arXiv subjects

W. Cassing

Publications and source records attributed to W. Cassing.

At least 145 records · Page 8Linked to original sources

arXiv2001

Medium effects in the production and pi0 gamma decay of omega-mesons in pA collisions in the GeV region

The omega-resonance production and its pi0 gamma decay in p-A reactions close to threshold is considered within the Intranuclear Cascade (INC) Model. The pi0 gamma invariant mass distribution shows two components which correspond to the omega decay 'inside' and 'outside' the nucleus, respectively. The 'inside' component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for the omega-meson and its decaying pion. The relative contribution of the 'inside' component is analyzed in detail for different kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between the pi0 and gamma momenta allows to separate events related to the 'inside' omega decay from different sources of background when uncorrelated pi0's and gamma's are produced.

nucl-th↗

arXiv2001

Open charm enhancement by secondary interactions in relativistic nucleus-nucleus collisions?

We calculate open charm production in $Pb+Pb$ reactions at SPS energies within the HSD transport approach - which is based on string, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom - including the production of open charm pairs from secondary 'meson'-'baryon' (or quark-diquark and antiquark-diquark) collisions. It is argued that at collision energies close to the $c\bar{c}$ pair threshold the dominant production mechanism is related to the two body (or quasi two body) reactions $πN \to \bar{D} (\bar{D^*}) Λ_c, (Σ_c)$. Estimates within the framework of the Quark-Gluon String model suggest cross sections of a few $μb$ for $πN \to \bar{D} Λ_c$ in the region of 1 GeV above threshold. The dynamical transport calculations for $Pb + Pb$ at 160 A$\cdot$GeV indicate that the open charm enhancement reported by the NA50 Collaboration might be due to such secondary reaction mechanisms.

hep-ph↗

arXiv2001

Deuteron photodisintegration within the Quark-Gluon Strings Model and QCD motivated nonlinear Regge trajectories

We investigate deuteron two-body photodisintegration within the framework of the Quark-Gluon Strings Model with nonlinear baryon Regge trajectories. Special attention is paid to the use of QCD motivated Regge trajectories of the logarithmic and square-root form which have been suggested recently by Brisudová, Burakovsky and Goldman. We find that the recent experimental data from TJNAF in the few GeV region can reasonably be described by the model. Angular distributions at different $γ$-energies are presented and the effect of a forward-backward asymmetry is discussed. Predictions for the energy dependence of $dσ/dt$ at higher energies and different $Θ_{cm}$ are presented, too.

hep-ph↗

arXiv2001

Open charm production in relativistic nucleus-nucleus collisions

We calculate excitation functions for open charm mesons in $Au+Au$ reactions from AGS to RHIC energies within the HSD transport approach which is based on string, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom. The open charm cross sections from $pN$ and $πN$ reactions are fitted to results from PYTHIA and scaled in magnitude to the available experimental data. From our dynamical calculations we find an approximate $m_T$-scaling for pions, kaons, $D$-mesons and $J/Ψ$ -- when discarding final state elastic scattering of kaons and $ϕ$-mesons with pions -- in central collisions of $Au + Au$ at 160 A$\cdot$GeV (with an apparent slope of 176 MeV) without employing the assumption of a Quark-Gluon Plasma (QGP). We demonstrate that this result is essentially due to a relative $m_T$-scaling in $pp$ collisions at $\sqrt{s} \approx$ 17.3 GeV. At lower bombarding energies of 25 A$\cdot$GeV a suppression of $D$-mesons by a factor of $\sim$ 10 relative to a global $m_T$-scaling with slope 143 MeV is expected. However, when incorporating attractive $D$-meson self energies as suggested by QCD sum rules, an approximate $m_T$-scaling is regained even at 25 A$\cdot$GeV. The effects of $D$-meson rescattering and charmonium absorption are discussed, furthermore, with respect to rapidity and transverse mass distributions in central collisions of $Au + Au$ at 25, 160 A$\cdot$GeV and 21.3 A$\cdot$TeV.

nucl-th↗

arXiv2000

Nonmesonic decay of the Lambda-hyperon in hypernuclei produced by p+Au collisions

The lifetime of the Lambda-hyperon for the nonmesonic decay Lambda N ---> N N has been determined by a measurement at COSY Juelich of the delayed fission of heavy hypernuclei produced in proton - Au collisions at T_p=1.9 GeV. It is found that heavy hypernuclei with mass numbers A= 180 +- 5 and atomic numbers Z= 74 +-2 fission with a lifetime 130ps +- 13ps (stat.) +- 15ps (syst.) . This value together with the results obtained for other heavy hypernuclei in previous investigations indicates (on the confidence level of 0.9) a violation of the phenomenological Delta I = 1/2 rule for Lambda N ---> NN transitions as known from the weak mesonic decays of kaons and hyperons. PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}

hep-ex↗

arXiv2000

Perspectives of $e^+e^-$ production in pp, pd and p Be reactions at SIS energies

We study dilepton production from pp, pd and p Be collisions from 1 - 5 GeV including the $π^0$, $η$, $ω$ and $Δ$ Dalitz decays, direct decays of vector mesons ($ρ$, $ω$) as well as subthreshold $ρ$ production via baryonic resonances (e.g. $D_{13}(1520), P_{11}(1710)$). Our calculations compare rather well with the pp and pd data from the DLS Collaboration, however, overestimate slightly the 'old' p Be data from that group. Futhermore, detailed predictions for differential dilepton spectra at SIS energies are made with a high mass resolution that can be controlled experimentally by the HADES Collaboration in near future.

