arXiv subjects

I. Antoniadis

Publications and source records attributed to I. Antoniadis.

At least 37 records · Page 2Linked to original sources

arXiv2011

One-loop adjoint masses for non-supersymmetric intersecting branes

We consider breaking of supersymmetry in intersecting D-brane configurations by slight deviation of the angles from their supersymmetric values. We compute the masses generated by radiative corrections for the adjoint scalars on the brane world-volumes. In the open string channel, the string two-point function receives contributions only from the infrared and the ultraviolet limits. The latter is due to tree-level closed string uncanceled NS-NS tadpoles, which we explicitly reproduce from the effective Born-Infeld action. On the other hand, the infrared region reproduces the one-loop mediation of supersymmetry breaking in the effective gauge theory, via messengers and their Kaluza-Klein excitations. In the toroidal set-up considered here, it receives contributions only from broken N=4 and N=2 supersymmetric configurations, and thus always leads at leading order to a tachyonic direction, in agreement with effective field theory expectations.

hep-th↗

arXiv2010

Brane singularities and their avoidance

The singularity structure and the corresponding asymptotic behavior of a 3-brane coupled to a scalar field or to a perfect fluid in a five-dimensional bulk is analyzed in full generality using the method of asymptotic splittings. In the case of the scalar field, it is shown that the collapse singularity at a finite distance from the brane can be avoided only at the expense of making the brane world-volume positively or negatively curved. In the case where the bulk field content is parametrized by an analogue of perfect fluid with an arbitrary equation of state P=γρbetween the `pressure' P and the `density' ρ, our results depend crucially on the constant fluid parameter γ: (i) For γ>-1/2, the flat brane solution suffers from a collapse singularity at finite distance, that disappears in the curved case. (ii) For γ<-1, the singularity cannot be avoided and it becomes of the big rip type for a flat brane. (iii) For -1<γ< or = -1/2, the surprising result is found that while the curved brane solution is singular, the flat brane is not, opening the possibility for a revival of the self-tuning proposal.

gr-qc↗

arXiv2010

N=4 Topological Amplitudes and Black Hole Entropy

We study the effects of N=4 topological string amplitudes on the entropy of black holes. We analyse the leading contribution associated to six-derivative terms and find one particular operator which can correct the entropy of N=4 black holes. This operator is BPS-like and appears in the effective action of type II string theory on K3 x T^2 or equivalently its heterotic dual on T^6. In both descriptions the leading contribution arises at one-loop, which we calculate explicitly on the heterotic side. We then consider whether this term has any consequences for the entropy of (large) N=4 black holes and find that it makes indeed a contribution at subleading order. Repeating the computation for small black holes with vanishing horizon area at the classical level, we prove that this coupling lifts certain flat directions in the entropy function thereby being responsible for the attractor equations of some moduli fields.

hep-th↗

arXiv2010

Deformed Topological Partition Function and Nekrasov Backgrounds

A deformation of the N=2 topological string partition function is analyzed by considering higher dimensional F-terms of the type W^{2g}*Upsilon^n, where W is the chiral Weyl superfield and each Upsilon factor stands for the chiral projection of a real function of N=2 vector multiplets. These terms generate physical amplitudes involving two anti-self-dual Riemann tensors, 2g-2 anti-self-dual graviphoton field strengths and 2n self-dual field strengths from the matter vector multiplets. Their coefficients F_{g,n} generalizing the genus g partition function F_{g,0} of the topological twisted type II theory, can be used to define a generating functional by introducing deformation parameters besides the string coupling. Choosing all matter field strengths to be that of the dual heterotic dilaton supermultiplet, one obtains two parameters that we argue should correspond to the deformation parameters of the Nekrasov partition function in the field theory limit, around the conifold singularity. Its perturbative part can be obtained from the one loop analysis on the heterotic side. This has been computed in [1] and in the field theory limit shown to be given by the radius deformation of c=1 CFT coupled to two-dimensional gravity. Quite remarkably this result reproduces the gauge theory answer up to a phase difference that may be attributed to the regularization procedure. The type II results are expected to be exact and should also capture the part that is non-perturbative in heterotic dilaton.

