3,253 research outputs found
The limit of massive gravity
Lorentz-invariant massive gravity is usually associated with a strong
coupling scale . By including non-trivial effects from the
Stueckelberg modes, we show that about these vacua, one can push the strong
coupling scale to higher values and evade the linear vDVZ-discontinuity. For
generic parameters of the theory and generic vacua for the Stueckelberg fields,
the -decoupling limit of the theory is well-behaved and free of any
ghost or gradient-like instabilities. We also discuss the implications for
nonlinear sigma models with Lorentzian target spaces.Comment: 38 pages, 1 figure, JHEP version, minor change
Massive Galileon Positivity Bounds
The EFT coefficients in any gapped, scalar, Lorentz invariant field theory
must satisfy positivity requirements if there is to exist a local, analytic
Wilsonian UV completion. We apply these bounds to the tree level scattering
amplitudes for a massive Galileon. The addition of a mass term, which does not
spoil the non-renormalization theorem of the Galileon and preserves the
Galileon symmetry at loop level, is necessary to satisfy the lowest order
positivity bound. We further show that a careful choice of successively higher
derivative corrections are necessary to satisfy the higher order positivity
bounds. There is then no obstruction to a local UV completion from
considerations of tree level 2-to-2 scattering alone. To demonstrate this we
give an explicit example of such a UV completion.Comment: 31 page
Positivity Bounds for Scalar Theories
Assuming the existence of a local, analytic, unitary UV completion in a
Poincar\'{e} invariant scalar field theory with a mass gap, we derive an
infinite number of positivity requirements using the known properties of the
amplitude at and away from the forward scattering limit. These take the form of
bounds on combinations of the pole subtracted scattering amplitude and its
derivatives. In turn, these positivity requirements act as constraints on the
operator coefficients in the low energy effective theory. For certain theories
these constraints can be used to place an upper bound on the mass of the next
lightest state that must lie beyond the low energy effective theory if such a
UV completion is to ever exist.Comment: 5 page
Phase Diagrams of Bi1-xSbx Thin Films with Different Growth Orientations
A closed-form model is developed to evaluate the band-edge shift caused by
quantum confinement for a two-dimensional non-parabolic carrier-pocket. Based
on this model, the symmetries and the band-shifts of different carrier-pockets
are evaluated for BiSb thin films that are grown along different crystalline
axes. The phase diagrams for the BiSb thin film systems with different growth
orientations are calculated and analyzed
Revisit of cosmic age problem
We investigate the cosmic age problem associated with 9 extremely old
globular clusters in M31 galaxy and 1 very old high- quasar APM 08279 + 5255
at . These 9 globular clusters have not been used to study the cosmic
age problem in the previous literature. By evaluating the age of the universe
in the CDM model with the observational constraints from the SNIa, the
BAO, the CMB, and the independent measurements, we find that the
existence of 5 globular clusters and 1 high- quasar are in tension (over
2 confidence level) with the current cosmological observations. So if
the age estimates of these objects are correct, the cosmic age puzzle still
remains in the standard cosmology. Moreover, we extend our investigations to
the cases of the interacting dark energy models. It is found that although the
introduction of the interaction between dark sectors can give a larger cosmic
age, the interacting dark energy models still have difficulty to pass the
cosmic age test.Comment: 11 pages, 5 figures, 1 table, accepted for publication in PR
Thermal Bremsstrahlung Radiation in a Two-Temperature Plasma
In the normal one-temperature plasma the motion of ions is usually neglected
when calculating the Bremsstrahlung radiation of the plasma. Here we calculate
the Bremsstrahlung radiation of a two-temperature plasma by taking into account
of the motion of ions. Our results show that the total radiation power is
always lower if the motion of ions is considered. We also apply the
two-temperature Bremsstrahlung radiation mechanism for an analytical
Advection-Dominated Accretion Flow (ADAF) model; we find the two-temperature
correction to the total Bremsstrahlung radiation for ADAF is negligible.Comment: 5 pages, 4 figures, accepted for publication in CHJAA. Some
discussions and references adde
Exploring the Latest Union2 SNIa Dataset by Using Model-Independent Parametrization Methods
We explore the cosmological consequences of the recently released Union2
sample of 557 Type Ia supernovae (SNIa). Combining this latest SNIa dataset
with the Cosmic microwave background (CMB) anisotropy data from the Wilkinson
Microwave Anisotropy Probe 7 year (WMAP7) observations and the baryon acoustic
oscillation (BAO) results from the Sloan Digital Sky Survey (SDSS) Data Release
7 (DR7), we measure the dark energy density function as a free function of redshift. Two
model-independent parametrization methods (the binned parametrization and the
polynomial interpolation parametrization) are used in this paper. By using the
statistic and the Bayesian information criterion, we find that the
current observational data are still too limited to distinguish which
parametrization method is better, and a simple model has advantage in fitting
observational data than a complicated model. Moreover, it is found that all
these parametrizations demonstrate that the Union2 dataset is still consistent
with a cosmological constant at 1 confidence level. Therefore, the
Union2 dataset is different from the Constitution SNIa dataset, which more
favors a dynamical dark energy.Comment: 11 pages, 8 figures, 2 tables, accepted for publication in PR
High-Field Shubnikov-de Haas Oscillations in the Topological Insulator BiTeSe
We report measurements of the surface Shubnikov de Haas oscillations (SdH) on
crystals of the topological insulator BiTeSe. In crystals with large
bulk resistivity (4 cm at 4 K), we observe 15 surface SdH
oscillations (to the = 1 Landau Level) in magnetic fields up to 45
Tesla. Extrapolating to the limit , we confirm the -shift
expected from a Dirac spectrum. The results are consistent with a very small
surface Lande -factor.Comment: Text expanded, slight changes in text, final version; Total 6 pages,
6 figure
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