BUU Transport Model Research Articles

Sensitivity of the evaporation residue observables to the symmetry energy

The static properties of the heaviest residue and unbound particles produced in central $^{64,58}Ni$ on $^{64,58}Ni$ collisions at 50 MeV/nucleon are predicted within the BUU transport model, in order to explore the sensitivity of those observables to the density dependence of the symmetry energy. We include fluctuations in the collision integral and use a meta-modelling for the mean-field which allows an independent variation of the different empirical parameters of the equation of state. We find that the isospin ratio of pre-equilibrium particles is a good estimator of the stiffness of the symmetry energy, in agreement with previous works. In addition to that, whatever be the functional form of the equation of state, we show that a higher symmetry energy at subsaturation densities leads to an increased size and isotopic ratio for the heaviest residue. This is understood in terms of energy sharing between the pre-equilibrium particles and the (quasi)fused system. The combination of the two observables might be an interesting tool to constrain the different density dependence below and above saturation, which is linked to the relatively poorly known parameter $K_{sym}$.

Angular dependence in proton-proton correlation functions in central40Ca + 40Ca and48Ca + 48Ca reactions

The angular dependence of proton-proton correlation functions is studied in central $^{40}Ca+^{40}Ca$ and $^{48}Ca+^{48}Ca$ nuclear reactions at E=80 MeV/A. Measurements were performed with the HiRA detector complemented by the 4$\pi$ Array at NSCL. A striking angular dependence in the laboratory frame is found within p-p correlation functions for both systems that greatly exceeds the measured and expected isospin dependent difference between the neutron-rich and neutron-deficient systems. Sources measured at backward angles reflect the participant zone of the reaction, while much larger sources observed at forward angles reflect the expanding, fragmenting and evaporating projectile remnants. The decrease of the size of the source with increasing momentum is observed at backward angles while a weaker trend in the opposite direction is observed at forward angles. The results are compared to the theoretical calculations using the BUU transport model.

Shear viscosity to entropy density ratio in BUU transport model

Shear viscosity (eta) is a basic transport coefficient of the medium. In this work, we calculate shear viscosity to entropy density ratio (eta/S) of an equilibrated system in intermediate energy heavy ion collisions within the framework of the Boltzmann-Uehling-Uhlenbeck model (BUU) model. After the equilibration of Au + Au system at central collision in a fixed volume is reached, temperature, pressure and energy density are extracted by the phase space information and then eta/S is calculated using the Green-Kubo formulas. The results show that eta/S drops with the incident energy and its value is not so drastically different from the RHIC results.

Fragment formation in proton induced reactions within a BUU transport model

The formation of fragments in proton-induced reactions at low relativistic energies within a combination of a covariant dynamical transport model and a statistical approach is investigated. In particular, we discuss in detail the applicability and limitations of such a hybrid model by comparing data on fragmentation at low relativistic SIS/GSI-energies.

Pion-induced double-charge exchange reactions in the Δ resonance region

We have applied the Giessen BUU (GiBUU) transport model to the description of the double-charge exchange (DCX) reaction of pions with different nuclear targets at incident kinetic energies of 120--180 MeV. The DCX process is highly sensitive to details of the interactions of pions with the nuclear medium and, therefore, represents a major benchmark for any model of pion scattering off nuclei at low and intermediate energies. The impact of surface effects, such as the neutron skins of heavy nuclei, is investigated. The dependence of the total cross section on the nuclear mass number is also discussed. We achieve a good quantitative agreement with the extensive data set obtained at LAMPF. Furthermore, we compare the solutions of the transport equations obtained in the test-particle ansatz using two different schemes: the full and the parallel ensemble methods.

