arXiv subjects

A. M. Uranga

Publications and source records attributed to A. M. Uranga.

At least 19 recordsLinked to original sources

arXiv2012

Discrete Gauge Symmetries in Discrete MSSM-like Orientifolds

Motivated by the necessity of discrete Z_N symmetries in the MSSM to insure baryon stability, we study the origin of discrete gauge symmetries from open string sector U(1)'s in orientifolds based on rational conformal field theory. By means of an explicit construction, we find an integral basis for the couplings of axions and U(1) factors for all simple current MIPFs and orientifolds of all 168 Gepner models, a total of 32990 distinct cases. We discuss how the presence of discrete symmetries surviving as a subgroup of broken U(1)'s can be derived using this basis. We apply this procedure to models with MSSM chiral spectrum, concretely to all known U(3)xU(2)xU(1)xU(1) and U(3)xSp(2)xU(1)xU(1) configurations with chiral bi-fundamentals, but no chiral tensors, as well as some SU(5) GUT models. We find examples of models with Z_2 (R-parity) and Z_3 symmetries that forbid certain B and/or L violating MSSM couplings. Their presence is however relatively rare, at the level of a few percent of all cases.

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arXiv2007

Instanton Induced Open String Superpotentials and Branes at Singularities

We study different aspects of the non-perturbative superpotentials induced by Euclidean E3-branes on systems of D3/D7-branes located at Abelian orbifold singularities. We discuss in detail how the induced couplings are consistent with the U(1) symmetries carried by the D3/D7-branes. We construct different compact and non-compact examples, and show phenomenologically relevant couplings like $μ$-terms or certain Yukawa couplings generated by these E3 instantons. Some other novel effects are described. We show an example where E3-instantons combine with standard gauge instantons to yield new multi-instanton effects contributing to superpotential, along the lines of ref.\cite{geu}. In the case of non-SUSY $Z_N$ tachyon-free singularities it is shown how E3-instantons give rise to non perturbative scalar couplings including exponentially suppressed scalar bilinears.

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arXiv2007

Instanton Induced Neutrino Majorana Masses in CFT Orientifolds with MSSM-like spectra

Recently it has been shown that string instanton effects may give rise to neutrino Majorana masses in certain classes of semi-realistic string compactifications. In this paper we make a systematic search for supersymmetric MSSM-like Type II Gepner orientifold constructions admitting boundary states associated with instantons giving rise to neutrino Majorana masses and other L- and/or B-violating operators. We analyze the zero mode structure of D-brane instantons on general type II orientifold compactifications, and show that only instantons with O(1) symmetry can have just the two zero modes required to contribute to the 4d superpotential. We however discuss how the addition of fluxes and/or possible non-perturbative extensions of the orientifold compactifications would allow also instantons with $Sp(2)$ and U(1) symmetries to generate such superpotentials. In the context of Gepner orientifolds with MSSM-like spectra, we find no models with O(1) instantons with just the required zero modes to generate a neutrino mass superpotential. On the other hand we find a number of models in one particular orientifold of the Gepner model $(2,4,22,22)$ with $Sp(2)$ instantons with a few extra uncharged non-chiral zero modes which could be easily lifted by the mentioned effects. A few more orientifold examples are also found under less stringent constraints on the zero modes. This class of $Sp(2)$ instantons have the interesting property that R-parity conservation is automatic and the flavour structure of the neutrino Majorana mass matrices has a simple factorized form.

