Showing papers on "Orientifold published in 2014"

Higgs Bundles and UV Completion in F-Theory

Abstract: F-theory admits 7-branes with exceptional gauge symmetries, which can be compactified to give phenomenological four-dimensional GUT models. Here we study general supersymmetric compactifications of eight-dimensional Yang–Mills theory. They are mathematically described by meromorphic Higgs bundles, and therefore admit a spectral cover description. This allows us to give a rigorous and intrinsic construction of local models in F-theory. We use our results to prove a no-go theorem showing that local SU(5) models with three generations do not exist for generic moduli. However we show that three-generation models do exist on the Noether–Lefschetz locus. We explain how F-theory models can be mapped to non-perturbative orientifold models using a scaling limit proposed by Sen. Further we address the construction of global models that do not have heterotic duals, considering models with base CP 3 or a blow-up thereof as examples. We show how one may obtain a contractible worldvolume with a two-cycle not inherited from the bulk, a necessary condition for implementing GUT breaking using fluxes.

Emergence of spontaneously broken supersymmetry on an anti-D3-brane in KKLT dS vacua

Abstract: The KKLT construction of de Sitter vacua includes an uplifting term coming from an anti-D3-brane. Here we show how this term can arise via spontaneous breaking of supersymmetry, based on the emergence of a nilpotent chiral supermultiplet on the world-volume of the anti-D3-brane. We establish and use the fact that both the DBI as well as the WZ term, with account of orientifolding, acquire a form of the Volkov-Akulov action. For an O3 orientifold involution of ℝ9,1 we demonstrate the cancellation between the fermionic parts of the DBI and WZ term for the D3-brane action. For the anti-D3-brane we show that the DBI action and the WZ action combine and lead to the emergence of the goldstino multiplet which is responsible for spontaneous breaking of supersymmetry. This provides a string theoretic explanation for the supersymmetric uplifting term in the KKLT effective supergravity model supplemented by a nilpotent chiral multiplet.

Emergence of Spontaneously Broken Supersymmetry on an Anti-D3-Brane in KKLT dS Vacua

Abstract: The KKLT construction of de Sitter vacua includes an uplifting term coming from an anti-D3-brane. Here we show how this term can arise via spontaneous breaking of supersymmetry, based on the emergence of a nilpotent chiral supermultiplet on the world-volume of the anti-D3-brane. We establish and use the fact that both the DBI as well as the WZ term, with account of orientifolding, acquire a form of the Volkov-Akulov action. For an O3 orientifold involution of $\mathbb{R}^{9,1}$ we demonstrate the cancellation between the fermionic parts of the DBI and WZ term for the D3-brane action. For the anti-D3-brane we show that the DBI action and the WZ action combine and lead to the emergence of the goldstino multiplet which is responsible for spontaneous breaking of supersymmetry. This provides a string theoretic explanation for the supersymmetric uplifting term in the KKLT effective supergravity model supplemented by a nilpotent chiral multiplet.

Anomalies and Invertible Field Theories

Abstract: We give a modern geometric viewpoint on anomalies in quantum field theory and illustrate it in a 1-dimensional theory: supersymmetric quantum mechanics. This is background for the resolution of worldsheet anomalies in orientifold superstring theory.

Higgs-otic Inflation and String Theory

Abstract: We propose that inflation is driven by a (complex) neutral Higgs of the MSSM extension of the SM, in a chaotic-like inflation setting. The SUSY breaking soft term masses are of order $10^{12}-10^{13}$ GeV, which is identified with the inflaton mass scale and is just enough to stabilise the SM Higgs potential. The fine-tuned SM Higgs has then a mass around 126 GeV, in agreement with LHC results. We point out that the required large field excursions of chaotic inflation may be realised in string theory with the (complex) inflaton/Higgs identified with a continuous Wilson line or D-brane position. We show specific examples and study in detail a IIB orientifold with D7-branes at singularities, with SM gauge group and MSSM Higgs sector. In this case the inflaton/Higgs fields correspond to D7-brane positions along a two-torus transverse to them. Masses and monodromy are induced by closed string $G_3$ fluxes, and the inflaton potential can be computed directly from the DBI+CS action. We show how this action sums over Planck suppressed corrections, which amount to a field dependent rescaling of the inflaton fields, leading to a linear potential in the large field regime. We study the evolution of the two components of the Higgs/inflaton and compute the slow-roll parameters for purely adiabatic perturbations. For large regions of initial conditions slow roll inflation occurs and 50-60 efolds are obtained with r>0.07, testable in forthcoming experiments. Our scheme is economical in the sense that both EWSB and inflation originate in the same sector of the theory, all inflaton couplings are known and reheating occurs efficiently.

