Dissertations / Theses on the topic 'String models'
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1
García, Pulido Ana Lucía. "Models and string topology." Thesis, University of Warwick, 2012. http://wrap.warwick.ac.uk/56787/.
Abstract:
This thesis concerns the study of string topology, a relatively new branch of algebraic topology. We begin with a survey of the background of string topology. In particular, this includes a summary of the papers [4] by Chas and Sulivan, [15] by Jones and [5] by Cohen and Jones that provide the background for the original work of this thesis. We then proceed to give a new e cient technique to do systematic computations of the full structure of the string topology for a large family of manifolds. For this, we rst use the results of Jones [15] and Cohen and Jones [5] to reduce the problem to calculating Hochschild homology and cohomology. Secondly, we use the concept of models to compute Hochschild homology and cohomology and obtain some further Hochschild structure. Thus, most of this work is devoted to developing this technique for calculating Hochschild homology and cohomology via models. This research contributes to the area by providing the rst general and systematic method of computing the full structure of string topology. In addition, we give multiple, transparent examples of our new theory.
2
Godazgar, Mohammad Hadi. "Dualities in string theory." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610466.
3
Lambert, Neil. "Massive sigma models and string solitons." Thesis, University of Cambridge, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627529.
4
Boyce, Michael M. (Michael MacField) Carleton University Dissertation Physics. "String inspired QCD and E6 models." Ottawa, 1996.
5
Bianchi, Lorenzo. "Perturbation theory for string sigma models." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://dx.doi.org/10.18452/17439.
Abstract:
In dieser Arbeit untersuchen wir Quanten-Aspekte des Green-Schwarz Superstrings in verschiedenen AdS-Hintergründen, die für die AdS/CFT Korrespondenz von Bedeutung sind, und geben einige Beispiele für perturbative Rechnungen in den entsprechenden integrablen Sigma-Modellen. Wir beginnen mit einer detaillierten Darstellung der Konstruktion der Wirkung des Typ-IIB-Superstrings auf dem AdS5 x S5-Hintergrund, die durch eine Supercoset-Sigma-Modell definiert wird, und zeigen die Grenzen dieser Herangehensweise für Hintergründe auf, die in niedrig-dimensionalen Beispielen der Eich/Gravitations-Dualität von Interesse sind. Daraufhin betrachten wir die Entwicklung um das BMN-Vakuum und die S-Matrix für die Streuung von Weltflächen-Anregungen. Um ihre Elemente effizient auszuwerten, entwickeln wir eine auf Unitarität basierende Methode für allgemeine massive, zweidimensionale Feldtheorien. Weiterhin betrachten wir den AdS-Lichtkegel eichfixierten String in AdS4 x CP3 in einer Entwicklung um das "null-cusp"-Vakuum. Die freie Energie dieses Modells hängt zusammen mit der anomalen Cusp-Dimension der Eichtheorie. Wir berechnen Korrekturen zur Zustandssumme des Superstring-Modells und leiten somit die anomale Cusp-Dimension der ABJM-Theorie bei starker Kopplung bis zur Zweischleifen-Ordnung her. Schlie\sslich berechnen wir auf Einschleifen-Ebene die Dispersionsrelation von Anregungen um das GKP-Vakuum. Unsere erfolgreiche Anwendung von auf Unitarität basierenden Cut-Techniken auf verschiedene Beispiele stützt die Vermutung, dass die S-Matrizen zweidimensionaler, integrabler Feldtheorien cut-konstruierbar sind. Weiterhin liefern unsere Ergebnisse wertvolle Daten, die die Konsistenz der String-Wirkung auf Quanten-Niveau belegen und stellen nicht-triviale stringente Tests der Quanten-Integrabilität der untersuchten Modelle dar.In this thesis we investigate quantum aspects of the Green-Schwarz superstring in various AdS backgrounds relevant for the AdS/CFT correspondence, providing several examples of perturbative computations in the corresponding integrable sigma-models. We start by reviewing in details the construction of the type IIB superstring action in AdS5 x S5 background defined as a supercoset sigma model, pointing out the limits of this procedure for backgrounds interesting in lower-dimensional examples of the gauge/gravity duality. We then consider the expansion about the BMN vacuum and the S-matrix for the scattering of worldsheet excitations. To evaluate its elements efficiently we develop a unitarity-based method for general massive two-dimensional field theories. We also analyze the AdS light-cone gauge fixed string in AdS4 x CP3 expanded around a “null cusp” vacuum. The free energy of this model is related to the cusp anomalous dimension of the gauge theory and, indirectly, to a non-trivial effective coupling entering all integrability-based calculations in AdS4/CFT3. We calculate corrections to the superstring partition function of the model, thus deriving the cusp anomalous dimension of ABJM theory at strong coupling up to two-loop order and giving support to a recent conjecture. Finally, we calculate at one-loop the dispersion relation of excitations about the GKP vacuum. Our successful application of unitarity-cut techniques on several examples supports the conjecture that S-matrices of two-dimensional integrable field theories are cut-constructible. Furthermore, our results provide valuable data in support of the quantum consistency of the string actions and furnish non-trivial stringent tests for the quantum integrability of the analyzed models.
6
Bernardo, Heliudson de Oliveira. "Cosmological models from string theory setups /." São Paulo, 2019. http://hdl.handle.net/11449/183612.
Abstract:
Orientador: Horatiu NastaseResumo: Nesta tese, discutimos três modelos cosmológicos que são baseados direta ou indiretamente em ideias advindas de teoria das cordas. Depois de uma revisão geral de cosmologia em teoria das cordas, um resumo de cosmologia e teoria das cordas é apresentado, com ênfase nos conceitos fundamentais e teóricos. Então descrevemos como o acoplamento camaleônico pode potencialmente afetar as predições de inflação cósmica com campo único, com tratamento cuidadoso dos modos de perturbação cosmológica adiabáticos e de entropia. Além disso uma nova abordagem para a dualidade-T em soluções cosmológicas de supergravidade bosônica é discutida no contexto de teoria dupla de campos. Por fim, propomos uma nova prescrição para o mapa holográfico em cosmologia que pode ser usado para conectar modelos fundamentais de cosmologia holográfica com outras abordagens fenomenológicas.
Doutor
7
Keyserlingk, Curt William Von. "String-net models in condensed matter systems." Thesis, University of Oxford, 2014. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669743.
8
Berndsen, Aaron. "String theoretic models of the early universe." Thesis, McGill University, 2008. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=18674.
