Publications (139)518.5 Total impact

Article: ${\cal R}^2$ inflation from scale invariant supergravity and anomaly free superstrings with fluxes
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ABSTRACT: The ${\cal R}^2$ scale invariant gravity theory coupled to conformally invariant matter is investigated. We show that in the nonsupersymmetric case the conformally coupled scalars belong to an $SO(1, 1+n)/SO(1+n)$ manifold, while in the supersymmetric case the scalar manifold becomes isomorphic to the K\"ahlerian space ${\cal M}_n$=$SU(1, 1+n)/ U(1)\times SU(1+n)$. In both cases when the underlying scale symmetry is preserved the vacuum corresponds to de Sitter space. Once the scale symmetry is broken by quantum effects, a transition to flat space becomes possible. We argue that the scale violating terms are induced by anomalies related to a $U(1)_R$ symmetry. The anomaly is resolved via the gauging of a PecceiQuinn axion shift symmetry. The theory describes an inflationary transition from de Sitter to flat Minkowski space, very similar to the Starobinsky inflationary model. The extension to metastable de Sitter superstring vacua is also investigated. The scalar manifold is extended to a much richer manifold, but it contains always ${\cal M}_n$ as a submanifold. In superstrings the metastability is induced by axions that cure the anomalies in chiral $N=1$ (or even $N=0$) supersymmetric vacua via a GreenSchwarz/PecceiQuinn mechanism generalized to four dimensions. We present some typical superstring models and discuss the possible stabilization of the noscale modulus.Fortschritte der Physik 09/2014; DOI:10.1002/prop.201400073 · 0.98 Impact Factor 
Article: Noncommutative/nonassociative IIA (IIB) geometries from Q and Rbranes and their intersections
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ABSTRACT: In this paper we discuss the construction of nongeometric Q and Rbranes as sources of nongeometric Q and Rfluxes in string compactifications. The nongeometric Qbranes, being obtained via Tduality from the NS 5brane or respectively from the KKmonopole, are still local solutions of the standard NS action, where however the background fields G and B possess nongeometric global monodromy properties. We show that using double field theory and redefined background fields $ \widetilde{G} $ and β as well as their corresponding effective action, the Qbranes are locally and globally well behaved solutions. Furthermore the Rbrane solution can be at least formally constructed using dual coordinates. We derive the associated nongeometric Q and Rfluxes and discuss that closed strings moving in the space transversal to the worldvolumes of the nongeometric branes see a noncommutative or a nonassociative geometry. In the second part of the paper we construct intersecting Q and Rbrane configurations as completely supersymmetric solutions of type IIA/B supergravity with certain SU(3) × SU(3) group structures. In the near horizon limit the intersecting brane configurations lead to type II backgrounds of the form AdS 4 × M 6, where the sixdimensional compact space M 6 is a torus fibration with various nongeometric Q and Rfluxes in the compact directions. It exhibits an interesting noncommutative and nonassociate geometric structure. Furthermore we also determine some of the effective fourdimensional superpotentials originating from the nongeometric fluxes.Journal of High Energy Physics 07/2013; 2013(7). DOI:10.1007/JHEP07(2013)048 · 6.22 Impact Factor 
Article: Basic Concepts of String Theory
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ABSTRACT: The book under review is a revision of the previous book of the second and the third author [Lectures on string theory. Berlin etc.: Springer Verlag (1989; Zbl 0716.53063)]. The length of this new version is more than twice of the previous one. The purpose of this new edition is the same as the first one: to prepare the reader for research in string theory. It is intended as a textbook in the sense that, starting from the basics, the material is presented in a pedagogical and selfcontained fashion. To make the book a pedagogical introduction, the authors had to make a selection of topics. The emphasis is on the worldsheet perspective of closed strings and of open strings ending on Dbranes, where twodimensional conformal field theory is the main tool. Compactifications of string theory, with and without fluxes, and string dualities are also discussed from the spacetime point of view, i.e., in geometric language. The major omissions are black holes in string theory, strings at finite temperature, string cosmology, anomalies in string theory, model building, matrix model description of Mtheory, string field theory, to name a few. The influence of the classical string monographs by Green, Schwarz, Witten, and Polchinski can be felt throughout most chapters. 
