46,708 research outputs found
Alternative experimental evidence for chiral restoration in excited baryons
Given existing empirical spectral patterns of excited hadrons it has been
suggested that chiral symmetry is approximately restored in excited hadrons at
zero temperature/density (effective symmetry restoration). If correct, this
implies that mass generation mechanisms and physics in excited hadrons is very
different as compared to the lowest states. One needs an alternative and
independent experimental information to confirm this conjecture. Using very
general chiral symmetry arguments it is shown that strict chiral restoration in
a given excited nucleon forbids its decay into the N \pi channel. Hence those
excited nucleons which are assumed from the spectroscopic patterns to be in
approximate chiral multiplets must only "weakly" decay into the N \pi channel,
(f_{N^*N\pi}/f_{NN\pi})^2 << 1. However, those baryons which have no chiral
partner must decay strongly with a decay constant comparable with f_{NN\pi}.
Decay constants can be extracted from the existing decay widths and branching
ratios. It turnes out that for all those well established excited nucleons
which can be classified into chiral doublets N_+(1440) - N_-(1535), N_+(1710) -
N_-(1650), N_+(1720) - N_-(1700), N_+(1680) - N_-(1675), N_+(2220) - N_-(2250),
N_+(?) - N_-(2190), N_+(?) - N_-(2600), the ratio is (f_{N^*N\pi}/f_{NN\pi})^2
~ 0.1 or much smaller for the high-spin states. In contrast, the only well
established excited nucleon for which the chiral partner cannot be identified
from the spectroscopic data, N(1520), has a decay constant into the N\pi
channel that is comparable with f_{NN\pi}. This gives an independent
experimental verification of the chiral symmetry restoration scenario.Comment: 4 pp. A new footnote with an alternative proof of impossibility of
parity doublet decay into pi + N is added. To appear in Phys. Rev. Let
Regge approach to charged-pion photoproduction at invariant energies above 2 GeV
A Regge model with absorptive corrections is employed in a global analysis of
the world data on positive and negative pion photoproduction for photon
energies from 3 to 8 GeV. In this region resonance contributions are expected
to be negligible so that the available experimental information on differential
cross sections and single polarization observables at -t \leq 2 GeV^2 allows us
to determine the non-resonant part of the reaction amplitude reliably. The
model amplitude is then used to predict observables for photon energies below 3
GeV. Differences between our predictions and data in this energy region are
systematically examined as possible signals for the presence of excited
baryons. We find that the data available for the polarized photon asymmetry
show promising resonance signatures at invariant energies around 2 GeV. With
regard to differential cross sections the analysis of negative pion
photoproduction data, obtained recently at JLab, indicates likewise the
presence of resonance structures around 2 GeVComment: misprint in Table 3 corrected; reference adde
Identification of the Beutler-Fano formula in eigenphase shifts and eigentime delays near a resonance
Eigenphase shifts and eigentime delays near a resonance for a system of one
discrete state and two continua are shown to be functionals of the Beutler-
Fano formulas using appropriate dimensionless energy units and line profile
indices. Parameters responsible for the avoided crossing of eigenphase shifts
and eigentime delays are identified. Similarly, parameters responsible for the
eigentime delays due to a frame change are identified. With the help of new
parameters, an analogy with the spin model is pursued for the S matrix and time
delay matrix. The time delay matrix is shown to comprise three terms, one due
to resonance, one due to a avoided crossing interaction, and one due to a frame
change. It is found that the squared sum of time delays due to the avoided
crossing interaction and frame change is unity.Comment: 17 pages, 3 figures, RevTe
Departure from the constant-period ephemeris for the transiting exoplanet WASP-12 b
Most hot Jupiters are expected to spiral in towards their host stars due to
transfering of the angular momentum of the orbital motion to the stellar spin.
