Young Theorists' Forum

Cosmic strings are a network of long, line-like objects stretching across space, that are likely to have formed in the early universe and may still exist today. Closed loops of cosmic string can break off from the original long strings. The loops will then begin to oscillate, emitting gravitational radiation signals that may be observable from earth. I shall discuss how we model the dynamics of cosmic string loops, and how their behaviour is affected when they move in different spacetime backgrounds, in turn affecting the signal we would detect. In particular, we look at their motion in spacetimes with extra dimensions, such as those predicted by string theory.

We show how the newly introduced formulation of gravity as a “Pure Connection” theory provides a natural framework for approaching the problem of graviton scattering. We review the latest developments in this novel approach and show how to compute scattering amplitudes using the so called "helicity methods." Furthermore we evaluate, in the new formulation, the MHV amplitude for a 4-gravitons scattering process and verify that it matches the known value coming from Einstein-Hilbert perturbative gravity.

One of the most fertile techniques in modern physics is that of dimensional reduction, which has matured greatly since its introduction by Kaluza, Klein, et al. at the beginning of the last century. It is now de rigueur for anyone working in string or M-theory as a way of reconciling the 10 or 11 spacetime dimensions required by these models to the 4 which most of us observe. I shall discuss some of the ways that one can use this technique and its reverse process, dimensional oxidation, to generate black objects in supergravity theories with diverse dimensionality and field content (of which black holes in GR are a familiar subclass).

For multi-field inflation, cosmic observables evolve during the super-horizon epoch and one interesting outcome of this is its capacity to generate large local non-Gaussianities. For a given inflationary model to become predictive it is necessary that one follows the evolution of observables until they become conserved. This may be possible if there exists and appropriate “adiabatic limit”. I will illustrate how this works in practice with numerous examples, graphs and give a basic cook-book for how to build your own model with large local non-Gaussianity, including showing the potential features needed in order to get positive or negative fNL.

There is currently a plethora of modified gravity and dark energy theories attempting to marry the observed apparent acceleration of the Universe with a gravitational theory. Most of these theories are constructed by "Lagrangian engineering", where explicit Lagrangian densities are written down, field equations computed and free parameters fitted or constrained by data. We will discuss a new framework where an effective gravitational action is written down which incorporates all possible theories with a particular field content; this is inspired by a particle physicists way of dealing with theories, where potentials are written down in generality under very few guiding symmetry considerations. The framework enables us to identify all possible free parameters which can be fit to data and produce observable predictions (at least in principle) -- this could allow us to immediately rule out entire classes of theories. We will give an overview of our formalism, in particular to our language and notation. We then proceed with some explicit examples: if the dark sector does not contain any extra fields, or if scalar fields are present.

The classical theory obtained in the low energy limit of Loop Quantum Gravity is notably sensitive to the "size" of the quanta of area and volume. Conformal Loop Quantum Gravity seeks to cure this pathology. In my talk, I was discuss the problem of extending the existing classical theory to a quantum theory by constructing a conformal loop representation. Tantalising links with twistor theory, geometric quantization and conformal geometry will be highlighted in particular.

The theory of inflation explains the appearance of the seeds of cosmic structure by the process of gravitational particle production. In string theory, inflation may be realised in the open string sector by the motion of D-branes, or in the closed string sector by the motion of light scalar fields that parameterise the geometry of the compact dimensions. A vector field naturally lives on the world-volume of the various D-branes which appear in these models, and in this talk I will discuss the role that this field may play in the cosmological evolution. In particular, the changing gravitational background induced by the inflaton may cause this vector field to undergo gravitational particle production, such that under suitable conditions, it may affect or generate the seeds of structure. This will give rise to interesting signatures in the spectrum and bispectrum of the curvature perturbation.

We propose a scheme to investigate whether motion degrades entanglement. We compute the degradation analytically for a Dirichlet scalar field in a Minkowski cavity for trajectories grafted from inertial and uniformly accelerated segments of small acceleration, giving explicit results for sample travel scenarios. Degradation of observable magnitude occurs for massless transverse quanta of optical wavelength at Earth gravity acceleration and for kaon mass quanta already at microgravity acceleration. We outline a space-based experiment for observing the effect and its gravitational counterpart.