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ABSTRACT: We discuss the phenomenology and cosmology of a Standardlike Model inspired by string theory, in which the gauge fields are localized on Dbranes wrapping certain compact cycles on an underlying geometry, whose intersection can give rise to chiral fermions. The energy scale associated with string physics is assumed to be near the Planck mass. To develop our program in the simplest way, we work within the construct of a minimal model with gaugeextended sector U(3)B×Sp(1)L×U(1)IR×U(1)L. The resulting U(1) content gauges the baryon number B, the lepton number L, and a third additional Abelian charge IR which acts as the third isospin component of an SU(2)R. All mixing angles and gauge couplings are fixed by rotation of the U(1) gauge fields to a basis diagonal in hypercharge Y and in an anomalyfree linear combination of IR and BL. The anomalous Z′ gauge boson obtains a string scale Stückelberg mass via a 4D version of the GreenSchwarz mechanism. To keep the realization of the Higgs mechanism minimal, we add an extra SU(2) singlet complex scalar, which acquires a VEV and gives a TeVscale mass to the nonanomalous gauge boson Z′′. The model is fully predictive and can be confronted with dijet and dilepton data from LHC8 and, eventually, LHC14. We show that MZ′′≈3–4 TeV saturates current limits from the CMS and ATLAS Collaborations. We also show that for MZ′′≲5 TeV, LHC14 will reach discovery sensitivity ≳5σ. After that, we demonstrate in all generality that Z′′ milliweak interactions could play an important role in observational cosmology. Finally, we examine some phenomenological aspects of the supersymmetric extension of the Dbrane construct.Physical review D: Particles and fields 06/2012; 86(6). DOI:10.1103/PhysRevD.86.066004 
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ABSTRACT: We discuss direct production of Regge excitations in the collisions of massless fourdimensional superstring states, focusing on the first excited level of open strings ending on Dbranes extending into higher dimensions. We construct covariant vertex operators and identify “universal” Regge states with the internal parts either trivial or determined by the worldsheet SCFT describing superstrings propagating on an arbitrary Calabi–Yau manifold. We evaluate the amplitudes involving one such massive state and up to three massless ones and express them in the helicity basis. The most important phenomenological applications of our results are in the context of lowmass string (and large extra dimensions) scenarios in which excited string states are expected to be produced at the LHC as soon as the string mass threshold is reached in the centerofmass energies of the colliding partons. In order to facilitate the use of partonic cross sections, we evaluate them and tabulate for all production processes: gluon fusion, quark absorbing a gluon, quark–antiquark annihilation and quark–quark scattering.Nuclear Physics B 02/2011; 843(3):570–601. DOI:10.1016/j.nuclphysb.2010.10.013 · 3.95 Impact Factor 
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ABSTRACT: Low string scale models might be tested at the LHC directly by their Regge resonances. For such models it is important to investigate the constraints of Standard Model precision measurements on the string scale. It is shown that highly suppressed FCNC processes like K0  [`(K)]0 {K^0}  {\overline K^0} oscillations or leptonic decays of the D 0meson provide nonnegligible lower bounds on both the perturbatively and surprisingly also nonperturbatively induced string theory couplings. We present both the Dbrane instanton formalism to compute such amplitudes and discuss various possible scenarios and their constraints on the string scale for (softly broken) supersymmetric intersecting Dbrane models. KeywordsRare Decays–Intersecting branes modelsJournal of High Energy Physics 02/2011; 2011(2):126. DOI:10.1007/JHEP02(2011)079 · 6.22 Impact Factor 
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ABSTRACT: We consider type II backgrounds of the form \mathbbR1,d  1 M10  d {\mathbb{R}^{1,d  1}} \times {\mathcal{M}_{10  d}} for even d, preserving 2 d/2 real supercharges; for d = 4, 6, 8 this is minimal supersymmetry in d dimensions, while for d = 2 it is N = ( 2,0 ) \mathcal{N} = \left( {2,0} \right) supersymmetry in two dimensions. For d = 6 we prove, by explicitly solving the Killingspinor equations, that there is a onetoone correspondence between background supersymmetry equations in purespinor form and Dbrane generalized calibrations; this correspondence had been known to hold in the d = 4 case. Assuming the correspondence to hold for all d, we list the calibration forms for all admissible Dbranes, as well as the background supersymmetry equations in purespinor form. We find a number of general features, including the following: The pattern of codimensions at which each calibration form appears exhibits a (mod 4) periodicity. In all cases one of the purespinor equations implies that the internal manifold is generalized CalabiYau. Our results are manifestly invariant under generalized mirror symmetry. KeywordsFlux compactifications–Dbranes–Superstring VacuaJournal of High Energy Physics 01/2011; 2011(1):128. DOI:10.1007/JHEP01(2011)063 · 6.22 Impact Factor 