On axionic dark matter in Type IIA string theory

Abstract: We investigate viable scenarios with various axions in the context of supersymmetric eld theory and in globally consistent D-brane models. The PecceiQuinn symmetry is associated with an anomalous U(1) symmetry, which acquires mass at the string scale but remains as a perturbative global symmetry at low energies. The origin of the scalar Higgs-axion potential from F-, D- and soft breaking terms is derived, and two Standard Model examples of global intersecting D6-brane models in Type II orientifolds are presented, which dier in the realisation of the Higgs sector and in the hidden sector, the latter of which is of particluar importance for the soft supersymmetry breaking terms.

Lobotomy of flux compactifications

Abstract: We provide the dictionary between four-dimensional gauged supergravity and type II compactifications on (6) with metric and gauge fluxes in the absence of supersymmetry breaking sources, such as branes and orientifold planes. Secondly, we prove that there is a unique isotropic compactification allowing for critical points. It corresponds to a type IIA background given by a product of two 3-tori with SO(3) twists and results in a unique theory (gauging) with a non-semisimple gauge algebra. Besides the known four AdS solutions surviving the orientifold projection to = 4 induced by O6-planes, this theory contains a novel AdS solution that requires non-trivial orientifold-odd fluxes, hence being a genuine critical point of the = 8 theory.

Brane inflation and moduli stabilization on twisted tori

Abstract: We consider supergravity compactifications on 6-dimensional twisted tori, which are 5-torus fibrations of the circle. The motion of branes on such manifolds can lead to power-law potentials at low energy, that may be useful for inflation. We classify the possible low energy potentials one can obtain by wrapping branes on different cycles of the fibre. Turning to the problem of moduli stabilization in such models, we prove a no-go result for solutions with parametrically small cosmological constant, under certain assumptions for the orientifolds and D-branes. We also consider the role of discrete Wilson lines in moduli stabilization on general closed manifolds, and show that gauge invariance restricts their contributions to the effective potential. We derive the allowed discrete Wilson lines in massive Type IIA supergravity on twisted tori. We conclude with a detailed example, computing the effective potentials in a class of models involving a twisted torus and an orientifold 6-plane.

A note on smeared branes in flux vacua and gauged supergravity

Abstract: In the known examples of flux vacua with calibrated spacetime-filling sources (branes or orientifold planes), one can smear the source in order to perform a standard KK reduction and obtain a lower-dimensional supergravity description. Furthermore, it is expected that the smeared and localized solution preserve equal amounts of supersymmetry. In this note we point out that the AdS7 solution discussed in arXiv:1111.2605 and arXiv:1309.2949 is a counterexample to this common lore. The solution is supersymmetric when the spacetime-filling D6-branes are localized but breaks supersymmetry in the smeared limit. By using the embedding tensor formalism we demonstrate that there is no gauged supergravity description for the solution, regardless of the source being smeared or not. We conjecture that for flux solutions with separation between the KK scale and AdS radius this cannot occur.

Pre-inflationary clues from String Theory?

Abstract: ``Brane supersymmetry breaking'' occurs in String Theory when the only available combinations of D-branes and orientifolds are not mutually BPS and yet do not introduce tree-level tachyon instabilities. It is characterized by the emergence of a steep exponential potential, and thus by the absence of maximally symmetric vacua. The corresponding low-energy supergravity admits intriguing spatially-flat cosmological solutions where a scalar field is forced to climb up toward the steep potential after an initial singularity, and additional milder terms can inject an inflationary phase during the ensuing descent. We show that, in the resulting power spectra of scalar perturbations, an infrared suppression is typically followed by a pre-inflationary peak that reflects the end of the climbing phase and can lie well apart from the approximately scale invariant profile. A first look at WMAP9 raw data shows that, while the χ2 fits for the low-l CMB angular power spectrum are clearly compatible with an almost scale invariant behavior, they display nonetheless an eye-catching preference for this type of setting within a perturbative string regime.

