19,500 research outputs found
Matching the (DR4)-R-6 interaction at two-loops
The coefficient of the interaction in the low energy
expansion of the two-loop four-graviton amplitude in type II superstring theory
is known to be proportional to the integral of the Zhang-Kawazumi (ZK)
invariant over the moduli space of genus-two Riemann surfaces. We demonstrate
that the ZK invariant is an eigenfunction with eigenvalue 5 of the
Laplace-Beltrami operator in the interior of moduli space. Exploiting this
result, we evaluate the integral of the ZK invariant explicitly, finding
agreement with the value of the two-loop interaction predicted
on the basis of S-duality and supersymmetry. A review of the current
understanding of the interactions in type II superstring
theory compactified on a torus with and is
included.Comment: 40 pages, various typos and coefficients corrected in version
Modular properties of two-loop maximal supergravity and connections with string theory
The low-momentum expansion of the two-loop four-graviton scattering amplitude
in eleven-dimensional supergravity compactified on a circle and a two-torus is
considered up to terms of order S^6R^4 (where S is a Mandelstam invariant and R
is the linearized Weyl curvature). In the case of the toroidal compactification
the coefficient of each term in the low energy expansion is generically a sum
of a number of SL(2,Z)-invariant functions of the complex structure of the
torus. Each such function satisfies a separate Poisson equation on moduli space
with particular source terms that are bilinear in coefficients of lower order
terms, consistent with qualitative arguments based on supersymmetry. Comparison
is made with the low-energy expansion of type II string theories in ten and
nine dimensions. Although the detailed behaviour of the string amplitude is not
generally expected to be reproduced by supergravity perturbation theory to all
orders, for the terms considered here we find agreement with direct results
from string perturbation theory. These results point to a fascinating pattern
of interrelated Poisson equations for the IIB coefficients at higher orders in
the momentum expansion which may have a significance beyond the particular
methods by which they were motivated.Comment: 79 pages, 4 figures. Latex format. v2: Small corrections made,
version to appear in JHE
On Schwinger Pair Creation in Gravity and in Closed Superstring Theory
We investigate the Schwinger pair creation process in the context of
gravitational models with the back reaction of the electric field included in
the geometry. The background is also an exact solution of type II superstring
theory, where the electric field arises by Kaluza-Klein reduction. We obtain a
closed formula for the pair creation rate that incorporates the gravitational
back reaction. At weak fields it has the same structure as the general
Schwinger formula, albeit pairs are produced by a combination of Schwinger and
Unruh effect, the latter due to the presence of a Rindler horizon. In four
spacetime dimensions, the rate becomes constant at strong electric fields. For
states with mass of Kaluza-Klein origin, the rate has a power-like dependence
in the electric field, rather than the familiar (non-perturbative) exponential
dependence. We also reproduce the same formula from the string partition
function for winding string states. Finally, we comment on the generalization
to excited string states.Comment: 21 page
Body image perception and body composition: assessment of perception inconsistency by a new index
BACKGROUND: A correct perception of the body image, as defined by comparison with actual anthropometric analyses, is crucial to ensure the best possible nutritional status of each individual. Bioimpedance analysis (BIA) represents a leading technique to assess body composition parameters and, in particular, the fat mass. This study examined the self-perception of body image at various levels of adiposity proposing a new index. METHODS: We investigated 487 young Italian adults (mean age of males: 21.9 ± 2.4 years; mean age of females: 21.0 ± 2.2 years). Each subject could choose, on the Contour Drawing Rating Scale, the silhouette that he/she considered most resembling his/her perceived body image as well as his/her ideal body image. On each subject, we performed anthropometric measurements and determined the values of Fat mass and út with BIA. A new index, FAIFAT (Feel fat status minus Actual fat status Inconsistency), was developed to evaluate possible fat status perception inconsistencies by BIA. RESULTS: Based on ideal and feel body image comparison, women showed higher dissatisfaction than men and preferred slimmer silhouettes. FAIFAT values indicated that the fat status perception was correct in the majority of the examined individuals and only three subjects showed a serious misperception. CONCLUSIONS: Our findings suggest that FAIFAT is an appropriate index for assessing the perceived fat status from the body image when compared with data obtained by BIA. In a population, the use of this index will allow the correct identification of groups at risk for eating disorders
Structural properties and quasiparticule energies of cubic SrO, MgO and SrTiO3
The structural properties and the band structures of the charge-transfer
insulating oxides SrO, MgO and SrTiO3 are computed both within density
functional theory in the local density approximation (LDA) and in the Hedin's
GW scheme for self-energy corrections, by using a model dielectric function,
which approximately includes local field and dynamical effects. The deep
valence states are shifted by the GW method to higher binding energies, in very
good agreement with photoemission spectra. Since in all of these oxides the
direct gaps at high-symmetry points of the Brillouin zone may be very sensitive
to the actual value of the lattice parameter a, already at the LDA level,
self-energy corrections are computed both at the theoretical and the
experimental a. For MgO and SrO, the values of the transition energies between
the valence and the conduction bands are improved by GW corrections, while for
SrTiO3 they are overestimated. The results are discussed in relation to the
importance of local field effects and to the nature of the electronic states in
these insulating oxides.Comment: 3 figures, accepted in J. Phys.: Condens. Matte
Eisenstein series for higher-rank groups and string theory amplitudes
Scattering amplitudes of superstring theory are strongly constrained by the
requirement that they be invariant under dualities generated by discrete
subgroups, E_n(Z), of simply-laced Lie groups in the E_n series (n<= 8). In
particular, expanding the four-supergraviton amplitude at low energy gives a
series of higher derivative corrections to Einstein's theory, with coefficients
that are automorphic functions with a rich dependence on the moduli. Boundary
conditions supplied by string and supergravity perturbation theory, together
with a chain of relations between successive groups in the E_n series,
constrain the constant terms of these coefficients in three distinct parabolic
subgroups. Using this information we are able to determine the expressions for
the first two higher derivative interactions (which are BPS-protected) in terms
of specific Eisenstein series. Further, we determine key features of the
coefficient of the third term in the low energy expansion of the
four-supergraviton amplitude (which is also BPS-protected) in the E_8 case.
This is an automorphic function that satisfies an inhomogeneous Laplace
equation and has constant terms in certain parabolic subgroups that contain
information about all the preceding terms.Comment: Latex. 38 pages. 1 figure. v2: minor changes and clarifications. v3:
minor corrections, version to appear in Communications in Number Theory and
Physics. v4: corrections to table
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