170 research outputs found
On the Perturbative Stability of Quantum Field Theories in de Sitter Space
We use a field theoretic generalization of the Wigner-Weisskopf method to
study the stability of the Bunch-Davies vacuum state for a massless,
conformally coupled interacting test field in de Sitter space. We find that in
theory the vacuum does {\em not} decay, while in
non-conformally invariant models, the vacuum decays as a consequence of a
vacuum wave function renormalization that depends \emph{singularly} on
(conformal) time and is proportional to the spatial volume. In a particular
regularization scheme the vacuum wave function renormalization is the same as
in Minkowski spacetime, but in terms of the \emph{physical volume}, which leads
to an interpretation of the decay. A simple example of the impact of vacuum
decay upon a non-gaussian correlation is discussed. Single particle excitations
also decay into two particle states, leading to particle production that
hastens the exiting of modes from the de Sitter horizon resulting in the
production of \emph{entangled superhorizon pairs} with a population consistent
with unitary evolution. We find a non-perturbative, self-consistent "screening"
mechanism that shuts off vacuum decay asymptotically, leading to a stationary
vacuum state in a manner not unlike the approach to a fixed point in the space
of states.Comment: 36 pages, 4 figures. Version to appear in JHEP, more explanation
A slice of AdS_5 as the large N limit of Seiberg duality
A slice of AdS_5 is used to provide a 5D gravitational description of 4D
strongly-coupled Seiberg dual gauge theories. An (electric) SU(N) gauge theory
in the conformal window at large N is described by the 5D bulk, while its
weakly coupled (magnetic) dual is confined to the IR brane. This framework can
be used to construct an N = 1 MSSM on the IR brane, reminiscent of the original
Randall-Sundrum model. In addition, we use our framework to study
strongly-coupled scenarios of supersymmetry breaking mediated by gauge forces.
This leads to a unified scenario that connects the extra-ordinary gauge
mediation limit to the gaugino mediation limit in warped space.Comment: 47 Pages, axodraw4j.st
Transplanckian axions !?
We discuss quantum gravitational effects in Einstein theory coupled to
periodic axion scalars to analyze the viability of several proposals to achieve
superplanckian axion periods (aka decay constants) and their possible
application to large field inflation models. The effects we study correspond to
the nucleation of euclidean gravitational instantons charged under the axion,
and our results are essentially compatible with (but independent of) the Weak
Gravity Conjecture, as follows: Single axion theories with superplanckian
periods contain gravitational instantons inducing sizable higher harmonics in
the axion potential, which spoil superplanckian inflaton field range. A similar
result holds for multi-axion models with lattice alignment (like the
Kim-Nilles-Peloso model). Finally, theories with axions can still achieve a
moderately superplanckian periodicity (by a factor) with no higher
harmonics in the axion potential. The Weak Gravity Conjecture fails to hold in
this case due to the absence of some instantons, which are forbidden by a
discrete gauge symmetry. Finally we discuss the realization of
these instantons as euclidean D-branes in string compactifications.Comment: 46 pages, 6 figures. Added references, clarifications, and missing
factor of 1/2 to instanton action. Conclusions unchange
Chiral Generations on Intersecting 5-branes in Heterotic String Theory
We show that there exist two 27 and one 27 bar of E6, net one D=4, N=1 chiral
matter supermultiplet as zero modes localized on the intersection of two
5-branes in the E8 x E8 heterotic string theory. The smeared intersecting
5-brane solution is used via the standard embedding to construct a heterotic
background, which provides, after a compactification of some of the transverse
dimensions, a five-dimensional Randall-Sundrum II like brane-world set-up in
heterotic string theory. As a by-product, we present a new proof of anomaly
cancellation between those from the chiral matter and the anomaly inflow onto
the brane without small instanton.Comment: 26 pages, 5 figures; references added, typo correcte
Deep Inelastic Scattering in Conformal QCD
We consider the Regge limit of a CFT correlation function of two vector and
two scalar operators, as appropriate to study small-x deep inelastic scattering
in N=4 SYM or in QCD assuming approximate conformal symmetry. After clarifying
the nature of the Regge limit for a CFT correlator, we use its conformal
partial wave expansion to obtain an impact parameter representation encoding
the exchange of a spin j Reggeon for any value of the coupling constant. The
CFT impact parameter space is the three-dimensional hyperbolic space H3, which
is the impact parameter space for high energy scattering in the dual AdS space.