Abstract:
This thesis comprises several manuscripts, each exploring aspects of the dynamics of the early Universe. The foundations of the work presented lies in the realm of cosmology, but draws heavily on string theory as a source of guidance. The thesis commences with a motivation for the research and provides an introduction to the contemporary views of Cosmology: following a historical perspective of cosmology, we motivate the inflationary paradigm of Big Bang Cosmology, and introduce several world-views promoted by string theory. The string-motivated models will address shortcomings of cosmologies based on General Relativity and the Standard Model, and will provide a comprehensive, coherent description of the early Universe that is expected to transition to our observed Universe. Two possibilities presented here include String Gas Cosmology (SGC) and the Brane World scenario. We provide an introduction to these two constructions, and subsequently report on the possibility of simultaneously stabilizing the dilaton and moduli fields in SGC stabilization models, a mechanism to solve the overshoot problem of racetrack inflation, and explore the possibility of long-lived relics in warped reheating.Cette th\èse explore la physique du jeune univers. Nous utilisons la th\'eorie des cordes pour nous aider \à comprendre l'\'epoque pr\'ec\'edant la nucl\'eosynth\èse. Deux manuscrits sont present\'es: un concernant la cosmologie de gaz de cordes (string gas cosmology, SGC), et un concernant le sc\'enario d'un univers membrane (brane world, BW). Nous pla\cons des contraintes sur la stabilization du moduli et du dilaton dans la construction SGC. Puis, nous \'etudions l\'epoque du r\'echauffement de l'univers dans la construction BW, et nous pla\c{c}ons des contraintes sur les param\ètres fundamentaux de la th\'eorie des cordes.
9
Panvel, Jal. "Exact string backgrounds and gauged WZW models." Thesis, University of Liverpool, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.241464.
10
Stepanchuk, Andrej. "Aspects of integrability in string sigma-models." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/28904.
Abstract:
The recent success in applying integrability-based methods to study examples of gauge/gravity dualities in highly (super)symmetric settings motivates the question of whether such methods can be carried over to more physical and less symmetric cases. In this thesis we consider two such examples of string sigma-models, which interpolate between integrable or solvable limits. First we consider classical string motion on curved p-brane backgrounds for which the sigma-model interpolates between the integrable flat space and AdS(k)xS(k) coset or WZW sigma-models. We find that while the equations for particle (i.e. geodesic) motion are integrable in these backgrounds, the equations for extended string motion are not. The second example we consider is string theory on AdS3xS3xT4 with mixed Ramond-Ramond (R-R) and Neveu-Schwarz-Neveu-Schwarz (NS-NS) 3-form fluxes, which interpolates between the integrable pure R-R and the pure NS-NS theory that can be solved using CFT methods. The dispersion relation and S-matrix for world-sheet excitations, which are the essential ingredients in solving for the string spectrum, are only partially fixed by integrability and symmetry arguments. By constructing the mixed flux generalisation of the dyonic giant magnon soliton, which we show can be interpreted as a bound-state of excitations, we determine the dispersion relation for massive excitations. We also construct the mixed flux generalisation of the folded string on AdS3xS1 and show that, at leading order in large angular momentum on AdS3, its energy is given by the pure R-R expression with the string tension rescaled by the R-R flux coefficient. Further, we derive the bound-state S-matrix and its 1-loop correction by considering the scattering of dyonic giant magnons and plane waves. From this we deduce the semiclassical and 1-loop dressing phases in the massive sector S-matrix, which we find to agree with recent proposals.
11
Macorra, Axel de la. "Supersymmetry breaking in 4D string theory." Thesis, University of Oxford, 1993. http://ora.ox.ac.uk/objects/uuid:0bc6b606-1a02-4d28-b68f-bd5c3ac11d04.
Abstract:
In this thesis we address the problem of supersymmetry breaking in four dimensional string theory. We derive an effective Lagrangian describing the low energy degrees of freedom including the Goldstone mode associated with the spontaneously broken R-symmetry when a gaugino condensate forms. We show the equivalence between our approach and those previously used for studying gaugino condensate in 4D string theory but we also show the need to include quantum effects due to the strong coupling constant in the hidden sector. We determine the vacuum structure of the complete scalar potential and show that supersymmetry is broken and a large mass hierarchy may develop with a single gaugino condensate. Realistic phenomenological values for the gauge coupling constant, unification scale and soft supersymmetric breaking terms can be obtained. Consistency with the minimal supersymmetric extension of the standard model requires the hidden gauge group to be SU(6) or SO(9).
12
Witkowski, Lukas Thomas. "Sequestering of Kähler moduli in type IIB string theory." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:6f5c8a99-26ca-401b-ad42-7bd3bf873f80.
Abstract:
In this thesis we employ string perturbation theory in toroidal orbifold models to study aspects of supersymmetry breaking in type IIB string theory. First, we determine the dependence of physical Yukawa couplings on blow-up moduli in models with D3-branes at orbifold singularities. Blow-up moduli are scalar fields describing the size of small blow-up cycles in the compactification geometry. In models implementing moduli stabilisation these fields can acquire F-terms and break supersymmetry. We examine the moduli-dependence of physical Yukawa couplings at string tree-level by computing disk correlation functions involving a Yukawa interaction of visible sector fields and an arbitrary number of blow-up moduli. We perform the calculation for one blow-up insertion explicitly and find that the correlation function vanishes if the blow-up modulus is associated with a small cycle distant to the visible sector. For more than one blow-up insertion we show that all such correlation functions are exponentially suppressed by the compactification volume. We explain how these results are relevant to suppressing soft terms to scales parametrically below the gravitino mass. Further, we determine corrections to holomorphic Yukawa couplings on D3-branes at an orbifold singularity due to non-perturbative effects such as gaugino condensation on a stack of D7-branes. This can be done by calculating a one-loop threshold correction to the gauge coupling on the D7-branes. We show that, if present, the new contributions to Yukawa couplings are not aligned with the tree-level couplings. As the new Yukawa couplings contribute to soft A-terms they are sources of flavour-changing neutral currents. Last we discuss an effect unrelated to supersymmetry breaking. We show that orbifold models with D3-branes at orbifold singularities can exhibit kinetic mixing of different massless Abelian factors. For this to be possible, the relevant U(1) factors have to be associated with more than one orbifold singularity.
13
Shaikh, Zain U. "Some mathematical structures arising in string theory." Thesis, University of Aberdeen, 2010. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=158375.
