Classification of symmetry breaking patterns in the theory of non-linear realizations

Symmetry is a crucial concept in physics. Many of the great revolutions in physics since the time of Newton were both based on principles of symmetry and led to new insights into the role of symmetry in physics. We may distinguish linearly realized symmetries from non-linearly realized symmetries, which lead to different consequences. Linearly realized symmetries determine what kind of particles we can expect to find in colliders and lead to important conserved quantities such as energy and momentum. Non-linearly realized symmetries lead to constraints on the dynamics of particles, but tell us nothing about how to classify fundamental particle states. The range of possibilities for linear symmetries was understood a long time ago by Coleman and Mandula. However, a classification of all possible non-linearly realized symmetries is lacking. In this thesis, we attempt to make progress in this direction. We focus on non-linear space-time symmetries in relativistic and supersymmetric field theories, using the theory of non-linear realizations. We are able to exhaustively classify so-called exceptional EFTs realized on the most commonly encountered particles of relativistic and supersymmetric physics

An Algebraic Classification of Exceptional EFTs

We classify four-dimensional effective field theories (EFTs) with enhanced soft limits, which arise due to non-linearly realised symmetries on the Goldstone modes of such theories. We present an algorithm for deriving all possible algebras that can be non-linearly realised on a set of Goldstone modes with canonical propagators, linearly realised Poincar\'{e} symmetries and interactions at weak coupling. We then perform a full classification of the cases with multiple scalars or multiple spin-$1/2$ fermions as the Goldstone modes. In each case there are only a small number of algebras consistent with field-dependent transformation rules, leading to the class of exceptional EFTs including the scalar sector of Dirac-Born-Infeld, Special Galileon and Volkov-Akulov theories. We also discuss the coupling of a $U(1)$ gauge vector to the exceptional scalar theories, showing that there is a Special Galileon version of the full Dirac-Born-Infeld theory. This paper is part I in a series of two papers, with the second providing an algebraic classification of supersymmetric theories

An Algebraic Classification of Exceptional EFTs Part II: Supersymmetry

We present a novel approach to classify supersymmetric effective field theories (EFTs) whose scattering amplitudes exhibit enhanced soft limits. These enhancements arise due to non-linearly realised symmetries on the Goldstone modes of such EFTs and we classify the algebras that these symmetries can form. Our main focus is on so-called exceptional algebras which lead to field-dependent transformation rules and EFTs with the maximum possible soft enhancement at a given derivative power counting. We adapt existing techniques for Poincar\'{e} invariant theories to the supersymmetric case, and introduce superspace inverse Higgs constraints as a method of reducing the number of Goldstone modes while maintaining all symmetries. Restricting to the case of a single Goldstone supermultiplet in four dimensions, we classify the exceptional algebras and EFTs for a chiral, Maxwell or real linear supermultiplet. Moreover, we show how our algebraic approach allows one to read off the soft weights of the different component fields from superspace inverse Higgs trees, which are the algebraic cousin of the on-shell soft data one provides to soft bootstrap EFTs using on-shell recursion. Our Lie-superalgebraic approach extends the results of on-shell methods and provides a complementary perspective on non-linear realisations

Internal Supersymmetry and Small-field Goldstini

The dynamics of the Goldstino mode of spontaneously broken supersymmetry is universal, being fully determined by the non-linearly realized symmetry. We investigate the small-field limit of this theory. This model non-linearly realizes an alternative supersymmetry algebra with vanishing anti-commutators between the fermionic generators, much like an internal supersymmetry. This Goldstino theory is akin to the Galilean scalar field theory that arises as the small-field limit of Dirac-Born-Infeld theory and non-linearly realizes the Galilean symmetry. Indeed, the small-field Goldstino is the partner of a complex Galilean scalar field under conventional supersymmetry. We close with the generalization to extended internal supersymmetry and a discussion of its higher-dimensional origin.Comment: 12 pages, v2: references added, typos corrected, discussion about a fermionic invariant improve

Moduli Backreaction on Inflationary Attractors

We investigate the interplay between moduli dynamics and inflation, focusing on the KKLT-scenario and cosmological $\alpha$-attractors. General couplings between these sectors can induce a significant backreaction and potentially destroy the inflationary regime; however, we demonstrate that this generically does not happen for $\alpha$-attractors. Depending on the details of the superpotential, the volume modulus can either be stable during the entire inflationary trajectory, or become tachyonic at some point and act as a waterfall field, resulting in a sudden end of inflation. In the latter case there is a universal supersymmetric minimum where the scalars end up, preventing the decompactification scenario. The gravitino mass is independent from the inflationary scale with no fine-tuning of the parameters. The observational predictions conform to the universal value of attractors, fully compatible with the Planck data, with possibly a capped number of e-folds due to the interplay with moduli.Comment: 23 pages, 13 figures. v2: minor clarifications and refs added. PRD versio

