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 $U(1)_Y$ 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

Option-pricing in incomplete markets: the hedging portfolio plus a risk premium-based recursive approach

Consider a non-spanned security $C_{T}$ in an incomplete market. We study the risk/return tradeoffs generated if this security is sold for an arbitrage-free price $\hat{C_{0}}$ and then hedged. We consider recursive "one-period optimal" self-financing hedging strategies, a simple but tractable criterion. For continuous trading, diffusion processes, the one-period minimum variance portfolio is optimal. Let $C_{0}(0)$ be its price. Self-financing implies that the residual risk is equal to the sum of the one-period orthogonal hedging errors, $\sum_{t\leq T} Y_{t}(0) e^{r(T -t)}$. To compensate the residual risk, a risk premium $y_{t}\Delta t$ is associated with every $Y_{t}$. Now let $C_{0}(y)$ be the price of the hedging portfolio, and $\sum_{t\leq T}(Y_{t}(y)+y_{t}\Delta t)e^{r(T-t)}$ is the total residual risk. Although not the same, the one-period hedging errors $Y_{t}(0) and Y_{t}(y)$ are orthogonal to the trading assets, and are perfectly correlated. This implies that the spanned option payoff does not depend on y. Let $\hat{C_{0}}-C_{0}(y)$. A main result follows. Any arbitrage-free price, $\hat{C_{0}}$, is just the price of a hedging portfolio (such as in a complete market), $C_{0}(0)$, plus a premium, $\hat{C_{0}}-C_{0}(0)$. That is, $C_{0}(0)$ is the price of the option's payoff which can be spanned, and $\hat{C_{0}}-C_{0}(0)$ is the premium associated with the option's payoff which cannot be spanned (and yields a contingent risk premium of sum $y_{t}\Delta$t$e^{r(T-t)}$ at maturity). We study other applications of option-pricing theory as well

A note on the RG flow in (N=1, D=4) Supergravity and applications to Z_3 orbifold/orientifold compactification

We apply the standard approach of RG flow for the gauge couplings in N=1 D=4 Supergravity to show how to match its results with the heterotic $Z_3$ orbifold and Type IIB ${Z_3}$ orientifold-based models. Using only supergravity, anomaly cancellation and the requirement of unification we determine the part of the K\"ahler potential of the model invariant under the symmetries of the model. For heterotic orbifolds/type IIB orientifolds, this shows that the lowest order K\"ahler term of the dilaton has the structure $-\ln(S+{\bar S})$ in agreement with string calculations. The structure of the holomorphic coupling is also found from arguments of unification and anomaly cancellation under the conjectured $SL(2,Z)_{T_i}$ symmetries of the models. A consequence of the latter is that in the case of the $Z_3$ orientifold the holomorphic coupling necessarily contains a part with coefficient proportional to the one loop beta function, in agreement with string calculations which do not however assume this symmetry. This gives circumstantial evidence for the existence of this symmetry at string level in $Z_3$ orientifold. Finally, we comment on the values of the unification scale and examine the possibility of mirage unification in which the effective unification scale may be situated far above the string scale.Comment: 17 pages, LaTe

String GUTs

Standard SUSY-GUTs such as those based on $SU(5)$ or $SO(10)$ lead to predictions for the values of $\alpha _s$ and $sin^2\theta _W$ in amazing agreement with experiment. In this article we investigate how these models may be obtained from string theory, thus bringing them into the only known consistent framework for quantum gravity. String models with matter in standard GUT representations require the realization of affine Lie algebras at higher levels. We start by describing some methods to build level $k=2$ orbifold string models with gauge groups $SU(5)$ or $SO(10)$. We present several examples and identify generic features of the type of models constructed. Chiral fields appropriate to break the symmetry down to the standard model generically appear in the massless spectrum. However, unlike in standard SUSY-GUTs, they often behave as string moduli, i.e., they do not have self-couplings. We also discuss briefly the doublet-triplet Higgs splitting. We find that, in some models, built-in sliding-singlet type of couplings exist.Comment: 40 pages, Latex. Two references modified, few other unimportant change

