358 research outputs found
Dynein structure and power stroke
Dynein ATPases are microtubule motors that are critical to diverse processes such as vesicle transport and the beating of sperm tails; however, their mechanism of force generation is unknown. Each dynein comprises a head, from which a stalk and a stem emerge. Here we use electron microscopy and image processing to reveal new structural details of dynein c, an isoform from Chlamydomonas reinhardtii flagella, at the start and end of its power stroke. Both stem and stalk are flexible, and the stem connects to the head by means of a linker approximately 10 nm long that we propose lies across the head. With both ADP and vanadate bound, the stem and stalk emerge from the head 10 nm apart. However, without nucleotide they emerge much closer together owing to a change in linker orientation, and the coiled-coil stalk becomes stiffer. The net result is a shortening of the molecule coupled to an approximately 15-nm displacement of the tip of the stalk. These changes indicate a mechanism for the dynein power stroke
D3-brane Potentials from Fluxes in AdS/CFT
We give a comprehensive treatment of the scalar potential for a D3-brane in a
warped conifold region of a compactification with stabilized moduli. By
studying general ultraviolet perturbations in supergravity, we systematically
incorporate `compactification effects' sourced by supersymmetry breaking in the
compact space. Significant contributions to the D3-brane potential, including
the leading term in the infrared, arise from imaginary anti-self-dual (IASD)
fluxes. For an arbitrary Calabi-Yau cone, we determine the most general IASD
fluxes in terms of scalar harmonics, then compute the resulting D3-brane
potential. Specializing to the conifold, we identify the operator dual to each
mode of flux, and for chiral operators we confirm that the potential computed
in the gauge theory matches the gravity result. The effects of four-dimensional
curvature, including the leading D3-brane mass term, arise directly from the
ten-dimensional equations of motion. Furthermore, we show that gaugino
condensation on D7-branes provides a local source for IASD flux. This flux
precisely encodes the nonperturbative contributions to the D3-brane potential,
yielding a promising ten-dimensional representation of four-dimensional
nonperturbative effects. Our result encompasses all significant contributions
to the D3-brane potential discussed in the literature, and does so in the
single coherent framework of ten-dimensional supergravity. Moreover, we
identify new terms with irrational scaling dimensions that were inaccessible in
prior works. By decoupling gravity in a noncompact configuration, then
systematically reincorporating compactification effects as ultraviolet
perturbations, we have provided an approach in which Planck-suppressed
contributions to the D3-brane effective action can be computed.Comment: 70 page
Fractional branes, warped compactifications and backreacted orientifold planes
The standard extremal p-brane solutions in supergravity are known to allow
for a generalisation which consists of adding a linear dependence on the
world-volume coordinates to the usual harmonic function. In this note we
demonstrate that remarkably this generalisation goes through in exactly the
same way for p-branes with fluxes added to it that correspond to fractional
p-branes. We relate this to warped orientifold compactifications by trading the
Dp-branes for Op-planes that solve the RR tadpole condition. This allows us to
interpret the worldvolume dependence as due to lower-dimensional scalars that
flow along the massless directions in the no-scale potential. Depending on the
details of the fluxes these flows can be supersymmetric domain wall flows. Our
solutions provide explicit examples of backreacted orientifold planes in
compactifications with non-constant moduli.Comment: 20 pages, incl. references. v2: small changes required for JHEP
publication. v3: few equation typos correcte
Scalar and vector mesons of flavor chiral symmetry breaking in the Klebanov-Strassler background
Recently, Dymarsky, Kuperstein and Sonnenschein constructed an embedding of
flavor D7- and anti-D7-branes in the Klebanov-Strassler geometry that breaks
the supersymmetry of the background, yet is stable. In this article, we study
in detail the spectrum of vector mesons in this new model of flavor chiral
symmetry breaking and commence an analytical analysis of the scalar mesons in
this setup.Comment: v1: 35 pages, 5 figures, 4 tables, includes self-contained review of
DKS construction; v2: corrected signs in eqs. (2.22) and (2.23), improved
discussion of scalar mesons in section 3.2; v3: major revision of the results
on scalar mesons, version submitted to JHEP; v4: version accepted by JHE
On Supersymmetric D7-branes in the Warped Deformed Conifold
We study the supersymmetric properties of D7-branes in the warped deformed
conifold. We consider the -symmetry conditions on D7-branes in this
specific warped background, taking into account the background NS-NS 2-form
flux. While any holomorphic embedding defines a supersymmetric D7-brane in the
