arXiv subjects

W. Cassing

Publications and source records attributed to W. Cassing.

209 records · Page 12Linked to original sources

arXiv1996

Heavy Meson Production in Proton-Nucleus Reactions with Empirical Spectral Functions

We study the production of $K^+, ρ, ω$ and $ϕ$ mesons in $p + ^{12}C$ reactions on the basis of empirical spectral functions. The high momentum, high removal energy part of the spectral function is found to be negligible in all cases close to the absolute threshold. Furthermore, the two-step process ($pN \rightarrow πN N; πN \rightarrow N + K^+, ρ, ω, ϕ$) dominates the cross section at threshold energies in line with earlier calculations based on the folding model.

nucl-th↗

arXiv1996

Probing Hadronic Polarizations with Dilepton Anisotropies

We investigate the production of $e^+e^-$ pairs from $p + Be$ and nucleus-nucleus collisions from 2 GeV/A to 200 GeV/A within a covariant transport approach and focus on the dilepton angular anisotropies as a function of the dilepton invariant mass. Whereas the low mass regime yields information about the Dalitz decays of the $Δ, η$ and $ω$, above M $\approx $0.5 GeV the angular anisotropies provide additional information about the $π^+π^- \to ρ^0 \to e^+e^-$ channel in the medium. The anisotropy coefficient is found to change its sign for M $>$ 0.5 GeV in case of nucleus-nucleus reactions when increasing the bombarding energy from 2 GeV/A to 200 GeV/A which sheds some light on the $ππ\to ρ$ dynamics.

nucl-th↗

arXiv1996

Dilepton anisotropy from $p + Be$ and $Ca + Ca$ collisions at BEVALAC energies

A full calculation of lepton-pair angular characteristics is carried out for $e^+e^-$ pairs created in $p + Be$ and $Ca + Ca$ collisions from 1.0 to 2.1 GeV/A. It is demonstrated that the dilepton decay anisotropy depends sensitively on the different sources and may be used for their disentangling. Due to the dominance of the $η$-and $Δ$-Dalitz decays and only a small anisotropy coefficient for $π^+π^-$ annihilation, the expected anisotropy coefficients show a decrease with invariant mass of the dilepton pair and change only moderately when comparing $p + Be$ and $Ca + Ca$ reactions at the same bombarding energy.

nucl-th↗

arXiv1996

Analysis of the HELIOS-3 $μ^+μ^-$ Data within a Relativistic Transport Approach

We present a nonperturbative dynamical study of $μ^+μ^-$ production in proton-nucleus and nucleus-nucleus collisions at SPS energies on the basis of the covariant transport approach HSD. For p + W reactions at 200 GeV bombarding energy the dimuon yield for invariant masses m $\leq $ 1.6 GeV is found to be dominated by the decays of the $η, ρ, ω$ and $Φ$ mesons. For 200 GeV/A S + W collisions, however, the dimuon yield shows an additional large contribution from $π^+π^-$, $K^+K^-$ and $πρ$ channels. We find that for `free' meson masses and form factors the experimental cross section is clearly underestimated for S + W in the invariant mass range 0.35 GeV $\leq$ m $\leq$ 0.65 GeV and that the HELIOS-3 data can only be reproduced within a hadronic scenario, if the $ρ/ω$-meson mass drops with baryon density. This finding suggests a partial restoration of chiral symmetry in S + W collisions at SPS energies.

hep-ph↗

arXiv1995

Decay Anisotropy of $e^+e^-$ Sources from $pN$ and $pd$ collisions

A full calculation of lepton-pair angular characteristics is carried out for $e^+e^-$ pairs created in $pp$, $pn$ and $pd$ collisions at intermediate energies. It is demonstrated that the proposed new observable, the dilepton decay anisotropy, quite sensitively changes for different sources and may be useful for their disentangling. The relevance of the dilepton decay anisotropy is shown in the context of a puzzling energy behavior for the ratio of the lepton yield from $pd$ to $pp$ reactions as observed at the BEVALAC.