Abstract:
This thesis is concerned with mathematical interpretations of some recent develop- ments in string theory. All theories are considered before quantisation. The rst half of the thesis investigates a large class of Lagrangians, L, that arise in the physics literature. Noether's famous theorem says that under certain conditions there is a bijective correspondence between the symmetries of L and the \conserved currents" or integrals of motion. The space of integrals of motion form a sheaf and has a bilinear bracket operation. We show that there is a canonical sheaf d1;0 J1( ) that contains a representation of the higher Dorfman bracket. This is the rst step to de ne a Courant algebroid structure on this sheaf. We discuss the existence of this structure proving that, for a re ned de nition, we have the necessary components. The pure spinor formalism of string theory involves the addition of the algebra of pure spinors to the data of the superstring. This algebra is a Koszul algebra and, for physicists, Koszul duality is string/gauge duality. Motivated by this, we investigate the intimate relationship between a commutative Koszul algebra A and its graded Lie superalgebra Koszul dual to A, U(g) = A!. Classically, this means we obtain the algebra of syzygies AS from the cohomology of a Lie subalgebra of g. We prove H (g 2;C) ' AS again and extend it to the notion of k-syzygies, which we de ne as H (g k;C). In particular, we show that H B er(A) ' H (g 3;C), where H Ber(A) is the Berkovits cohomology of A.
14
Rayner, David Andrew James. "Type I string phenomenology and extra dimensional models." Thesis, University of Southampton, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.249946.
15
Mohammedi, N. "Non-linear sigma models and string effective actions." Thesis, University of Southampton, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383437.
16
Constantin, Andrei. "Heterotic string models on smooth Calabi-Yau threefolds." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:30be3aee-ba9b-4417-9b00-ee26a6bd67c5.
Abstract:
This thesis contributes with a number of topics to the subject of string compactifications, especially in the instance of the E8 × E8 heterotic string theory compactified on smooth Calabi-Yau threefolds. In the first half of the work, I discuss the Hodge plot associated with Calabi-Yau threefolds that are hypersurfaces in toric varieties. The intricate structure of this plot is explained by the existence of certain webs of elliptic-K3 fibrations, whose mirror images are also elliptic-K3 fibrations. Such manifolds arise from reflexive polytopes that can be cut into two parts along slices corresponding to the K3 fiber. Any two half-polytopes over a given slice can be combined into a reflexive polytope. This fact, together with a remarkable relation on the additivity of Hodge numbers, give to the Hodge plot the appearance of a fractal. Moving on, I discuss a different type of web of manifolds, by looking at smooth Z3-quotients of Calabi-Yau three-folds realised as complete intersections in products of projective spaces. Non-simply connected Calabi-Yau three-folds provide an essential ingredient in heterotic string compactifications. Such manifolds are rare in the classical constructions, but they can be obtained as quotients of homotopically trivial Calabi-Yau three-folds by free actions of finite groups. Many of these quotients are connected by conifold transitions. In the second half of the work, I explore an algorithmic approach to constructing E8 × E8 heterotic compactifications using holomorphic and poly-stable sums of line bundles over complete intersection Calabi-Yau three-folds that admit freely acting discrete symmetries. Such Abelian bundles lead to N = 1 supersymmetric GUT theories with gauge group SU(5) × U(4) and matter fields in the 10, ⁻10, ⁻5, 5 and 1 representations of SU(5). The extra U(1) symmetries are generically Green-Schwarz anomalous and, as such, they survive in the low energy theory only as global symmetries. These, in turn, constrain the low energy theory and in many cases forbid the existence of undesired operators, such as dimension four or five proton decay operators. The line bundle construction allows for a systematic computer search resulting in a plethora of models with the exact matter spectrum of the Minimally Supersymmetric Standard Model, one or more pairs of Higgs doublets and no exotic fields charged under the Standard Model group. In the last part of the thesis I focus on the case study of a Calabi-Yau hypersurface embedded in a product of four CP1 spaces, referred to as the tetraquadric manifold. I address the question of the finiteness of the class of consistent and physically viable line bundle models constructed on this manifold. Line bundle sums are part of a moduli space of non-Abelian bundles and they provide an accessible window into this moduli space. I explore the moduli space of heterotic compactifications on the tetraquadric hypersurface around a locus where the vector bundle splits as a direct sum of line bundles, using the monad construction. The monad construction provides a description of poly-stable S(U(4) × U(1))–bundles leading to GUT models with the correct field content in order to induce standard-like models. These deformations represent a class of consistent non-Abelian models that has co-dimension one in Kähler moduli space.
17
Elghozi, Thomas André Claude. "Astrophysical and cosmological consequences of string-inspired models." Thesis, King's College London (University of London), 2017. https://kclpure.kcl.ac.uk/portal/en/theses/astrophysical-and-cosmological-consequences-of-stringinspired-models(da218836-7682-4995-9410-58738236fde3).html.
Abstract:
The standard model of cosmology, namely the CDM model, is based on Einstein's theory of General Relativity (GR) with a Cold Dark Matter (CDM) content and a positive cosmological constant , in addition to ordinary matter and radiation components. While it provides a paradigm in very good agreement with many observations, from Big Bang Nucleosynthesis (BBN) to Cosmic Microwave Background (CMB), several questions remain open and various theoretical extensions seem necessary in order to address them. An extensively studied ingredient of the CDM model is the inflationary scenario, which solves some of the issues associated with the initial conditions that the original hot Big Bang model cannot address, such as the homogeneity and flatness problems. Furthermore, it ts very well with current data, in particular, the spectrum of temperature anisotropies in the CMB. As we recall in Chapter 2, in some scenarii, the end of inflation may lead to the formation of Cosmic Strings (CS) or Cosmic SuperStrings (CSS), which can have a significant impact on some observables, even though they have been proven not to be the main source of CMB anisotropies. We focus on a particular phenomenological consequence of C(S)S, Gravitational Waves (GWs), which are becoming an important tool to gather new information on our universe. More specifically, energetic high frequency GW Bursts (GWB) are thought to be emitted by cusps, which are points on C(S)S temporarily reaching the speed of light. We investigate the occurrence of such phenomena in a particular setup where a light string is stretched between two heavy, almost fixed strings, as could appear in a C(S)S network. First, an analytical study allows us to draw simplifying hypotheses, such as the periodicity of the non-interacting movement of the string, and yields an effective rule to identify cuspy strings. In addition, we implement these assumptions in a numerical simulation, which settles the free parameter of this criterion. Also, the string and the network parameters are found to influence strongly the average number of cusps and thus the amount of energy released in the form of GWB. In particular, both the analytical and numerical studies demonstrate that the smaller the correlation length is (that is, the wavier the string is), the more cusps the string holds. String/M-theory yields a large variety of scenarii and thus a large phenomenological diversity, from inflation to Dark Matter (DM) candidates. It generally implies additional dimensions and additional ingredients, such as scalar fields (often involved with inflation) or extended objects (such as Cosmic SuperStrings). It can also provide a description of our universe, on which we focus in Chapter 3, in which all fields but the graviton live on a (3+1) brane, itself embedded in a larger-dimensional bulk. We consider a model where the bulk is populated with a gas of punctual, effectively 0-dimensional defects, which interact with our brane universe. Their collisions with open strings attached to the brane generate a recoil velocity of such D0-branes, later called D-particles. This additional vector eld acts as a new content of the universe, which from the low energy point of view behaves as a Dark Matter/Dark Energy (DE) mixture. The modifications of the graviton equations of motion are related to its squared field strength, which under certain circumstances condensate and plays the r^ole of an extra scalar field. This model, called the D-material universe, can not only give a mechanism for the growth of large scale structure but, as we show here, can also lead to a successful inflationary scenario, the condensate appearing as the slowly rolling inflaton. Moreover, it provides an effective DM fluid which fits restricted - by our model's hypotheses - lensing data, thus diminishing the need of conventional DM without overclosing the universe. Finally, this supplementary ingredient alters the graviton propagation as it brings in an effective mass term and affects the refractive index experienced by radiations. This study, which spans several cosmological eras and covers several length scales, leads to constraints on the free parameters of the model including the number density of D-particles and the string scale. Such analyses of models beyond the CDM model may provide important information | alternative exploration routes as well as additional possible bounds on the parameters | that would help us understand the dynamics of our universe.