To model physical systems it is often sensible to discretize space and time. Here we will take a look at quantum walks, which arise when the evolution operator for a single particle in discrete space-time is taken to be local and unitary. We will see that spin arises naturally and that in the continuum limit these particles behave like fermions.

One of the main challenges in Quantum Information is the reliable creation of sources of entanglement. Entanglement is a pivotal feature of Quantum Mechanics that allows us to make vast improvements in information processing over classical procedures. We show that when introducing a Relativistic "shaking" motion to Cavity QED, a strongly entangled two-mode squeezed state can be generated. This has strong links to the Dynamical Casimir Effect and could also be a way to test the equivalence principle of General Relativity using Quantum Field Theory.

Why study a theory which is patently wrong? What's wrong with good old Feynman diagrams? What even is a Momentum Twistor? These and several other questions you never thought to ask will be answered in this whistle-stop tour of scattering amplitudes in N=4 SYM. I will highlight some of the recent advances in understanding and uncovering of hidden structure in this theory, before finishing by looking at some analytic results in a toy model setting using powerful new tools.

The calculation of scattering amplitudes is important for the analysis of scattering processes at particle colliders as well as for our understanding of perturbation theory.

In this talk I will present a series of new relations for scattering amplitudes in quite general gauge theories at loop level.

The existence of these relations can be understood from the analysis of certain large momentum shifts of tree amplitudes and loop level integrands. As an example, a concrete relation for the integrand at one-loop will be discussed.

Great progress has been made in recent years in understanding the mathematical structure of scattering amplitudes and correlation functions in Planar N=4 sYM at strong and weak coupling. Form Factors in some weak sense interpolate between these quantities, and present a possible path to extend some results to other, less supersymmetric theories. I provide an introduction to their computation at weak coupling, the supersymmetric relations between them and their relation to deformations of the original theory.

Resummation methods allow to increase the precision of perturbative calculations in kinematic regions affected by large logarithmic contributions by resumming them to all orders in perturbation theory. We will show how large Sudakov logarithms appear in physical quantities such as the Drell-Yan and the DIS cross section and how resummed expressions can be derived based on the factorisation of these quantities into hard, collinear and soft contributions.

Theories involving "universal" extra spatial dimensions are natural extensions of the Standard Model with interesting phenomenology. I will explain the motivation for these models and discuss the minimal case, involving one extra dimension, in some detail. I will describe how I helped create a software implementation of this model to greatly ease calculations and finally how my collaborators and I are using data from LHC Higgs searches to constrain the model's parameter space.

The presence of Dark Matter in our universe is well-motivated by gravitational data, but the identity of this illusive substance remains largely a mystery. I review the current status of so-called Direct Detection experiments, which aim to detect and identify Dark Matter particles (commonly known as WIMPs) via non-gravitational means. My talk is intended for a general audience, and will hopefully prove both interesting and informative for all attendees, especially for model-builders for whom a good understanding of the current experimental status is vital.

In recent years there have been many developments in using on-shell methods to calculate scattering amplitudes at NLO and beyond. In this talk an overview of the developments in this area are given. Why these methods are useful in phenomenology, and how they are implemented is discussed along with outstanding problems and how they are being solved.

I explain how we identify the regions of the Constrained Minimal Supersymmetric Standard Model's parameter space that are in best agreement with experimental data, using Bayesian statistics. I show how we construct likelihood functions for experimental constraints, including the latest LHC 1/fb results. I present our probability maps on the (m0, m12) plane of the CMSSM, with and without the latest LHC 1/fb results.

One of the consequences of grand unification is the appearance of additional particles; beyond those seen in the standard model. With the development of the LHC we are now capable of producing collisions with sufficient energy that some of these additional beyond-the standard-model (BSM) particles can be produced. One particular type of BSM particle which could be produced is the leptoquark and its key interaction is to couple a quark directly with a lepton, an interaction which would provide new event signatures at the LHC.

Currently, within perturbative quantum field theory, most calculations of leptoquark production have only been done to leading order (LO). Thus an important area of research is to improve the accuracy of our theoretical predictions by performing these calculations at next-to-leading-order (NLO). The primary purpose of this talk will be to describe how these higher order calculations can be done.

The determination of accurate parton distribution functions from global fits to deep inelastic and hadronic collision data, is a key ingredient in the analysis of new experiments. In this talk I shall discuss the fitting methodology of the NNPDF (Neural Network parton distribution function) collaboration, and demonstrate a novel method of including new data into an existing NNPDF parton set, the reweighting method.

