8 research outputs found
Similarity between Kaluza-Klein and Open-string amplitudes in Diphoton Production
We calculate the tree-level open-string amplitudes for the scattering of four
massless particles with diphoton final states. These amplitudes are required to
reproduce those of standard model at the tree level in the low energy limit.
After low energy stringy corrections, we found that they have similar form to
the same processes induced by exchange of the Kaluza-Klein(KK) excitations of
graviton in ADD scenario. Using this similarity, we apply constraints on the KK
mass scale to the string scale . The results are consistent with
constraints from the 4-fermion scattering, about TeV.Comment: 10 pages, modified reference
Graviton production with 2 jets at the LHC in large extra dimensions
We study Kaluza-Klein (KK) graviton production in the large extra dimensions
model via 2 jets plus missing transverse momentum signatures at the LHC. We
make predictions for both the signal and the dominant Zjj and Wjj backgrounds,
where we introduce missing P_T-dependent jet selection cuts that ensure the
smallness of the 2-jet rate over the 1-jet rate. With the same jet selection
cuts, the distributions of the two jets and their correlation with the missing
transverse momentum provide additional evidence for the production of an
invisible massive object.Comment: 8 pages, 10 figures, 1 table; Version to be printed in JHE
Chameleon Gravity, Electrostatics, and Kinematics in the Outer Galaxy
Light scalar fields are expected to arise in theories of high energy physics
(such as string theory), and find phenomenological motivations in dark energy,
dark matter, or neutrino physics. However, the coupling of light scalar fields
to ordinary (or dark) matter is strongly constrained from laboratory, solar
system, and astrophysical tests of fifth force. One way to evade these
constraints in dense environments is through the chameleon mechanism, where the
field's mass steeply increases with ambient density. Consequently, the
chameleonic force is only sourced by a thin shell near the surface of dense
objects, which significantly reduces its magnitude.
In this paper, we argue that thin-shell conditions are equivalent to
"conducting" boundary conditions in electrostatics. As an application, we use
the analogue of the method of images to calculate the back-reaction (or
self-force) of an object around a spherical gravitational source. Using this
method, we can explicitly compute the violation of equivalence principle in the
outskirts of galactic haloes (assuming an NFW dark matter profile):
Intermediate mass satellites can be slower than their larger/smaller
counterparts by as much as 10% close to a thin shell.Comment: 17 pages, 3 figure
Classical and Quantum Consistency of the DGP Model
We study the Dvali-Gabadadze-Porrati model by the method of the boundary
effective action. The truncation of this action to the bending mode \pi
consistently describes physics in a wide range of regimes both at the classical
and at the quantum level. The Vainshtein effect, which restores agreement with
precise tests of general relativity, follows straightforwardly. We give a
simple and general proof of stability, i.e. absence of ghosts in the
fluctuations, valid for most of the relevant cases, like for instance the
spherical source in asymptotically flat space. However we confirm that around
certain interesting self-accelerating cosmological solutions there is a ghost.
We consider the issue of quantum corrections. Around flat space \pi becomes
strongly coupled below a macroscopic length of 1000 km, thus impairing the
predictivity of the model. Indeed the tower of higher dimensional operators
which is expected by a generic UV completion of the model limits predictivity
at even larger length scales. We outline a non-generic but consistent choice of
counterterms for which this disaster does not happen and for which the model
remains calculable and successful in all the astrophysical situations of
interest. By this choice, the extrinsic curvature K_{\mu\nu} acts roughly like
a dilaton field controlling the strength of the interaction and the cut-off
scale at each space-time point. At the surface of Earth the cutoff is \sim 1 cm
but it is unlikely that the associated quantum effects be observable in table
top experiments.Comment: 26 pages, 1 eps figur
Will we observe black holes at LHC?
