YTF 8

14 Jan 2016, 12:30 → 15 Jan 2016, 14:00 Europe/London

Durham University

Welcome to the eighth annual Young Theorists' Forum (YTF 8) taking place at Durham University on 14-15 January 2016. The purpose of YTF is to bring together postgraduate students working in theoretical physics, providing the opportunity to present their work to a sympathetic audience. Our aim is to foster development of early career researchers and encourage collaboration between different universities.

Topics to be discussed include:

• Beyond Standard Model Physics
• String Phenomenology
• Holography
• Cosmology
• Amplitudes
• Solitons
• Standard Model Phenomenology
• String Theory
• QCD
• Gravity

Thursday, 14 January

12:30 Lunch

13:30 Registration

14:00 Check-in

Parallel session: Cosmology and Dark Matter

Convener: Ms Jessica Turner (IPPP, Durham University)

1 Observing the String Axiverse

Axion-like particles (ALPs) populating many decades in mass (an "axiverse") are generically predicted by string theory. Depending on their mass and production mechanism, these ALPs may contribute to dark matter, dark energy and dark radiation. Such ALPs are potentially observable via their interaction with electromagnetism, leading to ALP-photon conversion in an external magnetic field. I will present predicted signals from axiverse dark matter and dark radiation in galaxies and galaxy clusters.

Speaker: Ms Francesca Day (University of Oxford)

Slides YTF.pdf

2 Charming Dark Matter

Dark matter models are often studied using the _Minimal Flavour Violation_ framework which restricts the new physics in various flavour measurements (e.g. meson mixing, rare decays). I will talk about an extension of minimal flavour violation that allows for sizeable contributions to flavour observables, and explain how neutral meson mixing can constrain certain dark matter models in this extended framework. I will also present an initial look at the constraints on my model from both flavour and dark matter observables.

Speaker: Mr Matthew Kirk (IPPP, Durham University)

Slides Matthew_Kirk_ytf8

3 Frame Covariant Formulation of Inflation Using a Generalised Approach to the Inflationary Attractor

Inflation has been a very successful generic explanation of the origin of cosmological anisotropies. However, the multitude of inflationary models, all with numerous different theoretical underpinnings, poses a challenge in determining the fundamental driving mechanism of inflation. With the aim of obtaining predictions from inflationary models in a concise and straightforward manner, we extend the potential slow-roll approximation and derive new, generalised forms for the potential slow-roll parameters. Thus, we are able to extract predictions for cosmological observables for a wide class of inflationary models (scalar-curvature theories). We furthermore show that, under the generalised potential approach, frame transformations (conformal transformations and inflaton reparametrisations) leave observable quantities invariant. We apply the generalised potential formalism to induced gravity inflation and Higgs inflation, and show that, since no approximations to the potential need be made, more accurate results for observable quantities are obtained.

Speaker: Mr Sotirios Karamitsos (University of Manchester)

Slides frame_covariant_formulation_of_inflation.pdf

4 Neutrinoless Double Beta Decay and High-Scale Baryogenesis

The constraints on baryogenesis models obtained from an observation of neutrinoless double beta decay will be discussed. The lepton number violating processes, which can underlie neutrinoless double beta decay, would together with sphaleron processes, which are effective in a wide range of energies, wash out a primordial baryon asymmetry of the universe. Typically, if a mechanism of neutrinoless double beta decay other than the standard light neutrino exchange is observed, typical scenarios of high-scale baryogenesis will be excluded. This can be achieved by different methods, e.g. through the observation in multiple isotopes or the measurement of the decay distribution. In addition, I will also highlight the connection with low energy lepton flavour violation and lepton number violation at the LHC.

Speaker: Mr Lukáš Gráf (University College London)

Slides lukas_graf_talk.pdf

Parallel session: Holography and Solitons

Convener: Mr Vaios Ziogas (Durham University)

5 Renormalized entanglement entropy

Entanglement entropy is a quantity that has received a large amount of attention recently in the literature, especially after the holographic prescription from Ryu and Takayanagi. It is a well known fact that the entanglement entropy is a UV divergent quantity and therefore should be renormalized. There have been attempts recently to define a renormalized version of the entanglement entropy, though previous attempts have all fallen short in one area or another. In this talk I will present recent work done with my supervisor, Marika Taylor, on formulating a renormalized holographic entanglement entropy using the standard holographic renormalization methods.