Their orbits can also precess due to planet-star interactions. Calculations
show that both effects could be detected for the very-hot exoplanet WASP-12 b
using the method of precise transit timing over a timespan of the order of 10
yr. We acquired new precise light curves for 29 transits of WASP-12 b,
spannning 4 observing seasons from November 2012 to February 2016. New
mid-transit times, together with literature ones, were used to refine the
transit ephemeris and analyse the timing residuals. We find that the transit
times of WASP-12 b do not follow a linear ephemeris with a 5 sigma confidence
level. They may be approximated with a quadratic ephemeris that gives a rate of
change in the orbital period of -2.56 +/- 0.40 x 10^{-2} s/yr. The tidal
quality parameter of the host star was found to be equal to 2.5 x 10^5 that is
comparable to theoretical predictions for Sun-like stars. We also consider a
model, in which the observed timing residuals are interpreted as a result of
the apsidal precession. We find, however, that this model is statistically less
probable than the orbital decay.Comment: Accepted for publication in A&A Letter
Zero Cosmological Constant and Nonzero Dark Energy from Holographic Principle
It is shown that the first law of thermodynamics and the holographic
principle applied to an arbitrary large cosmic causal horizon naturally demand
the zero cosmological constant and non-zero dynamical dark energy in the form
of the holographic dark energy. Semiclassical analysis shows that the
holographic dark energy has a parameter and an equation of state
comparable to current observational data, if the entropy of the horizon
saturates the Bekenstein-Hawking bound. This result indicates that quantum
field theory should be modified at large scale to explain dark energy. The
relations among dark energy, quantum vacuum energy and entropic gravity are
also discussed.Comment: Revtex 7 pages 2 fig
Nonuniqueness and derivative discontinuities in density-functional theories for current-carrying and superconducting systems
Current-carrying and superconducting systems can be treated within
density-functional theory if suitable additional density variables (the current
density and the superconducting order parameter, respectively) are included in
the density-functional formalism. Here we show that the corresponding conjugate
potentials (vector and pair potentials, respectively) are {\it not} uniquely
determined by the densities. The Hohenberg-Kohn theorem of these generalized
density-functional theories is thus weaker than the original one. We give
explicit examples and explore some consequences.Comment: revised version (typos corrected, some discussion added) to appear in
Phys. Rev.
Resonances in an external field: the 1+1 dimensional case
Using non-relativistic effective field theory in 1+1 dimensions, we
generalize Luescher's approach for resonances in the presence of an external
field. This generalized approach provides a framework to study the
infinite-volume limit of the form factor of a resonance determined in lattice
simulations.Comment: 13 pages, 2 postscript figure
Higgs Boson Sector of the Next-to-MSSM with CP Violation
We perform a comprehensive study of the Higgs sector in the framework of the
next-to-minimal supersymmetric standard model with CP-violating parameters in
the superpotential and in the soft-supersymmetry-breaking sector. Since the CP
is no longer a good symmetry, the two CP-odd and the three CP-even Higgs bosons
of the next-to-minimal supersymmetric standard model in the CP-conserving limit
will mix. We show explicitly how the mass spectrum and couplings to gauge
bosons of the various Higgs bosons change when the CP-violating phases take on
nonzero values. We include full one-loop and the logarithmically enhanced
two-loop effects employing the renormalization-group (RG) improved approach. In
addition, the LEP limits, the global minimum condition, and the positivity of
the square of the Higgs-boson mass have been imposed. We demonstrate the
effects on the Higgs-mass spectrum and the couplings to gauge bosons with and
without the RG-improved corrections. Substantial modifications to the allowed
parameter space happen because of the changes to the Higgs-boson spectrum and
their couplings with the RG-improved corrections. Finally, we calculate the
mass spectrum and couplings of the few selected scenarios and compare to the
previous results in literature where possible; in particular, we illustrate a
scenario motivated by electroweak baryogenesis.Comment: 40 pages, 49 figures; v2: typos corrected and references added; v3:
some clarification and new figures added, version published in PR
Ab-initio theory of NMR chemical shifts in solids and liquids
We present a theory for the ab-initio computation of NMR chemical shifts
(sigma) in condensed matter systems, using periodic boundary conditions. Our
approach can be applied to periodic systems such as crystals, surfaces, or
polymers and, with a super-cell technique, to non-periodic systems such as
amorphous materials, liquids, or solids with defects. We have computed the
hydrogen sigma for a set of free molecules, for an ionic crystal, LiH, and for
a H-bonded crystal, HF, using density functional theory in the local density
approximation. The results are in excellent agreement with experimental data.Comment: to appear in Physical Review Letter
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