The generalized uncertainty principle, motivated by string theory and
non-commutative quantum mechanics, suggests significant modifications to the
Hawking temperature and evaporation process of black holes. For
extra-dimensional gravity with Planck scale O(TeV), this leads to important
changes in the formation and detection of black holes at the the Large Hadron
Collider. The number of particles produced in Hawking evaporation decreases
substantially. The evaporation ends when the black hole mass is Planck scale,
leaving a remnant and a consequent missing energy of order TeV. Furthermore,
the minimum energy for black hole formation in collisions is increased, and
could even be increased to such an extent that no black holes are formed at LHC
energies.Comment: 5 pages, 2 figures. Minor changes to match version to appear in
Class. Quant. Gra
Black Hole Astrophysics in AdS Braneworlds
We consider astrophysics of large black holes localized on the brane in the
infinite Randall-Sundrum model. Using their description in terms of a conformal
field theory (CFT) coupled to gravity, deduced in Ref. [1], we show that they
undergo a period of rapid decay via Hawking radiation of CFT modes. For
example, a black hole of mass would shed most of its
mass in years if the AdS radius is mm,
currently the upper bound from table-top experiments. Since this is within the
mass range of X-ray binary systems containing a black hole, the evaporation
enhanced by the hidden sector CFT modes could cause the disappearance of X-ray
sources on the sky. This would be a striking signature of RS2 with a large AdS
radius. Alternatively, for shorter AdS radii, the evaporation would be slower.
In such cases, the persistence of X-ray binaries with black holes already
implies an upper bound on the AdS radius of L \la 10^{-2} mm, an order of
magnitude better than the bounds from table-top experiments. The observation of
primordial black holes with a mass in the MACHO range and an age comparable to the age of the universe would further
strengthen the bound on the AdS radius to L \la {\rm few} \times 10^{-6} mm.Comment: 14 pages, latex, no figures v2: added reference
Origami World
We paste together patches of to find solutions which describe two
4-branes intersecting on a 3-brane with non-zero tension. We construct
explicitly brane arrays with Minkowski, de Sitter and Anti-de Sitter geometries
intrinsic to the 3-brane, and describe how to generalize these solutions to the
case of , , where -branes intersect on a 3-brane. The
Minkowski and de Sitter solutions localize gravity to the intersection, leading
to 4D Newtonian gravity at large distances. We show this explicitly in the case
of Minkowski origami by finding the zero-mode graviton, and computing the
couplings of the bulk gravitons to the matter on the intersection. In de Sitter
case, this follows from the finiteness of the bulk volume. The effective 4D
Planck scale depends on the square of the fundamental 6D Planck scale, the
radius and the angles between the 4-branes and the radial
direction, and for the Minkowski origami it is . If this may account for the Planck-electroweak hierarchy even if , with a possibility for sub-millimeter corrections to the
Newton's law. We comment on the early universe cosmology of such models.Comment: plain LaTeX, 23 pages + 2 .eps figure
Signatures of modified gravity on the 21 cm power spectrum at reionisation
Scalar modifications of gravity have an impact on the growth of structure.
Baryon and Cold Dark Matter (CDM) perturbations grow anomalously for scales
within the Compton wavelength of the scalar field. In the late time Universe
when reionisation occurs, the spectrum of the 21cm brightness temperature is
thus affected. We study this effect for chameleon-f(R) models, dilatons and
symmetrons. Although the f(R) models are more tightly constrained by solar
system bounds, and effects on dilaton models are negligible, we find that
symmetrons where the phase transition occurs before z_* ~ 12 will be detectable
for a scalar field range as low as 5 kpc. For all these models, the detection
prospects of modified gravity effects are higher when considering modes
parallel to the line of sight where very small scales can be probed. The study
of the 21 cm spectrum thus offers a complementary approach to testing modified
gravity with large scale structure surveys. Short scales, which would be highly
non-linear in the very late time Universe when structure forms and where
modified gravity effects are screened, appear in the linear spectrum of 21 cm
physics, hence deviating from General Relativity in a maximal way.Comment: 22 pages, 8 figures, typos correcte