T-Duality for Orientifolds and Twisted KR-Theory

Abstract: D-brane charges in orientifold string theories are classified by the KR-theory of Atiyah. However, this is assuming that all O-planes have the same sign. When there are O-planes of different signs, physics demands a “KR-theory with a sign choice” which up until now has not been studied by mathematicians (with the unique exception of Moutuou, who did not have a specific application in mind). We give a definition of this theory and compute it for orientifold theories compactified on S1 and T2. We also explain how and why additional “twisting” is implemented. We show that our results satisfy all possible T-duality relationships for orientifold string theories on elliptic curves, which will be studied further in subsequent work.

Rigour and Rigidity: Systematics on particle physics D6-brane models on Z(2)xZ(6)

Abstract: We launch a systematic search for phenomenologically appealing string vacua with intersecting D-branes on the promising T6/Z(2)xZ(6)xOR orientifold with discrete torsion. The number of independent background lattices is reduced from six to two by new symmetries. The occurrence of USp(2N) and SO(2N) versus U(N) gauge groups is classified as well as D-branes without matter in the adjoint and/or symmetric representation. Supersymmetric fractional D6-branes allowing for RR tadpole cancellation are fully classified in terms of all possible values of the one complex structure modulus inherited from the underlying six-torus. We then systematically investigate the conditions for three particle generations at pairwise intersections of two D6-branes. Global SU(5) GUT models on T6/Z(2)xZ(6)xOR are excluded by demanding three generations and no exotic matter in the 15 representation. Two prototypes of global Pati-Salam models with a mild amount of vector-like exotic matter are found.

Exact probes of orientifolds

Abstract: We compute the exact vacuum expectation value of circular Wilson loops for Euclidean N = 4 super Yang-Mills with G = SO(N),Sp(N), in the fundamental and spinor representations. These field theories are dual to type IIB string theory compactified on AdS5 × RP 5 plus certain choices of discrete torsion, and we use our results to probe this holographic duality. We first revisit the LLM-type geometries having AdS5 × RP 5 as ground state. Our results clarify and refine the identification of these LLM-type geometries as bubbling geometries arising from fermions on a half harmonic oscillator. We further- more identify the presence of discrete torsion with the one-fermion Wigner distribution becoming negative at the origin of phase space. We then turn to the string world-sheet interpretation of our results and argue that for the quantities considered they imply two features: first, the contribution coming from world-sheets with a single crosscap is closely related to the contribution coming from orientable world-sheets, and second, world-sheets with two crosscaps don't contribute to these quantities.

Rigour and rigidity: Systematics on particle physics D6‐brane models on ℤ2 × ℤ6

Abstract: We launch a systematic search for phenomenologically appealing string vacua with intersecting D-branes on the promising T6/(ℤ2 × ℤ6 × Ω ℛ) orientifold with discrete torsion. The number of independent background lattices is reduced from six to two by new symmetries. The occurrence of USp(2N) and SO(2N) versus U(N) gauge groups is classified as well as D-branes without matter in the adjoint and/or symmetric representation. Supersymmetric fractional D6-branes allowing for RR tadpole cancellation are fully classified in terms of all possible values of the one complex structure modulus inherited from the underlying six-torus. We then systematically investigate the conditions for three particle generations at pairwise intersections of two D6-branes. Global SU(5) GUT models on T6/(ℤ2 × ℤ6 × Ω ℛ) are excluded by demanding three generations and no exotic matter in the 15 representation. Two prototypes of global Pati-Salam models with a mild amount of vector-like exotic matter are found.

