Heavy-flavour spectra in high-energy nucleus–nucleus collisions

The propagation of the heavy quarks produced in relativistic nucleus–nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors R _AA and the elliptic-flow coefficients v ₂ of electrons from heavy-flavor decay.     

Equilibrium and non-equilibrium effects in nucleus–nucleus collisions

Local thermal and chemical equilibration is studied for central A+A collisions at 10.7–160 AGeV in the Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model exhibits strong deviations from local equilibrium at the high density hadron–string phase formed during the early stage of the collision. Equilibration of the hadron–resonance matter is established in the central cell of volume V=125 fm³ at later stages, t≥10 fm/c, of the resulting quasi-isentropic expansion. The thermodynamical functions in the cell and their time evolution are presented. Deviations of the UrQMD quasi-equilibrium state from the statistical mechanics equilibrium are found. They increase with energy per baryon and lead to a strong enhancement of the pion number density as compared to statistical mechanics estimates at SPS energies.     

Rapidity dependence of multiplicity fluctuations and correlations in high-energy nucleus–nucleus interactions

The multiplicity fluctuations of the produced pions were studied using scaled variance method in ¹⁶O–AgBr interactions at 2.1 AGeV, ²⁴Mg–AgBr interactions at 4.5 AGeV, ¹²C–AgBr interactions at 4.5 AGeV, ¹⁶O–AgBr interactions at 60 AGeV and ³²S–AgBr interactions at 200 AGeV at two different binning conditions. In the first binning condition, the rapidity interval was varied in steps of one centring about the central rapidity until it reached 14. In the second case, the rapidity interval was increased in steps of 1.6 up to 14.4. Multiplicity distributions and their scaled variances were presented as a function of the dependence on the rapidity width for both the binning conditions. Multiplicity fluctuations were found to increase with the increase of rapidity interval and later found to saturate at larger rapidity window for all the interactions and in both the binning conditions. Multiplicity fluctuations were found to increase with the energy of the projectile beam. The values of the scaled variances were found to be greater than one in all the cases in both the binning conditions indicating the presence of correlation during the multiparticle production process in high-energy nucleus–nucleus interactions. Experimental results were compared with the results obtained from the Monte Carlo simulated events for all the interactions. The Monte Carlo simulated data showed very small values of scaled variance suggesting very small fluctuations for the simulated events. Experimental results obtained from ¹⁶O–AgBr interactions at 60 AGeV and ³²S–AgBr interactions at 200 AGeV were compared with the events generated by Lund Monte Carlo code (FRITIOF model). FRITIOF model failed to explain the multiplicity fluctuations of pions emitted from ¹⁶O–AgBr interactions at 60 AGeV for both the binning conditions. However, the experimental data agreed well with the FRITIOF model for ³²S–AgBr interactions at 200 AGeV.     

Folding model analysis of the nucleus–nucleus scattering based on Jacobi coordinates

This paper presents the results of scattering of ¹⁶O+²⁰⁹Bi interaction near the Coulomb barrier. The interaction potential between two nuclei is calculated using the double folding model with the effective nucleon–nucleon (NN) interaction. The calculations of the exchange part of the interaction were assumed to be of finite-range and the density dependence of the NN interaction is accounted for. Also the results are compared with the zero-range approximation. All these calculations are done using the wave functions of the two colliding nuclei in place of their density distributions. The wave functions are obtained by the D-dimensional wave equation using the hyperspherical calculations on the basis of Jacobi coordinates. The numerical results for the interaction potential and the differential scattering are in good agreement with the previous works.     