hep-th↗

arXiv2010

MSSM Higgs with dimension-six operators

We investigate an extension of the MSSM Higgs sector by including the effects of all dimension-five and dimension-six effective operators and their associated supersymmetry breaking terms. The corrections to the masses of the neutral CP-even and CP-odd Higgs bosons due to the d=5 and d=6 operators are computed. When the d=5 and d=6 operators are generated by the same physics (i.e. when suppressed by powers of the same scale M), due to the relative $\tanβ$ enhancement of the latter, which compensates their extra scale suppression (1/M), the mass corrections from d=6 operators can be comparable to those of d=5 operators, even for conservative values of the scale M. We identify the effective operators with the largest individual corrections to the lightest Higgs mass and discuss whether at the microscopic level and in the simplest cases, these operators are generated by "new physics" with a sign consistent with an increase of m_h. Simple numerical estimates easily allow an increase of m_h due to d=6 operators alone in the region of 10-30 GeV, while for a much larger increase light new states beyond MSSM may be needed, in which case the effective description is unreliable. Special attention is paid to the treatment of the effective operators with higher derivatives. These can be removed by non-linear field redefinitions or by an "unfolding" technique, which effectively ensure that any ghost degrees of freedom (of mass $\geqsim M$) are integrated out and absent in the effective theory at scales much smaller than M. Considering general coefficients of the susy operators with a scale of new physics above the LHC reach, it is possible to increase the tree-level prediction for the Higgs mass to the LEPII bound, thus alleviating the MSSM fine-tuning.

hep-ph↗

arXiv2009

Nonlinear Supersymmetry, Brane-bulk Interactions and Super-Higgs without Gravity

We derive the coupling of a hypermultiplet of N=2 global supersymmetry to the Dirac-Born-Infeld Maxwell theory with linear N=1 and a second nonlinear supersymmetry. At the level of global supersymmetry, this construction corresponds to the interaction with Maxwell brane fields of bulk hypermultiplets, such as the universal dilaton of type IIB strings compactified on a Calabi-Yau manifold. It displays in particular the active role of a four-form field. Constrained N=1 and N=2 superfields and the formulation of the hypermultiplet in its single-tensor version are used to derive the nonlinear realization, allowing a fully off-shell description. Exact results with explicit symmetries and supersymmetries are then obtained. The electric-magnetic dual version of the theory is also derived and the gauge structure of the interaction is exemplified with N=2 nonlinear QED of a charged hypermultiplet. Its Higgs phase describes a novel super-Higgs mechanism without gravity, where the goldstino is combined with half of the hypermultiplet into an N=1 massive vector multiplet.

hep-th↗

arXiv2009

A New Class of N=2 Topological Amplitudes

We describe a new class of N=2 topological amplitudes that compute a particular class of BPS terms in the low energy effective supergravity action. Specifically they compute the coupling F^2(λλ)^{g-2}(dϕ)^2 where F, λand ϕare gauge field strengths, gaugino and holomorphic vector multiplet scalars. The novel feature of these terms is that they depend both on the vector and hypermultiplet moduli. The BPS nature of these terms implies that they satisfy a holomorphicity condition with respect to vector moduli and a harmonicity condition as well as a second order differential equation with respect to hypermultiplet moduli. We study these conditions explicitly in heterotic string theory and show that they are indeed satisfied up to anomalous boundary terms in the world-sheet moduli space. We also analyze the boundary terms in the holomorphicity and harmonicity equations at a generic point in the vector and hyper moduli space. In particular we show that the obstruction to the holomorphicity arises from the one loop threshold correction to the gauge couplings and we argue that this is due to the contribution of non-holomorphic couplings to the connected graphs via elimination of the auxiliary fields.

hep-th↗

arXiv2008

MSSM with Dimension-five Operators (MSSM_5)

We perform a general analysis of the R-parity conserving dimension-five operators that can be present beyond the Minimal Supersymmetric Standard Model. Not all these operators are actually independent. We present a method which employs spurion-dependent field redefinitions that removes this "redundancy" and establishes the minimal, irreducible set of these dimension-five operators. Their potential effects on the MSSM Higgs sector are discussed to show that the tree level bound $m_h\leq m_Z$ cannot be easily lifted within the approximations used, and quantum corrections are still needed to satisfy the LEPII bound. An ansatz is provided for the structure of the remaining couplings in the irreducible set of D=5 operators, which avoids phenomenological constraints from flavor changing neutral currents. The minimal set of operators brings new couplings in the effective Lagrangian, notably "wrong"-Higgs Yukawa couplings and contact fermion-fermion-scalar-scalar interactions, whose effects are expected to be larger than those generated in the MSSM at loop or even tree level. This has implications in particular for LHC searches for supersymmetry by direct squark production.