The Sixth Superstring

The objective of this thesis is to discuss the possible new superstring proposed by Savdeep Sethi in April 2013. In order to do this, we give an introduction to string theory pretty much from the ground up - starting with the 26 dimensional bosonic string and then on to the five different flavours of ten dimensional superstring. Along the way we will discuss some of the dualities and transformations that relate these strings to each other. Savdeep Sethi's superstring will arise from just such a transformation: an orientifold of the type IIB superstring. Eventually we will start to hone in on the modern picture of string theory, which is that the superstrings are all perturbative limits of an 11-dimensional theory called 'M-theory'. We will discuss how Savdeep Sethi's superstring may fit into this web of theories. By construction, the new string looks very similar to the Type I superstring. We will have to find a way to distinguish the two from each other. By comparing the Kaluza-Klein towers that result from their M-theory descriptions, we will find a sharp distinction between Type I and the new superstring. However, we will be left with questions about the consistency of the new string and about its place in the M-theory web.

Internal supersymmetry and small-field Goldstini

The dynamics of the Goldstino mode of spontaneously broken supersymmetry is universal, being fully determined by the non-linearly realized symmetry. We investigate the small-field limit of this theory. This model non-linearly realizes an alternative supersymmetry algebra with vanishing anti-commutators between the fermionic generators, much like an internal supersymmetry. This Goldstino theory is akin to the Galilean scalar field theory that arises as the small-field limit of Dirac-Born-Infeld theory and non-linearly realizes the Galilean symmetry. Indeed, the small-field Goldstino is the partner of a complex Galilean scalar field under conventional supersymmetry. We close with the generalization to extended internal supersymmetry and a discussion of its higher-dimensional origin

An algebraic classification of exceptional EFTs. Part II. Supersymmetry

We present a novel approach to classify supersymmetric effective field theories (EFTs) whose scattering amplitudes exhibit enhanced soft limits. These enhancements arise due to non-linearly realised symmetries on the Goldstone modes of such EFTs and we classify the algebras that these symmetries can form. Our main focus is on so-called exceptional algebras which lead to field-dependent transformation rules and EFTs with the maximum possible soft enhancement at a given derivative power counting. We adapt existing techniques for Poincare invariant theories to the supersymmetric case, and introduce superspace inverse Higgs constraints as a method of reducing the number of Goldstone modes while maintaining all symmetries. Restricting to the case of a single Goldstone supermultiplet in four dimensions, we classify the exceptional algebras and EFTs for a chiral, Maxwell or real linear supermultiplet. Moreover, we show how our algebraic approach allows one to read off the soft weights of the different component fields from superspace inverse Higgs trees, which are the algebraic cousin of the on-shell soft data one provides to soft bootstrap EFTs using on-shell recursion. Our Lie-superalgebraic approach extends the results of on-shell methods and provides a complementary perspective on non-linear realisations

An algebraic classification of exceptional EFTs

We classify four-dimensional effective field theories (EFTs) with enhanced soft limits, which arise due to non-linearly realised symmetries on the Goldstone modes of such theories. We present an algorithm for deriving all possible algebras that can be non-linearly realised on a set of Goldstone modes with canonical propagators, linearly realised Poincaré symmetries and interactions at weak coupling. An important ingredient in our analysis is inverse Higgs trees which systematically incorporate the requirements for the existence of inverse Higgs constraints. These are the algebraic cousin of the on-shell soft data one provides for soft bootstrapping EFTs. We perform full classifications for single scalar and multiple spin-1/2 fermion EFTs and present a thorough analysis for multiple scalars. In each case there are only a small number of algebras consistent with field-dependent transformation rules, leading to the class of exceptional EFTs including the scalar sector of Dirac-Born-Infeld, Special Galileon and Volkov-Akulov theories. We also discuss the coupling of a U(1) gauge vector to the exceptional scalar theories, showing that there is a Special Galileon version of the full Dirac-Born-Infeld theory. This paper is part I in a series of two papers, with the second providing an algebraic classification of supersymmetric theories with non-linearly realised symmetries