348 research outputs found
Classification of the N=2, Z2 X Z2-symmetric type II orbifolds and their type II asymmetric duals
Using free world-sheet fermions, we construct and classify all the N=2, Z2 X
Z2 four-dimensional orbifolds of the type IIA/B strings for which the orbifold
projections act symmetrically on the left and right movers. We study the
deformations of these models out of the fermionic point, deriving the partition
functions at a generic point in the moduli of the internal torus T6=T2 X T2 X
T2. We investigate some of their perturbative and non-perturbative dualities
and construct new dual pairs of type IIA/type II asymmetric orbifolds, which
are related non-perturbatively and allow us to gain insight into some of the
non-perturbative properties of the type IIA/B strings in four dimensions. In
particular, we consider some of the (non-)perturbative gravitational
corrections.Comment: Latex, 47 pages, no figure
BPS States in N=3 Superstrings
The N = 3 string models are special solutions of the type II perturbative string theories. We present explicit expressions for the helicity supertraces,which count the number of the perturbative BPS multiplets. Assuming the non-perturbative duality of the heterotic string on T^6 and type II on K_3 x T^2 valid in N = 4 theories, we derive the N = 3 non-perturbative BPS mass formula by ``switching off" some of the N = 4 charges and ``fixing" to special values some of the N = 4 moduli. This operation corresponds to a well-defined Z_2 projection acting freely on the compactification manifold. The consistency of this projection and the precise connection of the N = 4 and N = 3 BPS spectrum is shown explicitly in several type II string constructions. The heterotic N = 3 and some asymmetric type II constructions turn out to be non-perturbative with the S moduli fixed at the self-dual point S=i. Some of the non-perturbative N = 3 type II are defined in the context of F-theory. The bosonic sector of the N=3 string effective action is also presented. This part can be useful for the study of 4d black holes in connection with the asymptotic density of BPS states in N = 3 string theory
R^2 Corrections and Non-perturbative Dualities of N=4 String ground states
We compute and analyse a variety of four-derivative gravitational terms in
the effective action of six- and four-dimensional type II string ground states
with N=4 supersymmetry. In six dimensions, we compute the relevant perturbative
corrections for the type II string compactified on K3. In four dimensions we do
analogous computations for several models with (4,0) and (2,2) supersymmetry.
Such ground states are related by heterotic-type II duality or type II-type II
U-duality. Perturbative computations in one member of a dual pair give a
non-perturbative result in the other member. In particular, the exact CP-even
R^2 coupling on the (2,2) side reproduces the tree-level term plus NS 5-brane
instanton contributions on the (4,0) side. On the other hand, the exact CP-odd
coupling yields the one-loop axionic interaction a.R\wedge R together with a
similar instanton sum. In a subset of models, the expected breaking of the
SL(2,Z)_S S-duality symmetry to a \Gamma(2)_S subgroup is observed on the
non-perturbative thresholds. Moreover, we present a duality chain that provides
evidence for the existence of heterotic N=4 models in which N=8 supersymmetry
appears at strong coupling.Comment: Latex2e, 51 pages, 1 figur
Non-Perturbative Gravitational Corrections in a Class of N=2 String Duals
We investigate the non-perturbative equivalence of some heterotic/type II dual pairs with N=2 supersymmetry. The perturbative heterotic scalar manifolds are respectively SU(1, 1)/U(1) x SO(2, 2+NV)/ SO(2) x SO(2+NV) and SO(4, 4+NH)/ SO(4) x SO(4+NH) for moduli in the vector multiplets and hypermultiplets. The models under consideration correspond, on the type II side, to self-mirror Calabi-Yau threefolds with Hodge numbers h(1,1)= NV +3= h(2,1)= NH +3, which are K3 fibrations. We consider three classes of dual pairs, with NV=NH=8, 4 and 2. The models with h(1,1)=7 and 5 provide new constructions, while the h(1,1)=11, already studied in the literature, is reconsidered here. Perturbative R2-like corrections are computed on the heterotic side by using a universal operator whose amplitude has no singularities in the (T,U) space, and can therefore be compared with the type II side result. We point out several properties connecting K3 fibrations and spontaneous breaking of the N=4 supersymmetry to N=2. As a consequence of the reduced S- and T- duality symmetries, the instanton numbers in these three classes are restricted to integers, which are multiples of 2, 2 and 4, for NV=8, 4 and 2, respectively
Soft Masses in Theories with Supersymmetry Breaking by TeV-Compactification
We study the sparticle spectroscopy and electroweak breaking of theories
where supersymmetry is broken by compactification (Scherk-Schwarz mechanism) at
a TeV. The evolution of the soft terms above the compactification scale and the
resulting sparticle spectrum are very different from those of the usual MSSM
and gauge mediated theories. This is traced to the softness of the
Scherk-Schwarz mechanism which leads to scalar sparticle masses that are only
logarithmically sensitive to the cutoff starting at two loops. As a result,
squarks and sleptons are naturally an order of magnitude lighter than gauginos.
In addition, the mechanism is very predictive and the sparticle spectrum
depends on just two new parameters. A significant advantage of this mechanism
relative to gauge mediation is that a Higgsino mass is
automatically generated when supersymmetry is broken. Our analysis applies
equally well to theories where the cutoff is near a TeV or or some
intermediate scale. We also use these observations to show how we may obtain
compactification radii which are hierarchically larger than the fundamental
cutoff scale.Comment: 26 pages, 1 figure, Late
Non-universal soft scalar masses in supersymmetric theories
The existence of non--universal soft masses is the most general situation in
supersymmetric theories. We study the consecuences that this situation has for
the low--energy sparticle spectrum. In particular, we analize in detail the
contribution to the scalar mass renormalization group equations of the
D--term. We obtain analytic expressions for the evolution of masses of the
three generations and these allow us to show that such a contribution can
produce important modifications on the spectrum. The necessity to avoid flavour
changing neutral currents does not constrain this result. Finally, we discuss a
realistic example in the context of string theory where the departure from
universality is large.Comment: Latex, 13 pages + 3 figs.(avalaible upon request
An Effective Supergravity for the Thermal Phases of N=4 Strings
A universal effective supergravity Lagrangian describing the thermal phases
of heterotic strings on T^4 x S^1, IIA and IIB strings on K^3 x S^1 is
constructed. The resulting non-perturbative phase structure is discussed.Comment: 9 pages, 6th Hellenic School and Workshop, Corfu, Greece, Sept. 9
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