nucl-th↗

arXiv1995

Dilepton Production at SPS Energies

We present a nonperturbative dynamical study of $e^+e^-$ production in proton- nucleus and nucleus-nucleus collisions at SPS energies on the basis of a covariant transport approach. For p + A reactions the dilepton yield for invariant masses m $\leq $ 1.2 GeV is found to be dominated by the decays of the $η, ρ, ω$ and $Φ$ mesons in line with the findings of the CERES collaboration. For S + Au collisions at 200 GeV/A the dilepton yield is, however, dominated by $π^+π^-$ annihilation due to the high pion densities achieved. Whereas for `free' meson masses and form factors the experimental cross section is slightly underestimated for 0.3 GeV $\leq$ m $\leq$ 0.45 GeV, different medium modifications of the $ρ$-meson appear compatible with current CERES data.

hep-ph↗

arXiv1994

Connected Green function approach to ground state symmetry breaking in $Φ^4_{1+1}$-theory

Using the cluster expansions for n-point Green functions we derive a closed set of dynamical equations of motion for connected equal-time Green functions by neglecting all connected functions higher than $4^{th}$ order for the $λΦ^4$-theory in $1+1$ dimensions. We apply the equations to the investigation of spontaneous ground state symmetry breaking, i.e. to the evaluation of the effective potential at temperature $T=0$. Within our momentum space discretization we obtain a second order phase transition (in agreement with the Simon-Griffith theorem) and a critical coupling of $λ_{crit}/4m^2=2.446$ as compared to a first order phase transition and $λ_{crit}/4m^2=2.568$ from the Gaussian effective potential approach.

hep-ph↗

arXiv1994

Two-Body Correlations in Pionic Systems

In close analogy to fermionic many-body theory the truncation of the bosonic BBGKY density matrix hierarchy on the two-body level leads to a coupled set of nonlinear equations of motion for the one-body density matrix and the two-body correlation function. These equations provide a nonperturbative description of the nonequilibrium time evolution of particle number conserving bosonic many-body systems including the dynamical resummation of parquet-like diagrams. Within this framework we study the properties of a pionic system as a function of temperature and density with focus on two-body quantities. For each temperature we find a related pion density for which the relative strength of the two-body correlation function assumes a maximum and the pionic system is far from the mean-field limit. Since these correlated phases up to $T$=200 MeV only appear at rather low pion density, the hot and dense pion gas as generated in ultrarelativistic nucleus-nucleus collisions should be well described within mean-field theory; i.e. the HBT analysis of pion sources from $π-π$ correlations should remain valid even in the case of strongly interacting pions.

nucl-th↗

arXiv1994

Constraint Correlation Dynamics of SU(N) Gauge Theories

A constraint correlation dynamics up to 4-point Green functions is proposed for SU(N) gauge theories which reduces the N-body quantum field problem to the two-body level. The resulting set of nonlinear coupled equations fulfills all conservation laws including fermion number, linear and angular momenta as well as the total energy. Apart from the conservation laws in the space-time degrees of freedom the Gauss law is conserved as a quantum expectation value identically for all times. The same holds for the Ward identities as generated by commutators of Gauss operators. The constraint dynamical equations are highly non-perturbative and thus applicable also in the strong coupling regime, as e.g. low-energy QCD problems.

nucl-th↗

arXiv1993

Compatibility of Correlation Dynamics of SU(N) Gauge Theories and Gauss Law in Temporal Gauge

A constraint correlation dynamics up to 4-point Green's functions is proposed for SU(N) gauge theories which reduces the N-body quantum field problem to the two-body level. The resulting set of nonlinear coupled equations fulfills all conservation laws including baryon number, linear and angular momenta as well as the total energy. Apart from the conservation laws in the space-time degrees of freedom the Gauss law is conserved identically for all times. The constraint dynamical equations are highly non-perturbative and thus applicable also in the strong coupling regime as e.g., low-energy QCD problems.

nucl-th↗

arXiv1993

Pion-nucleus reactions in a microscopic transport model

We analyse pion-nucleus reactions in a microscopic transport model of the BUU type, which propagates nucleons, pions, deltas and N(1440)-resonances explicitly in space and time. In particular we examine pion absorption and inelastic scattering cross sections for pion kinetic energies T(pi) =85-315MeV and various target masses. In general, the mass-dependence of the experimental data is well described for energies up to the delta-resonance (\approx 160 MeV) while the absorption cross sections are somewhat overestimated for the higher energies. In addition we study the possible dynamical effects of delta- and pion-potentials in the medium on various observables as well as alternative models for the in-medium delta-width.

nucl-th↗