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arXiv2007

Neutrino Majorana Masses from String Theory Instanton Effects

Finding a plausible origin for right-handed neutrino Majorana masses in semirealistic compactifications of string theory remains one of the most difficult problems in string phenomenology. We argue that right-handed neutrino Majorana masses are induced by non-perturbative instanton effects in certain classes of string compactifications in which the $U(1)_{B-L}$ gauge boson has a Stückelberg mass. The induced operators are of the form $e^{-U}ν_Rν_R$ where $U$ is a closed string modulus whose imaginary part transforms appropriately under $B-L$. This mass term may be quite large since this is not a gauge instanton and $Re U$ is not directly related to SM gauge couplings. Thus the size of the induced right-handed neutrino masses could be a few orders of magnitude below the string scale, as phenomenologically required. It is also argued that this origin for neutrino masses would predict the existence of R-parity in SUSY versions of the SM. Finally we comment on other phenomenological applications of similar instanton effects, like the generation of a $μ$-term, or of Yukawa couplings forbidden in perturbation theory.

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arXiv2004

Flux-induced SUSY-breaking soft terms on D7-D3 brane systems

We study the effect of RR and NSNS 3-form fluxes on the effective action of the worldvolume fields of Type IIB D7/D3-brane configurations. The D7-branes wrap 4-cycles on a local Calabi-Yau geometry. This is an extension of previous work on hep-th/0311241, where a similar analysis was applied to the case of D3-branes. Our present analysis is based on the D7- and D3-brane Dirac-Born-Infeld and Chern-Simons actions, and makes full use of the R-symmetries of the system, which allow us to compute explicitly results for the fields lying at the D3-D7 intersections. A number of interesting new properties appear as compared to the simpler case of configurations with only D3-branes. As a general result one finds that fluxes stabilize some or all of the D7-brane moduli. We argue that this is important for the problem of stabilizing Kahler moduli through non-perturbative effects in KKLT-like vacua. We also show that (0,3) imaginary self-dual fluxes, which lead to compactifications with zero vacuum energy, give rise to SUSY-breaking soft terms including gaugino and scalar masses, and trilinear terms. Particular examples of chiral MSSM-like models of this class of vacua, based on D3-D7 brane systems at orbifold singularities are presented.

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arXiv2004

Flux-induced SUSY-breaking soft terms

We describe the computation of SUSY-breaking terms on a D3-brane in a quite general type IIB supergravity background. We apply it to study the SUSY-breaking induced on the D3-brane world-volume by the presence of NSNS and RR 3-form fluxes. We provide explicit general formulae for the SUSY-breaking soft terms valid for the different types of fluxes, leading to different patterns of soft terms. Imaginary anti-selfdual fluxes with G_3 a pure (3,0)-form lead to soft terms corresponding to dilaton-dominated SUSY-breaking. More general SUSY-breaking patterns are discussed, arising from more general fluxes, or from distant anti-D3-branes. The known finiteness properties of dilaton-dominated soft terms are understood in terms of holography. The above results are interpreted in the context of the 4d effective supergravity theory, where flux components correspond to auxiliary fields of e.g. the 4d dilaton and overall volume modulus. We present semirealistic Type IIB orientifold examples with (meta)stable vacua leading to non-vanishing soft terms of the dilaton-domination type. Such models have many of the ingredients of the recent construction of deSitter vacua in string theory. We finally explore possible phenomenological applications of this form of SUSY-breaking, where we show that soft terms are of order M_s^2/M_p. Thus a string scale of order M_s=10^{10} GeV, and compactification scale three orders of magnitude smaller could explain the smallness of the weak scale versus the Planck mass.

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arXiv2003

Gauging Away the Strong CP Problem

We propose a new solution to the strong-CP problem. It involves the existence of an unbroken gauged $U(1)_X$ symmetry whose gauge boson gets a Stuckelberg mass term by combining with a pseudoscalar field $η(x)$. The latter has axion-like couplings to $F_{QCD}\wedge F_{QCD}$ so that the theta parameter may be gauged away by a $U(1)_X$ gauge transformation. This system leads to mixed gauge anomalies and we argue that they are cancelled by the addition of an appropriate Wess-Zumino term, so that no SM fermions need to be charged under $U(1)_X$. We discuss scenarios in which the above set of fields and couplings appear. The mechanism is quite generic, but a natural possibility is that the the $U(1)_X$ symmetry arises from bulk gauge bosons in theories with extra dimensions or string models. We show that in certain D-brane Type-II string models (with antisymmetric tensor field strength fluxes) higher dimensional Chern-Simons couplings give rise to the required D=4 Wess-Zumino terms upon compactification. In one of the possible string realizations of the mechanism the $U(1)_X$ gauge boson comes from the Kaluza-Klein reduction of the eleven-dimensional metric in M-theory.