18
Mehta, Viraf M. "Light U(1)s in heterotic string models." Thesis, University of Liverpool, 2013. http://livrepository.liverpool.ac.uk/14833/.
Abstract:
We present the construction of heterotic string models built using the free fermionic formulation, and focus on how additional U(1)s may arise. We motivate an anomaly free combination, U(1)_\zeta, as a proton lifetime preserving symmetry external to a left--right symmetric gauge group. This same combination is found to nullify lepton number in U(1)_{B-L} to form a leptophobic combination, also in left--right symmetric models, which we compare to other leptophobic U(1) combinations constructed in the context of different gauge groups. We then accommodate U(1)_\zeta as a proton lifeguard symmetry in an effective field theory. We present a comparative study of how gauge coupling unification constraints may be satisfied when SO(10)xU(1)_\zeta\not\subset E_6 and when the U(1)_\zeta charges do have an E_6 embedding. We show that without such an embedding, current values of \sin^2\theta_W(M_Z) and \alpha_3(M_Z) rule these models out. We go on to discuss how viable string models with this property included may be constructed.
19
Muia, Francesco <1987>. "Sequestered String Models: Supersymmetry Breaking and Cosmological Applications." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amsdottorato.unibo.it/7274/1/Muia_Francesco_Tesi.pdf.
Abstract:
In the present thesis I focused on the study of the phenomenology arising from a class of string models called sequestered compactifications, which were born with the aim of getting low-energy SUSY from strings. This is not an easy task if combined with cosmological constraints, since the mechanism of moduli stabilization fixes both the scale of supersymmetric particles and the scale of moduli, which tend to be of the same order. However, if on the one hand supersymmetric particles with TeV mass are desired in order to address the electroweak hierarchy problem, on the
other hand the cosmological moduli problem requires the moduli to be heavier than 100 TeV. The specific setup of sequestered compactifications makes this hierarchy
achievable, at least in principle: as in these models the visible sector is located on a stack of D3-branes at singularities, a physical separation between the visible degrees of freedom and the SUSY-breaking sources takes place. Such decoupling translates into a hierarchy between the scale of SUSY-breaking and the spectrum of supersymmetric particles. Interestingly, moduli are the four-dimensional manifestation of the existence of extra-dimensions. Since they are only gravitationally coupled,
they could decay late in the history of the universe, affecting in a significant way its cosmological evolution. Possible deviations of the cosmological observables from
the values predicted by the standard Hot Big Bang Theory constitute an interesting alternative for the discovery of new physics beyond the Standard Model, which is complementary to the particle physics search. For this reason in addition to SUSY-breaking in sequestered models, I also studied several cosmological scenarios arising
from them, such as production of non-thermal dark matter and dark radiation, reheating from moduli decay and inflation.
20
Muia, Francesco <1987>. "Sequestered String Models: Supersymmetry Breaking and Cosmological Applications." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amsdottorato.unibo.it/7274/.
Abstract:
In the present thesis I focused on the study of the phenomenology arising from a class of string models called sequestered compactifications, which were born with the aim of getting low-energy SUSY from strings. This is not an easy task if combined with cosmological constraints, since the mechanism of moduli stabilization fixes both the scale of supersymmetric particles and the scale of moduli, which tend to be of the same order. However, if on the one hand supersymmetric particles with TeV mass are desired in order to address the electroweak hierarchy problem, on the
other hand the cosmological moduli problem requires the moduli to be heavier than 100 TeV. The specific setup of sequestered compactifications makes this hierarchy
achievable, at least in principle: as in these models the visible sector is located on a stack of D3-branes at singularities, a physical separation between the visible degrees of freedom and the SUSY-breaking sources takes place. Such decoupling translates into a hierarchy between the scale of SUSY-breaking and the spectrum of supersymmetric particles. Interestingly, moduli are the four-dimensional manifestation of the existence of extra-dimensions. Since they are only gravitationally coupled,
they could decay late in the history of the universe, affecting in a significant way its cosmological evolution. Possible deviations of the cosmological observables from
the values predicted by the standard Hot Big Bang Theory constitute an interesting alternative for the discovery of new physics beyond the Standard Model, which is complementary to the particle physics search. For this reason in addition to SUSY-breaking in sequestered models, I also studied several cosmological scenarios arising
from them, such as production of non-thermal dark matter and dark radiation, reheating from moduli decay and inflation.
21
De, Martino Danilo. "String inflationary models with non-monotonic slow-roll and detectable tensor modes." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amslaurea.unibo.it/8391/.
Abstract:
The first chapter of this work has the aim to provide a brief overview of the history of our Universe, in the context of string theory and considering inflation as its possible application to cosmological problems. We then discuss type IIB string compactifications, introducing the study of the inflaton, a scalar field candidated to describe the inflation theory. The Large Volume Scenario (LVS) is studied in the second chapter paying particular attention to the stabilisation of the Kähler moduli which are four-dimensional gravitationally coupled scalar fields which parameterise the size of the extra dimensions. Moduli stabilisation is the process through which these particles acquire a mass and can become promising inflaton candidates. The third chapter is devoted to the study of Fibre Inflation which is an interesting inflationary model derived within the context of LVS compactifications. The fourth chapter tries to extend the zone of slow-roll of the scalar potential by taking larger values of the field φ. Everything is done with the purpose of studying in detail deviations of the cosmological observables, which can better reproduce current experimental data. Finally, we present a slight modification of Fibre Inflation based on a different compactification manifold. This new model produces larger tensor modes with a spectral index in good agreement with the date released in February 2015 by the Planck satellite.