Flux-induced soft terms on type IIB/F-theory matter curves and hypercharge dependent scalar masses

Abstract: Closed string fluxes induce generically SUSY-breaking soft terms on supersymmetric type IIB orientifold compactifications with D3/D7 branes. This was studied in the past by inserting those fluxes on the DBI+CS actions for adjoint D3/D7 fields, where D7-branes had no magnetic fluxes. In the present work we generalise those computations to the phenomenologically more relevant case of chiral bi-fundamental fields laying at 7-brane intersections and F-theory local matter curves. We also include the effect of 7-brane magnetic flux as well as more general closed string backgrounds, including the effect of distant $ \mathrm{D}3\left(\overline{D3}\right) $ -branes. We discuss several applications of our results. We find that squark/slepton masses become in general flux-dependent in F-theory GUT’s. Hypercharge-dependent non-universal scalar masses with a characteristic sfermion hierarchy m 2 < m 2 < m 2 < m 2 < m 2 are obtained. There are also flavor-violating soft terms both for matter fields living at intersecting 7-branes or on D3-branes at singularities. They point at a very heavy sfermion spectrum to avoid FCNC constraints. We also discuss the possible microscopic description of the fine-tuning of the EW Higgs boson in compactifications with a MSSM spectrum.

Natural inflation with and without modulations in type IIB string theory

Abstract: We propose a mechanism for the natural inflation with and without modulation in the framework of type IIB string theory on toroidal orientifold or orbifold. We explicitly construct the stabilization potential of complex structure, dilaton and Kahler moduli, where one of the imaginary component of complex structure moduli becomes light which is identified as the inflaton. The inflaton potential is generated by the gaugino-condensation term which receives the one-loop threshold corrections determined by the field value of complex structure moduli and the axion decay constant of inflaton is enhanced by the inverse of one-loop factor. We also find the threshold corrections can also induce the modulations to the original scalar potential for the natural inflation. Depending on these modulations, we can predict several sizes of tensor-to-scalar ratio as well as the other cosmological observables reported by WMAP, Planck and/or BICEP2 collaborations.

Exact partition functions on \mathbb{R}{{\mathbb{P}}^2} and orientifolds

Abstract: We consider gauged linear sigma models (GLSM) on $ \mathbb{R}{{\mathbb{P}}^2} $ , obtained from a parity projection of S 2. The theories admit squashing deformation, much like GLSM on S 2, which allows us to interpret the partition function as the overlap amplitude between the vacuum state and crosscap states. From these, we extract the central charge of Orientifold planes, and observe that the Gamma class makes a prominent appearance as in the recent D-brane counterpart. We also repeat the computation for the mirror Landau-Ginzburg theory, which naturally brings out the θ-dependence as a relative sign between two holonomy sectors on $ \mathbb{R}{{\mathbb{P}}^2} $ . We also show how our results are consistent with known topological properties of D-brane and Orientifold plane world-volumes, and discuss what part of the wrapped D-brane/Orientifold central charge should be attributed to the quantum volumes.

BPS domain walls from backreacted orientifolds

Abstract: Compactifications with D-brane and orientifold sources lead to standard gauged supergravity theories if the sources are smeared over the internal directions. It is therefore of interest to find how the solutions described by the gauged supergravity are altered by properly localising the sources. In this paper we analyse this for BPS domain wall solutions in the seven-dimensional gauged supergravity obtained from an O6 toroidal orientifold in massive IIA supergravity. This is one of the simplest no-scale supergravities that can be constructed and analysed in full detail. We find the BPS domain walls when the O6 planes are smeared. When the O6 planes are localised the domain wall solutions live in a warped compactification and we present the first-order equations these domain walls obey in 10 dimensions. In order to get explicit expressions we also consider the non-compact versions of the solutions for which the O6 planes have been traded for D6 branes and we recover the gauged supergravity expressions for the domain walls in the leading terms of the warp factor. Through T-duality we obtain partially localised solutions for compactifications to four dimensions using O3 planes with 3-form fluxes.