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arXiv2001

D=4 Chiral String Compactifications from Intersecting Branes

Intersecting Dp-branes often give rise to chiral fermions living on their intersections. We study the construction of four-dimensional chiral gauge theories by considering configurations of type II D(3+n)-branes wrapped on non-trivial n-cycles on T^{2n} x(R^{2(3-n)}/Z_N), for n=1,2,3. The gauge theories on the four non-compact dimensions of the brane world-volume are generically chiral and non-supersymmetric. We analyze consistency conditions (RR tadpole cancellation) for these models, and their relation to four-dimensional anomaly cancellation. Cancellation of U(1) gauge anomalies involves a Green-Schwarz mechanism mediated by RR partners of untwisted and/or twisted moduli. This class of models is of potential phenomenological interest, and we construct explicit examples of SU(3) x SU(2) x U(1) three-generation models. The models are non-supersymmetric, but the string scale may be lowered close to the weak scale so that the standard hierarchy problem is avoided. We also comment on the presence of scalar tachyons and possible ways to avoid the associated instabilities. We discuss the existence of (meta)stable configurations of D-branes on 3-cycles in (T^2)^3, free of tachyons for certain ranges of the six-torus moduli.

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arXiv2001

Intersecting Brane Worlds

It is known that chiral fermions naturally appear at certain intersections of branes at angles. Motivated by this fact, we propose a string scenario in which different standard model gauge interactions propagate on different (intersecting) brane worlds, partially wrapped in the extra dimensions. Quarks and leptons live at brane intersections, and are thus located at different positions in the extra dimensions. Replication of families follows naturally from the fact that the branes generically intersect at several points. Gauge and Yukawa couplings can be computed in terms of the compactification radii. Hierarchical Yukawa couplings appear naturally, since amplitudes involving three different intersections are proportional to exp{-A_{ijk}}, where A_{ijk} is the area of a string world-sheet extending among the intersections. The models are non-supersymmetric but the string scale may be lowered down to 1-10 TeV. The proton is however stable due to a set of discrete symmetries arising from world-sheet selection rules, exact to all orders in perturbation theory. The scenario has some distinctive features like the presence of KK, winding and other new excited states (`gonions'), with masses below the string scale and accessible to accelerators. The models contain scalar tachyons with the quantum numbers of standard SU(2) x U(1) Higgs doublets, and we propose that they induce electroweak symmetry breaking in a somewhat novel way. Specific string models with D4-branes wrapping on T^2 x (T^2)^2/Z_N, leading to three-family semirealistic spectra, are presented, in which the above properties are exemplified.

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arXiv2000

D-brane probes, RR tadpole cancellation and K-theory charge

We study RR charge cancellation consistency conditions in string compactifications with open string sectors, by introducing D-brane probes in the configuration. We show that uncancelled charges manifest as chiral gauge anomalies in the world-volume of suitable probes. RR tadpole cancellation can therefore be described as the consistency of the effective compactified theory not just in the vacuum, but also in all topological sectors (presence of D-brane probes). The result explains why tadpole cancellation is usually much stronger than anomaly cancellation of the compactified theory (in the vacuum sector). We use the probe criterion to construct consistent six-dimensional orientifolds of curved K3 spaces, where usual CFT techniques to compute tadpoles are not valid. As a last application, we consider compactifications where standard RR charge cancels but full K-theory charge does not. We show the inconsistency of such models manifests as a global gauge anomaly on the world-volume of suitable probes.