A search for ? meson nucleus bound state using antiproton annihilation on nucleus

The mass shift of the vector mesons in nuclei is known to be a powerful tool for investigating the mechanism of generating hadron mass from the QCD vacuum. The mechanism is known to be the spontaneous breaking of chiral symmetry. In 2007, KEK-PS E325 experiment reported about 3.4?% mass reduction of the ? meson in medium-heavy nuclei (Cu). This result is possibly one of the indications of the partial restoration of chiral symmetry in nuclei, however, unfortunately it is hard to make strong conclusions from the data. One of the ways to conclude the strength of the ? meson mass shift in nuclei will be by trying to produce only slowly moving ? mesons where the maximum nuclear matter effect can be probed. The observed mass reduction of the ? meson in the nucleus can be translated as the existence of an attractive force between ? meson and nucleus. Thus, one of the extreme conditions that can be achieved in the laboratory is indeed the formation of a ?-nucleus bound state, where the ? meson is ??trapped?? in the nucleus. The purpose of the experiment is to search for a ?-nucleus bound state and measure the binding energy of the system. We will demonstrate that a completely background-free missing-mass spectrum can be obtained efficiently by $(\bar{p}, \phi)$ spectroscopy together with K ^?+??? tagging, using the primary reaction channel $\bar{p} p \rightarrow \phi \phi$ . This paper gives an overview of the physics motivation and the detector concept, and explains the direction of the initial research and development effort.     

Empirical constraints on parton energy loss in nucleus–nucleus collisions at RHIC

We present empirical features of parton energy loss in nucleus–nucleus collisions at RHIC through studies of the spectra and nuclear modification factors (R_AA$R_{AA}$) for charged hadrons, neutral pions (π⁰${\pi }^0$) and non-photonic electrons. The flat distribution of R_AA$R_{AA}$ at high transverse momentum (pT$p_T$) for a given collision centrality is consistent with a scenario where parton energy loss ΔpT$\mathrm{\Delta }{p}_T$ is proportional to pT$p_T$. The centrality dependence of the parton energy loss indicates the absence of path length dependence in the magnitude of energy loss. The lack of strong path length dependence suggests a dynamical picture where the dense partonic medium undergoes rapid expansion and the density of the medium falls rapidly in the first a few Fermi interval, which may be much shorter than the full path length. Implications of the empirical constraints on the parton energy loss will also be discussed.     

Forward–backward emission of target evaporated fragments in high energy nucleus–nucleus collisions

The multiplicity distribution, multiplicity moment, scaled variance, entropy and reduced entropy of target evaporated fragments emitted in forward and backward hemispheres in 12 A Ge V4 He, 3.7 A Ge V16 O,60 A Ge V16 O, 1.7 A Ge V84 Kr and 10.7 A Ge V197Au-induced emulsion heavy target(Ag Br) interactions are investigated. It is found that the multiplicity distribution of target evaporated fragments emitted in both forward and backward hemispheres can be fitted by a Gaussian distribution. The multiplicity moments of target evaporated particles emitted in the forward and backward hemispheres increase with the order of the moment q, and the secondorder multiplicity moment is energy independent over the entire energy range for all the interactions in the forward and backward hemisphere. The scaled variance, a direct measure of multiplicity fluctuations, is close to one for all the interactions, which indicate a correlation among the produced particles. The entropy of target evaporated fragments emitted in both forward and backward hemispheres are the same within experimental errors.     

Probing the QCD equation of state with thermal photons in nucleus–nucleus collisions at RHIC

Thermal photon production at mid-rapidity in Au+Au reactions at is studied in the framework of a hydrodynamical model that describes efficiently the bulk identified hadron spectra at RHIC. The combined thermal plus NLO pQCD photon spectrum is in good agreement with the yields measured by the PHENIX experiment for all Au+Au centralities. Within our model, we demonstrate that the correlation of the thermal photon slopes with the charged hadron multiplicity in each centrality provides direct empirical information on the underlying degrees of freedom and on the form of the equation of state, s(T)/T³, of the strongly interacting matter produced in the course of the reaction. PACS 12.38.Mh, 24.10.Nz, 25.75.-q, 25.75.Nq     

Simulations of light antinucleus–nucleus interactions

Creations of light anti-nuclei (anti-deuterium, anti-tritium, anti-³He and anti-⁴He) are observed by collaborations at the LHC and RHIC accelerators. Some cosmic ray experiments are aimed to find the anti-nuclei in cosmic rays. To support the experimental studies of anti-nuclei a Monte Carlo simulation of anti-nuclei interactions with matter is implemented in the Geant4 toolkit. The implementation combines practically all known theoretical approaches to the problem of antinucleon-nucleon interactions.     