We determine the small-x structure functions associated to the exchange of a
Reggeon. We discuss unitarization from the point of view of scattering in AdS
and comment on the validity of the eikonal approximation.
We then focus on the weak coupling limit of the theory where the amplitude is
dominated by the exchange of the BFKL pomeron. Conformal invariance fixes the
form of the vector impact factor and its decomposition in transverse spin 0 and
spin 2 components. Our formalism reproduces exactly the general results predict
by the Regge theory, both for a scalar target and for gamma*-gamma* scattering.
We compute current impact factors for the specific examples of N=4 SYM and QCD,
obtaining very simple results. In the case of the R-current of N=4 SYM, we show
that the transverse spin 2 component vanishes. We conjecture that the impact
factors of all chiral primary operators of N=4 SYM only have components with 0
transverse spin.Comment: 44+16 pages, 7 figures. Some correction
Matrix Models for the Black Hole Information Paradox
We study various matrix models with a charge-charge interaction as toy models
of the gauge dual of the AdS black hole. These models show a continuous
spectrum and power-law decay of correlators at late time and infinite N,
implying information loss in this limit. At finite N, the spectrum is discrete
and correlators have recurrences, so there is no information loss. We study
these models by a variety of techniques, such as Feynman graph expansion, loop
equations, and sum over Young tableaux, and we obtain explicitly the leading
1/N^2 corrections for the spectrum and correlators. These techniques are
suggestive of possible dual bulk descriptions. At fixed order in 1/N^2 the
spectrum remains continuous and no recurrence occurs, so information loss
persists. However, the interchange of the long-time and large-N limits is
subtle and requires further study.Comment: 35 pages, 11 eps figures; v.2 minor typos fixe
The Intermediate Scale MSSM, the Higgs Mass and F-theory Unification
Even if SUSY is not present at the Electro-Weak scale, string theory suggests
its presence at some scale M_{SS} below the string scale M_s to guarantee the
absence of tachyons. We explore the possible value of M_{SS} consistent with
gauge coupling unification and known sources of SUSY breaking in string theory.
Within F-theory SU(5) unification these two requirements fix M_{SS} ~ 5 x
10^{10} GeV at an intermediate scale and a unification scale M_c ~ 3 x 10^{14}
GeV. As a direct consequence one also predicts the vanishing of the quartic
Higgs SM self-coupling at M_{SS} ~10^{11} GeV. This is tantalizingly consistent
with recent LHC hints of a Higgs mass in the region 124-126 GeV. With such a
low unification scale M_c ~ 3 x 10^{14} GeV one may worry about too fast proton
decay via dimension 6 operators. However in the F-theory GUT context SU(5) is
broken to the SM via hypercharge flux. We show that this hypercharge flux
deforms the SM fermion wave functions leading to a suppression, avoiding in
this way the strong experimental proton decay constraints. In these
constructions there is generically an axion with a scale of size f_a ~
M_c/(4\pi)^2 ~ 10^{12} GeV which could solve the strong CP problem and provide
for the observed dark matter. The prize to pay for these attractive features is
to assume that the hierarchy problem is solved due to anthropic selection in a
string landscape.Comment: 48 pages, 8 figures. v3: further minor correction
On the renormalization of multiparton webs
We consider the recently developed diagrammatic approach to soft-gluon
exponentiation in multiparton scattering amplitudes, where the exponent is
written as a sum of webs - closed sets of diagrams whose colour and kinematic
parts are entangled via mixing matrices. A complementary approach to
exponentiation is based on the multiplicative renormalizability of intersecting
Wilson lines, and their subsequent finite anomalous dimension. Relating this
framework to that of webs, we derive renormalization constraints expressing all
multiple poles of any given web in terms of lower-order webs. We examine these
constraints explicitly up to four loops, and find that they are realised
through the action of the web mixing matrices in conjunction with the fact that
multiple pole terms in each diagram reduce to sums of products of lower-loop