22
Anton, Fernando. "Half-string oscillator approach to closed string field theory and superconformal minimal models on the torus." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335904.
23
Ghosh, Rhea. "On intersecting 2-brane solutions in Type IIA supergravity theory /." Connect to online version, 2005. http://ada.mtholyoke.edu/setr/websrc/pdfs/www/2005/123.pdf.
24
Mayes, Van Eric. "Phenomenology of heterotic and type II orientifold string models." [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1597.
25
Cotellucci, Alessandro. "Flavour constraints on sequestered supersymmetry breaking in string models." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amslaurea.unibo.it/21709/.
Abstract:
The Standard Model (SM) is the best theory which describes Nature at the fundamental level. Even if it provides very accurate predictions, the SM features some open problems. A promising extension of the SM is the Minimal Supersymmetric Standard Model (MSSM) provided by supersymmetry. The MSSM can provide solutions for some of the open problems of the SM but it can suffer from the flavour supersymmetric problem associated with the presence of large Flavour Changing Neutral Currents (FCNCs). The flavour supersymmetric problem is related to the mechanism which mediates supersymmetry breaking from the hidden to the visible MSSM sector. A popular supersymmetry breaking mediation mechanism is gravity mediation but generic gravity mediated models lead to large FCNCs. The conditions to avoid FCNCs in gravity mediated models are called mirror mediation. These conditions can naturally be satisfied in 4D string compactifications. 4D string models introduce new scalar particles, the moduli, which interact gravitationally with matter and represent natural candidates to mediate supersymmetry breaking to the MSSM. Promising 4D string scenarios are sequestered models where the MSSM lives on branes at singularities and the visible sector is sequestered from the sources of supersymmetry breaking in the bulk of the extra dimensions. So one can realise low-energy supersymmetry and all moduli can be heavy enough to avoid any cosmological moduli problem. In this thesis we shall focus on 4D sequestered string scenarios and determine which models can reproduce mirror mediation without the production of large FCNCs. We will find two different classes of sequestered models where only one can be compatible with present flavour constraints on FCNCs. This comparison with observations will provide information on important details of the microscopic theory like the functional dependence on the extra-dimensional volume of the physical Yukawa couplings and the Kähler potential for matter fields.
26
Allen, Robin William. "Topics in non-linear sigma models and string theory." Thesis, University of Liverpool, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.232940.
27
Kagan, David. "Integrability and string theory : σ-models and spin chains." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613236.
28
Athanasopoulos, P. "Relations in the space of (2,0) heterotic string models." Thesis, University of Liverpool, 2016. http://livrepository.liverpool.ac.uk/3003839/.
Abstract:
Understanding better the landscape of string models and eventually finding, if possible, a dynamical way to select among them is one of the most interesting, open problems in string theory. In this thesis, we investigate aspects of the heterotic landscape and discuss relations among large classes of vacua. The first part of the thesis is devoted to the equivalence between free fermionic models and orbifolds. Free fermionic models and symmetric heterotic toroidal orbifolds both constitute exact backgrounds that can be used effectively for phenomenological explorations within string theory. It is widely believed that for Z_2xZ_2 orbifolds the two descriptions should be equivalent, but a detailed dictionary between both formulations was lacking. A detailed account of how the input data of both descriptions can be related to each other can be found in this thesis. In particular, we show that the generalized GSO phases of the free fermionic model correspond to generalized torsion phases used in orbifold model building. We illustrate our translation methods by providing free fermionic realizations for all Z_2xZ_2 orbifold geometries in six dimensions. In the second half of the thesis, we turn our attention to a novel idea called spinor-vector duality. In its original form, spinor-vector duality was limited to Z_2 structures. Here, we use the language of simple currents to generalize this idea to theories with arbitrary internal RCFTs. We also elucidate the underlying spectral flow structure. Even though the spectral flow has been traditionally used to relate states within a single model, we offer a new way to look at it, allowing relations between different models. Contrary to the equivalence between free fermionic models and orbifolds, many of the models related by the spectral flow are not physically equivalent. Nevertheless, the The idea of grouping together models into families according to the spectral flow orbit is quite important: the spectra of the models, though not identical, are related and we can make statements about models in the entire family by examining one representative. The grouping also offers a conceptual handle, acting as an organization principle in a vast landscape of models.
29
Matsuo, Yoshinori. "Nonperturbative effects in noncritical string theory and matrix models." 京都大学 (Kyoto University), 2007. http://hdl.handle.net/2433/136767.
30
Sakamoto, Junichi. "Integrable deformations of string sigma models and generalized supergravity." Kyoto University, 2019. http://hdl.handle.net/2433/242599.
31
Bedoya, Oscar Andrés Bedoya. "Invariância conforme no modelo sigma da corda/." São Paulo, 2004. http://hdl.handle.net/11449/132536.
Abstract:
Orientador: Nathan Jacob BerkovitsBanca: Victor de Oliveira Rivelles
Banca: Galen Mihaylov Sotkov
Resumo: Será feita uma revisão acerca das simetrias da teoria de corda bosônica e suas anomalias. Este estudo será feito tanto para o caso de cordas movendo-se num espaço-tempo tipo Minkowski, como no caso de cordas acopladas a campos tomando valores no espaço-tempo. Usando teoria de perturbações, será calculada a condição de cancelamento de anomalia a 1-loop. Esta condição estabelecerá vinculos interessantes entre os campos do espaço tempo através de uma equação do tipo da equação de Einstein. Finalmente, será mostrado como calcular condições de cancelamento de anomalia a ordens mais altas em teoria de perturbações usando métodos do grupo de renormalização
Abstract: A review will be made of the symmetries and the anomalies of the bosonic string. It will be considered the case of a string moving in a Minkowskian space- time and also, a string coupled to fields in space-time. Perturbation theory will allow calculation at the one-loop levei of the condition under which the anomaly vanishes. This condition will establish interesting constraints on the background fields since they obey an equation of the Einstein type. Finally, it will be shown how to calculate conditions for anomaly vanishing beyond the one-loop order using renormalization group methods
Mestre
32
Barone, Alessandro. "The cosmological moduli problem in multi-field string inflationary models." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amslaurea.unibo.it/18080/.
