264 research outputs found
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
Fluorescence enabled direct visual observation for diagnosis of ultrafiltration membrane fouling by bi-disperse submicron particle suspensions
Whilst direct observation (DO) methodologies can describe backâtransport of supraâmicron particles, present technologies are unable to discriminate submicron particles, which are primarily responsible for membrane fouling. In this study, we therefore introduce a fluorescence enabled direct visual observation (RLFâDVO) methodology to permit visual characterisation of submicron particle transport during crossâflow filtration. Particle discrimination was achievable for particle diameters exceeding 0.25 ”m; however, this was dependent upon particle concentration and the crossâflow velocity employed. Nevertheless, this is considerably below the detection limit of current techniques (around 3 ”m). During filtration of a binary dispersion comprised of submicron particles, deposition was observed before a change in transmembrane pressure was detected, which underpins the important role of DO for fouling diagnosis. Based on observations made during this study, recommendations are proposed that will further improve resolution. Importantly, this study demonstrates RLFâDVO can provide realâtime description of submicron particle transport during crossâflow filtration
Multifield Dynamics in Higgs-otic Inflation
In Higgs-otic inflation a complex neutral scalar combination of the and
MSSM Higgs fields plays the role of inflaton in a chaotic fashion. The
potential is protected from large trans-Planckian corrections at large inflaton
if the system is embedded in string theory so that the Higgs fields parametrize
a D-brane position. The inflaton potential is then given by a DBI+CS D-brane
action yielding an approximate linear behaviour at large field. The inflaton
scalar potential is a 2-field model with specific non-canonical kinetic terms.
Previous computations of the cosmological parameters (i.e. scalar and tensor
perturbations) did not take into account the full 2-field character of the
model, ignoring in particular the presence of isocurvature perturbations and
their coupling to the adiabatic modes. It is well known that for generic
2-field potentials such effects may significantly alter the observational
signatures of a given model. We perform a full analysis of adiabatic and
isocurvature perturbations in the Higgs-otic 2-field model. We show that the
predictivity of the model is increased compared to the adiabatic approximation.
Isocurvature perturbations moderately feed back into adiabatic fluctuations.
However, the isocurvature component is exponentially damped by the end of
inflation. The tensor to scalar ratio varies in a region ,
consistent with combined Planck/BICEP results.Comment: 35 pages, 11 figure
Holographic dilepton production in a thermalizing plasma
We determine the out-of-equilibrium production rate of dileptons at rest in
strongly coupled N=4 Super Yang-Mills plasma using the AdS/CFT correspondence.
Thermalization is achieved via the gravitational collapse of a thin shell of
matter in AdS_5 space and the subsequent formation of a black hole, which we
describe in a quasistatic approximation. Prior to thermalization, the dilepton
spectral function is observed to oscillate as a function of frequency, but the
amplitude of the oscillations decreases when thermal equilibrium is approached.
At the same time, we follow the flow of the quasinormal spectrum of the
corresponding U(1) vector field towards its equilibrium limit.Comment: 21 pages, 7 figures. v2: Version accepted for publication in JHEP;
minor modifications, added reference
Axion Protection from Flavor
The QCD axion fails to solve the strong CP problem unless all explicit PQ
violating, Planck-suppressed, dimension n<10 operators are forbidden or have
exponentially small coefficients. We show that all theories with a QCD axion
contain an irreducible source of explicit PQ violation which is proportional to
the determinant of the Yukawa interaction matrix of colored fermions.
Generically, this contribution is of low operator dimension and will
drastically destabilize the axion potential, so its suppression is a necessary
condition for solving the strong CP problem. We propose a mechanism whereby the
PQ symmetry is kept exact up to n=12 with the help of the very same flavor
symmetries which generate the hierarchical quark masses and mixings of the SM.