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ABSTRACT: We provide some evidence that closed string coordinates will become noncommutative turning on geometrical fluxes and/or Hfield flux background in closed string compactifications. This is in analogy to open string noncommutativity on the world volume of Dbranes with B and Ffield background. The class of 3dimensional backgrounds we are studying are twisted tori (fibrations of a 2torus over a circle) and the their Tdual Hfield, 3form flux backgrounds (Tfolds). The spatial noncommutativity arises due to the nontrivial monodromies of the toroidal Kähler resp. complex structure moduli fields, when going around the closed string along the circle direction. In addition we study closed string noncommutativity in the context of doubled geometry, where we argue that in general a noncommutative closed string background is Tdual to a commutative closed string background and vice versa. We also discuss the corresponding spatial uncertainty relations. Finally, in analogy to open string boundary conditions, we also argue that closed string momentum and winding modes define in some sense Dbranes in closed string doubled geometry. KeywordsFlux compactifications–NonCommutative Geometry–String DualityJournal of High Energy Physics 12/2010; 2010(12):128. DOI:10.1007/JHEP12(2010)084 · 6.22 Impact Factor 
Article: Strings at the LHC
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ABSTRACT: Assuming that the fundamental string mass scale is in the TeV range and the theory is weakly coupled, we discuss possible signals of string physics at the Large Hadron Collider (LHC).Fortschritte der Physik 07/2010; 58:833838. DOI:10.1002/prop.201000024 · 0.98 Impact Factor 
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ABSTRACT: We address the question how string compactifications with Dbranes are consistent with the black hole bound, which arises in any theory with number of particle species to which the black holes can evaporate. For the KaluzaKlein particles, both longitudinal and transversal to the Dbranes, it is relatively easy to see that the black hole bound is saturated, and the geometric relations can be understood in the language of speciescounting. We next address the question of the black hole evaporation into the higher string states and discover, that contrary to the naive intuition, the exponentially growing number of Regge states does not preclude the existence of semiclassical black holes of substringy size. Our analysis indicates that the effective number of string resonances to which such micro black holes evaporate is not exponentially large but is bounded by N = 1/gs2, which suggests the interpretation of the wellknown relation between the Planck and string scales as the saturation of the black hole bound on the species number. In addition, we also discuss some other issues in Dbrane compactifications with a low string scale of order TeV, such as the masses of light moduli fields.Fortschritte der Physik 06/2010; 58(6):505  527. DOI:10.1002/prop.201000008 · 0.98 Impact Factor 
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ABSTRACT: Threedimensional smooth, compact toric varieties (SCTV), when viewed as real sixdimensional manifolds, can admit Gstructures rendering them suitable for internal manifolds in supersymmetric flux compactifications. We develop techniques which allow us to systematically construct Gstructures on SCTV and read off their torsion classes. We illustrate our methods with explicit examples, one of which consists of an infinite class of toric \mathbbC\mathbbP1 \mathbb{C}{\mathbb{P}^1} bundles. We give a selfcontained review of the relevant concepts from toric geometry, in particular the subject of the classiffication of SCTV in dimensions ≤ 3. Our results open up the possibility for a systematic construction and study of supersymmetric flux vacua based on SCTV. KeywordsFlux compactificationsSuperstring VacuaJournal of High Energy Physics 05/2010; 2010(7):127. DOI:10.1007/JHEP07(2010)073 · 6.22 Impact Factor 
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ABSTRACT: We address the construction of nonsupersymmetric vacua in heterotic compactifications with intrinsic torsion and background fluxes. In particular, we implement the approach of domainwall supersymmetry breaking (DWSB) previously developed in the context of type II flux compactifications. This approach is based on considering backgrounds where probe NS5branes wrapping internal threecycles and showing up as fourdimensional domainwalls do not develop a BPS bound, while all the other BPS bounds characterizing the N = 1 \mathcal{N} = 1 supersymmetric compactifications are preserved at treelevel. Via a scalar potential analysis we provide the conditions for these backgrounds to solve the tendimensional equations of motion including order α′ corrections. We also consider backgrounds where some of the NS5domainwalls develop a BPS bound, show their relation to noscale SUSYbreaking vacua and construct explicit examples via elliptic fibrations. Finally, we consider backgrounds with a nontrivial gaugino condensate and discuss their relation to supersymmetric and nonsupersymmetric vacua in the present context. KeywordsFlux compactificationsSupersymmetry BreakingSuperstrings and Heterotic StringsSuperstring VacuaJournal of High Energy Physics 04/2010; 2010(6):156. DOI:10.1007/JHEP06(2010)090 · 6.22 Impact Factor 