Abstract:
The 4D low-energy limit of string compactifications is characterised by the ubiquitous presence of string moduli which are new gravitationally coupled scalar fields which develop mass via supersymmetry breaking effects. During inflation it is generically expected that these fields receive large contributions to their mass of order the Hubble constant. They are therefore shifted from their minimum and after the end of inflation start oscillating around it behaving as non-relativistic matter. Given that matter redshifts slower than radiation, the moduli quickly come to dominate the energy density of the universe. Hence, when they decay, they would dilute anything that has been produced before. It is thus crucial to require that the moduli decay before Big-bang nucleosynthesis in order to preserve the successful prediction for the abundances of the light elements. This condition sets a lower bound on their masses of order 50 TeV. This potential problem goes under the name of “cosmological moduli problem”. In this thesis, we shall study this problem in promising multi-field inflationary models which naturally emerge in type IIB compactifications. In particular, we will explore if the presence of a large number of spectator fields during inflation can reduce the initial misalignment of the moduli. We shall also explore if the dynamics of the system can forbid a period of moduli domination after the end of inflation.
33
Okada, Takashi. "Matrix Models in String and M-theory and Exact Results." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/175112.
34
Mota, Luis Antonio C. P. da. "Symmetry breaking and masses in supersymmetric and compactified string models." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334814.
35
Nooij, Sander E. M. "Classification of the chiral Zâ‚‚ x Zâ‚‚ heterotic string models." Thesis, University of Oxford, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409178.
36
Dawson, Peter Dale. "Effective actions and charges of D-branes in curved space-time." Title page, abstract and contents only, 2003. http://web4.library.adelaide.edu.au/theses/09PH/09phd2727.pdf.
Abstract:
Bibliography: leaves 181-190. This thesis firstly investigates K-invariant and supersymmetric actions of D-branes in curved space-time. Following this, research into charges of D-branes in a group manifold are studied. In particular, the charge groups are determined for the symmetry preserving (or untwisted) D-branes on a compact, simple, connected, simply connected group manifold. The purpose of this research is to determine these charge groups in order that they can be compared to the charge groups predicted by twisted K-theory for D-branes in a group manifold, thus providing a future important check to the theorem that D-brane charges are determined by twisted K-theory, one of the most important recent ideas in string theory.
37
Foakes, A. P. "Three loop calculations of #beta#-functions for string associated non-linear sigma models." Thesis, University of Southampton, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.234191.
38
Blair, Christopher David Andrew. "Duality and extended geometry in string theory and M-theory." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709364.
39
Jusinskas, Renann Lipinski [UNESP]. "Exploring the properties of the pure spinor b ghost." Universidade Estadual Paulista (UNESP), 2014. http://hdl.handle.net/11449/123696.
Abstract:
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000826754.pdf: 1966505 bytes, checksum: 8923fad36f756ab17c9bb0de6d0871a9 (MD5)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Esta tese é baseada em parte do meu trabalho de doutoramento e tem como objetivo apresentar uma análise detalhada de algumas propriedades recém abordadas do fantasma b composto no formalismo de espinores puros. Primeiramente será feita uma revisão dos formalismos mínimo e não-mínimo. Em seguida, será apresentada a construção do fantasma b passo a passo, inlcuindo correções quânticas. Por fim, serão estudadas em detalhes suas propriedades fundamentais, ue vão desde a nilpotência até a definição de um possível conjugado, o fantasma c
This thesis is based in part of my work during the Ph.D. and aims to present a detailed analysis of some newly studied properties of the composite non-minimal pure spinor b ghost. First, a review of the minimal and non-minimal pure spinor formalisms will be presented. Then, the construction of the non-minimal b ghost will be done step-by-step, including quantum corrections. Finally, some of its fundamental properties will be studied i detail, ranging from nilpotency until the definition of a possible canonical conjugate, the c ghost
FAPESP: 2009/17516-4
40
Jusinskas, R. L. (Renann Lipinski). "Exploring the properties of the pure spinor b ghost /." São Paulo, 2014. http://hdl.handle.net/11449/123696.
Abstract:
Orientador: Nathan Jacob BerkovitsBanca: Horatiu Nastase
Banca: Andrei Mikhailov
Banca: Victor de Oliveira Rivelles
Banca: Vladimir Perchin
Resumo: Esta tese é baseada em parte do meu trabalho de doutoramento e tem como objetivo apresentar uma análise detalhada de algumas propriedades recém abordadas do fantasma b composto no formalismo de espinores puros. Primeiramente será feita uma revisão dos formalismos mínimo e não-mínimo. Em seguida, será apresentada a construção do fantasma b passo a passo, inlcuindo correções quânticas. Por fim, serão estudadas em detalhes suas propriedades fundamentais, ue vão desde a nilpotência até a definição de um possível conjugado, o fantasma c
Abstract: This thesis is based in part of my work during the Ph.D. and aims to present a detailed analysis of some newly studied properties of the composite non-minimal pure spinor b ghost. First, a review of the minimal and non-minimal pure spinor formalisms will be presented. Then, the construction of the non-minimal b ghost will be done step-by-step, including quantum corrections. Finally, some of its fundamental properties will be studied i detail, ranging from nilpotency until the definition of a possible canonical conjugate, the c ghost
Doutor
41
De, Notariis Kevin. "Light hyperweak new gauge bosons from kinetic mixing in string models." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amslaurea.unibo.it/19491/.
Abstract:
String theory is at the moment our best candidate for a unified quantum theory of gravity, aiming to reconcile all the known (and unknown) interactions with gravity as well as provide insights for currently mysterious phenomena that the Standard Model and the modern Cosmology are not able to explain. In fact, it is believed that most of the problems associated to the Standard Model can indeed be resolved in string theory. Supersymmetry is supposed to be an elegant solution to the Hierarchy problem (even though more and more stringent bounds in this direction are being placed by the fact that we have been unable to experimentally find supersymmetry yet), while all the axions that compactifications bring into play can be used to resolve the strong CP problem as well as provide good candidates for Dark Matter. Inflationary models can also be constructed in string theory, providing, then, the most diffused solution to the Horizon problem. This work, in particular, is formulated in type IIB string theory compactified on an orientifolded Calabi-Yau three-fold in LARGE Volume Scenario (LVS) and focuses on the stabilisation of all the moduli in play compatible with the construction of a hidden gauge sector whose gauge boson kinetically mixes to the visible sector U(1), acquiring a mass via a completely stringy process resulting in the St{\"u}ckelberg mechanism. The "compatibility" regards the fact that certain experimental bounds should be respected combined with recent data extrapolated by Coherent Elastic Neutrino-Nucleus Scattering (CE$\nu$NS) events at the Spallation Neutron Source at Oak Ridge National Laboratory. We are going to see that in this context we will be able to fix all the moduli as well as present a brane and fluxes set-up reproducing the correct mass and coupling of the hidden gauge boson. We also get a TeV scale supersymmetry, since the gravitino in this model will be of order O(TeV), with an uplifted vacuum to reproduce a de Sitter universe as well.
42
Wilshin, Simon. "Identifiable branes and their realisation in toy models of string theory." Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534180.
