1,220 research outputs found

### Grassland carbon sequestration and emissions following cultivation in a mixed crop rotation

Grasslands are potential carbon sinks to reduce unprecedented increase in atmospheric CO2. Effect of age (1 to 4-yr-old) and management (slurry, grazing multispecies mixture) of a grass phase mixed crop rotation on carbon sequestration and emissions upon cultivation was compared with 17-yr-old grassland and a pea field as reference. Aboveground and root biomass were determined and soils were incubated to study CO2 emissions after soil disturbance. Aboveground biomass was highest in 1-yr-old grassland with slurry application and lowest in 4-yr-old grassland without slurry application. Root biomass was highest in 4-yr-old grassland, but all 1 to 4-yr-old grasslands were in between the pea field (0.81Â±0.094 g kg-1 soil) and the 17-yr-old grassland (3.17Â±0.22 g kg-1 soil). Grazed grasslands had significantly higher root biomass than cut grasslands. There was no significant difference in the CO2 emissions within 1 to 4-yr-old grasslands. Only the 17-yr-old grassland showed markedly higher CO2 emissions (4.9 Â± 1.1 g CO2 kg-1 soil). Differences in aboveground and root biomass did not affect CO2 emissions, and slurry application did not either. The substantial increase in root biomass with age but indifference in CO2 emissions across the age and management in temporary grasslands, thus, indicates potential for long-term sequestration of soil C

### Type IIA Orientifold Limit of M-Theory on Compact Joyce 8-Manifold of Spin(7)-Holonomy

We show that M-theory compactified on a compact Joyce 8-manifold of
$Spin(7)$-holonomy, which yields an effective theory in $D = 3$ with $\N$ = 1
supersymmetry, admits at some special points in it moduli space a description
in terms of type IIA theory on an orientifold of compact Joyce 7-manifold of
$G_2$-holonomy. We find the evidence in favour of this duality by computing the
massless spectra on both M-thory side and type IIA side. For the latter, we
compute the massless spectra by going to the orbifold limit of the Joyce
7-manifold.Comment: 26 pages, 2 eps figures, Latex file, two references and one footnote
added, corrected some typo

### Sitting on the Domain Walls of N=1 Super Yang-Mills

In pure N=1 supersymmetric Yang-Mills with gauge group SU(N), the domain
walls which separate the N vacua have been argued, on the basis of string
theory realizations, to be D-branes for the confining string. In a certain
limit, this means that a configuration of k parallel domain walls is described
by a 2+1-dimensional U(k) gauge theory. This theory has been identified by
Acharya and Vafa as the U(k) gauge theory with 4 supercharges broken by a
Chern-Simons term of level N in such a way that 2 supercharges are preserved.
We argue further that the gauge coupling of the domain wall gauge theory goes
like g^2 ~ Lambda/N, for large N. In the case of two domain walls, we show that
the U(2) world-volume theory generates a quadratic potential on the Coulomb
branch at two loops in perturbation theory which is consistent with there being
a supersymmetric bound state of the two wall system. A mass gap of order
Lambda/N is generated around the supersymmetric minimum and we estimate the
size of the bound-state to be order Lambda/ \sqrt N. At large distance the
potential reaches a constant that can qualitatively account for the binding
energy of the two walls even though stringy effects are not, strictly speaking,
decoupled.Comment: 18 pages, LaTex. 6 figure

### The Baryon-Dark Matter Ratio Via Moduli Decay After Affleck-Dine Baryogenesis

Low-scale supersymmetry breaking in string motivated theories implies the
presence of O(100) TeV scale moduli, which generically lead to a significant
modification of the history of the universe prior to Big Bang Nucleosynthesis.
Such an approach implies a non-thermal origin for dark matter resulting from
scalar decay, where the lightest supersymmetric particle can account for the
observed dark matter relic density. We study the further effect of the decay on
the baryon asymmetry of the universe, and find that this can satisfactorily
address the problem of the over-production of the baryon asymmetry by the
Affleck-Dine mechanism in the MSSM. Remarkably, there is a natural connection
between the baryon and dark matter abundances today, which leads to a solution
of the `Cosmic Coincidence Problem'.Comment: 12 pages, no figure. v2: references adde