Some properties concerning the quasi-inverse of a T-norm

Some properties of the quasi-inverse operators are presented. They are basic tools in order to reduce complex expressions involving several of such operators. An effective calculation for the quasi-inverse of a continuous t-norm is also provided.Peer ReviewedPostprint (published version

Orbifold-induced μ\mu term and electroweak symmetry breaking

It is known that a Higgs $\mu$ term can be naturally generated through the K\"ahler potential in orbifold string models in which one of the three compactified complex planes has order two. In this class of models explicit expressions for both the $\mu$ parameter and the soft SUSY-breaking parameters can be obtained under the assumption that the goldstino is an arbitrary linear combination of the fermionic partners of the dilaton $S$ and all the moduli $T_i,U_i$. We apply this picture to the MSSM and explore the consistency of the obtained boundary conditions with radiative gauge symmetry breaking. We find that consistency with the measured value of the top-quark mass can only be achieved if the goldstino has a negligible dilatino component and relevant components along the $T_3,U_3$ moduli associated to the order-two complex plane.Comment: 9 pages, LaTeX fil

Numerical study of current wing-tip devices for commercial aircraft

It will be carried out numerical simulations of new designs of wing tip devices for commercial aircrafts. The performance of this devices will be estimated and it will be compared with the performance of some classical solutions

Threshold Corrections to Gauge Couplings in Orbifold Compactifications

We derive the moduli dependent threshold corrections to gauge couplings in toroidal orbifold compactifications. The underlying six dimensional torus lattice of the heterotic string theory is not assumed ---as in previous calculations--- to decompose into a direct sum of a four--dimensional and a two--dimensional sublattice, with the latter lying in a plane left fixed by a set of orbifold twists. In this more general case the threshold corrections are no longer automorphic functions of the modular group, but of certain congruence subgroups of the modular group. These groups can also be obtained by studying the massless spectrum; moreover they have larger classes of automorphic functions. As a consequence the threshold corrections cannot be uniquely determined by symmetry considerations and certain boundary conditions at special points in the moduli space, as was claimed in previous publications.Comment: 20 pages (Latex) (There was a minor Latex error in the original version. This is removed now

Constraints on discrete symmetries from anomaly cancellation in compactified superstring theories

Compactified string theories give rise to discrete symmetries which are essential if they are to provide a realistic low energy theory. We find that in a class of four dimensional string theories these symmetries are constrained by similar conditions to those discrete anomaly cancellation conditions found in the case the discrete symmetry is a residue of a spontaneously broken gauge symmetry. Such conditions strongly constrain the allowed form of the low energy effective theory.Comment: 8 pages, OUTP-93-14

Duality Anomaly Cancellation, Minimal String Unification and the Effective Low-Energy Lagrangian of 4-D Strings

We present a systematic study of the constraints coming from target-space duality and the associated duality anomaly cancellations on orbifold-like 4-D strings. A prominent role is played by the modular weights of the massless fields. We present a general classification of all possible modular weights of massless fields in Abelian orbifolds. We show that the cancellation of modular anomalies strongly constrains the massless fermion content of the theory, in close analogy with the standard ABJ anomalies. We emphasize the validity of this approach not only for (2,2) orbifolds but for (0,2) models with and without Wilson lines. As an application one can show that one cannot build a ${\bf Z}_3$ or ${\bf Z}_7$ orbifold whose massless charged sector with respect to the (level one) gauge group $SU(3)\times SU(2) \times U(1)$ is that of the minimal supersymmetric standard model, since any such model would necessarily have duality anomalies. A general study of those constraints for Abelian orbifolds is presented. Duality anomalies are also related to the computation of string threshold corrections to gauge coupling constants. We present an analysis of the possible relevance of those threshold corrections to the computation of $\sin^2\theta_W$ and $\alpha_3$ for all Abelian orbifolds. Some particular {\it minimal} scenarios, namely those based on all ${\bf Z}_N$ orbifolds except ${\bf Z}_6$Comment: 69 page