43
Görlich, Lars. "N = 1 and non-supersymmetric open string theories in six and four space-time dimensions." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2003. http://dx.doi.org/10.18452/14978.
Abstract:
Die vorliegende Arbeit beinhaltet ein einführendes Kapitel über Orbifold-Konstruktionen in dem neben rudimentären Grundlagen bereits speziellere Themen wie Diskrete Torsion und asymmetrische Orbifold-Gruppen behandelt werden. Als Beispiele für Orbifolde werden Kompaktifizierungen auf Tori sowie das asymmetrische T^4/Z(3)^L x Z(3)^R Orbifold behandelt. Danach wird eine allgemein gehaltene Einführung in Orientifolde gegeben, einschließlich des offenen String Sektors samt Chan-Paton Freiheitsgraden. Die darauf folgenden Kapitel 4-7 behandeln von mir durchgeführte Forschungsarbeiten. Kapitel 4 beschäftigt sich mit der Quantisierung des offenen Strings mit linearen Randbedingungen, wie sie bei Strings in elektro-magnetischen Feldern auftreten. Weiterhin wird die Quantisierung der Null- und Impuls-Moden des offenen Strings in Torus-Kompaktifizierungen durchgeführt. Außerdem wird für den Fall allgemeiner konstanter Hintergrund Neveu-Schwarz U(1)-Hintergrundfelder der Kommutator der Stringkoordinaten berechnet. Dieser stützt bisherige Resultate zur Nicht-Kommutativität von offenen Stringtheorien in Neveu-Schwarz Hintergründen. Kapitel 5 gibt, zusammen mit einigen neuen Erkenntnissen, Resultate von [1] über asymmetrische Orientifolde, insbesondere deren D-Branen Inhalt wieder. Kapitel 6 faßt die Veröffentlichung [2] zusammen, in der untersucht wurde, inwieweit sich phänomenolgisch interessante Modelle in Orientifolden von Torus-Kompaktifizierungen finden lassen. Insbesondere tragen die D9-Branen magnetische Flüsse, womit chirale Fermionen im Spektrum auftreten. Die Rechnungen werden größtenteils im gleichwertigen, T-dualen Bild ausgeführt. In diesem ist die Anzahl der chiralen Fermionen durch die topologische Schnittzahl der D-Branen gegeben. Existieren auf Torus-Kompaktifizierungen entweder nur nicht-chirale oder nicht-supersymmetrische Modelle, so lassen sich auf gewissen Orbifolden beide Eigenschaften miteinander vereinbaren. Kapitel 7 behandelt das "sigma Omega"-Orientifold auf einem T^6/Z(4) Orbifold. Als besonders interessantes Beispiel wird ein supersymmetrisches U(4) x U(2)^3_L x U(2)^3_R Modell vorgestellt, daß durch Einschalten geeigneter Hintergrundfelder in der effektiven Niederenergie-Wirkung auf ein Modell gebrochen wird, daß dem MSSM (minimalem supersymmetrischen Standard Modell) sehr ähnlich ist. Dieses Kapitel basiert auf unserer Publikation [3]. Ferner ist der Arbeit ein Anhang beigefügt, der einige der verwendeten Formeln sowie Beweise zu zwei Sätzen enthält, die im Text verwendet wurden.This thesis contains an introductory chapter on orbifolds. Besides rudimentary basics we discuss more advanced topics like discrete torsion and asymmetric orbifold groups. As examples we investigate torus compactifications and an asymmetric T^4/Z(3)^L x Z(3)^R orbifold. The following chapter explains the foundations of orientifolds, including open strings with Chan-Paton degrees of freedom. Chapters 4-7 present own research. In chapter 4 we quantize open strings with linear boundary conditions, as they show up in electro-magnetic fields. We quantize the zero- and momentum-modes for toroidal compactifications, too. As an application we calculate the commutator of the coordinate fields in the case of general constant Neveu-Schwarz U(1)-field strengths. Thereby we confirm previous results on non-commutativity of open string theories in Neveu-Schwarz backgrounds. Chapter 5 reviews the results of a former publication [1] on asymmetric orientifolds, supplemented by some recent insights in connection with the preceeding chapter. Chapter 6 is a summary of [2]. In this publication we investigated to what extend one can build phenomenologically interesting models from toroidal orientifolds. By turning on magnetic fluxes on D9-branes we induce chiral fermions. Most calculations are performed in an (equivalent) T-dual picture. Here the number of chiral fermions is given by the topological intersection number of D-branes. In orientifolds of toroidal compactifications one obtains either non-chiral or non-supersymmetric orientifold solutions. However both properties can be reconciled in orientifolds that are obtained from specific supersymmetric orbifold compactifications. In chapter 7 we present the "sigma Omega"-Orientifold on a T^6/Z(4) orbifold. As a very attractive example we investigate a supersymmetric U(4) x U(2)^3_L x U(2)^3_R model that is broken to an MSSM-like model by switching on suitable background fields in the low energy effective action. This chapter is based on our publication [3]. The thesis is supplemented by an appendix with formulas applied in the text, as well as proofs to two theorems that were used as well.
44
Krippendorf, Sven-Ludwig. "D-brane model building and moduli stabilisation in type IIB string theory." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609632.
45
Barnaby, Neil. "Cosmological instabilities." Thesis, McGill University, 2007. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=103365.
Abstract:
Though historically the word "tachyon" has been used to describe hypothetical particles which propagate faster than the speed of light, in a more modern context the term has been recycled to refer to certain unstable states in field theory. This thesis explores the role of tachyonic instabilities in cosmology considering tachyons which arise in string theory and also more conventional, field theoretic instabilities. Our study of such instabilities is, in part, motivated by attempts to embed inflation into string theory. We will argue that the study of string theory models of inflation is well-motivated and may provide a rare potential observational window into string physics.After reviewing the necessary background material concerning inflation, cosmological perturbation theory and tachyonic instabilities we study in detail the dynamics of the tachyonic instability which marks the end of a particular string theory model of inflation, focusing on the processes of reheating and cosmic string production. We show that the peculiar dynamics of the open string tachyon leads to various novelties in these processes and consider also potential observational consequences.
We consider tachyonic preheating at the end of hybrid inflation in a conventional field theory setting and show that the preheating process can leave an observable imprint on the Cosmic Microwave Background, either through n = 4 contamination of the power spectrum or else through large nongaussian signatures. The possibility of large nongaussianity is particularly interesting since it demonstrates that hybrid inflation provides one of the few well-motivated models which can generate an observable nongaussian signature.
Finally, we study a novel string theoretic model of inflation, p-adic inflation. This model is nonlocal, however, it is free of the usual problems (such as ghosts) which plague nonlocal theories. Furthermore, the nonlocal structure of the theory leads to a variety of unexpected dynamics including the possiblity of a slowly rolling inflaton, despite an extremely steep potential.