integrals. Relevant singularities of multi-eikonal amplitudes up to three loops
are calculated in dimensional regularization using an exponential infrared
regulator. Finally, we formulate a new conjecture for web mixing matrices,
involving a weighted sum over column entries. Our results form an important
step in understanding non-Abelian exponentiation in multiparton amplitudes, and
pave the way for higher-loop computations of the soft anomalous dimension.Comment: 60 pages, 15 figure
The influence of foot geometry on the calcaneal osteotomy angle based on two-dimensional static force analyses
Background: Malalignment of the hindfoot can be corrected with a calcaneal osteotomy (CO). A well-selected osteotomy angle in the sagittal plane will reduce the shear force in the osteotomy plane while walking. The purpose was to determine the presence of a relationship between the foot geometry and loading of the calcaneus, which influences the choice of the preferred CO angle. Methods A static free body force analysis was made of the posterior calcaneal fragment in the second half of the stance phase to determine the main loads: the plantar apeunorosis (PA) and Achilles tendon (AT). The third load is on the osteotomy surface which should be oriented such that the shear component of the force is zero. The force direction of the PA and AT was measured on 58 MRIs of the foot, and the force ratio between both structures was taken from the literature. In addition the PA-to-AT force ratio was estimated for different foot geometries to identify the relationship. Results: Based on the wish to minimize the shear force during walking, a mean CO angle was determined to be 33º (SD8) relative to the foot sole. In pes planus foot geometry, the angle should be higher than the mean. In pes cavus foot geometry, the angle should be smaller. Conclusion: Foot geometry, in particular the relative foot heights is a determinant for the individual angle in performing the sliding calcaneal osteotomy. It is recommended to take into account the foot geometry (arch) when deciding on the CO angle for hindfoot correction.Biomechanical EngineeringMechanical, Maritime and Materials Engineerin
Fluorescence-based high-throughput functional profiling of ligand-gated ion channels at the level of single cells
Ion channels are involved in many physiological processes and are attractive targets for therapeutic intervention. Their functional properties vary according to their subunit composition, which in turn varies in a developmental and tissue-specific manner and as a consequence of pathophysiological events. Understanding this diversity requires functional analysis of ion channel properties in large numbers of individual cells. Functional characterisation of ligand-gated channels involves quantitating agonist and drug dose-response relationships using electrophysiological or fluorescence-based techniques. Electrophysiology is limited by low throughput and high-throughput fluorescence-based functional evaluation generally does not enable the characterization of the functional properties of each individual cell. Here we describe a fluorescence-based assay that characterizes functional channel properties at single cell resolution in high throughput mode. It is based on progressive receptor activation and iterative fluorescence imaging and delivers >100 dose-responses in a single well of a 384-well plate, using α1-3 homomeric and αβ heteromeric glycine receptor (GlyR) chloride channels as a model system. We applied this assay with transiently transfected HEK293 cells co-expressing halide-sensitive yellow fluorescent protein and different GlyR subunit combinations. Glycine EC values of different GlyR isoforms were highly correlated with published electrophysiological data and confirm previously reported pharmacological profiles for the GlyR inhibitors, picrotoxin, strychnine and lindane. We show that inter and intra well variability is low and that clustering of functional phenotypes permits identification of drugs with subunit-specific pharmacological profiles. As this method dramatically improves the efficiency with which ion channel populations can be characterized in the context of cellular heterogeneity, it should facilitate systems-level analysis of ion channel properties in health and disease and the discovery of therapeutics to reverse pathological alterations
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