nucl-th↗

arXiv2000

Squeeze-out of nuclear matter in peripheral heavy-ion collisions and momentum-dependent effective interactions

We perform a systematic study of in-plane and out-of-plane proton and neutron flow from nucleus-nucleus collisions within the BUU transport approach employing different parameter sets for the mean-field potentials that can be characterized by the nuclear incompressibility and the stiffness of the momentum-dependent potential. We find that a simultaneous description of the experimental data from the BEVALAC and the SIS on both the nucleon squeeze-out $v_2$ and the in-plane flow $F$ at beam energies $E_{lab} = 0.15 ÷2$ AGeV requires a mean field with strong momentum dependence, i.e. an effective Landau mass $m^* \simeq 0.68 m_0$ at normal nuclear matter density $ρ_0=0.17$ fm$^{-3}$, where $m_0=0.938$ GeV is the bare nucleon mass. Some experimental data on the squeeze-out require an even stiffer momentum dependence ($m^* \approx 0.62 m_0$). All systems investigated are found to be compatible with $m^*/m_0 = 0.65 \pm 0.03$.

nucl-th↗

arXiv2000

Low energy kaon photoproduction from nuclei

We study $K^+$-meson production in $γ{A}$ interaction at energies below the reaction threshold in free space. The Thomas-Fermi and spectral function approaches are used for the calculations of the production process. It is found that the measurement of the differential spectra may allow to reconstruct the production mechanism and to investigate the dispersion relations entering the production vertex. It is shown that the contribution from secondary pion induced reactions to the total kaon photoproduction is negligible for $E_γ{<}$1.2 GeV so that strangeness production at low energies is sensitive to the nuclear spectral function.

nucl-th↗

arXiv2000

Production of $a_0$-mesons in the reactions $πN \to a_0 N$ and $p p \to d a_0^+$ at GeV energies

We investigate the reactions $πN \to a_0 N$ and $p p \to d a_0^+$ near threshold and at medium energies. An effective Lagragian approach and the Regge pole model are applied to analyze different contributions to the cross section of the reaction $πN \to a_0 N$. These results are used to calculate the differential and total cross sections of the reaction $p p \to d a_0^+$ within the framework of the two-step model in which two nucleons produce an $a_0$-meson via $π$ -meson exchange and fuse to a deuteron. The necessity of new measurements on $a_0$ production and branching fractions (of its decay to the $K\bar K$ and $πη$ channels) is emphasized for clarifying the $a_0$ structure. Detailed predictions for the reaction $pp \to d a_0^+$ are presented for the energy regime of the proton synchrotron COSY-Jülich.

nucl-th↗

arXiv2000

Thermal and chemical equilibration of hadronic matter

We study thermal and chemical equilibration in 'infinite' hadron matter as well as in finite size relativistic nucleus-nucleus collisions using a BUU cascade transport model with resonance and string degrees-of-freedom. The 'infinite' hadron matter is simulated within a cubic box employing periodic boundary conditions. The various equilibration times depend on baryon density and energy density and are much shorter for particles consisting of light quarks then for particles including strangeness. For kaons and antikaons the chemical equilibration time is found to be larger than $\simeq$ 40 fm/c for all baryon and energy densities considered. The inclusion of continuum excitations, i.e. hadron 'strings', leads to a limiting temperature of $T_s\simeq$ 150 MeV.

nucl-th↗

arXiv2000

Perspectives of scalar- and vector-meson production in hadron-nucleus reactions

The production and decay of vector mesons ($ρ, ω$) in $pA$ reactions at COSY energies is studied with particular emphasis on their in-medium spectral functions. It is explored within transport calculations, if hadronic in-medium decays like $π^+π^-$ or $π^0 γ$ might provide complementary information to their dilepton ($e^+e^-$) decays. Whereas the $π^+ π^-$ signal from the $ρ$-meson is found to be strongly distorted by pion rescattering, the $ω$-meson Dalitz decay to $π^0 γ$ appears promising even for more heavy nuclei. The perspectives of scalar meson ($f_0, a_0$) production in $pp$ reactions are investigated within a boson-exchange model indicating that the $f_0$-meson might hardly be detected in these collisions in the $K \bar{K}$ or $ππ$ decay channels whereas the exclusive channel $pp \to d a_0^+$ looks very promising.