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arXiv2000

Type IIB Orientifolds without Untwisted Tadpoles, and non-BPS D-branes

We discuss the construction of six- and four-dimensional Type IIB orientifolds with vanishing untwisted RR tadpoles, but generically non-zero twisted RR tadpoles. Tadpole cancellation requires the introduction of D-brane systems with zero untwisted RR charge, but non-zero twisted RR charges. We construct explicit models containing branes and antibranes at fixed points of the internal space, or non-BPS branes partially wrapped on it. The models are non-supersymmetric, but are absolutely stable against decay to supersymmetric vacua. For particular values of the compactification radii tachyonic modes may develop, triggering phase transitions between the different types of non-BPS configurations of branes, which we study in detail in a particular example. As an interesting spin-off, we show that the $\IT^6/\IZ_4$ orientifold without vector structure, previously considered inconsistent due to uncancellable twisted tadpoles, can actually be made consistent by introducing a set of brane-antibrane pairs whose twisted charge cancels the problematic tadpole.

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arXiv2000

D-Branes at Singularities : A Bottom-Up Approach to the String Embedding of the Standard Model

We propose a bottom-up approach to the building of particle physics models from string theory. Our building blocks are Type II D-branes which we combine appropriately to reproduce desirable features of a particle theory model: 1) Chirality ; 2) Standard Model group ; 3) N=1 or N=0 supersymmetry ; 4) Three quark-lepton generations. We start such a program by studying configurations of D=10, Type IIB D3-branes located at singularities. We study in detail the case of Z_N, N=1,0 orbifold singularities leading to the SM group or some left-right symmetricextension. In general, tadpole cancellation conditions require the presence of additional branes, e.g. D7-branes. For the N=1 supersymmetric case the unique twist leading to three quark-lepton generations is Z_3, predicting $\sin^2θ_W=3/14=0.21$. The models obtained are the simplest semirealistic string models ever built. In the non-supersymmetric case there is a three-generation model for each Z_N, N>4, but the Weinberg angle is in general too small. One can obtain a large class of D=4 compact models by considering the above structure embedded into a Calabi Yau compactification. We explicitly construct examples of such compact models using Z_3 toroidal orbifolds and orientifolds, and discuss their properties. In these examples, global cancellation of RR charge may be achieved by adding anti-branes stuck at the fixed points, leading to models with hidden sector gravity-induced supersymmetry breaking. More general frameworks, like F-theory compactifications, allow completely $\NN=1$ supersymmetric embeddings of our local structures, as we show in an explicit example.

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arXiv1999

Tachyon-free Non-supersymmetric Type IIB Orientifolds via Brane-Antibrane Systems

We derive the rules to construct type IIB compact orientifolds in six and four dimensions including D-branes and anti-D-branes. Even though the models are non-supersymmetric due to the presence of the anti-D-branes, we show that it is easy to construct large classes of models free of tachyons. Brane-antibrane annihilation can be prevented for instance by considering models with branes and antibranes stuck at different fixed points in the compact space. We construct several anomaly-free and tachyon-free six-dimensional orientifolds containing D9-branes and anti-D5-branes. This setup allows to construct four-dimensional chiral models with supersymmetry unbroken in the bulk and in some D-brane sectors, whereas supersymmetry is broken (at the string scale) in some `hidden' anti-D-brane sector. We present several explicit models of this kind. We also comment on the role of the non-cancelled attractive brane-antibrane forces and the non-vanishing cosmological constant, as providing interesting dynamics for the geometric moduli and the dilaton, which may contribute to their stabilization.

