87 research outputs found
Open-charm systems in cold nuclear matter
We study the spectral distributions of charmed meson with quantum
numbers in cold nuclear matter applying a self-consistent and covariant
many-body approach established previously for the nuclear dynamics of kaons. At
leading orders the computation requires as input the free-space two-body
scattering amplitudes only. Our results are based on the s-wave meson-nucleon
amplitudes obtained recently in terms of a coupled-channel approach. The
amplitudes are characterized by the presence of many resonances in part so far
not observed. This gives rise to an intriguing dynamics of charmed mesons in
nuclear matter. At nuclear saturation density we predict a pronounced two-mode
structure of the mesons with a main branch pushed up by about 32 MeV. The
lower branch reflects the coupling to two resonance-hole states that are almost
degenerate. For the we obtain a single mode pushed up by about 18 MeV
relative to the vacuum mode. Most spectacular are the results for the
meson. The presence of an exotic resonance-hole state gives rise to a rather
broad and strongly momentum dependent spectral distribution.Comment: 11 pages, 3 figure
On causality, unitarity and perturbative expansions
We present a pedagogical case study how to combine micro-causality and
unitarity based on a perturbative approach. The method we advocate constructs
an analytic extrapolation of partial-wave scattering amplitudes that is
constrained by the unitarity condition. Suitably constructed conformal mappings
help to arrive at a systematic approximation of the scattering amplitude. The
technique is illustrated at hand of a Yukawa interaction. The typical case of a
superposition of strong short-range and weak long-range forces is investigated.Comment: 12 pages, 12 figure
Electromagnetic transitions in an effective chiral Lagrangian with the eta-prime and light vector mesons
We consider the chiral Lagrangian with a nonet of Goldstone bosons and a
nonet of light vector mesons. The mixing between the pseudoscalar mesons eta
and eta-prime is taken into account. A novel counting scheme is suggested that
is based on hadrogenesis, which conjectures a mass gap in the meson spectrum of
QCD in the limit of a large number of colors. Such a mass gap would justify to
consider the vector mesons and the eta-prime meson as light degrees of freedom.
The complete leading order Lagrangian is constructed and discussed. As a first
application it is tested against electromagnetic transitions of light vector
mesons to pseudoscalar mesons. Our parameters are determined by the
experimental data on photon decays of the omega, phi and eta-prime meson. In
terms of such parameters we predict the corresponding decays into virtual
photons with either dielectrons or dimuons in the final state.Comment: 17 pages, extended discussion on mixin
On baryon resonances and chiral symmetry
We study J^P=(3/2)^- baryon resonances as generated by chiral coupled-channel
dynamics. Parameter free results are obtained in terms of the Weinberg-Tomozawa
term predicting the leading s-wave interaction strength of Goldstone bosons
with baryon-decuplet states. In the 'heavy' SU(3) limit with m_\pi = m_K \sim
500 MeV the resonances turn into bound states forming a decuplet and octet
representation of the SU(3) group. Using physical masses the mass splitting are
remarkably close to the empirical pattern.Comment: revised version: includes two additional references, gives improved
discussions and eliminted some misprint
On kinematical constraints in boson-boson systems
We consider the scattering of two-bosons with negative parity and spin 0 or
1. Starting from helicity partial-wave scattering amplitudes we derive
transformations that eliminate all kinematical constraints. Such amplitudes are
expected to satisfy partial-wave dispersion relations and therefore provide a
suitable basis for data analysis and the construction of effective field
theories. Our derivation relies on a decomposition of the various scattering
amplitudes into suitable sets of invariant functions. A novel algebra was
developed that permits the efficient computation of such functions in terms of
computer algebra codes.Comment: 14 pages, 8 table
Baryon resonances from chiral coupled-channel dynamics
We discuss the formation of s- and d-wave baryon resonances as predicted by
the chiral SU(3) symmetry of QCD. Based on the leading order term of the chiral
Lagrangian a rich spectrum of molecules is generated, owing to final-state
interactions. The spectrum of s- and d-wave baryon states with zero and
non-zero charm is remarkably consistent with the empirical pattern. In
particular, the recently announced Sigma_c(2800) of the BELLE collaboration is
reproduced with realistic mass and width parameters. Similarly, the d-wave
states Lambda_c(2625) and Xi_c(2815) are explained naturally to be chiral
excitations of J^P=3/2^+ states. In the open-charm sector exotic multiplet
structures are predicted. These findings support a radical conjecture: meson
and baryon resonances that do not belong to the large-N_c ground state of QCD
should be viewed as hadronic molecular state
Migdal's short range correlations in a covariant model
We construct a covariant model for short range correlations of a pion emerged
in nuclear matter. Once the delta-hole contribution is considered an additional
and so far neglected channel opens that leads to significant modifications in
the vicinity of the kinematical region defined by \omega \sim |\vec q |. We
speculate that this novel effect should be important for the quantitative
interpretation of charge exchange reactions like C(He,t).Comment: correction of minor misprint
Antikaons and hyperons in nuclear matter with saturation
We evaluate the antikaon and hyperon spectral functions in a self-consistent
and covariant many-body approach. The computation is based on coupled-channel
dynamics derived from the chiral SU(3) Lagrangian. A novel subtraction scheme
is developed that avoids kinematical singularities and medium-induced power
divergencies all together. Scalar and vector mean fields are used to model
nuclear binding and saturation. The effect of the latter is striking for the
antikaon spectral function that becomes significantly more narrow at small
momenta. Attractive mass shifts of about 30 and 40 MeV are predicted for the
Lambda(1405) and Sigma(1385) resonances. Once scalar and vector mean fields for
the nucleon are switched on the Lambda(1520) resonances dissolves almost
completely in nuclear matter. All together only moderate attraction is
predicted for the nuclear antikaon systems at saturation density. However, at
larger densities we predict a sizable population of soft antikaon modes that
arise from the coupling of the antikaon to a highly collective Lambda(1115)
nucleon-hole state. This may lead to the formation of exotic nuclear systems
with strangeness and antikaon condensation in compact stars at moderate
densities.Comment: 49 pages, 13 figures, The revised manuscript contains additional
material at twice nuclear saturation density. An unexpected and novel
mechanism is unravelled that may have dramatic implications on the formation
of exotic nuclear systems with strangeness and antikaon condensation in
compact star
Baryon self energies in the chiral loop expansion
We compute the self energies of the baryon octet and decuplet states at the
one-loop level applying the manifestly covariant chiral Lagrangian. It is
demonstrated that expressions consistent with the expectation of power counting
rules arise if the self energies are decomposed according to the
Passarino-Veltman scheme supplemented by a minimal subtraction. This defines a
partial summation of the chiral expansion. A finite renormalization required to
install chiral power counting rules leads to the presence of an infrared
renormalization scale. Good convergence properties for the chiral loop
expansion of the baryon octet and decuplet masses are obtained for natural
values of the infrared scale. A prediction for the strange-quark matrix element
of the nucleon is made.Comment: 36 pages, 4 figures, 8 tables. The revised manuscript contains a
proof that given any one-loop integral that arises when computing one-baryon
processes it is sufficient to renormalize the scalar master-loop functions of
the Passarino-Veltman reduction in a manner that the latter are compatible
with the expectation of chiral counting rule
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