Speaker: Mr William Woodhead (University of Southampton)

6 Hot Multiboundary Wormholes from Bipartite Entanglement

Many beautiful, recent results in the field of holography have made manifest that poignant aspects of bulk geometry are encoded in the boundary theory as quantum information. In particular, Susskind's recently conjectured ER=EPR conjecture pertains to the view that there is an intimate connection between entanglement entropy in the boundary theory and connectedness of the dual, bulk geometry. We investigated the spatial dependence of the entanglement entropy for 2D boundary CFT states dual to 3D multi-boundary wormholes. In this context, one can probe the entanglement structure with the RT conjecture by simply computing and comparing lengths of geodesics. We found that when the temperatures of the boundary CFTs tend to infinity the spatial dependence of the entanglement entropy supporting to the wormhole is extremely simple and purely bipartite.

Speaker: Mr Alexander Peach (Durham University)

7 Supergravity Couplings from CFT Correlation Functions

The gauge/gravity correspondence offers a unique tool for understanding strongly coupled quantum field theories and quantum gravity, which made it one of the leading research fields in theoretical physics in the last 15 years. Over the years the correspondence has been refined and many lessons have been learned. However, with the complexity of the duality there are still a huge number of questions to answer and conjectures to be tested. We would like to pick up some of these, and try to complete our understanding of the supergravity couplings by inferring knowledge about the field theory on the other side of the duality. This also provides a way of testing the correspondence itself.

Speaker: Mr Stanislav Schmidt (University of Southampton)

Slides durham_talk_main.pdf

8 Baby Skyrmions in AdS

Studies in holographic contexts, such as holographic QCD and holographic superconductors, have motivated the investigation of solitons in AdS and AdS-like spacetimes. In this talk we will investigate the baby Skyrme model in a pure AdS background without a pion mass term, and numerically find soliton and multi-soliton solutions. We find that ring-like structures appear to be energetically favourable, and we construct a point-particle approximation to predict the energy-minimising configurations at higher topological charges.

Speaker: Mr Matthew Elliot-Ripley (Durham University)

Slides MatthewElliotRipley_YTFtalk.pdf

16:10 Tea and Coffee

Parallel session: QCD

Convener: Mr Robin Linten (IPPP Durham)

9 Next-to-Leading Log Contributions to Jet Production with High Energy Jets

In its embryonic form, High Energy Jets (HEJ) is a perturbative framework which allows for the resummation of QCD processes to Leading Log (LL) accuracy. This talk will briefly outline how the framework is derived and the key features of the resummation technique. Following on from this, we present how the formalism could be improved to Next-to-Leading Log (NLL) accuracy. We focus specifically on the inclusion of contributions which are formally sub-leading in jet production but LL in a particular partonic subprocess, extending the range of HEJ's applicability.

Speaker: Mr James Cockburn (University of Edinburgh)

Slides YTF_2016.pdf

10 NNLO Soft Functions and Non-Cusp Soft Anomalous Dimensions in SCET

I will present an algorithm for the automated calculation of NNLO dijet soft functions, and show novel results for a variety of collider observables. I will also detail the extraction of the associated two-loop, non-cusp soft anomalous dimensions necessary for resummations at NNLL accuracy in the Soft Collinear Effective Theory (SCET).

Speaker: Mr Jim Talbert (University of Oxford)

Slides JimTalbert_YTFtalk.pdf

11 Higher Dimensional Theories and the Banks-Zaks Fixed Point of QCD.

QCD and other asymptotically free field theories possess the unique characteristic of the beta-function having a non-trivial fixed point. This is known as the Banks-Zaks fixed point and is believed to have connections with the chiral symmetry phase transition. Using perturbative quantum field theory techniques we can construct higher dimensional extensions of scalar and gauge theories. These can be linked to 4-dimensional models via a non-trivial fixed point. The aim of this talk will be to introduce the framework of building towers of theories over multiple dimensions, while also demonstrating how UV properties of one QFT could potentially drive the IR dynamics of another. I will also discuss the renormalization scheme invariance of QCD critical exponents that can be calculated using the Banks-Zaks fixed point.

