YTF 24

18 Dec 2024, 14:00 → 19 Dec 2024, 18:10 Europe/London

Centre for Particle Theory

Welcome to YTF 2024! 

YTF exists to bring together postgraduate students working in theoretical physics/applied maths and encourage them to share their research. YTF 24 is a great opportunity for presenting your work, while also engaging and collaborating with students from different universities. YTF 24 will feature:

• 20 minute talks, with 5 minutes of questions 
• Gong Show Talks (shorter 5 minute talks)
• Evening poster session  

There will also be a talk from a plenary speaker, this year it will be Claudia de Rham from Imperial College London!

YTF 24 will be held at Durham University over two days: Wednesday 18th and Thursday 19th December 2024, registration is on a first-come first-served basis.

We want to make sure YTF is as inclusive as possible. All attendees must abide by the code of conduct and any breaches or concerns can be reported to the EDI officer who will be present during the conference. A quiet room will be provided at the venue. All presenters are encouraged to consider using colour-blind friendly styles for their materials.

If you have any childcare needs during YTF please feel free to contact us so we can help you in this regard.

In addition, please let us know through the registration form of any access requirements you have.

For information regarding travel and accommodation, please review the travel and accommodation tab.

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Topics expected to be discussed include:

• Amplitudes
• Astroparticle physics
• BSM physics
• Cosmology
• Flavour physics
• Generalised symmetries
• Gravity
• Holography
• Lattice QCD
• Machine learning
• Non-perturbative QFT
• Phenomenology
• QCD
• String theory
• SUSY

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If you have any questions, do get in touch with our communications officer Elliot at elliot.fox@durham.ac.uk. You can also follow us on social media on Twitter (X) @Durham_YTF or Instagram @ytf_durham to keep up-to-date with the latest from the team. 

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We could not hope to run this conference every year without the generous support of our sponsors to whom we are extremely grateful. For YTF 24 we are proud to be supported by:

• Fundamental Physics UK (FPUK) Virtual Theory Centre
• UK Lattice Field Theory (UKLFT) Centre
• IOP Early Career Members
• IOP Mathematical and Theoretical Physics
• IOP Astroparticle Physics

YTF2024Poster_retro.pdf

Wednesday 18 December

14:00 → 14:10 General Address: Welcome & Introduction

Welcome & Introduction: Elliot Fox & Ery McPartland
Thursday Welcome: Elliot Fox
Farewell Address: Elliot Fox & Malina Rosca

YTF 2024 Welcome Slides.pptx

14:10 → 15:10 Gong Show Talks

Chair: Yunji Ha

14:10 The need for event-shape resummation

Speaker: Darcy Peake

DarcyPeake_YTF Presentation 2024.pdf

14:20 Quantum black-to-white holes: Covariant non-perturbative corrections

Holonomy corrections, which are integral to loop quantum gravity (LQG), have recently been consistently incorporated for the classical Schwarzschild black hole using a novel framework that lets one incorporate non-perturbative quantum geometry corrections in a generally covariant manner. The vacuum spherically symmetric solutions, solved in different gauges, are shown to describe the same physical spacetime. The global structure is that of a non-singular wormhole region connecting the black hole to a white hole. Our result is valid for an arbitrary scale-dependent holonomy parameter which demonstrates the robustness of LQG black holes with respect to quantization ambiguities. The semiclassical analysis of a scalar field on such quantum black hole backgrounds shows that the Hawking thermal distribution is regained, and the LQG corrections enter through the sub-leading greybody factors.

Speaker: Idrus Husin Belfaqih (University of Edinburgh)

14:30 The Resonance Aware POWHEG Method

In this talk I will start by introducing the POWHEG method for matching fixed order calculations to a parton shower. I will show how the double counting is removed by POWHEG. Then I will go on to look at an extension to this which is resonance aware, POWHEG-RES. This extension to POWHEG adds the complexity of the matching algorithm keeping track of the productions channels associated with a given final state. I aim to show why this method improves the accuracy of the matching algorithm and is necessary in future precision computations.

Speaker: Patrick Hurley (University of Sussex)

PH - YTF Gong Talk.pdf

14:40 Partial N3LL + NNLO Resummed Predictions for the Drell-Yan Process in Rapidity Dependent Jet Veto Observables

Jet vetoes are important tools that are frequently used to cut away backgrounds or separate different hard scattering processes. Rapidity dependent jet vetoes with a tight veto at central rapidities and a loose veto at forward rapidities can reduce sensitivity to jets from pile-up and the underlying event. Applying tight cuts on such variables requires resummation of large logarithms of the hard scale over the jet veto scale. I will discuss the resummation of two different rapidity dependent jet veto observables, and present new results at partial N3LL + NNLO for the Drell-Yan process with these two jet vetoes applied.

Based on work done with Shireen Gangal and Jonathan Gaunt.

Speaker: Thomas Clark (University of Manchester)

Presentation TC YTF 2024.pdf

14:50 The (Conformal) Bootstrap Philosophy

Speaker: Hector Puerta-Ramisa (Durham University)

YTF_24_talk.pdf

15:10 → 16:10 Plenary Speaker: Professor Claudia de Rham

Chair: Malina Rosca

15:10 Positivity in Research & in the Sky

I will discuss the subtle interplay between the behaviour of gravity as we observe it in our “everyday experiments”, and its embedding within an ultimate high energy completion. In particular, I will emphasize how the notion of causality is manifested with gravity, a consideration which is particularly relevant for putting constraints on the physical theories we use to describe observations and make future predictions. By considering low-energy processes including effects from Standard Model particles, I will also argue how we can get an insight on some properties of physics at high energy, well beyond what is accessible with current and future observations and experiments.

