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W. Cassing

Publications and source records attributed to W. Cassing.

At least 127 records · Page 7Linked to original sources
arXiv2003

Nonmesonic decay of the Lambda hyperon in nuclear matter - implications on the weak Lambda-N interaction

The lifetime of the Lambda hyperon in heavy hypernuclei as measured by the COSY-13 Collaboration in proton - Au, Bi and U collisions has been analysed to yield tau(Lambda) = (145 +- 11) ps. This value for tau(Lambda) is compatible with the lifetime extracted from antiproton annihilation on Bi and U targets, however, much more accurate. We find that the dependence of the lifetime tau(Lambda) on the mass of hypernuclei indicates a violation of the phenomenological Delta I = 1/2 rule known from the weak mesonic decays of strange particles. PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}

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arXiv2002

In-medium spectral functions of charmonia studied by $\bar p + A$ reactions

We study the perspectives of resonant charmonium production in $\bar{p} + A$ reactions within the Multiple Scattering Monte Carlo (MSMC) approach. We calculate the production of the resonances $Ψ(2S)$ and $Ψ(3770)$ on various nuclei, their propagation and decay to dileptons and $D+ \bar{D}$ in the medium and vacuum, respectively, employing parametrizations for the $D,\bar{D}$ self energies taken from QCD sum rule studies. The elastic and inelastic interactions of the charmonia and open charm mesons in the medium are taken into account, too. It is found that the $D, \bar{D}$ invariant mass spectra from light and heavy nuclei are not sufficiently sensitive to the in-medium properties of the $Ψ(2S)$ and $Ψ(3770)$. However, a 'suppression' of low mass dileptons from the $Ψ(3770)$ might be seen experimentally as well as a small broadening of the $Ψ(2S)$ dilepton spectra.

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arXiv2002

Hadronic observables from SIS to SPS energies - anything strange with strangeness ?

We calculate $p, π^\pm, K^\pm$ and $Λ$(+$Σ^0$) rapidity distributions and compare to experimental data from SIS to SPS energies within the UrQMD and HSD transport approaches that are both based on string, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom. The two transport models do not include any explicit phase transition to a quark-gluon plasma (QGP). It is found that both approaches agree rather well with each other and with the experimental rapidity distributions for protons, $Λ$'s, $π^\pm$ and $K^\pm$. Inspite of this apparent agreement both transport models fail to reproduce the maximum in the excitation function for the ratio $K^+/π^+$ found experimentally between 11 and 40 A$\cdot$GeV. A comparison to the various experimental data shows that this 'failure' is dominantly due to an insufficient description of pion rapidity distributions rather than missing 'strangeness'. The modest differences in the transport model results -- on the other hand -- can be attributed to different implementations of string formation and fragmentation, that are not sufficiently controlled by experimental data for the 'elementary' reactions in vacuum.

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arXiv2002

Exclusive and semi-inclusive strangeness and charm production in $πN$ and $NN$ reactions

Using the Quark-Gluon Strings Model (QGSM) combined with Regge phenomenology we consider the reactions $π^- p \to K^0 Λ$ and $π^- p \to D^- Λ_c^+$ which are dominated by the contributions of the $K^*$ and $D^*$ Regge trajectories, respectively. The spin structure of the amplitudes is described by introducing Reggeized Born terms. It is found that the existing data for the reaction $π^- p \to K^0 Λ$ are in reasonable agreement with the model predictions. To describe the absolute values of the cross sections it is necessary to introduce also suppression factors which can be related to absorption corrections. Furthermore, assuming the SU(4) symmetry to hold for Regge residues and the universality of absorption corrections we calculate the cross section of the reaction $π^- p \to D^- Λ_c^+$. Employing the latter results from $π^- p$ reactions we then estimate the contributions of the pion exchange mechanism to the cross sections of the reactions $NN \to NKΛ$ and $NN \to N\bar D Λ_c$ and compare them with the contributions of the $K$ and $D$ exchanges. We find that the $NN$ reactions are dominated not by pion exchange but by $K$ and $D$ exchanges, respectively. Moreover, assuming the SU(4) symmetry to hold approximately for the coupling constants $g_{ND Λ_c}$ = $g_{NK Λ}$ we analyze also the production of leading $Λ_c$ hyperons in the reaction $NN \to Λ_c X$. It is shown that the non-perturbative mechanism should give an essential contribution to the $Λ_c$ yield for $x \geq 0.5$.

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arXiv2002

Transport analysis of in-medium hadron effects in pA and AA collisions

The production and decay of vector mesons ($ρ, ω$) in pA and AA reactions 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 in $γA$ and pA reactions. Furthermore, the influence of nucleon and kaon/antikaon potentials on the $K^\pm$ yields and spectra in pA collisions is calculated and compared to the recent data from the ANKE Collaboration.