We discuss Grand Unification Theories (GUTs), focussing on the standard SU(5), SO(10) and E6 candidates. We will review the matter content of the representations and examine the mass spectrum of the first and second generation of squarks and sleptons. We will also provide a testable comparison of the alternative group structures and discuss how the first and second generation can be used to constrain the third generation. Constraints on the parameter space coming from Charge and Colour Breaking minima (CCB) and Unbounded From Below (UFB) conditions on the scalar potential will also be addressed.

I will discuss BPS solutions in the field theory on two D4-branes, specifically instantons with a non-zero scalar field. I'll describe the moduli space of all possible BPS solutions and find it explicitly for two instantons. The dynamics of slow moving instanton-like fields can then be approximated by geodesic motion in this space, although the scalar field will also induce a potential.

Some of the simplest backgrounds for Supergravity are the inhomogeneous hyperboloids, obtained by analytically continuing the famous sphere reductions. We can consider two very different sets of fluctuations about these backgrounds. The first is a rare example of a Pauli reduction, i.e. a consistent truncation to the complete set of Kaluza-Klein zero modes. Though these reductions are very interesting for a variety of reasons, the hyperboloids are non-compact, so don't make much physical sense: in particular, they don't reproduce 4D gravity. Performing a careful analysis of 4D gravitational waves, we are led to a second, very peculiar set of fluctuations that give the correct Newton's Law. The problem can be recast in the language of 1D Quantum Mechanics, with a seemingly pathological potential. I'll comment on the interpretation of both sets of fluctuations in this language.

We will construct an Heterotic string model that augments the Standard Model gauge group with an additional U(1) symmetry. A discussion of how U(1)s occur in free fermionic models will be followed by an explicit construction where the additional symmetry forbids Proton Decay Mediating Operators, induced in many other extensions of the Standard Model, while allowing for light neutrino masses through a Type-III seesaw mechanism.

The D-brane interpretation of monopoles gives physical insight into the Nahm transform which constructs them. The Nahm equation is an equation for the D1-brane fields and the Nahm transform switches perspective to the D3-brane worldvolume, where the monopole lives. In M-theory a similar situation occurs when an M2-brane ends on an M5-brane along a one dimensional intersection, the 3-form field strength on the M5-brane is hodge-dual to itself, hence the name 'selfdual string'.

Recently, there has been considerable discussion in the literature about applications of AdS/CFT to condensed matter systems. In this talk, I'll focus on one of them, namely the striped phase. Starting off with an enriched Einstein-Maxwell theory, one can study the stability properties of electrically or magnetically charged AdS-RN black holes in the top-down and bottom-up approach. It turns out that one can find instabilities corresponding to a spatially modulated phase transitions.

The M5-brane remains a mysterious object. Its worldvolume description is known to arise from a six-dimensional CFT with (2,0) supersymmetry. However, very little is known about six-dimensional quantum field theories. In this talk I will outline work to construct a non-Abelian system of equations that furnish a representation of the (2,0) supersymmetry tensor multiplet. The resulting on-shell conditions lead to a novel system of equations which propagate in one null and four space directions. Following more recent work, I will then show how these equations reduce to motion on instanton moduli space. Subsequently quantising the system leads directly to a previous light-cone proposal of the (2,0) theory.

Skyrmions arise as topological soliton solutions in the Skyrme model which has been shown to be a low energy effective theory of QCD in the large N_C limit. If quantized suitably, they can be used to describe structure and properties of nucleons and nuclei. We argue that the semiclassical rigid-body quantization, commonly used in the literature, has several shortcomings and that removing this simplification may result in a more accurate description of nucleons and nuclei. We present an extensive numerical study of spinning, arbitrarily deforming Skyrmions of topological charge B=1-4. Some preliminary results on spinning Skyrme solutions of higher topological charge will be discussed as well.

The accessible mass spectra at the low scale in SUSY models with Majorana gaugino masses is severely restricted by the form of the RG equations. In particular, if it is assumed that the observable spectrum is due to the evolution of some theory defined at the high scale, then a 'forbidden zone' of low scale mass configurations arises. Interestingly, it may be possible to access such configurations if instead, the gauginos are allowed to acquire a Dirac mass.

