686 research outputs found
Lattice supersymmetry, superfields and renormalization
We study Euclidean lattice formulations of non-gauge supersymmetric models
with up to four supercharges in various dimensions. We formulate the conditions
under which the interacting lattice theory can exactly preserve one or more
nilpotent anticommuting supersymmetries. We introduce a superfield formalism,
which allows the enumeration of all possible lattice supersymmetry invariants.
We use it to discuss the formulation of Q-exact lattice actions and their
renormalization in a general manner. In some examples, one exact supersymmetry
guarantees finiteness of the continuum limit of the lattice theory. As a
consequence, we show that the desired quantum continuum limit is obtained
without fine tuning for these models. Finally, we discuss the implications and
possible further applications of our results to the study of gauge and
non-gauge models.Comment: 44 pages, 1 figur
Is Barbero's Hamiltonian formulation a Gauge Theory of Lorentzian Gravity?
This letter is a critique of Barbero's constrained Hamiltonian formulation of
General Relativity on which current work in Loop Quantum Gravity is based.
While we do not dispute the correctness of Barbero's formulation of general
relativity, we offer some criticisms of an aesthetic nature. We point out that
unlike Ashtekar's complex SU(2) connection, Barbero's real SO(3) connection
does not admit an interpretation as a space-time gauge field. We show that if
one tries to interpret Barbero's real SO(3) connection as a space-time gauge
field, the theory is not diffeomorphism invariant. We conclude that Barbero's
formulation is not a gauge theory of gravity in the sense that Ashtekar's
Hamiltonian formulation is. The advantages of Barbero's real connection
formulation have been bought at the price of giving up the description of
gravity as a gauge field.Comment: 12 pages, no figures, revised in the light of referee's comments,
accepted for publication in Classical and Quantum Gravit
Charged Particles in a 2+1 Curved Background
The coupling to a 2+1 background geometry of a quantized charged test
particle in a strong magnetic field is analyzed. Canonical operators adapting
to the fast and slow freedoms produce a natural expansion in the inverse square
root of the magnetic field strength. The fast freedom is solved to the second
order.
At any given time, space is parameterized by a couple of conjugate operators
and effectively behaves as the `phase space' of the slow freedom. The slow
Hamiltonian depends on the magnetic field norm, its covariant derivatives, the
scalar curvature and presents a peculiar coupling with the spin-connection.Comment: 22 page
Inflation in Realistic D-Brane Models
We find successful models of D-brane/anti-brane inflation within a string
context. We work within the GKP-KKLT class of type IIB string vacua for which
many moduli are stabilized through fluxes, as recently modified to include
`realistic' orbifold sectors containing standard-model type particles. We allow
all moduli to roll when searching for inflationary solutions and find that
inflation is not generic inasmuch as special choices must be made for the
parameters describing the vacuum. But given these choices inflation can occur
for a reasonably wide range of initial conditions for the brane and antibrane.
We find that D-terms associated with the orbifold blowing-up modes play an
important role in the inflationary dynamics. Since the models contain a
standard-model-like sector after inflation, they open up the possibility of
addressing reheating issues. We calculate predictions for the CMB temperature
fluctuations and find that these can be consistent with observations, but are
generically not deep within the scale-invariant regime and so can allow
appreciable values for as well as predicting a potentially
observable gravity-wave signal. It is also possible to generate some admixture
of isocurvature fluctuations.Comment: 39 pages, 21 figures; added references; identified parameters
combining successful inflation with strong warping, as needed for consistency
of the approximation
1/2-BPS Correlators as c=1 S-matrix
We argue from two complementary viewpoints of Holography that the 2-point
correlation functions of 1/2-BPS multi-trace operators in the large-N (planar)
limit are nothing but the (Wick-rotated) S-matrix elements of c=1 matrix model.
On the bulk side, we consider an Euclideanized version of the so-called
bubbling geometries and show that the corresponding droplets reach the
conformal boundary. Then the scattering matrix of fluctuations of the droplets
gives directly the two-point correlators through the GKPW prescription. On the
Yang-Mills side, we show that the two-point correlators of holomorphic and
anti-holomorphic operators are essentially equivalent with the transformation
functions between asymptotic in- and out-states of c=1 matrix model. Extension
to non-planar case is also discussed.Comment: 28 pages, 3 figures, corrected typos, version to appear in JHE
Generating Optimal Topologies in Structural Design Using a Homogenization Method,"
This paper proposes a basic method for designing light and rigid structures that have a maximum natural frequency for a designated mode. A design variable "density," related to the material properties of a three-dimensional solid element, is introduced into the finite element method ( FEM ). Thus, a structure is expressed as a density distribution inside its design domain, and the optimal structure is obtained by searching for the most suitable such distribution
Uplifting and Inflation with D3 Branes
Back-reaction effects can modify the dynamics of mobile D3 branes moving
within type IIB vacua, in a way which has recently become calculable. We
identify some of the ways these effects can alter inflationary scenarios, with
the following three results: (1) By examining how the forces on the brane due
to moduli-stabilizing interactions modify the angular motion of D3 branes
moving in Klebanov-Strassler type throats, we show how previous slow-roll
analyses can remain unchanged for some brane trajectories, while being modified
for other trajectories. These forces cause the D3 brane to sink to the bottom
of the throat except in a narrow region close to the D7 brane, and do not
ameliorate the \eta-problem of slow roll inflation in these throats; (2) We
argue that a recently-proposed back-reaction on the dilaton field can be used
to provide an alternative way of uplifting these compactifications to Minkowski
or De Sitter vacua, without the need for a supersymmetry-breaking anti-D3
brane; and (3) by including also the D-term forces which arise when
supersymmetry-breaking fluxes are included on D7 branes we identify the 4D
supergravity interactions which capture the dynamics of D3 motion in D3/D7
inflationary scenarios. The form of these potentials sheds some light on recent
discussions of how symmetries constrain D term interactions in the low-energy
theory.Comment: JHEP.cls, 35 pages, 3 .eps figure
Racetrack Inflation
We develop a model of eternal topological inflation using a racetrack
potential within the context of type IIB string theory with KKLT volume
stabilization. The inflaton field is the imaginary part of the K\"ahler
structure modulus, which is an axion-like field in the 4D effective field
theory. This model does not require moving branes, and in this sense it is
simpler than other models of string theory inflation. Contrary to
single-exponential models, the structure of the potential in this example
allows for the existence of saddle points between two degenerate local minima
for which the slow-roll conditions can be satisfied in a particular range of
parameter space. We conjecture that this type of inflation should be present in
more general realizations of the modular landscape. We also consider
`irrational' models having a dense set of minima, and discuss their possible
relevance for the cosmological constant problem.Comment: 23 pages 7 figures. The final version with minor modifications, to
appear in JHE
Self-Excited Vibration of Statically Unloaded Pads in Tilting-Pad Journal Bearings
load characteristics are satisfactory if ^m«0.66ap, but these pads do not have tendencies to loose their stability
- …