arXiv subjects

E. L. Bratkovskaya

Publications and source records attributed to E. L. Bratkovskaya.

At least 109 records · Page 6Linked to original sources

arXiv2001

$e^+e^-$ production in $p A$ reactions at SIS energies

Detailed predictions for dilepton production from $p A$ reactions at SIS energies are presented within a semi-classical BUU transport model that includes the off-shell propagation of vector mesons nonperturbatively and calculates 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.

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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.

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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.

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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.

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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

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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arXiv2000

Excitation functions of hadronic observables from SIS to RHIC energies

We calculate excitation functions for various dynamical quantities as well as experimental observables from SIS 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 without including any explicit phase transition to a quark-gluon plasma (QGP). It is argued that the failure of this more 'conventional' approach in comparison to experimental data should indicate the presence of a different phase which might be either attributed to space-time regions of vanishing scalar quark condensate ($ $ = 0) or to the presence of a QGP phase with strongly interacting partons. We study the $K/π$ ratio, the low mass dilepton enhancement in the invariant mass regime from 0.2 -- 1.2 GeV as well as charmonium suppression for central Au + Au collisions as a function of the bombarding energy and present predictions for these observables as well as hadron rapiditiy distributions at RHIC energies. Whereas all observables studied within HSD show smooth increasing/decreasing excitation functions, the experimental $K^+/π^+$ ratio indicates a maximum at 11 A$\cdot$GeV (or above) which is interpreted as a signature for a chirally restored phase in the course of the reaction.

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arXiv1999

e^+e^- Pair Production from $γ$ A Reactions

We present a calculation of e^+e^- production in $γA$ reactions at MAMI and TJNAF energies within a semi-classical BUU transport model. Dilepton invariant mass spectra for $γ$C, $γ$Ca and $γ$Pb are calculated at 0.8, 1.5 and 2.2 GeV. We focus on observable effects of medium modifications of the $ρ$ and $ω$ mesons. The in-medium widths of these mesons are taken into account in a dynamical, consistent way. We discuss the transport theoretical treatment of broad resonances.

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arXiv1999

e^+e^- production from pp reactions at BEVALAC energies

We have performed a detailed study of dilepton production from pp collisions including the subthreshold $ρ$ production via baryonic resonances (N(1520), N(1700)) in addition to the conventional dilepton sources as $π^0, η, ω$ and $Δ$ Dalitz decays and direct decays of vector mesons ($ρ, ω$). The role of baryonic resonances in $ρ$ production from nucleon-nucleon collisions is studied in comparison to the DLS data which are well described.

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arXiv1999

e+ e- pairs from pi- A reactions

We calculate dilepton production for the reactions $π^-$C and $π^-$Pb at 1.3 GeV within a semi-classical BUU transport model and compare our results to a previously published calculation. We show that a modified treatment of the $ρ$-meson production and propagation gives substantially different results. We, furthermore, discuss uncertainties related to the electromagnetic decay of the $ρ$-meson and the elementary $π^- N \to e^+ e^- X$ channels.

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arXiv1998

$e^+e^-$ pairs from $π^-$A reactions

We present a dynamical study of $e^+e^-$ production in $π^-$A reactions at 1.3 and 1.7 GeV on the basis of the Coupled-Channel-BUU approach. The contributions from vector mesons ($ρ,ω$) are calculated taking into account the collisional broadening effect and are compared to background sources in the dilepton spectrum from the Dalitz decays of $ω$ and $η$ mesons produced in the reaction. Two possible scenarios for the medium modifications of the vector mesons are investigated, i.e. the 'dropping mass' scheme for the $ρ$ and $ω$ and a momentum dependent $ρ$-meson spectral function that includes the polarization of the $ρ$-meson due to resonant $ρ-N$ scattering.

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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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arXiv1998

Low-mass dileptons and dropping rho meson mass

Using the transport model, we have studied dilepton production from heavy-ion collisions at Bevalac energies. It is found that the enhanced production of low-mass dileptons observed in the experiment by the DLS collaboration cannot be explained by the dropping of hadron masses, in particular the $ρ$-meson mass, in dense matter.