This "axion flavor protection" is straightforwardly realized in theories which
employ radiative fermion mass generation and grand unification. A universal
feature of this construction is that the heavy quark Yukawa couplings are
generated at the PQ breaking scale.Comment: 16 pages, 2 figure
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
Basal body stability and ciliogenesis requires the conserved component Poc1
Centrioles are the foundation for centrosome and cilia formation. The biogenesis of centrioles is initiated by an assembly mechanism that first synthesizes the ninefold symmetrical cartwheel and subsequently leads to a stable cylindrical microtubule scaffold that is capable of withstanding microtubule-based forces generated by centrosomes and cilia. We report that the conserved WD40 repeat domainâcontaining cartwheel protein Poc1 is required for the structural maintenance of centrioles in Tetrahymena thermophila. Furthermore, human Poc1B is required for primary ciliogenesis, and in zebrafish, DrPoc1B knockdown causes ciliary defects and morphological phenotypes consistent with human ciliopathies. T. thermophila Poc1 exhibits a protein incorporation profile commonly associated with structural centriole components in which the majority of Poc1 is stably incorporated during new centriole assembly. A second dynamic population assembles throughout the cell cycle. Our experiments identify novel roles for Poc1 in centriole stability and ciliogenesis
Wavefunctions and the Point of E8 in F-theory
In F-theory GUTs interactions between fields are typically localised at
points of enhanced symmetry in the internal dimensions implying that the
coefficient of the associated operator can be studied using a local
wavefunctions overlap calculation. Some F-theory SU(5) GUT theories may exhibit
a maximum symmetry enhancement at a point to E8, and in this case all the
operators of the theory can be associated to the same point. We take initial
steps towards the study of operators in such theories. We calculate
wavefunctions and their overlaps around a general point of enhancement and
establish constraints on the local form of the fluxes. We then apply the
general results to a simple model at a point of E8 enhancement and calculate
some example operators such as Yukawa couplings and dimension-five couplings
that can lead to proton decay.Comment: 46 page
DWSB in heterotic flux compactifications
We address the construction of non-supersymmetric vacua in heterotic
compactifications with intrinsic torsion and background fluxes. In particular,
we implement the approach of domain-wall supersymmetry breaking (DWSB)
previously developed in the context of type II flux compactifications. This
approach is based on considering backgrounds where probe NS5-branes wrapping
internal three-cycles and showing up as four-dimensional domain-walls do not
develop a BPS bound, while all the other BPS bounds characterizing the N=1
supersymmetric compactifications are preserved at tree-level. Via a scalar
potential analysis we provide the conditions for these backgrounds to solve the
ten-dimensional equations of motion including order \alpha' corrections. We
also consider backgrounds where some of the NS5-domain-walls develop a BPS
bound, show their relation to no-scale SUSY-breaking vacua and construct
explicit examples via elliptic fibrations. Finally, we consider backgrounds
with a non-trivial gaugino condensate and discuss their relation to
supersymmetric and non-supersymmetric vacua in the present context.Comment: 56 pages, 1 figur
Radiative Fermion Masses in Local D-Brane Models
In the context of D-brane model building, we present a realistic framework
for generating fermion masses that are forbidden by global symmetries. We show
that the string theoretical Large volume scenario circumvents the standard lore
that fermion masses generated by loop effects are too small in generic gravity
mediated scenarios. We argue that the fact that in toric singularity models,
the up quark masses have always a zero eigenvalue, corresponding to the
lightest generation, is due to the presence of approximate global symmetries
that we explicitly identify in del Pezzo singularities. These symmetries are
broken by global effects and therefore proportional to inverse powers of the
volume. We estimate the generic size of radiative corrections to fermion masses
in different phenomenological manifestations of the Large volume scenario.
Concrete realizations in terms of flavor violating soft-terms are estimated and
contrasted with current bounds on flavour changing neutral currents.
Contributions from generic extra Higgs-like fields set bounds on their masses
close to the GUT scale to produce realistic fermion masses.Comment: 21 pages. v2: minor rewording and reference adde
- âŠ