absence of background H-flux, most of the D7-brane embeddings considered in the
literature do not preserve supersymmetry for the warped deformed conifold
without also including brane worldvolume flux. For the simplest such embedding,
we construct numerically the worldvolume fluxnecessary to restore
supersymmetry. We also comment on the dual field theory descriptions in terms
of cascading N=1 supersymmetric gauge theories with flavors. Finally, we
discuss some possible applications of our results to moduli stabilization and
vacuum energy uplifting, gauge/gravity duality, and string inflationary model
building.Comment: 37 pages, 2 figures, v3:many clarifications and improvements to susy
restoring flu
On thermodynamics of N=6 superconformal Chern-Simons theory
We study thermodynamics of N=6 superconformal Chern-Simons theory by
computing quantum corrections to the free energy. We find that in weakly
coupled ABJM theory on R(2) x S(1), the leading correction is non-analytic in
the 't Hooft coupling lambda, and is approximately of order lambda^2
log(lambda)^3. The free energy is expressed in terms of the scalar thermal mass
m, which is generated by screening effects. We show that this mass vanishes to
1-loop order. We then go on to 2-loop order where we find a finite and positive
mass squared m^2. We discuss differences in the calculation between Coulomb and
Lorentz gauge. Our results indicate that the free energy is a monotonic
function in lambda which interpolates smoothly to the N^(3/2) behaviour at
strong coupling.Comment: 29 pages. v2: references added. v3: minor changes, references added,
published versio
Spinflation with Angular Potentials
We investigate in detail the cosmological consequences of realistic angular
dependent potentials in the brane inflation scenario. Embedding a warped throat
into a compact Calabi-Yau space with all moduli stabilized breaks the no-scale
structure and induces angular dependence in the potential of the probe
D3-brane. We solve the equations of motion from the DBI action in the warped
deformed conifold including linearized perturbations around the imaginary
self-dual solution. Our numerical solutions show that angular dependence is a
next to leading order correction to the dominant radial motion of the brane,
however, just as angular motion typically increases the amount of inflation
(spinflation), having additional angular dependence also increases the amount
of inflation. We also derive an analytic approximation for the number of
e-foldings along the DBI trajectory in terms of the compactification
parameters.Comment: 20 pages, 10 figures. Revised to published version: minor errors
corrected, references and discussion adde
Correlation Functions of Large N Chern-Simons-Matter Theories and Bosonization in Three Dimensions
We consider the conformal field theory of N complex massless scalars in 2+1
dimensions, coupled to a U(N) Chern-Simons theory at level k. This theory has a
't Hooft large N limit, keeping fixed \lambda = N/k. We compute some
correlation functions in this theory exactly as a function of \lambda, in the
large N (planar) limit. We show that the results match with the general
predictions of Maldacena and Zhiboedov for the correlators of theories that
have high-spin symmetries in the large N limit. It has been suggested in the
past that this theory is dual (in the large N limit) to the Legendre transform
of the theory of fermions coupled to a Chern-Simons gauge field, and our
results allow us to find the precise mapping between the two theories. We find
that in the large N limit the theory of N scalars coupled to a U(N)_k
Chern-Simons theory is equivalent to the Legendre transform of the theory of k
fermions coupled to a U(k)_N Chern-Simons theory, thus providing a bosonization
of the latter theory. We conjecture that perhaps this duality is valid also for
finite values of N and k, where on the fermionic side we should now have (for
N_f flavors) a U(k)_{N-N_f/2} theory. Similar results hold for real scalars
(fermions) coupled to the O(N)_k Chern-Simons theory.Comment: 49 pages, 16 figures. v2: added reference
Soft branes in supersymmetry-breaking backgrounds
We revisit the analysis of effective field theories resulting from
non-supersymmetric perturbations to supersymmetric flux compactifications of
the type-IIB superstring with an eye towards those resulting from the
backreaction of a small number of anti-D3-branes. Independently of the
background, we show that the low-energy Lagrangian describing the fluctuations
of a stack of probe D3-branes exhibits soft supersymmetry breaking, despite
perturbations to marginal operators that were not fully considered in some
previous treatments. We take this as an indication that the breaking of
supersymmetry by anti-D3-branes or other sources may be spontaneous rather than
explicit. In support of this, we consider the action of an anti-D3-brane
probing an otherwise supersymmetric configuration and identify a candidate for
the corresponding goldstino.Comment: 36+5 pages. References added, minor typos correcte
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