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ABSTRACT: We consider instanton solutions of Euclidean HořavaLifshitz gravity in four dimensions satisfying the detailed balance condition. They are described by geometric flows in three dimensions driven by certain combinations of the Cotton and Ricci tensors as well as the cosmologicalconstant term. The deformation curvature terms can have competing behavior leading to a variety of fixed points. The instantons interpolate between any two fixed points, which are vacua of topologically massive gravity with Λ > 0, and their action is finite. Special emphasis is placed on configurations with SU(2) isometry associated with homogeneous but generally nonisotropic Bianchi IX model geometries. In this case, the combined RicciCotton flow reduces to an autonomous system of ordinary differential equations whose properties are studied in detail for different couplings. The occurrence and stability of isotropic and anisotropic fixed points are investigated analytically and some exact solutions are obtained. The corresponding instantons are classified and they are all globally \mathbbR S3 \mathbb{R} \times {S^3} and complete spaces. Generalizations to higherdimensional gravities are also briey discussed. KeywordsSolitons Monopoles and InstantonsDifferential and Algebraic GeometryClassical Theories of GravityJournal of High Energy Physics 01/2010; 2010(4):158. DOI:10.1007/JHEP04(2010)131 · 6.22 Impact Factor 
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ABSTRACT: The Large Hadron Collider presents an unprecedented opportunity to probe the realm of new physics in the TeV region and shed light on some of the core unresolved issues of particle physics. These include the nature of electroweak symmetry breaking, the origin of mass, the possible constituent of cold dark matter, new sources of CP violation needed to explain the baryon excess in the universe, the possible existence of extra gauge groups and extra matter, and importantly the path Nature chooses to resolve the hierarchy problem – is it supersymmetry or extra dimensions. Many models of new physics beyond the standard model contain a hidden sector which can be probed at the LHC. Additionally, the LHC will be a top factory and accurate measurements of the properties of the top and its rare decays will provide a window to new physics. Further, the LHC could shed light on the origin of neutralino masses if the new physics associated with their generation lies in the TeV region. Finally, the LHC is also a laboratory to test the hypothesis of TeV scale strings and D brane models. An overview of these possibilities is presented in the spirit that it will serve as a companion to the Technical Design Reports (TDRs) by the particle detector groups ATLAS and CMS to facilitate the test of the new theoretical ideas at the LHC. Which of these ideas stands the test of the LHC data will govern the course of particle physics in the subsequent decades. 
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ABSTRACT: We extend the study of scattering amplitudes presented in “The LHC String Hunter's Companion” [1] to the case of fivepoint processes that may reveal the signals of low mass strings at the LHC and are potentially useful for detailed investigations of fundamental Regge excitations. In particular, we compute the fullfledged string disk amplitudes describing all 2→3 parton scattering subprocesses leading to the production of three hadronic jets. We cast our results in a form suitable for the implementation of stringy partonic cross sections in the LHC data analysis. We discuss the universal, modelindependent features of multiparton processes and point out the existence of even stronger universality relating Ngluon amplitudes to the amplitudes involving N−2 gluons and one quark–antiquark pair. We construct a particularly simple basis of two functions describing all universal fivepoint amplitudes. We also discuss modeldependent amplitudes involving four fermions and one gauge boson that may be relevant for studying jets associated to Drell–Yan pairs and other processes depending on the spectrum of Kaluza–Klein particles, thus on the geometry of compact dimensions.Nuclear Physics B 08/2009; DOI:10.1016/j.nuclphysb.2009.11.008 · 3.95 Impact Factor 
Article: Can we see strings at the LHC?
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ABSTRACT: In this article we will overview possible model independent LHC signatures of intersecting brane models.Fortschritte der Physik 06/2009; 57(57):434445. DOI:10.1002/prop.200900065 · 0.98 Impact Factor 
Article: LHC phenomenology for string hunters
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ABSTRACT: We consider extensions of the standard model based on open strings ending on Dbranes, with gauge bosons due to strings attached to stacks of Dbranes and chiral matter due to strings stretching between intersecting Dbranes. Assuming that the fundamental string mass scale is in the TeV range and the theory is weakly coupled, we discuss possible signals of string physics at the Large Hadron Collider (LHC). In previous works, direct channel excitations of Regge recurrences in parton–parton scattering supplied the outstanding new signature. The present work considers the deviation from standard model expectations for the 4fermion processes qq→qq and qq′→qq′, in which the schannel excitation of string resonances is absent. In this case, we find that Kaluza–Klein recurrences at masses somewhat less than the string scale generate effective 4fermion contact terms which can significantly enhance the dijet R ratio above its QCD value of about 0.6. The simultaneous observation of a nearby resonant structure in the dijet mass spectrum would provide a “smoking gun” for TeV scale string theory. In this work, we also show that (1) for Mstring<3.5 TeV, the rates for various topologies arising from the pp→Z0+jet channel could deviate significantly from standard model predictions and (2) that the sizeable cross sections for Regge recurrences can allow a 6σ discovery for string scales as large as 3 TeV after about 1 year of LHC operation at and .Nuclear Physics B 04/2009; DOI:10.1016/j.nuclphysb.2009.06.023 · 3.95 Impact Factor 