I will motivate the MRSSM (Minimal R-symmetric Supersymmetric Standard Model), where all fermions, including the gauginos and Higgsinos, are Dirac particles. I will then outline how the extra fields required by the MRSSM can be understood as completing an N=2 gauge and Higgs sector.

The E6SSM is a promising model based on the SU(3)xSU(2)xU(1)xU(1)' subgroup of E_6. It gives a solution to the MSSM mu-problem without introducing massless axions, gauge anomalies or cosmological domain walls. The model contains three families of complete 27's of E_6, giving a richer phenomenology than the MSSM. In my talk I will present a study of typical gluino decays. The E6SSM generically has gluino cascade decay chains which are about 2 steps longer than the MSSM's due to the presence of several light neutralino states. This implies less missing (and more visible) transverse momentum in collider experiments and kinematical distributions such as M_eff and m_T2 are different. Scans of parameter space and MC analysis suggest that current SUSY search strategies and exclusion limits has to be reconsidered for models with longer decay chains such as the E6SSM.

The matching of superconformal indices is an important test of Seiberg duality. After a short introduction to Seiberg duality, I will introduce the index and methods for calculating it before moving on to the problem of testing index matching. I will finish by describing the relationship between the classic field theoretic tests of Seiberg duality and index matching.

Lattice nonrelativistic QCD (NRQCD) is an effective theory that can be used to describe heavy quarks. The upsilon spectrum contains many states that can be determined using NRQCD, which can be compared to experiment. An understanding of b quarks on the lattice, along with experimental results, is important in the determination of elements of the Cabibbo-Kobayashi-Maskawa matrix - a further incentive for lattice upsilon studies, as the same action can be used for results in B physics. I will, in a talk suitable for those with little exposure to lattice field theory, discuss recent work that has taken place on the upsilon spectrum using NRQCD on lattice configurations with 2+1+1 quark flavors in the sea.

Weak interactions between different flavours of quark are described by the CKM matrix. Because quarks are bound inside hadrons, we require non-perturbative lattice QCD calculations of the strong interaction between the constituent quarks to extract CKM elements from flavour physics experiments. I will describe our lattice calculation of the vector and axial vector form factors in Ds to phi semileptonic decays, compare our results to BaBar's experimental results and explain how they can be used to extract a value of Vcs.

This project is to determine whether by adding the charm quark to the sea in Domain Wall Fermion lattice simulations do we gain more accuracy over error, or do any improvements get swamped by the error. We simulate charmed mesons and baryons with valence charm quarks and calculating many quantities on the lattice. By analysing these quantities we can determine the error as a function of the mass of the valence charm and determine if it is feasible to push it up to its physical value.

Abstract: Contradictory results for the charmonium potential have been computed within QCD phenomenology. I will outline the method by which quark--anti-quark potentials can be computed using Nambu-Bethe-Salpeter wavefunctions and the Schroedinger equation. I will also present some preliminary results for the charmonium potential using this method for different temperatures on the lattice.

We explore the phase diagram of the SU(2) Yang-Mills theory in 5 dimensions by numerical simulations. The lattice system shows a dimensionally-reduced phase where the extra dimension is small compared to the four-dimensional correlation length. In this phase, at low energies, this system behaves like a four-dimensional gauge theory coupled to an adjoint scalar field. By tuning the bare parameters of the lattice model, we identify lines of constant physics, and analyse the behaviour of the adjoint scalar mass as a function of the compactification and the cut-off scales. The perturbative prediction that the effective theory contains a light particle with a mass that is independent of the cut-off is tested against nonpertubative results.

Technicolor (TC) is one of the main extensions of the Standard Model. The internal symmetry group of the SM SU(3)xSU(2)xU(1) is enlarged with a new TC group and the Higgs sector is replaced by a strong interacting and confining sector with matter fields (techniquarks) coupled to the TC gauge fields. Chiral symmetry breaking in the TC sector triggers EW symmetry breaking, providing masses to the weak bosons. Extended Technicolor (ETC) interactions provide masses to the SM fermions. Phenomenologically allowed candidates for TC models must display a beta function such that for a large range of scales the coupling constant evolves slowly or "walks" (Walking Technicolor). Lattice Gauge Theory provides a perfect framework for the study of the non-perturbative evolution of the coupling constant.