### On Mirror Symmetry for Manifolds of Exceptional Holonomy

We consider Type II string theories on ${\bf T^n}/{{\bf Z_2}^m}$ Joyce
orbifolds. This class contains orbifolds which can be desingularised to give
manifolds of $G_2$ $({\bf n}$$=$$7)$ and $Spin(7)$ holonomy $({\bf n}$$=$$8)$.
In the $G_2$ holonomy case we present two types of $T$-duality transformation
which are clearly generalisations of the $T$-duality/mirror transformation in
Calabi-Yau spaces. The first maps Type IIA theory on one such space from this
class to Type IIB theory on another such space. The second maps Type IIA (IIB)
to Type IIA (IIB). In the case of $Spin(7)$ holonomy we present a $T$-duality
transformation which maps Type IIA (IIB) theory on one such space to Type IIA
(IIB) on another such space. As orbifold conformal field theories these
$T$-dual target spaces are related via the inclusion/exclusion of discrete
torsion and the $T$-duality is proven to genus $g$ in string perturbation
theory. We then apply a Strominger, Yau, Zaslow type argument which suggests
that manifolds of $G_2$ holonomy which have a ``mirror'' of the first (second)
type admit supersymmetric ${\bf T^3}$ (${\bf T^4}$) fibrations and that
manifolds of $Spin(7)$ holonomy for which a mirror exists admit fibration by
supersymmetric $4$-tori. Further evidence for this suggestion is given by
examining the moduli space structure of wrapped D-branes.Comment: 20 Pages, Late

### Rotating membranes on G_2 manifolds, logarithmic anomalous dimensions and N=1 duality

We show that the $E-S \sim \log S$ behaviour found for long strings rotating
on $AdS_5\times S^5$ may be reproduced by membranes rotating on $AdS_4\times
S^7$ and on a warped $AdS_5$ M-theory solution. We go on to obtain rotating
membrane configurations with the same $E-K \sim \log K$ relation on $G_2$
holonomy backgrounds that are dual to ${\mathcal{N}}=1$ gauge theories in four
dimensions. We study membrane configurations on $G_2$ holonomy backgrounds
systematically, finding various other Energy-Charge relations. We end with some
comments about strings rotating on warped backgrounds.Comment: 1+44 pages. Latex. No figures. Minor corrections to make all membrane
configurations consistent. One configuration is now noncompac

### Features of deSitter Vacua in M-Theory

We compute the masses of all moduli in the unstable deSitter vacua arising in
the toy model of cosmological M-theory flux compactifications on the G2
holonomy manifolds of [1]. The slow-roll parameters in the tachyonic directions
are shown to be too large to be useful for conventional models of inflation.
However, it appears that we can find fast roll regimes which could, under
certain conditions, account for the current dark energy driven accelerated
expansion of the universe.Comment: 14 pages, 1 figur

### Affleck-Dine baryogenesis in inflating curvaton scenario with O($10-10^2$TeV) mass moduli curvaton

We study the Affleck-Dine (AD) baryogenesis in the inflating curvaton
scenario, when the curvaton is a moduli field with O($10-10^2$TeV) mass. A
moduli field with such mass is known to be free from the Polonyi problem, and
furthermore its decay products can explain the present cold dark matter
abundance. In our scenario, it further explains the primordial curvature
perturbation and the present baryon density all together. The current
observational bound on the baryon isocurvature perturbation, which severely
constrains the AD baryogenesis with the original oscillating moduli curvaton
scenario, is shown to put practically negligible constraint if we replace the
oscillating curvaton with the inflating curvaton.Comment: 1+21pages v2: minor correction v3: included short reviews, added
refs, fixed typo

### Higher Dimensional Analogues of Donaldson-Witten Theory

We present a Donaldson-Witten type field theory in eight dimensions on
manifolds with $Spin(7)$ holonomy. We prove that the stress tensor is BRST
exact for metric variations preserving the holonomy and we give the invariants
for this class of variations. In six and seven dimensions we propose similar
theories on Calabi-Yau threefolds and manifolds of $G_2$ holonomy respectively.
We point out that these theories arise by considering supersymmetric Yang-Mills
theory defined on such manifolds. The theories are invariant under metric
variations preserving the holonomy structure without the need for twisting.
This statement is a higher dimensional analogue of the fact that
Donaldson-Witten field theory on hyper-K\"ahler 4-manifolds is topological
without twisting. Higher dimensional analogues of Floer cohomology are briefly
outlined. All of these theories arise naturally within the context of string
theory.Comment: 23 Pages, Latex. Our statement that these theories are independent of
the metric is corrected to the statement that the theories are invariant
under deformations that preserve the holonomy structure of the manifold. We
also include more details of the construction of a higher dimensional
analogue of Floer theory. Three references are adde

### Scherk-Schwarz reduction of M-theory on G2-manifolds with fluxes

We analyse the 4-dimensional effective supergravity theories obtained from
the Scherk--Schwarz reduction of M-theory on twisted 7-tori in the presence of
4-form fluxes. We implement the appropriate orbifold projection that preserves
a G2-structure on the internal 7-manifold and truncates the effective field
theory to an N=1, D=4 supergravity. We provide a detailed account of the
effective supergravity with explicit expressions for the Kaehler potential and
the superpotential in terms of the fluxes and of the geometrical data of the
internal manifold. Subsequently, we explore the landscape of vacua of M-theory
compactifications on twisted tori, where we emphasize the role of geometric
fluxes and discuss the validity of the bottom-up approach. Finally, by reducing
along isometries of the internal 7-manifold, we obtain superpotentials for the
corresponding type IIA backgrounds.Comment: 43 pages, Latex; v3 typos corrected, one reference added, JHEP
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