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arXiv2002

Polarization observables in high-energy deuteron photodisintegration within the Quark-Gluon Strings Model

Deuteron two-body photodisintegration is analysed within the framework of the Quark-Gluon Strings Model. The model describes fairly well the recent experimental data from TJNAF in the few GeV region. Angular distributions at different $γ$-energies are presented and the effect of a forward-backward asymmetry is discussed. New results from the QGSM for polarization observables from 1.5 -- 6 GeV are presented and compared with the available data.

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arXiv2002

Differential Flow of Protons in Au+Au Collisions at AGS Energies

We study the proton sideward and elliptic differential flow for Au+Au collisions at AGS energies (2 -- 8 A cdot GeV) in a microscopic relativistic transport model that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher hadronic excitations. In order to explore the sensitivity of the various differential flows to the nuclear equation of state (EoS) we use three different parameterizations of the scalar- and vector mean-fields, i.e. NL2 (soft), NL23 (medium) and NL3 (hard), with their momentum dependence fitted to the experimental Schrodinger equivalent potential (at normal nuclear matter density rho_0) up to kinetic energies of 1 GeV. We calculate the excitation function of sideward and elliptic flow within these parameter sets for Au+Au collisions and compare with the recent data from the E895 Collaboration as a function of rapidity, impact parameter and transverse momentum, respectively. We find that the best description of the differential data is provided by a rather 'stiff' EoS at 2 A cdot GeV (NL3) while at higher bombarding energies (4--8 A cdot GeV) a 'medium' EoS leads to the lowest chi^2 with respect to the data. However, the differences in the transverse and elliptic flows (from the different parameter sets) become of minor significance at 4--8 A cdot GeV. We attribute this insensitivity to a similar reduction of the vector potential in all models and to the dominance of string degrees of freedom at these bombarding energies.

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arXiv2002

Production of $a_0$-mesons in pp and pn reactions

We investigate the cross section for the reaction $NN \to NNa_0$ near threshold and at medium energies. An effective Lagrangian approach with one-pion exchange is applied to analyze different contributions to the cross section for different isospin channels. The Reggeon exchange mechanism is also considered. The results are used to calculate the contribution of the $a_0$ meson to the cross sections and invariant $K \bar K$ mass distributions of the reactions $pp\to pn K^+\bar K^0$ and $pp\to pp K^+K^-$. It is found that the experimental observation of $a_0^+$ mesons in the reaction $pp\to pn K^+\bar K^0$ is much more promising than the observation of $a_0^0$ mesons in the reaction $pp\to pp K^+K^-$.

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arXiv2002

Near-threshold production of $a_0(980)$-mesons in $πN$ and NN collisions and $a_0/f_0$-mixing

We consider near-threshold $a_0(980)$-meson production in $πN$ and $NN$ collisions. An effective Lagrangian approach with one-pion exchange is applied to analyze different contributions to the cross section for different isospin channels. The Reggeon exchange mechanism is also evaluated for comparison. The results from $πN$ reactions are used to calculate the contribution of the $a_0$ meson to the cross sections and invariant $K \bar K$ mass distributions of the reactions $pp\to pn K^+\bar K^0$ and $pp\to pp K^+K^-$. It is found that the experimental observation of $a_0^+$ mesons in the reaction $pp\to pn K^+\bar K^0$ is much more promising than the observation of $a_0^0$ mesons in the reaction $pp\to pp K^+K^-$. Effects of isospin violation in the reactions $pN \to d a_0$, $pd \to \mathrm{^3He/^3H} a_0$, and $ dd \to \mathrm{^4He} a_0$, which are induced by $a_0(980)$--$f_0(980)$ mixing, are also analyzed.

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arXiv2002

Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei

The ratio of forward K+ production on copper, silver and gold targets to that on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as a function of the kaon momentum p_K using the ANKE spectrometer at COSY-Juelich. The strong suppression in the ratios observed for p_K<200-250 MeV/c can be ascribed to a combination of Coulomb and nuclear repulsion in the K+A system. This opens a new way to investigate the interaction of K+-mesons in the nuclear medium. Our data are consistent with a K+A nuclear potential of V_K~20 MeV at low kaon momenta and normal nuclear density. Given the sensitivity of the data to the kaon potential, the current experimental precision might allow one to determine V_K to better than 3 MeV.

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arXiv2002

Probing hadronic formation times with antiprotons in p+A reactions at AGS energies

The production of antiprotons in $p+A$ reactions is calculated in a microscopic transport approach employing hadronic and string degrees of freedom (HSD). It is found that the abundancies of antiprotons as observed by the E910 Collaboration in $p+A$ reactions at 12.3 GeV/c as well as 17.5 GeV/c can approximately be described on the basis of primary proton-nucleon and secondary meson-baryon production channels for all targets. The transport calculations demonstrate that the antiproton rapidity distributions for heavy targets are sensitive to the $\bar{p}$ (or hadron) formation time in the nuclear medium. Within our analysis the data from the E910 Collaboration are reasonably described with a formation time of $ 0.4-0.8$ fm/c in the hadron rest frame.