hep-ph↗

arXiv2008

Higher Dimensional Operators in the MSSM

The origin and the implications of higher dimensional effective operators in 4-dimensional theories are discussed in non-supersymmetric and supersymmetric cases. Particular attention is paid to the role of general, derivative-dependent field redefinitions which one can employ to obtain a simpler form of the effective Lagrangian. An application is provided for the Minimal Supersymmetric Standard Model extended with dimension-five R-parity conserving operators, to identify the minimal irreducible set of such operators after supersymmetry breaking. Among the physical consequences of this set of operators are the presence of corrections to the MSSM Higgs sector and the generation of "wrong"-Higgs Yukawa couplings and fermion-fermion-scalar-scalar interactions. These couplings have implications for supersymmetry searches at the LHC.

hep-ph↗

arXiv2008

Nonlinear N=2 Supersymmetry, Effective Actions and Moduli Stabilization

Nonlinear supersymmetry is used to compute the general form of the effective D-brane action in type I string theory compactified to four dimensions in the presence of internal magnetic fields. In particular, the scalar potential receives three contributions: (1) a nonlinear part of the D-auxiliary component, associated to the Dirac-Born-Infeld action; (2) a Fayet-Iliopoulos (FI) D-term with a moduli-dependent coefficient; (3) a D-auxiliary independent (but moduli dependent) piece from the D-brane tension. Minimization of this potential leads to three general classes of vacua with moduli stabilization: (i) supersymmetric vacua allowing in general FI terms to be cancelled by non-trivial vacuum expectation values (VEV's) of charged scalar fields; (ii) anti-de Sitter vacua of broken supersymmetry in the presence of a non-critical dilaton potential that can be tuned at arbitrarily weak string coupling; (iii) if the dilaton is fixed in a supersymmetric way by three-form fluxes and in the absence of charged scalar VEV's, one obtains non supersymmetric vacua with positive vacuum energy.

hep-th↗

arXiv2008

Supersymmetric Models with Higher Dimensional Operators

In 4D renormalisable theories, integrating out massive states generates in the low energy effective action higher dimensional operators (derivative or otherwise). Using a superfield language it is shown that a 4D N=1 supersymmetric theory with higher derivative operators in either the Kahler or the superpotential part of the Lagrangian and with an otherwise arbitrary superpotential, is equivalent to a 4D N=1 theory of second order (i.e. without higher derivatives) with additional superfields and renormalised interactions. We provide examples where a free theory with trivial supersymmetry breaking provided by a linear superpotential becomes, in the presence of higher derivatives terms and in the second order version, a non-trivial interactive one with spontaneous supersymmetry breaking. The couplings of the equivalent theory acquire a threshold correction through their dependence on the scale of the higher dimensional operator(s). The scalar potential in the second order theory is not necessarily positive definite, and one can in principle have a vanishing potential with broken supersymmetry. We provide an application to MSSM and argue that at tree-level and for a mass scale associated to a higher derivative term in the TeV range, the Higgs mass can be lifted above the current experimental limits.

hep-th↗

arXiv2007

Topics on String Phenomenology

These lectures present some topics of string phenomenology and contain two parts. In the first part, I review the possibility of lowering the string scale in the TeV region, that provides a theoretical framework for solving the mass hierarchy problem and unifying all interactions. The apparent weakness of gravity can then be accounted by the existence of large internal dimensions, in the submillimeter region, and transverse to a braneworld where our universe must be confined. I review the main properties of this scenario and its implications for observations at both particle colliders, and in non-accelerator gravity experiments. In the second part, I discuss a simple framework of toroidal string models with magnetized branes, that offers an interesting self-consistent setup for string phenomenology. I will present an algorithm for fixing the geometric parameters of the compactification, build calculable particle physics models such as a supersymmetric SU(5) Grand Unified Theory with three generations of quarks and leptons, and implement low energy supersymmetry breaking with gauge mediation that can be studied directly at the string level.

hep-th↗

arXiv2007

Harmonicity in N=4 supersymmetry and its quantum anomaly

The holomorphicity property of N=1 superpotentials or of N=2 F-terms involving vector multiplets is generalized to the case of N=4 1/2-BPS effective operators defined in harmonic superspace. The resulting harmonicity equations are shown to control the moduli dependence of the couplings of higher dimensional operators involving powers of the N=4 Weyl superfield, computed by N=4 topological amplitudes. These equations can also be derived on the string side, exhibiting an anomaly from world-sheet boundary contributions that leads to recursion relations for the non-analytic part of the couplings.

