this approach: These reactions lead to the abundant production of multi-strange nuclei and new hypernuclear states. A broad distribution of predicted hypernuclei in masses and isospin allows for investigating properties of exotic hypernuclei, as well as the hypermatter both at high and low temperatures. There is a saturation of the hypernuclei production at high energies, therefore, the optimal way to pursue this experimental research is to use the accelerator facilities of intermediate energies, like FAIR (Darmstadt) and NICA (Dubna).

Entropy and Multifractality in Relativistic Ion-Ion Collisions

Entropy production in multiparticle systems is investigated by analyzing the experimental data on ion-ion collisions at AGS and SPS energies and comparing the findings with those reported earlier for hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions. It is observed that the entropy produced in limited and full phase space, when normalized to maximum rapidity, exhibits a kind of scaling which is nicely supported by Monte Carlo model HIJING. Using Rényi’s order q information entropy, multifractal characteristics of particle production are examined in terms of generalized dimensions, Dq. Nearly the same values of multifractal specific heat, c, observed in hadronic and ion-ion collisions over a wide range of incident energies suggest that the quantity c might be used as a universal characteristic of multiparticle production in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions. The analysis is extended to the study of spectrum of scaling indices. The findings reveal that Rényi’s order q information entropy could be another way to investigate the fluctuations in multiplicity distributions in terms of spectral function f(α), which has been argued to be a convenient function for comparison sake not only among different experiments but also between the data and theoretical models.

Initial temperature and extent of chemical equilibration of partons in relativistic collisions of heavy nuclei

We study the consequences of a premise that, if a thermalized and chemically equilibrating quark gluon plasma is formed in relativistic collisions of heavy nuclei, then a knowledge of energy and entropy densities of the plasma fixes the initial temperature and the product of gluon fugacity and formation time uniquely, provided we know the relative fugacities of quarks and gluons. Thus, a smaller formation time would imply a larger fugacity, independent of the initial temperature. Next, we explore the limits of chemical equilibration of partons during the initial stages in relativistic collisions of heavy nuclei. Experimentally measured rapidity densities of transverse energy and charged particle multiplicities at RHIC and LHC energies are used to estimate energy and number densities with the assumption of the formation of a kinetically equilibrated plasma that may not be chemically equilibrated for quarks and gluons. The estimates are found to be very sensitive to the correction factor used to multiply the Bjorken energy density to get the initial energy density. The evolution of the chemical equilibration during the quark gluon plasma phase is inferred by solving master equations, including the processes and along with expansion and cooling of the plasma. Possible consequences for the invariant mass distribution of intermediate mass dileptons radiated from the plasma are discussed.

Relativistic collisions as Yang–Baxter maps

We prove that one-dimensional elastic relativistic collisions satisfy the set-theoretical Yang–Baxter equation. The corresponding collision maps are symplectic and admit a Lax representation. Furthermore, they can be considered as reductions of a higher dimensional integrable Yang–Baxter map on an invariant manifold. In this framework, we study the integrability of transfer maps that represent particular periodic sequences of collisions.

Overview of ATLAS results

The heavy-ion programme in the ATLAS experiment at the Large Hadron Collider aims to probe and characterise hot and dense matter created in relativistic lead-lead collisions. Moreover, smaller collision systems involving nuclei and hadrons are of interest to disentangle initial- from final-state effects. This report presents new results based on lead-lead and proton-proton data collected at sNN=5.02 TeV in 2015, including measurements of bulk collectivity, charged-particle production, electroweak bosons, photon-jet correlations, and quarkonium suppression. First attempts to measure electromagnetic processes in ultra-peripheral collisions are also discussed.

Fast calculator for X-ray emission due to Radiative Recombination and Radiative Electron Capture in relativistic heavy-ion atom collisions

In experiments with highly charged, fast heavy ions the Radiative Recombination (RR) and Radiative Electron Capture (REC) processes have significant cross sections in an energy range of up to a few GeV/u. They are some of the most important charge changing processes in collisions of heavy ions with atoms and electrons, leading to the emission of a photon along with the formation of the ground and excited atomic states. Hence, for the understanding and planning of experiments, in particular for X-ray spectroscopy studies, at accelerator ring facilities, such as FAIR, it is crucial to have a good knowledge of these cross sections and the associated radiation characteristics. In the frame of this work a fast calculator, named RECAL, for the RR and REC process is presented and its capabilities are demonstrated with the analysis of a recently conducted experiment at the Experimental Storage Ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. A method is presented to determine unknown X-ray emission cross sections via normalization of the recorded spectra to REC cross sections calculated by RECAL.

Azimuthal correlations of D-mesons in p+p and p+Pb collisions at LHC energies

We study the correlations of D mesons produced in $p$+$p$ and $p$+Pb collisions. These are found to be sensitive to the effects of the cold nuclear medium and the transverse momentum ($p_T$) regions we are looking into. In order to put this on a quantitative footing, as a first step we analyse the azimuthal correlations of D meson-charged hadron(Dh), and then predict the same for D meson -anti D meson ($D\overline{D}$) pairs in $p$+$p$ and $p$+Pb collisions with strong coupling at leading order $\cal{O}$($\alpha_{s}^{2}$) and next to leading order $\cal{O}$($\alpha_{s}^{3}$) which includes space-time evolution (in both systems), as well cold nuclear matter effects (in $p$+Pb). This also sets the stage and baseline for the identification and study of medium modification of azimuthal correlations in relativistic collision of heavy nuclei at the Large Hadron Collider.

Production of dimeson atoms in high-energy collisions

The production of two-meson electromagnetic bound states and free meson pairs \( \pi^+\pi^-\) , \( K^+K^-\) , \( \pi^+K^{\mp}\) in relativistic collisions has been considered. It is shown that using of exact Coulomb wave functions for dimeson atom (DMA) allows one to calculate the yield of discrete states with the desired accuracy. The relative probabilities of production of DMA and meson pairs in the free state are estimated. The amplitude of DMA transition from 1S to 2P state, which is essential for the pionium Lamb shift measurements, has been obtained.