933 research outputs found
From meson-nucleon scattering to vector mesons in nuclear matter
We employ meson-nucleon scattering data to deduce the properties of the
low-mass vector mesons in nuclear matter, and present results for the
and in-medium spectral functions. The corresponding thermal emission
rate for lepton pairs is also discussed.Comment: Talk given at 28th International Workshop on Gross Properties of
Nuclei and Nuclear Excitations, Hirschegg, Austria, 16-22 Jan. 200
Scattering of vector mesons off nucleons
We construct a relativistic and unitary approach to 'high' energy pion- and
photon-nucleon reactions taking the , , ,
final states into account. Our scheme dynamically
generates the s- and d-wave nucleon resonances N(1535), N(1650) and N(1520) and
isobar resonances and in terms of quasi-local
interaction vertices. The description of photon-induced processes is based on a
generalized vector-meson dominance assumption which directly relates the
electromagnetic quasi-local 4-point interaction vertices to the corresponding
vertices involving the and fields. We obtain a satisfactory
description of the elastic and inelastic pion- and photon-nucleon scattering
data in the channels considered. The resulting s-wave - and
-nucleon scattering amplitudes are presented. Using these amplitudes we
compute the leading density modification of the and energy
distributions in nuclear matter. We find a repulsive energy shift for the
meson at small nuclear density but predict considerable strength in
resonance-hole like -meson modes. Compared to previous calculations our
result for the -meson spectral function shows a significantly smaller
in-medium effect. This reflects a fairly small coupling strength of the N(1520)
resonance to the channel.Comment: 78 pages, 19 figures, moderately revised version, accepted in Nucl.
Phys.
From meson- and photon-nucleon scattering to vector mesons in nuclear matter
We present a relativistic and unitary approach to pion- and photon-nucleon
scattering taking into account the , , , ,
, and channels. Our scheme dynamically
generates the s- and d-wave nucleon resonances N(1535), N(1650), N(1520) and
N(1700) and isobar resonances and in terms of
quasi-local two-body interaction terms. We obtain a fair description of the
experimental data relevant for slow vector-meson propagation in nuclear matter.
The s-wave - and -meson nucleon scattering amplitudes, which
define the leading density modification of the - and -meson
spectral functions in nuclear matter, are predicted.Comment: 6 pages, 1 figure; contribution to the Int. Workshop XXX on Gross
Properties of Nuclei and Nuclear Excitations: Ultrarelativistic Heavy Ion
Collisions, Hirschegg, Jan. 13-19, 200
The reaction close to the vector meson production threshold
The and reaction cross sections
are calculated below and in the vicinity of the vector meson (,
) production threshold. These processes are largely responsible for the
emission of pairs in pion-nucleus reactions and contribute to the
dilepton spectra observed in relativistic heavy ion collisions. They are
dominated by the decay of low-lying baryon resonances into vector meson-nucleon
channels. The vector mesons materialize subsequently into pairs. Using
and amplitudes calculated in the
center of mass energy interval GeV, we compute the and reaction cross sections in these
kinematics. Below the vector meson production threshold, the
interference in the channel appears largely destructive for the
cross section and constructive for the cross section. The pion beam and the HADES detector at GSI offer a unique
possibility to measure these effects. Such data would provide strong
constraints on the coupling of vector meson-nucleon channels to low-lying
baryon resonances.Comment: Talk given at the Budapest'02 Workshop on Quark and Hadron Dynamics,
Budapest (Hungary), March 3rd-7th, 200
Charge fluctuations in chiral models and the QCD phase transition
We consider the Polyakov loop-extended two flavor chiral quark--meson model
and discuss critical phenomena related with the spontaneous breaking of the
chiral symmetry. The model is explored beyond the mean-field approximation in
the framework of the functional renormalisation group. We discuss properties of
the net-quark number density fluctuations as well as their higher cumulants. We
show that with the increasing net-quark number density, the higher order
cumulants exhibit a strong sensitivity to the chiral crossover transition. We
discuss their role as probes of the chiral phase transition in heavy-ion
collisions at RHIC and LHC.Comment: 4 pages, 3 figures, to appear in the proceedings of Quark Matter
2011, 23-28 May 2011, Annecy, Franc
Protist predation can favour cooperation within bacterial species
Here, we studied how protist predation affects cooperation in the opportunistic pathogen bacterium Pseudomonas aeruginosa, which uses quorum sensing (QS) cell-to-cell signalling to regulate the production of public goods. By competing wild-type bacteria with QS mutants (cheats), we show that a functioning QS system confers an elevated resistance to predation. Surprisingly, cheats were unable to exploit this resistance in the presence of cooperators, which suggests that resistance does not appear to result from activation of QS-regulated public goods. Instead, elevated resistance of wild-type bacteria was related to the ability to form more predation-resistant biofilms. This could be explained by the expression of QS-regulated resistance traits in densely populated biofilms and floating cell aggregations, or alternatively, by a pleiotropic cost of cheating where less resistant cheats are selectively removed from biofilms. These results show that trophic interactions among species can maintain cooperation within species, and have further implications for P. aeruginosa virulence in environmental reservoirs by potentially enriching the cooperative and highly infective strains with functional QS system
Probing the QCD Critical Point with Higher Moments of Net-proton Multiplicity Distributions
Higher moments of event-by-event net-proton multiplicity distributions are
applied to search for the QCD critical point in the heavy ion collisions. It
has been demonstrated that higher moments as well as moment products are
sensitive to the correlation length and directly connected to the thermodynamic
susceptibilities computed in the Lattice QCD and Hadron Resonance Gas (HRG)
model. In this paper, we will present measurements for kurtosis (),
skewness () and variance () of net-proton multiplicity
distributions at the mid-rapidity () and GeV/ for
Au+Au collisions at =19.6, 39, 62.4, 130 and 200 GeV, Cu+Cu
collisions at =22.4, 62.4 and 200 GeV, d+Au collisions at
=200 GeV and p+p collisions at =62.4 and 200 GeV.
The moment products and of net-proton
distributions, which are related to volume independent baryon number
susceptibility ratio, are compared to the Lattice QCD and HRG model
calculations. The and of net-proton
distributions are consistent with Lattice QCD and HRG model calculations at
high energy, which support the thermalization of the colliding system.
Deviations of and for the Au+Au collisions at
low energies from HRG model calculations are also observed.Comment: 10 pages, 8 figures, Proceedings of 27th Winter Workshon on Nuclear
Dynamics. Feb. 6-13 (2011
The Spectral Function in a Relativistic Resonance Model
We calculate the spectral function of the meson in nuclear
matter. The calculation is performed in the {\it low density} approximation,
where the in-medium self energy is completely determined by the
vacuum forward scattering amplitude. This amplitude is derived from a
relativistic resonance model. In comparison to previous non-relativistic
calculations we find a much weaker momentum dependence of ,
especially in the transverse channel. Special attention is paid to
uncertainties in the model. Thus, we compare the impact of different coupling
schemes for the interaction on the results and discuss various
resonance parameter sets.Comment: 47 pages, 19 figures, some discussion and formulae added, minor typos
removed, accepted for publication in Nuclear Physics
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