710 research outputs found
Gauss-Bonnet gravity renders negative tension braneworlds unstable
We show that the Gauss-Bonnet correction to Einstein gravity induces a
gravitational tachyon mode, namely an unstable spin 2 fluctuation, in the
Randall-Sundrum I model. We demonstrate that this instability is generically
related to the presence of a negative tension brane in the set-up, with or
without -symmetry across it. Indeed it is shown that the tachyon mode is a
bound state localised on any negative tension brane of co-dimension one,
embedded in anti-de Sitter background. We discuss the possible resolution of
this instability by the inclusion of induced gravity terms on the branes or by
an effective four-dimensional cosmological constant.Comment: published versio
Spontaneous Lorentz Breaking and Massive Gravity
We study a theory where the presence of an extra spin-two field coupled to
gravity gives rise to a phase with spontaneously broken Lorentz symmetry. In
this phase gravity is massive, and the Weak Equivalence Principle is respected.
The newtonian potentials are in general modified, but we identify an
non-perturbative symmetry that protects them. The gravitational waves sector
has a rich phenomenology: sources emit a combination of massless and massive
gravitons that propagate with distinct velocities and also oscillate. Since
their velocities differ from the speed of light, the time of flight difference
between gravitons and photons from a common source could be measured.Comment: 4 page
Strong coupling in massive gravity by direct calculation
We consider four-dimensional massive gravity with the Fierz-Pauli mass term.
The analysis of the scalar sector has revealed recently that this theory
becomes strongly coupled above the energy scale \Lambda = (M_{Pl}^2 m^4)^{1/5}
where m is the mass of the graviton. We confirm this scale by explicit
calculations of the four-graviton scattering amplitude and of the loop
correction to the interaction between conserved sources.Comment: 9 pages, 3 figures, some clarifications adde
DBI with Primordial Magnetic Field in the Sky
In this paper, we study the generation of a large scale magnetic field with
amplitude of order G in an inflationary model which has been introduced in
hep-th/0310221. This inflationary model based on existence of a speed limit for
inflaton field. Generating a mass for inflaton at scale above the ,
breaks the conformal triviality of the Maxwell equation and causes to originate
a magnetic field during the inflation. The amplitude strongly depends on the
details of reheating stage and also depends on the e-foldings parameter N. We
find the amplitude of the primordial magnetic field at decoupling time in this
inflationary background using late time behavior of the theory.Comment: 12 pages, no figure, typos correcte
Optical Line Emission from z6.8 Sources with Deep Constraints on Ly Visibility
We analyze a sample of -dropout galaxies in the CANDELS GOODS South and
UDS fields that have been targeted by a dedicated spectroscopic campaign aimed
at detecting their Ly line. Deep IRAC observations at 3.6 and 4.5
m are used to determine the strength of optical emission lines affecting
these bands at z6.5-6.9 in order to i) investigate possible physical
differences between Ly emitting and non-emitting sources; ii) constrain
the escape fraction of ionizing photons; iii) provide an estimate of the
specific star-formation rate at high redshifts. We find evidence of strong
[OIII]+H emission in the average (stacked) SEDs of galaxies both with
and without Ly emission. The blue IRAC [3.6]-[4.5] color of the stack
with detected Ly line can be converted into a rest-frame equivalent
width EW([OIII]+H)=1500 assuming a flat intrinsic
stellar continuum. This strong optical line emission enables a first estimate
of f20% on the escape fraction of ionizing photons from
Ly detected objects. The objects with no Ly line show less
extreme EW([OIII]+H)=520 suggesting different
physical conditions of the HII regions with respect to Ly-emitting
ones, or a larger f. The latter case is consistent with a combined
evolution of f and the neutral hydrogen fraction as an explanation of
the lack of bright Ly emission at z6. A lower limit on the specific
star formation rate, SSFR9.1 for galaxies at these redshifts can be derived from the
spectroscopically confirmed sample.Comment: 10 pages, 7 figures; The Astrophysical Journal in press; matched to
the published versio
On brane-induced gravity in warped backgrounds
We study whether modification of gravity at large distances is possible in
warped backgrounds with two branes and a brane-induced term localized on one of
the branes. We find that there are three large regions in the parameter space
where the theory is weakly coupled up to high energies. In one of these regions
gravity on the brane is four-dimensional at arbitrarily large distances, and
the induced Einstein term results merely in the renormalization of the 4d
Planck mass. In the other two regions the behavior of gravity changes at
ultra-large distances; however, radion becomes a ghost. In parts of these
regions, both branes have positive tensions, so the only reason for the
appearance of the ghost field is the brane-induced term. In between these three
regions, there are domains in the parameter space where gravity is strongly
coupled at phenomenologically unacceptable low energy scale.Comment: 12 pages, 2 fig, JHEP3 style required, typos correcte
Stability of the Higgs mass in theories with extra dimensions
We analyze the ultraviolet stability of the Higgs mass in recently proposed
Kaluza-Klein models compactified on S_1/Z_2 or S_1/(Z_2\times Z_2'), both at
the field theory and string theory level. Fayet-Iliopoulos terms of U(1)
hypercharge are shown to be of vital importance for this discussion. Models
with a single Higgs doublet seem to be generically affected by quadratic
divergences.Comment: Contribution to the Proceedings of Durham IPPP meeting May 2001.(12
pages, LaTeX
The electromagnetic calorimeter of the AMS-02 experiment
The electromagnetic calorimeter (ECAL) of the AMS-02 experiment is a
3-dimensional sampling calorimeter, made of lead and scintillating fibers. The
detector allows for a high granularity, with 18 samplings in the longitudinal
direction, and 72 sampling in the lateral direction. The ECAL primary goal is
to measure the energy of cosmic rays up to few TeV, however, thanks to the fine
grained structure, it can also provide the separation of positrons from
protons, in the GeV to TeV region. A direct measurement of high energy photons
with accurate energy and direction determination can also be provided.Comment: Proceedings of SF2A conference 201
A case of sole i(4)(p10) in myelodysplastic syndrome
Case report on a case of sole i(4)(p10) in myelodysplastic syndrome
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