Circling the square: deforming fractional D-branes in type II/Ω \( \mathrm{\mathcal{R}} \) orientifolds

Abstract: We study complex structure deformations of special Lagrangian cycles associated to fractional D-branes at $$ \mathbb{Z} $$ 2 singularities in Type II/Ω $$ \mathrm{\mathcal{R}} $$ orientifold models. By means of solving hypersurface constraints, we show how to compute the volumes of the most simple D-brane configurations. These volumes are given as a function of the deformation parameters depending on the D-brane position relative to the smoothed out singularity. We observe which cycles keep the special Lagrangian property in various deformation scenarios and what orientifold involutions are allowed. As expected, the volume and thus the tree level value of the gauge coupling hardly change for D-branes not wrapping the exceptional cycle on the deformed singularity, whereas the volume of D-branes passing through the singularity depends on the deformation parameter by some power law.

Circling the Square: Deforming fractional D-branes in Type II/$\Omega \mathcal{R}$ orientifolds

Abstract: We study complex structure deformations of special Lagrangian cycles associated to fractional D-branes at $\mathbb{Z}_2$ singularities in Type II/$\Omega \mathcal{R}$ orientifold models. By means of solving hypersurface constraints, we show how to compute the volumes of the most simple D-brane configurations. These volumes are given as a function of the deformation parameters depending on the D-brane position relative to the smoothed out singularity. We observe which cycles keep the special Lagrangian property in various deformation scenarios and what orientifold involutions are allowed. As expected, the volume and thus the tree level value of the gauge coupling hardly change for D-branes not wrapping the exceptional cycle on the deformed singularity, whereas the volume of D-branes passing through the singularity depends on the deformation parameter by some power law.

Intersecting D6-branes on the $ \mathbb{Z} $ 12-II orientifold

Abstract: Much work has been done by a number of authors with the aim of constructing the supersymmetric Standard Model in type IIA intersecting-brane theories compactified on an orientifold with various $ \mathbb{Z} $ N or $ \mathbb{Z} $ M × $ \mathbb{Z} $ N point groups. Here we consider the $ \mathbb{Z} $ 12 point group which has previously received comparatively little attention. We consider intersecting D6-branes that wrap 3-cycles consisting of a 2-cycle of the 4-dimensional lattice upon which the $ \mathbb{Z} $ 12 is realised times a 1-cycle of the remaining 2-torus. Our discussion is restricted to the case when these 2-cycles are “factorisable” in the sense discussed in section 3. Although it is possible to find models with the correct supersymmetric Standard Model quark-doublet content, we have not found it possible to obtain the correct quark-singlet content.

String theory on elliptic curve orientifolds and KR-theory

Abstract: We analyze the brane content and charges in all of the orientifold string theories on space-times of the form E x R^8, where E is an elliptic curve with holomorphic or anti-holomorphic involution. Many of these theories involve "twistings" coming from the B-field and/or sign choices on the orientifold planes. A description of these theories from the point of view of algebraic geometry, using the Legendre normal form, naturally divides them into three groupings. The physical theories within each grouping are related to one another via sequences of T-dualities. Our approach agrees with both previous topological calculations of twisted KR-theory and known physics arguments, and explains how the twistings originate from both a mathematical and a physical perspective.

On finiteness of type IIB compactifications: magnetized branes on elliptic Calabi-Yau threefolds

Abstract: The string landscape satisfies interesting finiteness properties imposed by supersymmetry and string-theoretical consistency conditions. We study $$ \mathcal{N} $$ = 1 supersymmetric compactifications of Type IIB string theory on smooth elliptically fibered Calabi-Yau threefolds at large volume with magnetized D9-branes and D5-branes. We prove that supersymmetry and tadpole cancellation conditions imply that there is a finite number of such configurations. In particular, we derive an explicitly computable bound on the number of magnetic flux quanta, as well as the number of D5-branes, which is independent of the continuous moduli of the setup. The proof applies if a number of easy to check geometric conditions of the twofold base are met. We show that these geometric conditions are satisfied for the almost Fano twofold bases given by each toric variety associated to a reflexive two-dimensional polytope as well as by the generic del Pezzo surfaces dP n with n = 0,…,8. Physically, this finiteness proof shows that there exist a finite collection of four-dimensional gauge groups and chiral matter spectra in the 4D supergravity theories realized by these compactifications. As a by-product we explicitly construct all generators of the Kahler cones of dP n and work out their relation to representation theory.