Speaker: Ms Rebecca Simms (University of Liverpool)

Slides RebeccaSimmsYTF2016.pdf

Parallel session: String Theory

Convener: Mr Felix Haehl (Durham University)

12 Estimation for 2 dimensional free fermion negativity

Quantum entanglement is a has played an increasingly important role in string theory.In this presentation, I will talk about negativity, a measure of entanglement for tripartite systems. In particularly, the negativity of a 2 dimensional free fermion, which is still a open question. I will discuss the main difficulty and current knowledge of the free fermion negativity and introduce a path integral which is a convenient tool to help us study the feature of negativity.

Speaker: Mrs Yihong Wang (Stonybrook University)

Slides slides_YihongWang.pdf

13 Engineering F-Theory GUTs

Attempts have been made at constructing D-brane GUT models which suffer from a number of difficulties like the lack of spinor representation for the SO(10) or the perturbative vanishing of the top quark Yukawa couplings for the SU(5) models. In fact, these difficulties arose as the past constructions relied heavily on local 7-branes. Such obstacles can be avoided by considering mutually non-local 7-branes. This enlarged class of models goes under the name of F-theory. F-theory encodes the physics of 7-branes in higher-dimensional geometry. Moreover, in certain limits F-theory is dual to the heterotic E8xE8 and M-theory. As a matter of fact, one of the most convenient ways to think about F-theory is via the duality with M-theory. This method is used to construct global F-theory GUTs. The aim of the talk will be to introduce some of the key ideas in engineering F-theory GUTs. We will look at some aspects of the SU(5) (extensive work has been done in this case) and the SO(10) GUT like matter curves and Yukawa points. To conclude, GUT breaking methods will be discussed briefly and work in progress will be mentioned.

Speaker: Mr Johar Ashfaque (University of Liverpool)

Slides JM_ASHFAQUE.pdf

14 Shift-Symmetries at one-loop

A feature of the low energy theories of certain string models is the existence of a shift-symmetry in the Kähler potential at tree level. In this talk I will consider the fate of this shift-symmetry at one-loop and show that while it does not generally hold at this order, there is a certain limit of compactification where the symmetry is restored. I will then argue why we expect this restoration to hold to all orders in perturbation theory.

Speaker: Mr Richard Stewart (University of Durham)

Slides RichardStewartYTF2016.pdf

17:55 Break

Plenary talk

15 Quantum field theory of fluids

I discuss the quantization of a perfect fluid. This differs from textbook quantum field theory, because of the presence of vortex modes, which map to an infinite collection of quantum mechanical free particles rather than harmonic oscillators. As a result, the theory is plagued by infra-red divergences. I argue that there exists, nevertheless, a consistent effective field theory description, valid at large distances and times.

Speaker: Dr Ben Gripaios (Cambridge)

Slides BenGripaios_YTFplenarytalk.pdf

Poster and Pizza

16 Charming Dark Matter

Speaker: Mr Matthew Kirk (IPPP, Durham University)

17 Discovering Z' bosons in Minimal Walking Technicolor

Speaker: Ms Azaria Coupe (University of Southampton)

18 Falsifying Baryogenesis with Neutrinoless Double Beta Decay.

Speaker: Mr Lukáš Gráf (University College London)

19 Searches for NMSSM Signatures in CMS

Speaker: Mr Alexander Titterton (University of Southampton)

Friday, 15 January

Parallel session: Gravity

Convener: Dr Alan Reynolds (Durham University)

20 Numerical General Relativity and Asymmetric Bubble Collapse

I introduce GRChombo: a new numerical relativity code which incorporates full adaptive mesh refinement (AMR). The AMR capability permits the study of a range of new physics which has previously been computationally infeasible in a full 3+1 setting, including the study of critical phenomena in the collapse of scalar field bubbles in asymmetric configurations. I will present some preliminary results of such collapses, and suggest areas of interest for future study.