Atom as a “dressed” nucleus

We show that the electrostatic potential of an atomic nucleus “seen” by a fast charged projectile at short distances is quantum mechanically smeared due to nucleus motion around the atomic center of inertia. For example, the size of the “positive charge cloud” in the Hydrogen ground state is much larger than the proper proton size. For target atoms in excited initial states, the effect is even larger. The elastic scattering at large angles is generally weaker than the Rutherford scattering since the effective potential at short distances is softer than the Colombian one due to a natural “cutoff”. In addition, the large-angle scattering leads to target atom excitations due to pushing the nucleus (⇒ inelastic processes). The Rutherford cross section is in fact inclusive rather than elastic. These results are analogous to those from QED. Non-relativistic atomic calculations are presented. The difference and the value of these calculations arise from nonperturbatively (exact) nucleus “dressing” that immediately leads to correct physical results and to significant technical simplifications. In these respects a nucleus bound in an atom is a simple but rather realistic model of a “dressed” charge in the QFT. This idea is briefly demonstrated on a real electron model (electronium) which is free from infinities.      

Second-order configuration mixing in an N≠Z nucleus

The effect of the neutron excess on the effective magnetic dipole operator is studied using the second-order perturbation theory. A numerical calculation is performed for a nucleon in the 0f shell under the presence of the ⁴⁰Ca and ⁴⁸Ca core, respectively. The results are presented in terms of the effective nucleon g-factors for a neutron and a proton separately.     

Λααα cluster model for the Λ 13 C nucleus

The _Λ ¹³ C hypernucleus is treated as a (1/2)⁺ bound state of the Λααα system. The s-wave model is used on the basis of differential equations for the corresponding Yakubovsky components. No account is taken of 2+2 clustering in the system. Phenomenological potentials are used to simulate the αα and αΛ interactions. The system as a whole is bound owing to the additional potential of three-body interaction between the alpha-particle clusters. The differential equations for the Yakubovsky components are solved numerically by the cluster-reduction method. The binding energies are calculated for the ground and the first excited state of the _Λ ¹³ C hypernucleus. It is shown that the dominant type of clustering in the system is (Λαα)α.     

16O nucleus in the 4α cluster model

The ¹⁶O nucleus is treated as a bound state of the four-alpha-particle system showing 3α + α clustering. The pair interaction of the alpha particles involved is simulated by a phenomenological potential. Additional three-particle potentials are introduced in order that the entire system and its three-particle subsystems be bound. The parameters of these potentials are determined by fitting the experimental values of the binding energies and the root-mean-square radii of the ¹²C and ¹⁶O nuclei. The calculations are performed on the basis of the s-wave differential equations for the Faddeev and Yakubovsky components. The ground and the first excited state of the ¹⁶O nucleus are investigated. The most probable spatial arrangement of the alpha-particle clusters in the system is determined. The charge form factors are calculated for the ¹²C and ¹⁶O nuclei. The results of our model calculations comply well with experimental data.     

Characteristics of Λ andK 0 particles produced in central nucleus nucleus collisions at a 4.5 GeV/c momentum per incident nucleon

Data on Λ's andK ⁰'s, produced in central nucleus-nucleus collisions (C?C, C?Ne, O?Ne, C?Cu, C?Zr, C?Pb, O?Pb) at a 4.5 GeV/c momentum per incident nucleon obtained in the streamer chamber spectrometer SKM-200, are presented. Multiplicities, transverse momenta, rapidities and other characteristics are considered and compared with those for inelastic He?Li interactions. The polarization of Λ's was found to be consistent with zero. The upper limit of \(\bar \Lambda /\Lambda \) production ratio was estimated to be ≤10^?2 with a 90% confidence level. The results are compared with data of other experiments and some model calculations.     

A simplified Glauber model for hadron–nucleus cross sections

A model for hadron–nucleus cross sections based on a simplified Glauber approach is proposed. The predictions of the model are compared with experimental data for the inelastic and the total hadron–nucleus cross sections from available databases. The model has been implemented in the framework of the Gent4 toolkit.