arXiv subjects
Publications and source records attributed to Luis E. Ibanez.
Recent developemnts in string compactifications in the presence of antisymmetric field backgrounds suggest a new simple and predictive structure for soft terms in the MSSM depending only on two parameters. They give rise to a positive definite scalar potential, a solution to the $μ$-problem, flavor universality and absence of a SUSY-CP problem.
We use compact D=4, N=1, Type IIB orientifolds as a testing ground for recent ideas about precocious gauge coupling unification and a low energy string scale. We find that certain such orientifolds have the interesting property that gauge couplings receive moduli-dependent corrections which mimic the effect of field theoretical logarithmic running. The effective cut-off scale for the logarithmic correction is not $M_{string}$ but rather $M_X=\sqrtαM_{Planck}M_c/M_{string}$, where $M_c$ is the compactification scale. Thus there is just normal logarithmic running up to $M_{string}$ and extra moduli dependent corrections which behave as if there was further running to a higher virtual scale $M_X$. In this mechanism a prominent role is played by anomalous U(1)'s with moduli dependent Fayet-Iliopoulos terms. A vanishing FI-term fixes the modulus dependence of the corrected gauge coupling. We discuss possible ways to implement this mechanism in the context of a simple extension of the MSSM.
After a review of some topics concerning the phenomenological applications of perturbative string theory, I discuss to what extent all of it is affected by the recent developements in string dualities.
Supersymmetric N=1, D=4 string vacua are known to be finite in perturbation theory. However, the effective low energy D=4, N=1 field theory lagrangian does not yield in general finite theories. In this note we present the first (to our knowledge) such an example. It may be constructed in three dual ways: i) as a $Z_3$, SO(32) heterotic orbifold; ii) as a Type -IIB, $Z_3$ orientifold with only ninebranes and a Wilson line or iii) as a Type-IIB, $Z_6$ orientifold with only fivebranes. The gauge group is $SU(4)^3$ with three chiral generations. Although chiral, a subsector of the model is continuosly connected to a model with global N=4 supersymmetry. From the $Z_6$, Type IIB orientifold point of view the above connection may be understood as a transition of four dynamical fivebranes from a fixed point to the bulk. The N=1 model is thus also expected to be S-dual. We also remark that, using the untwisted dilaton and moduli fields of these constructions as spurion fields, yields soft SUSY-breaking terms which preserve finiteness even for N=0.
I discuss several issues concerning the use of string models as unified theories of all interactions. After a short review of gauge coupling unification in the string context, I discuss possible motivations for the construction of $SU(5)$ and $SO(10)$ String-GUTs. I describe the construction of such String-GUTs using different orbifold techniques and emphasize those properties which could be general. Although $SO(10)$ and $SU(5)$ String-GUTs are relatively easy to build, the spectrum bellow the GUT scale is in general bigger than that of the MSSM and includes colour octets and $SU(2)$ triplets. The phenomenological prospects of these theories are discussed. I then turn to discuss soft SUSY-breaking terms obtained under the assumption of dilaton/moduli dominance in SUSY-breaking string schemes. I underline the unique finiteness pr of the soft terms induced by the dilaton sector. These improved finiteness properties seem to be related to the underlying $SU(1,1)$ structure of the dilaton couplings. I conclude with an outlook and some speculations regarding $N=1$ duality.
The structure of the quark and lepton masses and mixing angles provides one of the few windows we have on the underlying physics generating the \sm. In an attempt to identify the underlying symmetry group we look for the simplest gauge extension of the SUSY standard model capable of generating the observed structure. We show that the texture structure and hierarchical form found in the (symmetric) quark and lepton mass matrices follows if one extends the gauge group of the standard model to include an horizontal $U(1)$ gauge factor, constrained by the need for anomaly cancellation. This $U(1)$ symmetry is spontaneously broken slightly below the unification/string scale leaving as its only remnant the observed structure of masses and mixings. Anomaly cancellation is possible only in the context of superstring theories via the Green Schwarz mechanism with $sin^2(θ_W)=3/8$.
I discuss some new trends in particle theory beyond the standard model. Some topics briefly covered include elecroweak baryogenesis, gauge versus non-gauge discrete symmetries, the strong CP problem, dynamical symmetry breaking scenarios, supersymmetric grand unification, new aspects in low energy supersymmetry and superstring phenomenology.
