Jörg Aichelin

Study of fragmentation using clusterization algorithm with realistic binding energies

We here study fragmentation using a simulated annealing clusterization algorithm (SACA) with binding energy at a microscopic level. In an earlier version, a constant binding energy (4 MeV/nucleon) was used. We improve this binding energy criterion by calculating the binding energy of different clusters using a modified Bethe–Weizsacker mass (BWM) formula. We also compare our calculations with experimental data of the ALADiN group. Nearly no effect of this modification is visible.

What determines the K- multiplicity at energies around (1-2)A GeV?

In heavy ion reactions at energies around (1-2)A GeV the measured K- yields appear rather high as compared to pp collisions as shown by the KaoS Collaboration. Employing quantum molecular dy-namics simulations, we show that this is caused by the fact that the dominant production channel is not BB-->BBK+K- but the mesonic Lambda(Sigma)pi-->K-B reaction. Because the Lambda (Sigma) stem from the reaction BB-->Lambda(Sigma)K+B, the K+ and the K- yield are strongly correlated, i.e., the K(-)/K(+) ratio occurs to be nearly independent of the impact parameter as found experimentally. The final K- yield is strongly influenced by the K+N [due to their production via the Lambda(Sigma)] but very little by the K-N potential.

Quantum molecular-dynamics approach to heavy-ion collisions with Brueckner G-matrix cross sections

Abstract We calculate the nucleon-nucleon cross section in the nuclear medium with the Brueckner G -matrix and apply it in the quantum molecular-dynamics (QMD) approach to the C-C and Nb-Nb reactions. QMD describes successfully the many-body dynamics of heavy-ion collisions using so far an isotropic NN cross section of 40 mb. Comparing the momentum transfer in transverse and longitudinal direction and the number of emitted particles calculated with the G -matrix cross section and the isotropic cross section, we find for these inclusive data only minor differences between the results in the C-C case but larger ones in the Nb-Nb case. A good agreement is obtained with the experimental double-differential cross section for charged-particle emission in the C-C reaction at E lab =84MeV/nucleon.

Quantum molecular dynamics a microscopic model from UNILAC to CERN energies

Abstract We demonstrate that the microscopic QMD approach is useful to study heavy ion collisions from fusion fussion phenomena to the quest for signals of the quark gluon plasma. We discuss the possibilities and difficulties to determine the nuclear equation of state from heavy ion collisions. We investigate the influence of momentum dependent interactions and of in medium corrections to the nucleon-nucleon cross sections in the framework of the QMD model. The model is extended to low energies by including a Pauli potential in the nucleon-nucleon interaction. We show that it is possible to extract information on the effective cross sections from the experimental rapidity distributions of the fragments. We also investigate the transverse momentum of complex fragments with and without in medium corrections. The experimental data yield evidence for a stiff equation of state. A covariant extension of the QMD model is presented, which is applied to very high energy (10…200 AGeV) heavy ion collisions. Particle production and decay of heavy resonances are included. Predictions of the stopping power at AGS and SPS are presented. The importance of secondary scattering and nuclear stopping up to the highest energies is demonstrated. This is particularly important for the recently observed enhancement of strangeness production, which was proposed as a signal for QGP formation.

Sensitivity of the transverse flow to the symmetry energy

We study the sensitivity of transverse flow to symmetry energy in the Fermi energy region as well as at high energies. We find that transverse flow is sensitive to symmetry energy and its density dependence in the Fermi energy region. We also show that the transverse flow can address the symmetry energy at densities about twice the saturation density; however, it shows insensitivity to the symmetry energy at densities {rho}/{rho}{sub 0}>2. The mechanism for the sensitivity of transverse flow to symmetry energy and its density dependence is also discussed.

Study of balance energy in central collisions for heavier nuclei

Thirty years ago it was predicted by Scheid and Greiner [1] that in heavy ion reactions the nuclei will be compressed and heated and that this yields for non central reactions to in-plane flow 〈p x 〉. More than a decade later, this conjecture was confirmed by the Plastic Ball group [2]. In the following investigations it turned out that this in-plane flow carries information on the nuclear equation of state [3]. If the nuclear equation of state is stiffer, more compressional energy will be stored in semicentral reactions and, when released, more in-plane flow will be given to the nucleons. The maximal density which is reached in a reaction depends on the beam energy as well as on the system size. The lower the beam energy the less is the compression. At very low energies, the repulsive part of the nuclear equation of state, which appears at densities above the normal nuclear matter density, is not tested anymore and the nucleons feel only the attractive mean field. A typical example is the deep inelastic reactions in which the two nuclei rotate around a common center. This rotation creates in-plane flow as well but in opposite direction: Due to the common rotation the nucleons stick together for a while and will be emitted into the direction opposite to the impact parameter whereas the in-plane flow which is caused by compression will be in the direction of the impact parameter. There is a beam energy at which the in-plane flow disappears when changing from the direction into that opposite to the impact parameter. It has been shown in the simulation of heavy ion reactions that this beam energy called balance energy, Ebal, [4, 5, 6] depends on the nucleon-nucleon (nn) cross-section in the medium as well as on the potential [5, 6]. With the very recently measured Ebal in Au + Au collisions [7] (earlier only estimated values were available [8]), a renewed interest has emerged in the field [9]. In addition to the Au system, balance energies Ebal of C + C [10], Ne + Al [10], Ar + Al [11], Ar + Al [12], Ar + Sc [7, 10, 13], Ar + V [6, 14], Zn + Al [15], Ar + Ni [9], Zn + Ti [11], Ni + Ni [7, 9], Zn + Ni [11], Kr + Nb [7, 10], Nb + Nb [4], Xe + Sn [9] and La + La [4] are also available. It is worth mentioning that most of the above studies were for the central collisions only. A few, however, also searched for the impact parameter dependence of the balance energy [7, 12, 13, 15]. Apart from the directed in-plane flow, differential as well as elliptic flow has also been predicted very recently [16]. The measurements of the balance energy over wide range of system sizes provide an excellent opportunity to pin down the role of the mass dependence, where only preliminary studies [7, 10] have been performed yet. These preliminary studies suggest a power law dependence ∝ A of the balance energy on the mass number of the system. Interestingly, most of the theoretical studies are done within the Boltzmann-Uehling-Uhlenbeck (BUU) model [4, 5, 6, 7, 10, 12, 15, 16, 17, 18, 19, 20, 21]. Some attempts, however, also exist within the framework of Quantum Molecular Dynamics (QMD) model [13, 22, 23, 24]. Heavy systems are rather rarely analyzed in these approaches. Our present aim is therefore to study the mass dependence of the balance energy in heavy colliding nuclei and to predict for the first time the disappearance of the collective in-plane flow in central U + U collision. We shall show that the mass dependence of Ebal for heavier nuclei scales approximately more as 1 √ A rather than as

Elliptic and triangular flow of heavy flavor in heavy-ion collisions

We investigate the elliptic and the triangular flow of heavy mesons in ultrarelativistic heavy-ion collisions at RHIC and the LHC. The dynamics of heavy quarks is coupled to the locally thermalized and fluid dynamically evolving quark-gluon plasma. The elliptic flow of

Nonthermal p / π Ratio at LHC as a Consequence of Hadronic Final State Interactions

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Heavy quarkonium hadron cross-section in QCD at leading twist

mesons and the centrality dependence measured at the LHC is well reproduced for purely collisional and bremsstrahlung interactions. Due to the event-by-event fluctuating initial conditions from the EPOS2 model, the

The multifragmentation of spectator matter

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