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A. Sibirtsev

Publications and source records attributed to A. Sibirtsev.

At least 91 records · Page 5Linked to original sources
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.

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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.

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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.

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arXiv2000

Novel features of $J/Ψ$ dissociation in matter

We make a detailed study of the effect that the recently predicted modification of the in-medium masses of charmed mesons would have on $J/Ψ$ dissociation on pion and $ρ$-meson comovers in relativistic heavy ion collisions. We find a substantial dependence of the $J/Ψ$ absorption rates on the density and temperature of the nuclear matter. This suggests that a quantitative analysis of $J/Ψ$ dissociation in nucleus nucleus collisions should include the effects of the modification of meson masses in dense matter.

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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.

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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.

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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.

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arXiv1999

Studying the $ωN$ elastic and inelastic cross section with nucleons

We explore the possibility to measure the elastic and inelastic $ωN$ cross section in $p{+}d{\to}d{+}ω{+}p_{sp}$ and $p{+}A$ reactions. Our studies indicate that the elastic scattering cross sections can be determined for $ω$ momenta above 1 GeV/c in $p{+}d$ reactions by gating on high proton spectator momenta whereas the $ωN$ absorption cross section down to low relative $ω$ momenta is most effectively studied in $p{+}A$ reactions at beam energies 2.0--2.7 GeV

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arXiv1999

On Studying Charm in Nuclei through Antiproton Annihilation

We examine the production of open charm in antiproton annihilation on finite nuclei. The enhancement of the subthreshold production cross section, even in a nucleus as light as carbon, should provide a clean signature of the reduction in the masses of these mesons in-medium. We also show that a careful analysis of the $D^+$ and $D^-$ spectra can yield important information on the cross section for $D^{\pm}N$ scattering.

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arXiv1999

Probing the antikaon potential by K$^-$A elastic scattering

We extract the antikaon potential from differential data on elastic $K^-A$ scattering within Glauber theory at an antikaon momentum of 800 MeV/c. The $K^-N$ cross section at densities up to 0.1 $fm^3$ is found to be close to the value given in free space $K^-p$ and $K^-n$ interactions (averaged over the number of protons and neutrons) whereas the ratio $α$ of the real to imaginary part of the forward scattering amplitude $f(0)$ differs from the ratio $α$ in free space indicating a dropping of $Re f(0)$ at finite nuclear density. The extrapolated $K^-$ potential at a momentum of 0.8 GeV/c amounts to $\approx $--28 MeV at density $ρ_0$ which is substantially less than the values extracted from kaonic atoms at zero momentum or heavy-ion data at 300 MeV/c${\leq}p_K{\leq}$600 MeV/c. We, furthermore, investigate the perspectives of scattering experiments on $^{12}C$ at low ($\approx$300 MeV/c) and high ($\approx$1.2 GeV/c) momenta.

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arXiv1999

On the search for a narrow penta-quark Z$^+$ baryon in NN interactions

The possibility for an observation of a narrow penta-quark $Z^+$ baryon in $NN$ reactions is discussed. It is shown that the $pp{\to}nΣ^+K^+$ reaction at excess energies around 100 MeV above threshold provides optimal conditions for $Z^+$ baryon detection by an analysis of the $nK^+$ invariant mass spectrum, if the $Z^+$ mass is located around 1.5 GeV involving a rather narrow width.

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arXiv1999

The antikaon potential in nuclear matter at finite momentum

We study the antikaon potential at finite momentum for proton and neutron nuclear matter. Our approach is based on the momentum dependence of the real part of the total $K^-N$ scattering amplitude, which is fixed in free space by experimental data, using the dispersion relation approach. We decompose the contributions to the scattering amplitude from different processes and consider the $Λ$(1405) and $Σ$(1385) to be dissolved in nuclear matter at density $ρ_0$. Within this approach the $K^-$ potential is found to be attractive up to high kaon momenta. Our result is in line with the data on the antikaon potential evaluated from the analysis of kaonic atoms (low momenta), heavy-ion (interdmediate momenta) and proton-nucleus (high momenta) collisions.