Low-energy pions in nuclear matter and ππ photoproduction within a BUU transport model

A description of the low-energy scattering of pions and nuclei within a BUU transport model is presented. Implementing different scenarios of medium modifications, the mean free path of pions in nuclear matter at low momenta and pion absorption reactions on nuclei have been studied and compared to data and to results obtained via quantum-mechanical scattering theory. We show that even in a regime of a long pionic wavelength the semi-classical transport model is still a reliable framework for pion kinetic energies greater than ≈ 20-30MeV. Results are presented on π-absorption cross-sections in the regime of 10 MeV ⩽ T π kin ⩽ 130MeV and on photon-induced ππ production at incident beam energies of 400-500MeV.

Dilepton production at SIS energies

In this short review the results of detailed studies for dilepton production from p+A andA+A reactions at SIS energies are presented. The calculations are based on a semi-classical BUU transport model that includes the off-shell propagation of vector mesons and evaluates the width of the vector mesons dynamically. Different scenarios of in-medium modifications of vector mesons, such as collisional broadening and dropping vector meson masses, are investigated and the possibilities for an experimental observation of in-medium effects in p+A reactions at 1–4 GeV are discussed for a variety of nuclear targets.

Modelling nuclear effects in neutrino scattering

We have developed a model to describe the interactions of neutrinos with nucleons and nuclei via charged and neutral currents, focusing on the region of the quasielastic and Δ ( 1232 ) peaks. For ν N collisions a fully relativistic formalism is used. The extension to finite nuclei has been done in the framework of a coupled-channel BUU transport model where we have studied exclusive channels taking into account in-medium effects and final state interactions.

Calculations of K ± and ϕ Production in Near-Threshold Proton–Nucleus Collisions

K± and ϕ meson production in proton-nucleus (pA) collisions has been calculated within a BUU transport model. It is shown that the nucleon-hyperon strangeness transfer channel is essential. The role of three-body reactions has been investigated within the medium. The target mass dependence of ϕ production is predicted to give important information on the in-medium properties of all three mesons.

Isospin effects in two-particle correlation functions: A probe for the isospin dependence of the nuclear equation of state

Small-angle two-particle correlation functions, involving neutrons and protons, have been measured for E/A = 61 MeV Ar + Sn-112,Sn-124 collisions, i.e. systems similar in size but with different isospin asymmetry. The correlation strength of neutron-proton (np) and proton-proton (pp) pairs is stronger for the more neutron-rich system, indicating a shorter average emission time for this system. Comparisons with isospin-dependent BUU transport model simulations are presented, and their implications for the density dependence of the nuclear symmetry energy are discussed.

Hadron Attenuation at HERMES and Jefferson Lab

We investigate the attenuation of hadrons in deep inelastic lepton-nucleus scattering in the kinematical regime of the HERMES and Jefferson Lab experiments. The calculation is carried out in the framework of a BUU transport model. Our results indicate a strong influence of (pre)hadronic final state interactions on the observed multiplicity ratios.

HADRON ATTENUATION AT HERMES BY (PRE)HADRONIC FSI

We investigate hadron formation in high energy electroproduction off complex nuclei in the framework of a BUU transport model. Our approach combines a quantum mechanical treatment of the photon's initial state interactions with a semi-classical coupled channel simulation of the (pre)hadronic final-state interactions (FSI). This allows us to study the hadron attenuation observed at HERMES and to get information on the space-time picture of hadron formation separately for π±, π0, K±, p and [Formula: see text].

LOW ENERGY DOUBLE-PION PHOTOPRODUCTION IN NUCLEI

We have investigated π0π0 and π±π0 photoproduction off complex nuclei. Simulations have been performed by means of a semi-classical BUU transport model including a full coupled-channel treatment of the final state interactions. We show that the target mass dependence of the π0π0 invariant mass spectrum as measured by the TAPS collaboration can be explained without introducing medium effects beyond absorption and quasi-elastic scattering of the final state particles. On the other hand, we find considerable discrepancies with the data in the π±π0 channel, which are not understood.