Mirage models confront the LHC. II. Flux-stabilized type IIB string theory

Abstract: We continue the study of a class of string-motivated effective supergravity theories in light of current data from the CERN Large Hadron Collider (LHC). In this installment we consider type IIB string theory compactified on a Calabi-Yau orientifold in the presence of fluxes, in the manner originally formulated by Kachru et al. We allow for a variety of potential uplift mechanisms and embeddings of the Standard Model field content into $D3$-and $D7$-brane configurations. We find that an uplift sector independent of the K\"ahler moduli, as is the case with anti-$D3$-branes, is inconsistent with data unless the matter and Higgs sectors are localized on $D7$ branes exclusively, or are confined to twisted sectors between $D3$-and $D7$-branes. We identify regions of parameter space for all possible $D$-brane configurations that remain consistent with Planck observations on the dark matter relic density and measurements of the $CP$-even Higgs mass at the LHC. Constraints arising from LHC searches at $\sqrt{s}=8\text{ }\text{ }\mathrm{TeV}$ and the LUX dark matter detection experiment are discussed. The discovery prospects for the remaining parameter space at dark matter direct-detection experiments are described, and signatures for detection of superpartners at the LHC with $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ are analyzed.

Model independent bounds on tensor modes and stringy parameters from CMB

Abstract: In this paper we will derive bounds on tensor-to-scalar ratio, $r$, string coupling, $g_s$ and compactification volume, ${\cal V}_E$, by demanding the validity of an effective field theory - the inflationary scale and the Hubble parameter during inflation must be well below the Kaluza-Klein (KK) mass scale, string scale, and $4$ dimensional Planck mass. Within type IIB orientifold compactifications, we can put further constraints on the parameters by invoking the hierarchy between gravitino mass in $4$ dimensions and inflationary scale.

Brane Curvature Corrections to the $\mathcal{N}=1$ Type II/F-theory Effective Action

Abstract: We initiate a study of corrections to the Kahler potential of $\mathcal{N}=1$ type II/F-theory compactifications that arise from curvature terms in the action of D-branes and orientifold planes. We first show that a recently proposed correction to the Kahler coordinates, which was argued to appear at order $\alpha^{\prime 2}g_s$ and be proportional to the intersection volume of D7-branes and O7-planes, is an artifact of an inconvenient field basis in the dual M-theory frame and can be removed by a field redefinition of the 11D metric. We then analyze to what extent curvature terms in the DBI and WZ action may still lead to corrections of a similar kind and identify two general mechanisms that can potentially modify the volume dependence of the Kahler potential in the presence of D-branes and O-planes. The first mechanism is related to an induced Einstein-Hilbert term on warped brane worldvolumes, which leads to a shift in the classical volume of the compactification manifold. The resulting corrections are generic and can appear at one-loop order on branes and O-planes of various dimensions and for configurations with or without intersections. We discuss in detail the example of intersecting D7-branes/O7-planes, where a correction can appear already at order $\alpha^{\prime 2}g_s^2$ in the Kahler potential. Due to an extended no-scale structure, however, it is then still subleading in the scalar potential. We also discuss a second mechanism, which is due to an induced D3-brane charge in the WZ action of D7-branes. Contrary to the first type of corrections, it appears at open string tree-level and shifts the definition of the Kahler coordinates in terms of the classical volume but leaves the volume itself uncorrected. Our work has implications for moduli stabilization and model building and suggests interesting generalizations to F-theory.