Speaker: Ms Katy Clough (Kings College London)

Slides YTF8_Jan_2016_Talk_K_Clough.pdf

21 First integrals of affine connections on surfaces

Given a pseudo-Riemannian manifold, there is a natural notion of geodesics defined by the Levi-Civita connection. But the geodesic equations can be written just in terms of the Christoffel symbols $\Gamma$: \begin{equation} \ddot x^a+\Gamma^a_{bc} \dot x^b\dot x^c=0, \end{equation} where $x^a$ are local coordinates. Therefore, geodesics can be defined for any symmetric affine connection (not necessarily metric). The same is true for Killing forms $K_a$, which are defined to be solutions to $$ \nabla_{(a}K_{b)}:=\frac{1}{2}(\nabla_{a}K_{b}-\nabla_{b}K_{a})=0,$$ and correspond to first integrals of the geodesic equations linear in the momenta: $K_a \dot x^a$. I will present the method of prolongation and Frobenius theorem to determine necessary and sufficient conditions for an affine connection on a two-dimensional manifold (not necessarily endowed with a metric) to admit 0, 1, 2 or 3 Killing forms. Reference: F. Contatto, M. Dunajski. (2015) First integrals of affine connections and Hamiltonian systems of hydrodynamic type. [ arXiv:1510.01906 ]

Speaker: Mr Felipe Contatto (Department of Applied Maths and Theoretical Physics - Cambridge)

Paper http://arxiv.org/pdf/1510.01906v1.pdf

22 Lorentz-violating matter dispersion relations from modified quantum gravity

We consider "classical" fermions and scalars coupled to a 4-d diffeomorphism breaking gravity model, and we derive the one-loop Lorentz-violating effective dispersion relation for matter, after integrating out gravitons. The modified gravity model is the z=3 non-projectable Horava-Lifshitz gravity which, due to its improved UV behaviour, involves logarithmic divergences only. We find that with generic values for the different parameters, the model leads to $10^{10}$ GeV as the typical scale above which the current upper bounds on Lorentz violation can no longer be satisfied. On the other hand, it is always possible to fine-tune the parameters of the model, such that the effective maximum speed seen by particles is consistent with Special Relativity.

Speaker: Mr Julio Leite (King's College London)

23 Looking for signatures of a modified speed of sound for the tensor modes.

Higher derivative interactions might induce an effective speed of sound for the tensor modes. Such speed can be removed by a disformal transformation which is a change in the light cone slope. This transformation rescale the coordinates in such a way that the scalar modes get a effective speed. We study the effects of this transformation for CMB photons propagation. We find that by using this method it is possible to impose heavy constraints to an effective theory for gravitational waves.

Speaker: Mr Sebastian Cespedes (DAMTP, University of Cambridge)

Slides Durham.pdf

Parallel session: Standard Model Phenomenology

Convener: Mr Simon Armstrong (IPPP)

24 Reconstructing dileptonic $t\bar{t}$ events (and doing stuff with them)

Top pair production is one of the most widely studied interactions at the LHC and offers opportunities both to improve our understanding of Standard Model "parameters" like parton density functions and constrain or potentially discover new physics beyond the Standard Model. An interesting family of observables in top pair production are angular correlations which can probe the spin correlations between the two tops and the polarisations of the individual tops themselves, both of which can be sensitive to new physics effects. The dileptonic decay channel is useful for studying spin correlation observables since the two leptons are experimentally very clean and straightforward to interpret. However for studying polarisations we need to fully reconstruct the individual top momenta which presents a challenge since we expect two hard neutrinos in the final state, which we only can detect as a single missing transverse momentum vector. We are therefore forced to figure out how to get reliable solution(s) for the two neutrinos out of this underconstrained kinematic system if we want to fully reconstruct the individual top momenta, keeping in mind that the method must be robust against the considerable noise present in real world LHC data. In this talk I will focus on the so-called "MT2 Assisted On-Shell" (MAOS) neutrino reconstruction method, with some brief remarks about other methods currently in use. I will introduce the MT2 observable itself and show you how to calculate it. If time permits I will also briefly discuss some potential phenomenological applications of using this reconstruction to calculate observables.