hep-th↗

arXiv2007

A New Gauge Mediation Theory

We propose a class of models with gauge mediation of supersymmetry breaking, inspired by simple brane constructions, where R-symmetry is very weakly broken. The gauge sector has an extended N=2 supersymmetry and the two electroweak Higgses form an N=2 hypermultiplet, while quarks and leptons remain in N=1 chiral multiplets. Supersymmetry is broken via the D-term expectation value of a secluded U(1) and it is transmitted to the Standard Model via gauge interactions of messengers in N=2 hypermultiplets: gauginos thus receive Dirac masses. The model has several distinct experimental signatures with respect to ordinary models of gauge or gravity mediation realizations of the Minimal Supersymmetric Standard Model (MSSM). First, it predicts extra states as a third chargino that can be observed at collider experiments. Second, the absence of a D-flat direction in the Higgs sector implies a lightest Higgs behaving exactly as the Standard Model one and thus a reduction of the `little' fine-tuning in the low tan(beta) region. This breaking of supersymmetry can be easily implemented in string theory models

hep-ph↗

arXiv2007

Magnetic fluxes and moduli stabilization

Stabilization of closed string moduli in toroidal orientifold compactifications of type IIB string theory are studied using constant internal magnetic fields on D-branes and 3-form fluxes that preserve N=1 supersymmetry in four dimensions. Our analysis corrects and extends previous work by us, and indicates that charged scalar VEV's need to be turned on, in addition to the fluxes, in order to construct a consistent supersymmetric model. As an explicit example, we first show the stabilization of all Kahler class and complex structure moduli by turning on magnetic fluxes on different sets of D9-branes that wrap the internal space T^6 in a compactified type I string theory, when a charged scalar on one of these branes acquires a non-zero VEV. The latter can also be determined by adding extra magnetized branes, as we demonstrate in a subsequent example. In a different model with magnetized D7-branes, in a IIB orientifold on T^6/Z_2, we show the stabilization of all the closed string moduli, including the axion-dilaton at weak string coupling g_s, by turning on appropriate closed string 3-form fluxes.

hep-th↗

arXiv2007

Topological Amplitudes and Physical Couplings in String Theory

In these lectures, we review the main properties of the topological theory obtained by twisting the N=2 two-dimensional superconformal algebra, associated to supersymmetric string compactifications. In particular, we describe a set of physical quantities in string theory that are computed by topological amplitudes. These are in general higher-dimensional F-terms in the low energy effective supergravity, or fermion masses after supersymmetry breaking. We discuss N=2 compactifications of type II strings, N=1 compactifications of heterotic and type I strings, as well as N=4 string vacua. Particular emphasis is put on alternative string dual representations allowing calculability, and on the generalization of N=2 holomorphicity and its anomaly.

hep-th↗

arXiv2006

Living with Ghosts and their Radiative Corrections

The role of higher derivative operators in 4D effective field theories is discussed in both non-supersymmetric and supersymmetric contexts. The approach, formulated in the Minkowski space-time, shows that theories with higher derivative operators do not always have an improved UV behaviour, due to subtleties related to the analytical continuation from the Minkowski to the Euclidean metric. This continuation is further affected at the dynamical level due to a field-dependence of the poles of the Green functions of the particle-like states, for curvatures of the potential of order unity in ghost mass units. The one-loop scalar potential in lambda*phi^4 theory with a single higher derivative term is shown to have infinitely many counterterms, while for a very large mass of the ghost the usual 4D renormalisation is recovered. In the supersymmetric context of the O'Raifeartaigh model of spontaneous supersymmetry breaking with a higher derivative (supersymmetric) operator, it is found that quadratic divergences are present in the one-loop self-energy of the scalar field. They arise with a coefficient proportional to the amount of supersymmetry breaking and suppressed by the scale of the higher derivative operator. This is also true in the Wess-Zumino model with higher derivatives and explicit soft breaking of supersymmetry. In both models, the UV logarithmic behaviour is restored in the decoupling limit of the ghost.

hep-th↗

arXiv2006

N=4 Topological Amplitudes and String Effective Action

Certain scattering amplitudes in the gravitational sector of type II string theory on K3 x T^2 are found to be computed by correlation functions of the N=4 topological string. This analysis extends the already known results for K3 by Berkovits and Vafa, which correspond to six-dimensional terms in the effective action, involving four Riemann tensors and 4g-4 graviphotons, R^4T^{4g-4}, at genus g. We find two additional classes of topological amplitudes that use the full internal SCFT of K3 x T^2. One of these string amplitudes is mapped to a 1-loop contribution in the heterotic theory, and is studied explicitly. It corresponds to the four-dimensional term R^2(ddΦ)^2T^{2g-4}, with Φa Kaluza-Klein graviscalar from T^2. Finally, the generalization of the harmonicity relation for its moduli dependent coupling coefficient is obtained and shown to contain an anomaly, generalizing the holomorphic anomaly of the N=2 topological partition function F_g.

hep-th↗