Stringlike structures in the real and complex Kerr-Schild geometry

Abstract: Four-dimensional Kerr-Schild (KS) geometry displays remarkable relationships with quantum world and theory of superstrings. In particular, the Kerr-Newman (KN) solution has gyromagnetic ratio g = 2, as that of the Dirac electron and represents a consistent background for gravitational and electromagnetic field of the electron. As a consequence of very big spin/mass ratio, black hole horizons disappear, exposing the naked Kerr singular ring. We consider four-decade history of development of this structure which took finally the form of a point-string-membrane-bubble complex which is reminiscent of the enhancon model of string/M-theory. A complex string obtained in the complex structure of the Kerr geometry gives an extra dimension to the world-sheet of the real Kerr string, forming a membrane by analogue with the string/M-theory unification. By analysis of the orientifold parity of the complex Kerr string, we obtain that the determined by the Kerr theorem principal null congruence of the Kerr geometry is described by a quartic equation in projective twistor space CP3, and therefore, it creates the known Calabi- Yau twofold (K3 surface) in twistor space of the 4d KS geometry. We connect it with N=2 superstring which has (complex) critical dimension two and may be embedded into complex KS geometry.

String Theory clues for the low-$\ell$ CMB ?

Abstract: "Brane Supersymmetry Breaking" is a peculiar string-scale mechanism that can unpair Bose and Fermi excitations in orientifold models. It results from the simultaneous presence, in the vacuum, of collections of D-branes and orientifolds that are not mutually BPS, and is closely tied to the scale of string excitations. It also leaves behind, for a mixing of dilaton and internal breathing mode, an exponential potential that is just too steep for a scalar to emerge from the initial singularity while descending it. As a result, in this class of models the scalar can generically bounce off the exponential wall, and this dynamics brings along, in the power spectrum, an infrared depression typically followed by a pre-inflationary peak. We elaborate on a possible link between this type of bounce and the low-$\ell$ end of the CMB angular power spectrum. For the first 32 multipoles, one can reach a 50 % reduction in $\chi^{\,2}$ with respect to the standard $\Lambda$CDM setting.

Topics in Supergravity Phenomenology

Abstract: The first part of the review article is devoted to investigation of impor- tant phenomenological and particle-cosmology-related issues in the context of Type IIB string compactifications. After undertaking a brief review of (split) supersym- metry in the context of Beyond Standard Model Physics, we discuss the possibility of realizing "µ-split-like SUSY" scenario from a phenomenological model, which we show could be realizable locally as the large volume limit of a type IIB Swiss-Cheese Calabi-Yau orientifold involving a mobile space-time filling D3-brane localized at a nearly special Lagrangian three-cycle embedded in the "big" divisor (hence the local nature of the model's realization) and multiple fluxed stacks of space-time filling D7- branes wrapping the same "big" divisor. Naturally realizing split-SUSY scenario of N. Arkani-Hamed and S. Dimopoulos in our model, we show that the mass of one of the Higgs formed by a linear combination of two Higgs doublets (related to the D3- brane position moduli), can be produced to be of the order of 125 GeV whereas other Higgs as well as higgsino mass parameter to be very heavy- the "µ-split-like SUSY" scenario. The squarks'/sleptons' (the Wilson line moduli on D7-branes' world vol- ume) masses also turn out to be very heavy. Motivated by the fact that the gravitino appears as the Lightest Supersymmetric Particle (LSP) in our model, we explore the possibility of the gravitino as a viable cold dark matter candidate by showing its life time to be of the order of or greater than the age of the universe whereas lifetimes of decays of the co-NLSPs (the first generation squark/slepton and the lightest neu- tralino) to the LSP (the gravitino) turns out to be too short to disturb predictions of

A second look at gauged supergravities from fluxes in M-theory

Abstract: We investigate reductions of M-theory beyond twisted tori by allowing the presence of KK6 monopoles (KKO6-planes) compatible with N = 4 supersymmetry in four dimensions. The presence of KKO6-planes proves crucial to achieve full moduli stabilisation as they generate new universal moduli powers in the scalar potential. The resulting gauged supergravities turn out to be compatible with a weak G2 holonomy at N = 1 as well as at some non-supersymmetric AdS4 vacua. The M-theory flux vacua we present here cannot be obtained from ordinary type IIA orientifold reductions including background fluxes, D6-branes (O6-planes) and/or KK5 (KKO5) sources. However, from a four-dimensional point of view, they still admit a description in terms of so-called non-geometric fluxes. In this sense we provide the M-theory interpretation for such non-geometric type IIA flux vacua.