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arXiv1999

$ωN$ final state interactions and $ω$-meson production from heavy-ion collisions

We calculate the elastic and inelastic $ωN\to ωN, \to πN, \to ρN, \to ρπN, \to ππN, \to σN$ reactions within a boson exchange approximation where the $ωρπ$ coupling constant and form factor are fixed by the reaction $πN \to ωN$ in comparison to the experimental data. We find rather large $ωN$ cross sections at low relative momenta of the $ω$-meson which leads to a substantial broadening of the $ω$-meson width in nuclear matter. The implications of the $ωN$ final state interactions are studied for $ω$ production in $^{12}C{+}^{12}C, ^{40}Ca{+}^{40}Ca$ and $^{58}Ni{+}^{58}Ni$ reactions at about 2 A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general m_T-scaling (for $π^0$ and $η$ mesons) might be controlled experimentally by the TAPS Collaboration.

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arXiv1999

Final State Interactions in Near Threshold Meson Production from pp Collisions

We analyse the experimental data on near threshold $π$, $η$, $ω$, $η'$ and $K^+$ production from $pp$ collisions and show that all information gained so far is compatible with approximately constant production matrix elements when including the rescattering between the baryons in the final states. Different methods to include the final state interactions are discussed and their range of validity is indicated. We, furthermore, show that Dalitz plots for the proton-meson invariant mass spectra at different energies should be suited to distinguish between final state interactions and resonant production amplitudes.

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arXiv1999

Resonance model study of kaon production in baryon baryon reactions for heavy ion collisions

The energy dependence of the total kaon production cross sections in baryon baryon ($N$ and $Δ$) collisions are studied in the resonance model, which is a relativistic, tree-level treatment. This study is the first attempt to complete a systematic, consistent investigation of the elementary kaon production reactions for both the pion baryon and baryon baryon reactions. Our model suggests that the magnitudes of the isospin-averaged total cross sections for the $N N \to N Y K$ and $ΔN \to N Y K$ ($Y = Λ$ or $Σ$) reactions are almost equal at energies up to about 200 MeV above threshold. However, the magnitudes for the $ΔN$ reactions become about 6 times larger than those for the $N N$ reactions at energies about 1 GeV above threshold. Furthermore, the magnitudes of the isospin-averaged total cross sections for the $N N \to ΔY K$ reactions turn out to be comparable to those for the $N N \to N Y K$ reactions at $N N$ invariant collision energies about 3.1 GeV, and about 5 to 10 times larger at $N N$ invariant collision energies about 3.5 GeV. The microscopic cross sections are parametrized in all isospin channels necessary for the transport model studies of kaon production in heavy ion collisions. These cross sections are then applied in the relativistic transport model to study the sensitivity to the underlying elementary kaon production cross sections.

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arXiv1998

The role of the $P_{11}(1710)$ in the $NN{\to}NΣK$ reaction

Using the resonance model, which was successfully applied for the study of the $pp{\to}pΛK^+$ reaction, we investigate $NN{\to}NΣK$ reactions that should provide cleaner information about resonance excitations and meson exchange contributions. For this purpose we demonstrate that the invariant mass distribution for the $ΣK$ system, as well as the Dalitz plot for the $NN{\to}NΣK$ reaction, provide direct information about the $ΣK$ production mechanism, which can be tested in the near future by experiments at COSY.

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arXiv1998

On the possibility of $f_0$ observation in low energy $pp$ collisions

Within the meson-exchange model we calculate $f_0$-meson production cross section in $\piN$ and $NN$ reactions and investigate the possibility for $f_0$ observation via the $K\bar{K}$ decay mode in $pp$ collisions. Our studies indicate that an extraction of the $f_0$ signal is unlikely due to the large background from other reaction channels.

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