nucl-th↗

arXiv2000

On the current status of OZI violation in $πN$ and pp reactions

The available data on $ω$ and $ϕ$ production from $πN$ and $pp$ collisions are reanalyzed with respect to an OZI rule violation on the basis of transition matrix elements. The data are found to be compatible with a constant ratio $R$ up to excess energies $ε{\approx}300$ MeV, however, the ratio $R$ deviates substantially from the SU(3) prediction based on the present knowledge of the $ϕ-ω$ mixing angle.

nucl-th↗

arXiv2000

On the $Λ$ to $Σ^0$ ratio from proton-proton collisions

We compare the recent COSY data on the total $pp{\to}pΛ{K^+}$ and $pp{\to}pΣ^0{K^+}$ cross sections with the predictions from two boson exchange models that either are based solely on $π$ and $K$ exchange or include $π$, $η$ and $ρ$ exchange as well as the virtual excitation of intermediate baryon resonances. Both models are found to reproduce the data after the inclusion of final state hyperon-nucleon interactions. Thus, within the experimental uncertainties, both models also roughly reproduce the strong dependence of the $Λ$ to $Σ^0$ ratio as a function of the excess energy near threshold, as well as at higher energies.

nucl-th↗

arXiv2000

Studying the $ω$ properties in pA collisions via the $ω{\to}π^0γ$ decay

Within transport calculations we study the production and decay of $ω$-mesons in $pA$ reactions at COSY energies including elastic and inelastic $ωN$ rescattering, the $ω{\to}π^0γ$ Dalitz decay as well as $π^0 N$ rescattering. The resulting invariant $π^0 γ$ mass distributions indicate that in-medium modifications of the $ω$-meson may be observed experimentally.

nucl-th↗

arXiv2000

Aspects of thermal and chemical equilibration of hadronic matter

We study thermal and chemical equilibration in 'infinite' hadron matter as well as in finite size relativistic nucleus-nucleus collisions using a BUU cascade transport model that contains resonance and string degrees-of-freedom. The 'infinite' hadron matter is simulated within a cubic box with periodic boundary conditions. The various equilibration times depend on baryon density and energy density and are much shorter for particles consisting of light quarks then for particles including strangeness. For kaons and antikaons the chemical equilibration time is found to be larger than $\simeq$ 40 fm/c for all baryon and energy densities considered. The inclusion of continuum excitations, i.e. hadron 'strings', leads to a limiting temperature of $T_s\simeq$ 150 MeV. We, furthermore, study the expansion of a hadronic fireball after equilibration. The slope parameters of the particles after expansion increase with their mass; the pions leave the fireball much faster then nucleons and accelerate subsequently heavier hadrons by rescattering ('pion wind'). If the system before expansion is close to the limiting temperature $T_s$, the slope parameters for all particles after expansion practically do not depend on (initial) energy and baryon density. Finally, the equilibration in relativistic nucleus-nucleus collision is considered. Since the reaction time here is much shorter than the equilibration time for strangeness, a chemical equilibrium of strange particles in heavy-ion collisions is not supported by our transport calculations. However, the various particle spectra can approximately be described within the blast model.

nucl-th↗

arXiv2000

Equilibration within a semiclassical off-shell transport approach

Equilibration times for nuclear matter configurations -- modelling intermediate and high energy nucleus-nucleus collisions -- are evaluated within the semiclassical off-shell transport approach developed recently. The transport equations are solved for a finite box in coordinate space employing periodic boundary conditions. The off-shell transport model is shown to give proper off-shell equilibrium distributions in the limit $t \to \infty$ for the nucleon and $Δ$-resonance spectral functions. We find that equilibration times within the off-shell approach are only slightly enhanced as compared to the on-shell limit for the momentum configurations considered.

nucl-th↗

arXiv2000

Dileptons and hadron spectral functions in heavy-ion collisions

We briefly review the dilepton experiments at BEVALAC/SIS and SPS energies for p+p, p+A and A+A collisions as well as our present understanding of the data within transport theoretical simulations. Since dileptons from p+A and A+A collisions in particular probe the in-medium spectral functions of vector mesons, a novel semiclassical off-shell transport approach is introduced on the basis of the Kadanoff-Baym equations that describes the dynamical evolution of broad hadron spectral functions. The implications of the off-shell dynamics -- relative to the conventional on-shell transport dynamics -- are discussed for proton spectra, high energy $γ$, pion, kaon and antikaon production from GANIL to AGS energies in comparison to experimental data.

nucl-th↗

arXiv2000

Semiclassical transport of hadrons with dynamical spectral functions in A+A collisions at SIS/AGS energies

The transport of hadrons with dynamical spectral functions A_h(X,\vec{P},M^2) is studied for nucleus-nucleus collisions at SIS and AGS energies in comparison to the conventional quasi-particle limit and the available experimental data within the recently developed off-shell HSD transport approach. Similar to reactions at GANIL energies the off-shell effects show up in high momentum tails of the particle spectra, however, at SIS and AGS energies these modifications are found to be less pronounced than at lower energies due to the high lying excitations of the nucleons in the collision zone.

nucl-th↗