Low-energy ππ photoproduction off nuclei

In the present Letter we investigate π0π0 and π±π0 photoproduction off complex nuclei at incident beam energies of 400–460 MeV. Simulations of two pion photoproduction on protons and nuclei are performed by means of a semi-classical BUU transport model including a full coupled-channel treatment of the final state interactions. Elastic scattering of the final state pions with the nucleons in the surrounding nuclear medium is found to yield a downward shift of the ππ invariant mass distribution. We show that the target mass dependence of the π0π0 invariant mass spectrum as measured by the TAPS Collaboration can be explained without introducing medium effects beyond absorption and quasi-elastic scattering of the final state particles. On the other hand, we find considerable discrepancies with the data in the π±π0 channel, which are not understood.

Hadron formation and attenuation in deep inelastic lepton scattering off nuclei

We investigate hadron formation in deep inelastic lepton scattering on N, Kr and Xe nuclei in the kinematic regime of the HERMES experiment. The elementary electron–nucleon interaction is described within the event generator PYTHIA while a full coupled-channel treatment of the final state interactions is included by means of a BUU transport model. We find a good agreement with the measured charged hadron multiplicity ratio RMh for N and Kr targets by accounting for the deceleration and absorption of the primarily produced particles as well as for the creation of secondary hadrons in the final state interactions.

Inclusive $\omega$ photoproduction off nuclei

We investigate inclusive omega photoproduction off complex nuclei, concentrating on the feasibility to examine a possible in-medium change of the omega meson properties by observing the pi^0 gamma invariant mass spectrum. The simulations are performed by means of a BUU transport model including a full coupled-channel treatment of the final state interactions. In-medium changes of the omega spectral density are found to yield a moderate modification of the observables as compared to the situation in free space. Also the effects of a momentum dependence of the strong omega potential are discussed.

ηelectroproduction on nuclei in the nucleon resonance region

We investigate eta electroproduction on nuclei for Q^2=2.4 and 3.6 GeV^2 in the framework of a coupled-channel BUU transport model. We analyze the importance of final state interactions and side feeding and compare with findings drawn from eta photoproduction. It is shown that in contrast to photoproduction the influence of etas stemming from secondary processes becomes important at high Q^2.

In-medium modifications of theS11(1535)resonance andηphotoproduction

We investigate the influence of possible in-medium modifications of the S11(1535) in eta photoproduction in nuclei. Besides Fermi motion, Pauli blocking and binding effects also collisional broadening is accounted for. The in-medium width is obtained from a realistic resonance-hole model. Results on eta photoproduction are obtained from a semi-classical BUU transport model and compared with data. We find that calculations including a momentum dependent nucleon and resonance potential are in agreement with the recent KEK data. In contrast, collisional broadening has only little influence.

ρ/ω properties from dilepton spectra in pA reactions at 12 GeV

The dilepton production from $pC$ and $pCu$ collisions at $T_{lab}=12$ GeV is calculated using the semi-classical BUU transport model, that includes the off-shell propagation of vector mesons nonperturbatively and calculates the width of the vector mesons dynamically. It is found that the collisional broadening of the vector meson width and dropping vector meson masses lead to a small enhancement of the dilepton yield below the $\omega$ meson mass pole in $p Cu$ collisions compared to $p C$, which is, however, not sufficient to explain the enhanced production of dileptons as observed in the experiment by the KEK-PS E325 collaboration. It is argued that such in-medium effects are expected to be small due to kinematical reasons -- the dominant part of vector mesons radiate dileptons and decay outside the nucleus due to the finite formation time for $\rho$ and $\omega$ mesons, which is large by Lorentz covariance since the produced vector mesons move with high velocity relative to the target nucleus.

Photon and electron induced particle production on nuclei

We discuss several particle production reactions on nuclei within a BUU transport model. After a short presentation of the model we show our results for the transparency ratio for the nucleon knock-out reaction in different nuclei. The influence of the nucleon spectral function on subthreshold particle production is discussed.