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arXiv1998

Charmonium Suppression with cc~ Dissociation by Strings

We study the production of cc~ pairs in nuclear reactions at SPS energies within the covariant transport approach HSD. The production of cc~ is treated perturbatively employing experimental cross sections while the interactions of cc~ pairs with baryons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the cc~-baryon cross sections the data on J/Ψsuppression in p+A reactions are reproduced in line with calculations based on the Glauber model. Additionally the dissociation of the cc~ pairs by strings is included in a purely geometrical way. We find good agreement with experimental data from the NA38 and NA50 collaboration with an estimate for the string radius of ~0.2-0.3 fm.

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arXiv1998

Dilepton production and $m_T$-scaling at BEVALAC/SIS energies

We present a dynamical study of $e^+e^-$ production in C + C and Ca + Ca collisions at BEVALAC/SIS energies on the basis of the covariant transport approach HSD employing momentum-dependent $ρ$-meson spectral functions that include the pion modifications in the nuclear medium as well as the polarization of the $ρ$-meson due to resonant $ρ- N$ scattering. We find that the experimental data from the DLS collaboration cannot be described within the $ρ$-meson spectral function approach. A dropping $η$-mass scenario leads to a good reproduction of the DLS dilepton data, however, violates the $m_T$-scaling of $π^0$ and $η$ spectra as observed by the TAPS collaboration as well as $η$ photoproduction on nuclei.

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arXiv1997

Meson $m_T$-scaling in heavy-ion collisions at SIS energies

We perform systematical studies on transverse-mass spectra of $π^0$, $η$, $ω$, $ϕ$, $K^+$ and $K^-$ mesons for C + C, Ni + Ni, Au + Au collisions at SIS energies within the HSD transport approach. We find that the $m_T$-spectra sensitively reflect the in-medium properties of the mesons. The 'bare mass' scenario leads to a general scaling behaviour for the meson $m_T$-spectra when including a mass shift due to the associate strangeness production threshold for kaons and antikaons whereas a 'dropping' mass scheme violates this $m_T$-scaling. The relative slope parameters, however, provide valuable information about the average meson potentials in the nuclear medium.

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arXiv1997

Probing the Rho Spectral Function in Hot and Dense Nuclear Matter by Dileptons

We present a dynamical study of $e^+e^-$ and $μ^+ μ^-$ production in proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis of the covariant transport approach HSD employing a momentum-dependent $ρ$-meson spectral function that includes the pion modifications in the nuclear medium as well as the polarization of the $ρ$-meson due to resonant $ρ$$-N$ scattering. We find that the experimental data from the CERES and HELIOS-3 Collaborations can be described equally well as within the dropping $ρ$-mass scenario. Whereas corresponding dilepton $q_T$-spectra are found to be very similar, the inclusive dilepton yield in the invariant mass range $0.85 \leq M \leq 1.0$ GeV should allow to disentangle the two scenarios experimentally.

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arXiv1997

Production and absorption of $c \bar{c}$ pairs in nuclear collisions at SPS energies

We study the production of $c \bar{c}$ pairs and dimuons from hard collisions in nuclear reactions within the covariant transport approach HSD, which describes successfully both hadronic and electromagnetic observables from p+A and A+A collisions from SIS to SPS energies. The production of $c \bar{c}$ and Drell-Yan pairs is treated perturbatively employing experimental cross sections while the interactions of $c\bar{c}$ pairs with hadrons are included by conventional cascade-type two-body collisions. Adopting 6mb for the $c \bar{c}$-baryon cross sections the data on $J/Ψ$ suppression in p+A reactions are reproduced in line with calculations based on the Glauber model. We study different models for $c \bar{c}$ dissociation on mesons in comparison with the experimental data of the HELIOS-3, NA38 and NA50 collaborations. Adopting absorption cross sections with mesons above the $D\bar{D}$ threshold in the order of 1.5 - 3mb we find that all data on $J/Ψ$ suppression from both proton-nucleus and nucleus-nucleus collisions can be described without assuming the formation of a quark-gluon plasma in these collisions.

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