46
Steinke, Ronald. "The derivation of an effective string theory from a field theory containing vortex solutions, and its application to Regge trajectories /." Thesis, Connect to this title online; UW restricted, 2000. http://hdl.handle.net/1773/9804.
47
Hoyle, Charles D. "Sub-millimeter tests of the gravitational inverse-square law /." Thesis, Connect to this title online; UW restricted, 2001. http://hdl.handle.net/1773/9782.
48
Walton, Mark 1960. "Two scale compactification of the E(8)xE(8) heterotic string." Thesis, McGill University, 1987. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75346.
Abstract:
A simple two scale compactification scheme for the E(8) x E(8) heterotic string is studied. The internal space used is a direct product of two compact spaces, each with its own length scale. Compactification on the smaller 4-dimensional (4d) manifold is carried out to obtain 6d theories with simple supersymmetry (SUSY). Assuming the background torsion vanishes, we show that this manifold must be K3. Compactification on K3 is studied in detail. Also analyzed are the two possible torsion-free compactifications on the orbifold K3$ sp prime$ (the limit of the manifold K3). The compactification from 6d to 4d on the larger scale 2d manifold results in Grand Unified Theories (GUT's) with broken SUSY. We show that it is not possible to generate a realistic theory using our scheme. Strings exclude what is conceivable from the perspective of point field theories: getting a realistic GUT from a 6d theory with simple SUSY.
49
Guidetti, Veronica. "Axion-like particles and the 3.5 keV line in 4D string models." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/10346/.
Abstract:
This work is focused on axions and axion like particles (ALPs) and their possible relation with the 3.55 keV photon line detected, in recent years, from galaxy clusters and other astrophysical objects. We focus on axions that come from string compactification and we study the vacuum structure of the resulting low energy 4D N=1 supergravity effective field theory.
We then provide a model which might explain the 3.55 keV line through the following processes.
A 7.1 keV dark matter axion decays in two light axions, which, in turn, are transformed into photons thanks to the Primakoff effect and the existence of a kinetic mixing between two U(1)s gauge symmetries belonging respectively to the hidden and the visible sector.
We present two models, the first one gives an outcome inconsistent with experimental data, while the second can yield the desired result.
50
Vescovi, Edoardo. "Perturbative and non-perturbative approaches to string sigma-models in AdS/CFT." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://dx.doi.org/10.18452/17612.
Abstract:
Diese Doktorarbeit behandelt quantentheoretische Aspekte von Typ II Superstringtheorien in AdS5xS5- und AdS4xCP3-Hintergründen für die AdS/CFT Korrespondenz. Dabei werden perturbative Methoden und Verfahren der Gitterfeldtheorie benutzt. Die Konstruktion des Supercoset-Sigma-Modell für Strings im AdS5xS5wird skizziert, während die generelle Quantendynamik des Superstring in AdS4xCP3 mit Hilfe einer dimensionaler Reduktion der Supermembrane-Wirkung in AdS4xS7 beschrieben wird. Ein manifest kovarianter Formalismus zur semi-klassischen Quantisierung von Strings um beliebige Minimalflächen in AdS5xS5 wird präsentiert, der durch die Darstellung der Fluktuationsoperatoren in Form von intrinsischen und extrinsischen Varianten der Hintergrundgeometrie erreicht wird. Eine Verallgemeinerung der Lame-Differentialgleichung zur 4. Ordnungwird mit Hilfe der Spektralmethode exakt gelöst. Dadurch konnten in 1-Schleifennäherung die Energie des String im SU(2)-Sektor, im Grenzfall beschrieben durch ein Quantum-Landau-Lifshitz-Modell, und der bosonische Beitrag zur Energie des in AdS5 und S5 rotierenden String bestimmt werden. In ähnlicher Weise erhält man in 1-Schleifennäherung den ¼-BPS-Latitude Wilson-Loop in N=4-SYM-Theorie normiert durch den ½-BPS-Loop. Das Regularisierungsschema reproduziert die Vorhersage aus supersymmetrischer Lokalisierung bis auf einen Rest, welcher diskutiert wird. Außerdem wird die Wirkung des AdS4xCP3-String, entwickelt um den null-cusp Hintergrund, studiert und die Cusp-Anomalie in 2-Schleifennäherung berechnet. Das Ergebnis stimmt mit dem vermuteten exakten Ausdruck, der ABJM-Interpolationsfunktion, überein. Schließlich wird die AdS5xS5-Superstringtheorie in AdS-Lichtkegel-Eichung diskretisiert und mit Monte-Carlo-Algorithmen Gittersimulationen bei endlicher Kopplung durchgeführt. Dabei werden die Stringwirkung, aus der mit Hilfe von AdS/CFT die null-cusp anomale Dimension von N=4 SYM extrahiert werden kann, und die Masse einer AdS Anregung bestimmt.This thesis discusses quantum aspects of type II superstring theories in AdS5xS5 and AdS4xCP3 backgrounds relevant for the AdS/CFT correspondence, using perturbative methods at large string tension and lattice field theory techniques inspired by a work of Roiban and McKeown. We review the construction of the supercoset sigma-model for strings in the AdS5xS5 background, whereas the general quantum dynamics of the superstring in AdS4xCP3 is described by a double dimensional reduction of the supermembrane action in AdS4xS7. We present a manifestly covariant formalism for semiclassical quantization of strings around arbitrary minimal-area surfaces in AdS5xS5, expressing the fluctuation operators in terms of intrinsic and extrinsic invariants of the background geometry. We exactly solve the spectral problem for a fourth-order generalization of the Lame'' differential equation with doubly periodic coefficients in a complex variable. This calculates the one-loop energy of the (J1,J2)-string in the SU(2) sector in the limit described by a quantum Landau-Lifshitz model and the bosonic contribution to the energy of the (S,J)-string rotating in AdS5 and S5. Similar techniques calculate the 1/4-BPS latitude Wilson loops in N=4 SYM theory at one loop, normalized to the 1/2-BPS circular loop. Our regularization scheme reproduces the next-to-leading order predicted by supersymmetric localization, up to a remainder function that we discuss upon. We also study the AdS4xCP3 string action expanded around the null cusp background and compute the cusp anomaly up to two loops. This agrees with an all-loop conjectured expression of the ABJM interpolating function. We finally discretize the AdS5xS5 superstring theory in the AdS light-cone gauge and perform lattice simulations at finite coupling with a Monte Carlo algorithm. We measure the string action, from which we extract the null cusp anomalous dimension of N=4 SYM as derived from AdS/CFT, as well as the mass of an AdS excitation.