Standard (level one) heterotic string models with standard model gauge group predict the unification of $SU(3)$ and $SU(2)$ gauge couplings whereas the $U(1)$ factor is unified modulo an unknown normalization factor $k_1$. On the other hand the unification mass is known. I argue that this situation is quite analogous (though opposite) to that in SUSY-GUTs in which the $U(1)$ normalization is known ($k_1=5/3$) but the unification mass $M_X$ is unknown. I emphasize that $k_1$ should be taken as a free parameter in the string approach (quite in the same way as $M_X$ is taken as a free parameter in SUSY-GUTs). If this is done, the success of the string approach concerning gauge coupling unification is comparable to that in SUSY-GUTs
Four-dimensional strings with the standard model gauge group $SU(3)\times SU(2)\times U(1)$ give model-dependent predictions for the tree level weak mixing-angle. In the presence of an extra pseudo-anomalous gauged- ${U(1)}_X$, the value of the weak angle may be computed purely in terms of the charges of the massless fermions of the theory, independently of the details of the massive string sector. I present the simplest such $ {U(1)}_X$ which leads to the canonical result $sin^2θ_W=3/8$ in the supersymmetric standard model. This is a sort of gauged Peccei-Quinn symmetry which requires the presence of just the minimal set of Higgs doublets and forbids dimension-four $B$ and $L$-violating terms. In this approach the cancellation of the $U(1)_X$ anomalies through a Green-Schwarz mechanism plays a crucial role. In a different context (that of non-string low-energy supersymmetric models) I briefly discuss wether this type of anomaly cancellation mechanism could be of phenomenological relevance close to the electroweak scale. (Talk at the 23-th Workshop of the INFN Eloisatron Project on Properties of Susy particles, Erice, October 1992)
We discuss two independent issues about the baryon asymmetry of the universe. First, assuming that it is generated by an unspecified source at high temperatures, we study the effects of non-perturbative $SU(2)_W$ dynamics above the electroweak scale, in the context of supersymmetric models. We find that there is a substantial difference with the nonsupersymmetric case with the net effect of relaxing previous bounds on B and L violating interactions. In particular supersymmetry allows neutrino masses as large as 10 eV (preferred by solar neutrino and COBE data and measurable at future neutrino oscillation experiments). Second, we argue that the existence of a mixed lepton number-gravitational anomaly in the standard model will induce B-L violating interactions. These transitions would be catalized by Einstein-Yang-Mills instantons or sphalerons and could create a primordial B-L asymmetry at Planck temperatures or lower. Gravity (and the anomaly structure of the standard model) could then be the ultimate source of the baryon asymmetry. We analyze the viability of the presently known gravitational instantons and sphalerons to realize this scenario. (Talk presented by FQ at the Texas/Pascos Conference, Berkeley Dec.1992.)
I discuss and extend several results concerning the cancellation of discrete gauge anomalies. I show how heavy fermions do not decouple in the presence of discrete gauge anomalies. As a consequence, in general, cancellation of discrete gauge anomalies cannot be described merely in terms of low energy operators involving only the light fermions. I also discuss cancellation of discrete gauge anomalies through a discrete version of the Green-Schwarz (GS) mechanism as well as the possibility of discrete gauge R-symmetries and their anomalies. Finally, some phenomenological applications are discussed. This includes symmetries guaranteeing absence of FCNC in two-Higgs models and generalized matter parities stabilizing the proton in the supersymmetric standard model. In the presence of a discrete GS mechanism or/and gauge R-symmetries, new possibilities for anomaly free such symmetries are found.
It has been argued that any primordial B+L asymmetry existing at very high temperatures can be subsequently erased by anomalous electroweak effects. We argue that this is not necessarily the case in the supersymmetric standard model because, apart from B and/or L, there are, above a certain temperature $T_{SS}$, two other anomalous U(1) currents. As a consequence, anomalous electroweak effects are only able to partially transform a B+L excess into a generation of primordial sparticle (e.g. gaugino) density. This relaxes recent bounds on B,L-violating non-renormalizable couplings by several orders of magnitude. In particular, dimension-5 couplings inducing neutrino masses may be 4 orders of magnitude larger than in the non-supersymmetric case, allowing for neutrino masses of the order of 10 eV. These values are consistent with a MSW+see-saw explanation of the solar-neutrino data and also with possible neutrino oscillations measurable at accelerators. Cosmological bounds on other rare processes, such as neutron-antineutron oscillations get also relaxed by several orders of magnitude compared with previous estimates.
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 $\sin^2θ_W$ and $α_3$ for all Abelian orbifolds. Some particular {\it minimal} scenarios, namely those based on all ${\bf Z}_N$ orbifolds except ${\bf Z}_6$
I discuss several aspects of strings as unified theories. After recalling the difficulties of the simplest supersymmetric grand unification schemes I emphasize the distinct features of string unification. An important role in constraining the effective low energy physics from strings is played by $duality$ symmetries. The discussed topics include the unification of coupling constants (computation of $\sin ^2θ_W$ and $α_s$ at the weak scale), supersymmetry breaking through gaugino condensation, and properties of the induced SUSY-breaking soft terms. I remark that departures from universality in the soft terms are (in contrast to the minimal SUSY model) generically expected.