Speaker: Mr Karl Nordstrom (University of Glasgow)

Slides nordstrom_ytf8.pdf

25 Muon g-2: data combination, fitting and systematic bias

The anomalous magnetic moment of the muon, $a_{\mu}$, stands as an enduring precision test of the Standard Model. With the hadronic contribution providing the largest uncertainty to the standing $3.3\sigma$ discrepancy between theoretical and experimental estimates, the combination of $e^{+}e^{-} \rightarrow$ hadrons cross section data in the determination of $a_{\mu}^{\text{had, LOVP}}$ must be statistically reliable to achieve a precise estimate to $a_{\mu}^{\text{SM}}$. Recent studies have highlighted the prospect of systematic biases that can arise through the fitting of global normalisation uncertainties. An analysis of the existing fitting method shows that although it has the potential to incur a bias, previous results are still reliable. However, it necessary to produce a new, unbiased iterative fitting procedure: the $R_m^I$ method. Applying this new method in the determination of the dominant $e^{+}e^{-} \rightarrow \pi^{+}\pi^{-}$ hadronic contribution yields an unbiased result of $a_{\mu}^{\pi^{+}\pi^{-}}(0.305 \leq \sqrt{s} \leq 2 \ \text{GeV}) = (504.42 \pm 2.24) \times 10^{-10}$. We then apply the $R_m^I$ method to other dominant hadronic channels as an indication of the impact on a full analysis of the determination of $a_{\mu}^{\text{had, LOVP}}$

Speaker: Mr Alex Keshavarzi (University of Liverpool)

Slides Keshavarzi_Gm2_YTF2016.pdf

26 Unitarity bound in the composite two Higgs doublet model

We discuss a composite two Higgs doublet model based on a symmetry breaking SO(6) × U (1)_x → SO(4) ×SO(2) × U (1)_x at a scale f, and explain how the effective kinetic terms and Yukawa interactions are obtained. The coupling of the Higgs boson as a pseudo Nambu-Goldstone boson to the standard model fields can deviate from that of the standard model Higgs boson due to the non-linear feature. These deviations cause to unitarity violation at high energies because the cancellation of the quadratic energy terms does not happen. We will examine the constrains on parameter space from perturbative unitarity with a fixed energy scale in the composite two Higgs doublet model.

Speaker: Ms Emine Yildirim (University of Southampton)

Slides YTF_2016.pdf

10:40 Tea and Coffee

Parallel session: Amplitudes

27 Double copy of classical bremsstrahlung solutions

The BCJ double copy relates scattering amplitudes of gauge and gravity theories. In an attempt to study this duality in a non-perturbative context, it was recently found that Kerr-Schild solutions in general relativity are somehow a double copy of linearised Yang-Mills solutions. In this talk I apply the solutions double copy procedure to bremsstrahlung systems and explain its relation to the scattering amplitudes double copy.

Speaker: Mr Andrés Luna-Godoy (University of Glasgow)

Slides preYTF2016.pdf

28 Tales of the Unexpected: One-Loop Soft Theorems via Hidden Symmetries

Controlling infrared divergences in QFT cross-sections has been an important phenomenological problem for many decades. Recently, there has been renewed theoretical interest, with the observation that soft theorems emerge as the Ward identities of asymptotic symmetries, at least at tree level. I shall discuss recent work probing the one-loop corrections to subleading soft theorems in the context of N=4 super-Yang-Mills theory. In particular, I shall outline how hidden symmetries may be used to constrain the form of such corrections. Further, I shall present new computations which determine the corrections precisely, revealing them to have a surprisingly simple form. These results may find fruitful application in QCD resummation and the development of holographic theories. The talk is based on arXiv:1511.06716.