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arXiv2002

Probing the interactions of charmed mesons with nuclei in $\bar p$ induced reactions

We study the perspectives of resonant and nonresonant charmed meson production in $\bar{p} + A$ reactions within the Multiple Scattering Monte Carlo (MSMC) approach. We calculate the production of the resonances $Ψ(3770), Ψ(4040)$ and $Ψ(4160)$ on various nuclei, their propagation and decay to $D, \bar{D}, D^*, \bar{D}^*, D_s, \bar{D}_s$ in the medium and vacuum, respectively. The modifications of the open charm vector mesons in the nuclear medium are found to be rather moderate or even small such that dilepton spectroscopy will require an invariant mass resolution of a few MeV. Furthermore, the elastic and inelastic interactions of the open charm mesons in the medium are taken into account, which can be related to $(u,d)$-, $s$- or $c$-quark exchange with nucleons. It is found that by studying the $D/\bar{D}$ ratio for low momenta in the laboratory ($\leq 2-2.2$ GeV/c) as a function of target mass $A$ stringent constraints on the $c$-quark exchange cross section can be obtained. On the other hand, the ratios $D^-_s/D^+_s$ as well as $D/D^-_s$ and $D/D^+_s$ at low momenta as a function of $A$ will permit to fix independently the strength of the $s$-quark exchange reaction in $D^-_s N$ scattering.

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arXiv2002

K$^+$ production in proton-nucleus reactions and the role of momentum-dependent potentials

The production of $K^+$ mesons in proton-nucleus collisions from 1.0 to 2.5 GeV is analyzed with respect to one-step nucleon-nucleon $(NN\to N Y K^+$) and two-step $Δ$-nucleon $(ΔN \to K^+ Y N$) or pion-nucleon $(πN \to K^+ Y $) production channels on the basis of a coupled-channel transport approach (CBUU) including the kaon final state interactions. The influence of momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are studied as well as the importance of $K^+$ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental $K^+$ spectra taken at LBL Berkeley, SATURNE, CELSIUS, GSI and COSY-Jülich. It is found that the momentum-dependent baryon potentials effect the excitation function of the $K^+$ cross section; at low bombarding energies of $\sim $ 1.0 GeV the attractive baryon potentials in the final state lead to a relative enhancement of the kaon yield whereas the net repulsive potential at bombarding energies $\sim$ 2 GeV causes a decrease of the $K^+$ cross section. Furthermore it is pointed out, that especially the $K^+$ spectra at low momenta (or kinetic energy $T_K$) allow to determine the in-medium $K^+$ potential almost model independently due to a relative shift of the $K^+$ spectra in kinetic energy that arises from the acceleration of the kaons when propagating out of the nuclear medium to free space, i.e. converting potential energy to kinetic energy of the free kaon.

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arXiv2001

Antibaryon production in hot and dense nuclear matter

The production of antibaryons is calculated in a microscopic transport approach employing multiple meson fusion reactions according to detailed balance relations with respect to baryon-antibaryon annihilation. It is found that the abundancies of antiprotons as observed from peripheral to central collisions of $Pb + Pb$ at the SPS and $Au + Au$ at the AGS can approximately be described on the basis of multiple interactions of 'formed' hadronic states which drive the system to chemical equilibrium by flavor exchange or quark rearrangement reactions.

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arXiv2001

The lifetime of the Lambda hyperon bound in hypernuclei produced by p+U collisions

The nonmesonic decay of the Lambda hyperon has been investigated by observation of delayed fission from heavy hypernuclei produced in proton-U collisions at Tp = 1.9 GeV. The lifetime of heavy hypernuclei with masses A approximately 220 obtained in the present work, i.e. tau = 138 +- 6 (stat.) +-m 17 (syst.) ps, is the most accurate result for heavy hypernuclei produced in proton and antiproton induced collisions on a U target so far. PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}

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arXiv2001

Quenching of resonance production in heavy-ion collisions at 1-2 A GeV

We study the pion production in heavy-ion collisions at SIS energies within the coupled resonance BUU model. The standard calculation overpredicts the pion multiplicity in the central Au+Au collision at 1 A GeV by about a factor of 2, which is interpreted as a signature for the quenching of the resonance production/absorption in nucleon-nucleon collisions at high nuclear density. Results of the calculations taking into account a specific form of quenching are presented for pion spectra from light and heavy systems as well as for the pion in-plane and out-of-plane flow.

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arXiv2001

Studying the $ω$-mass in-medium in $γ+ A\to π^0γ+X$ reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium $ω$ mass exploiting the process $γ+A\toπ^0γ+X$ for C, Ca and Nb nuclei. The distortions due to final-state interactions of the $π^0$ and background contributions from the reaction $γ+A\toπ^0π^0+X$ are found to be small in the mass range of interest ($0.6<M_{π^0γ}<0.8$ GeV). Furthermore, the effect of the detector resolution on the $π^0γ$-mass determination is discussed.

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