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arXiv1999

Orientifolding the conifold

In this paper we study the N=1 supersymmetric field theories realized on the world-volume of type IIB D3-branes sitting at orientifolds of non-orbifold singularities (conifold and generalizations). Several chiral models belong to this family of theories. These field theories have a T-dual realization in terms of type IIA configurations of relatively rotated NS fivebranes, D4-branes and orientifold six-planes, with a compact $x^6$ direction, along which the D4-branes have finite extent. We compute the spectrum on the D3-branes directly in the type IIB picture and match the resulting field theories with those obtained in the type IIA setup, thus providing a non-trivial check of this T-duality. Since the usual techniques to compute the spectrum of the model and check the cancellation of tadpoles, cannot be applied to the case orientifolds of non-orbifold singularities, we use a different approach, and construct the models by partially blowing-up orientifolds of C^3/(Z_2 x Z_2) and C^3/(Z_2 x Z_3) orbifolds.

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arXiv1999

N=1 Type IIA brane configurations, Chirality and T-duality

We consider four-dimensional N=1 field theories realized by type IIA brane configurations of NS-branes and D4-branes, in the presence of orientifold six-planes and D6-branes. These configurations are known to present interesting effects associated to the appearance of chiral symmetries and chiral matter in the four-dimensional field theory. We center on models with one compact direction (elliptic models) and show that, under T-duality, the configurations are mapped to a set of type IIB D3-branes probing N=1 orientifolds of C^2/Z_N singularities. We explicitly construct these orientifolds, and show the field theories on the D3-brane probes indeed reproduces the field theories constructed using the IIA brane configurations. This T-duality map allows to understand the type IIB realization of several exotic brane dynamics effects on the type IIA side: Flavour doubling, the splitting of D6-branes and O6-planes in crossing a NS-brane and the effect of a non-zero type IIA cosmological constant turn out to have surprisingly standard type IIB counterparts.

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arXiv1999

Tadpole versus anomaly cancellation in D=4,6 compact IIB orientifolds

It is often stated in the literature concerning D=4,6 compact Type IIB orientifolds that tadpole cancellation conditions i) uniquely fix the gauge group (up to Wilson lines and/or moving of branes) and ii) are equivalent to gauge anomaly cancellation. We study the relationship between tadpole and anomaly cancellation conditions and qualify both statements. In general the tadpole cancellation conditions imply gauge anomaly cancellation but are stronger than the latter conditions in D=4, N=1 orientifolds. We also find that tadpole cancellation conditions in Z_N D=4,6 compact orientifolds do not completely fix the gauge group and we provide new solutions different from those previously reported in the literature.

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arXiv1999

Sigma-model anomalies in compact type IIB orientifolds and Fayet-Iliopoulos terms

Compact Type IIB D=4, N=1 orientifolds have certain U(1) sigma model symmetries at the level of the effective Lagrangian. These symmetries are generically anomalous. We study the particular case of $Z_N$ orientifolds and find that these anomalies may be cancelled by a generalized Green-Schwarz mechanism. This mechanism works by the exchange of twisted RR-fields associated to the orbifold singularities and it requires the mixing between twisted and untwisted moduli of the orbifold. As a consequence, the Fayet-Iliopoulos terms which are present for the gauged anomalous U(1)'s of the models get an additional untwisted modulus dependent piece at the tree level.

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arXiv1998

A Note on Superconformal N=2 theories and Orientifolds

We construct the T duals of certain type IIA brane configurations with one compact dimension (elliptic models) which contain orientifold planes. These configurations realize four-dimensional $\NN=2$ finite field theories. For elliptic models with two negatively charged orientifold six-planes, the T duals are given by D3 branes at singularities in the presence of O7-planes and D7-branes. For elliptic models with two oppositely charged orientifold planes, the T duals are D3 branes at a different kind of orientifold singularities, which do not require D7 branes. We construct the adequate orientifold groups, and show that the cancellation of twisted tadpoles is equivalent to the finiteness of the corresponding field theory. One family of models contains orthogonal and symplectic gauge factors at the same time. These new orientifolds can also be used to define some six-dimensional RG fixed points which have been discussed from the type IIA brane configuration perspective.

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