Speaker: Mr Edward Hughes (Queen Mary University of London)

Slides EdwardHughestalk.pdf

29 Applying Colour-Kinematics Duality to the Unitarity Method

Colour-kinematics duality in tree-level Yang-Mills amplitudes (BCJ relations) invites new possibilities for loop-level computations. In this talk I shall briefly introduce the generalised unitarity approach at one loop. Then, in the context of a 5-gluon, 2-loop QCD amplitude, I will demonstrate how the BCJ relations can be applied to unitarity cuts. This allows leading colour information to be recycled into nonleading colour.

Speaker: Mr Gustav Mogull (University of Edinburgh)

Slides YTFPresentation.pdf

30 Blackhat - Efficient Calculation of NLO amplitudes for Higgs + many jets

I will give a short intro to the blackhat library which I have been working on, why we need efficient high multiplicity NLO amplitudes, how we can calculate them using generalised unitarity, BCFW and how to not need 6 or more dimensions.

Speaker: Mr Simon Armstrong (IPPP)

Slides pres.pdf

Parallel session: BSM

Convener: Mr Andres Olivares del Campo (IPPP, Durham University)

31 Drell-Yan production of multi-Z's at the LHC

The Drell-Yan di-lepton production at hadron colliders is by far the preferred channel to search for new heavy spin-1 particles. Traditionally, such searches have exploited the Narrow Width Approximation (NWA) for the signal, thereby neglecting the effect of the interference between the additional Z'-bosons and the Standard Model Z and γ. Recently, it has been established that both finite width and interference effects can be taken into account in experimental searches while still retaining the model independent approach ensured by the NWA. This assessment has been made for the case of popular single-Z' models currently probed at the Large Hadron Collider (LHC). In this talk I review the scope of the CERN machine in relation to the above respects for the case of some benchmark multi-Z' models. In particular, we consider Non-Universal Extra Dimensional (NUED) scenarios and the 4-Dimensional Composite Higgs Model (4DCHM), both predicting a multi-Z' peaking structure.

Speaker: Mr Juri Fiaschi (University of Southampton)

Slides Fiaschi_MultiZ.pdf

32 BSM Physics with Sherpa

We present a fully automated framework as part of the Sherpa event generator for the computation of tree-level cross sections in beyond Standard Model scenarios, making use of model information given in the Universal FeynRules Output format. Elementary vertices are implemented into C++ code automatically and provided to the matrix-element generator Comix at runtime. Widths and branching ratios for unstable particles are computed from the same building blocks. The corresponding decays are simulated with spin correlations. Parton showers, QED radiation and hadronization are added by Sherpa, providing a full simulation of arbitrary BSM processes at the hadron level.

Speaker: Mr Silvan Kuttimalai (IPPP Durham)

33 A smörgåsbord of flavour in GUTs

There are many open questions in high energy physics today, ranging from the origin of neutrino mass to the strong CP problem. In particular, there is no current frontrunner for a theory which explains the flavour mixing and mass hierarchies within the SM, though some of the most complete and realistic attempts are found among Flavour GUTs. I will review two such models, based on SU(5) and SO(10) with discrete flavour symmetries, and show how they may simultaneously resolve several open questions related to flavour, gauge coupling unification, and cosmology.

Speaker: Mr Fredrik Björkeroth (University of Southampton)

Slides bjorkeroth_ytf8.pdf

34 Higgs decays in SM Effective Field Theory at one-loop

Standard Model Effective Field Theory (SMEFT) is a method to parametrise the impact of new physics which may become accessible at higher energies without specifying its UV origin. The new physics is said to be integrated out, leaving behind effective non-renormalisable operators. In this talk, we supplement the Standard Model with all (baryon number conserving) operators which appear at dimension-6 and calculate, to one-loop, the amplitudes for $h \rightarrow b\bar{b}$ and $h \rightarrow \tau \bar{\tau}$ in the limit of vanishing gauge couplings. Special attention will be given to the set-up and renormalisation of the amplitudes in the context of SMEFT.

Speaker: Mr Darren Scott (IPPP, Durham University)

Slides smeft_short.pdf

12:50 Lunch