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ABSTRACT: We consider string theory compactifications of the form AdS with orientifold sixplanes, where is a sixdimensional compact space that is either a nilmanifold or a coset. For all known solutions of this type we obtain the fourdimensional lowenergy effective theory by computing the superpotential, the Kähler potential and the mass spectrum for the light moduli. For the nilmanifold examples we perform a crosscheck on the result for the mass spectrum by calculating it alternatively from a direct Kaluza–Klein reduction and find perfect agreement. We show that in all but one of the coset models all moduli are stabilized at the classical level. As an application we show that all but one of the coset models can potentially be used to bypass a recent nogo theorem against inflation in type IIA theory.Classical and Quantum Gravity 01/2009; 26(2):025014. DOI:10.1088/02649381/26/2/025014 · 3.10 Impact Factor 
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ABSTRACT: We update the D3/D7brane inflation model on K3×T2/Z2 with branes and fluxes. For this purpose, we study the low energy theory including gs corrections to the gaugino condensate superpotential that stabilizes the K3 volume modulus. The gauge kinetic function is verified to become holomorphic when the original N=2 supersymmetry is spontaneously broken to N=1 by bulk fluxes. From the underlying classical N=2 supergravity, the theory inherits a shift symmetry which provides the inflaton with a naturally flat potential. We analyze the fate of this shift symmetry after the inclusion of quantum corrections. The field range of the inflaton is found to depend significantly on the complex structure of the torus but is independent of its volume. This allows for a large kinematical field range for the inflaton. Furthermore, we show that the D3/D7 model may lead to a realization of the recent CMB fit by Hindmarsh et al. with an 11% contribution from cosmic strings and a spectral index close to ns=1. On the other hand, by a slight change of the parameters of the model one can strongly suppress the cosmic string contribution and reduce the spectral index ns to fit the WMAP5 data in the absence of cosmic strings. We also demonstrate that the inclusion of quantum corrections allows for a regime of eternal D3/D7 inflation.Nuclear Physics B 01/2009; DOI:10.1016/j.nuclphysb.2008.07.033 · 3.95 Impact Factor 
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ABSTRACT: Assuming that the fundamental string mass scale is in the TeV range and the theory is weakly coupled, we discuss possible signals of string physics at the Large Hadron Collider (LHC). In Dbrane constructions, the dominant contributions to fullfledged string amplitudes for all the common QCD parton subprocesses leading to dijets are completely independent of the details of compactification, and can be evaluated in a parameterfree manner. We make use of these amplitudes evaluated near the first resonant pole to determine the discovery potential of LHC for the first Regge excitations of the quark and gluon. Remarkably, the reach of LHC after a few years of running can be as high as 6.8 TeV. Even after the first 100 pb(1) of integrated luminosity, string scales as high as 4.0 TeV can be discovered. Data on pp>directgamma + jet can provide corroboration for string physics at scales as high as 5 TeV.Physical Review Letters 12/2008; 101(24):241803. DOI:10.1103/PhysRevLett.100.171603 · 7.73 Impact Factor
Publication Stats
6k  Citations  
518.50  Total Impact Points  
Top Journals
Institutions

2005–2014

Max Planck Institute of Physics
München, Bavaria, Germany


1982–2013

LudwigMaximilianUniversity of Munich
 Arnold Sommerfeld Center for Theoretical Physics (ASC)
München, Bavaria, Germany


1993–2004

HumboldtUniversität zu Berlin
 Department of Physics
Berlín, Berlin, Germany


1999

University of California, Santa Barbara
 Kavli Institute for Theoretical Physics
Santa Barbara, California, United States


1992

Universidad Autónoma de Madrid
 Department of Theoretical physics
Madrid, Madrid, Spain


1989–1992

CERN
 Physics Department (PH)
Genève, Geneva, Switzerland


1991

Central University of Venezuela
 Facultad de Ciencias
Caracas, Capital, Venezuela 
Northeastern University
 Department of Physics
Boston, Massachusetts, United States


1986–1988

California Institute of Technology
Pasadena, California, United States


1984

Max Planck Institute for Astrophysics
Arching, Bavaria, Germany


1983

University Hospital München
München, Bavaria, Germany
