1,003 research outputs found
Direct Instantons and Nucleon Magnetic Moments
We calculate the leading direct-instanton contributions to the operator
product expansion of the nucleon correlator in a magnetic background field and
set up improved QCD sum rules for the nucleon magnetic moments. Remarkably, the
instanton contributions are found to affect only those sum rules which had
previously been considered unstable. The new sum rules show good stability and
reproduce the experimental values of the nucleon magnetic moments with values
of , the quark condensate magnetic susceptibility, consistent with other
estimates.Comment: 15 pages, 2 figure
Resonances in radiative hyperon decays
The importance of resonances for the radiative hyperon decays is examined in
the framework of chiral perturbation theory. Low lying baryon resonances are
included into the effective theory and tree contributions to these decays are
calculated. We find significant contributions to both the parity-conserving and
parity-violating decay amplitudes and a large negative value for the asymmetry
parameter in polarized Sigma^+ -> p gamma is found, in agreement with the
experimental result alpha(p Sigma^+) = -0.76 +/- 0.08.Comment: 14 pages, 2 figure
Quantum corrections to the entropy of charged rotating black holes
Hawking radiation from a black hole can be viewed as quantum tunneling of
particles through the event horizon. Using this approach we provide a general
framework for studying corrections to the entropy of black holes beyond
semiclassical approximations. Applying the properties of exact differentials
for three variables to the first law thermodynamics, we study charged rotating
black holes and explicitly work out the corrections to entropy and horizon area
for the Kerr-Newman and charged rotating BTZ black holes. It is shown that the
results for other geometries like the Schwarzschild, Reissner-Nordstr\"{o}m and
anti-de Sitter Schwarzschild spacetimes follow easily
Anomaly analysis of Hawking radiation from Kaluza-Klein black hole with squashed horizon
Considering gravitational and gauge anomalies at the horizon, a new method
that to derive Hawking radiations from black holes has been developed by
Wilczek et al. In this paper, we apply this method to non-rotating and rotating
Kaluza-Klein black holes with squashed horizon, respectively. For the rotating
case, we found that, after the dimensional reduction, an effective U(1) gauge
field is generated by an angular isometry. The results show that the gauge
current and energy-momentum tensor fluxes are exactly equivalent to Hawking
radiation from the event horizon.Comment: 15 pages, no figures, the improved version, accepted by Eur. Phys. J.
Role of surface roughness in hard x-ray emission from femtosecond laser produced copper plasmas
The hard x-ray emission in the energy range of 30-300 keV from copper plasmas
produced by 100 fs, 806 nm laser pulses at intensities in the range of
10 W cm is investigated. We demonstrate that surface
roughness of the targets overrides the role of polarization state in the
coupling of light to the plasma. We further show that surface roughness has a
significant role in enhancing the x-ray emission in the above mentioned energy
range.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Point symmetries in the Hartree-Fock approach: Symmetry-breaking schemes
We analyze breaking of symmetries that belong to the double point group
D2h(TD) (three mutually perpendicular symmetry axes of the second order,
inversion, and time reversal). Subgroup structure of the D2h(TD) group
indicates that there can be as much as 28 physically different, broken-symmetry
mean-field schemes --- starting with solutions obeying all the symmetries of
the D2h(TD) group, through 26 generic schemes in which only a non-trivial
subgroup of D2h(TD) is conserved, down to solutions that break all of the
D2h(TD) symmetries. Choices of single-particle bases and the corresponding
structures of single-particle hermitian operators are discussed for several
subgroups of D2h(TD).Comment: 10 RevTeX pages, companion paper in nucl-th/991207
Aspects of meson properties in dense nuclear matter
We investigate the modification of meson spectral densities in dense nuclear
matter at zero temperature. These effects are studied in a fully relativistic
mean field model which goes beyond the linear density approximation and also
includes baryon resonances. In particular, the role of N*(1520) and N*(1720) on
the rho meson spectral density is highlighted. Even though the nucleon-nucleon
loop and the nucleon-resonance loop contribute with the opposite sign, an
overall reduction of rho meson mass is still observed at high density.
Importantly, it is shown that the resonances cause substantial broadening of
the rho meson spectral density in matter and also induces non-trivial momentum
dependence. The spectral density of the a0 meson is also shown. We study the
dispersion relations and collective oscillations induced by the rho meson
propagation in nuclear matter together with the influence of the mixing of rho
with the a0 meson. The relevant expression for the plasma frequency is also
recovered analytically in the appropriate limit.Comment: 19 pages, 17 figure
The large-N(c) nuclear potential puzzle
An analysis of the baryon-baryon potential from the point of view of
large-N(c) QCD is performed. A comparison is made between the N(c)-scaling
behavior directly obtained from an analysis at the quark-gluon level to the
N(c)-scaling of the potential for a generic hadronic field theory in which it
arises via meson exchanges and for which the parameters of the theory are given
by their canonical large-N(c) scaling behavior. The purpose of this comparison
is to use large-N(c) consistency to test the widespread view that the
interaction between nuclei arises from QCD through the exchange of mesons.
Although at the one- and two-meson exchange level the scaling rules for the
potential derived from the hadronic theory matches the quark-gluon level
prediction, at the three- and higher-meson exchange level a generic hadronic
theory yields a potential which scales with N(c) faster than that of the
quark-gluon theory.Comment: 17 pages, LaTeX, 5 figure
Nucleon-Nucleon Scattering under Spin-Isospin Reversal in Large-N_c QCD
The spin-flavor structure of certain nucleon-nucleon scattering observables
derived from the large N_c limit of QCD in the kinematical regime where
time-dependent mean-field theory is valid is discussed. In previous work, this
regime was taken to be where the external momentum was of order N_c which
precluded the study of differential cross sections in elastic scattering. Here
it is shown that the regime extends down to order N_c^{1/2} which includes the
higher end of the elastic regime. The prediction is that in the large N_c
limit, observables describable via mean-field theory are unchanged when the
spin and isospin of either nucleon are both flipped. This prediction is tested
for proton-proton and neutron-proton elastic scattering data and found to fail
badly. We argue that this failure can be traced to a lack of a clear separation
of scales between momentum of order N_c^{1/2} and N_c^1 when N_c is as small as
three. The situation is compounded by an anomalously low particle production
threshold due to approximate chiral symmetry.Comment: 5 pages, 1 figur
G\"{o}del black hole, closed timelike horizon, and the study of particle emissions
We show that a particle, with positive orbital angular momentum, following an
outgoing null/timelike geodesic, shall never reach the closed timelike horizon
(CTH) present in the -dimensional rotating G\"{o}del black hole
space-time. Therefore a large part of this space-time remains inaccessible to a
large class of geodesic observers, depending on the conserved quantities
associated with them. We discuss how this fact and the existence of the closed
timelike curves present in the asymptotic region make the quantum field
theoretic study of the Hawking radiation, where the asymptotic observer states
are a pre-requisite, unclear. However, the semiclassical approach provides an
alternative to verify the Smarr formula derived recently for the rotating
G\"{o}del black hole. We present a systematic analysis of particle emissions,
specifically for scalars, charged Dirac spinors and vectors, from this black
hole via the semiclassical complex path method.Comment: 13 pages; minor changes, references adde
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