Speaker: Prof. Claudia de Rham (Imperial College London)

YTF plenary talks Durham Dec 2024.pptx

16:10 → 16:40 Break

Cold drinks and biscuits

16:40 → 18:10 Beyond the Standard Model (BSM) Physics

Chair: Tom Stone

16:40 Lower Mass Bound on Neutrino Mass for Leptogenesis from Dark Matter with large coupling Hierarchy

Davidson and Ibarra showed that for successful matter anti-matter asymmetry generation from extending the Standard Model to include Dirac and Majorana Neutrino mass contributions, the lightest Right Handed Neutrino (RHN) mass possible is $10^9$ GeV, known as the Davidson Ibarra (DI) bound. Falkowski et al introduced a new interaction to this standard Leptogenesis model resulting in two sector Leptogenesis from dark matter. This involves asymmetry generation in both the visible sector and the new dark sector from which the dark sector has two new couplings from Heavy RHN’s. They show that the DI bound of $10^9$ GeV is still not broken. This presents a problem as a large RHN mass contributes to the electroweak hierarchy problem; lowering it alleviates this tension. The point below which this tension is alleviated is approximately $3 \times 10^7$ GeV also called the Vessani bound. However, the effect of a very large hierarchy in the dark sector couplings has not been studied. We have explored the effect of large hierarchy in the dark couplings. We found that the RHN mass can be lowered from $10^9$ GeV to approximately $3 \times 10^7$ GeV. Washout prevented it from going lower. However, after further exploration of the effects of washout suppression from a large dark scalar mass we found that the mass of the lightest RHN is at $5 \times 10^4$ GeV.

Speaker: Henry McKenna (The University of Liverpool)

Henry McKenna YTF 2024.pdf

17:10 Non-decoupling scalars at future colliders

This work extends the Standard Model with a set of scalar electroweak multiplets that gain the majority of their mass via the Higgs mechanism. These particles are non-decoupling, providing a pattern of deviations to low energy observables that require a HEFT description. Considering their minimal gauge and Higgs couplings, I'll show that almost any additional scalar to the SM with these properties are discoverable at a future lepton collider. Furthermore, these scalars can induce a strongly first order phase transition, generating gravitational-waves detectable by future interferometers such as LISA.

Speaker: Graeme Crawford (University of Glasgow)

loryonsGC.pdf

17:40 BSM Multi-Higgs: Collider Phenomenology and Electroweak Phase Transitions

In this talk, I shall discuss the phenomenological prospects of neutral triple Higgs production compared to di-Higgs production across various Higgs-sector extensions (R2HDM, C2HDM and N2HDM), all within the context of a strong first-order electroweak phase transition. Our analysis reveals that scalar sector resonance contributions can significantly enhance triple Higgs production, despite the small Standard Model (SM) baseline expectation. Notably, one can identify potential enhancements up to 40 times the SM predictions, underscoring the importance and feasibility of experimental efforts at the High-Luminosity LHC (HL-LHC) and FCC-hh. This investigation not only motivates experimental pursuits but also sheds light on the thermal history of our universe, offering valuable insights into fundamental physics and the evolution of the cosmos.

Speaker: Wrishik Naskar (University of Glasgow)

WN_YTF24.pdf

16:40 → 18:10 Cosmology

Chair: Hector Puerta Ramisa

16:40 Asymptotically Safe Cosmology

Asymptotic Safety was proposed by Weinberg as a novel way to deal with quantum gravity. Einstein theory of gravity is in principle not renormalizable, however, one need not run the couplings to Planck scale if there is a non-trivial fixed point for the couplings. All the beta functions of the theory should vanish at this fixed point rendering signs for a conformal theory. Hence, the beta functions for the gravitational theory are related to the conformal anomalies. In this talk, I will discuss how ideas from asymptotic safety can be handy in our understanding of the early universe, be it the scale invariance of the CMB power spectrum or the conformal singularity at the bang. This is based on work with Latham Boyle and Neil Turok at the University of Edinburgh.

Speaker: Mr Vatsalya Vaibhav (University of Edinburgh)

Asymptotically_Safe_Cosmology.pdf

17:10 Scalar Field Effective Potentials in de Sitter Space

In this work, we present a detailed computation of the standard and constraint effective potentials in 4-dimensional de Sitter spacetime. Second to the pathologies arising in the perturbation theory of "lighter-than-Hubble" scalars, the stochastic approach is commonly used in the literature to describe the dynamics of scalar degrees of freedom. However, some ambiguities appear in the set-up of this theory. Chiefly, the potential term in the stochastic equation of motion does not have a straightforward generalisation from the Minkowski case, as in curved geometries its definition becomes thermodynamic ensemble-dependent.

Starting from Euclidean path integral methods, we obtain novel, closed-form, analytical expressions for both potentials and examine whether either one is infrared (IR) and ultraviolet (UV) safe at one-loop in the asymptotically light limit, $\frac{m^2}{H^2} \rightarrow 0$. This work warrants the study of the dynamics, vacuum structure and stability of light cosmological spectators in a well-defined mathematical set-up.

Speaker: Mr Lucas Vicente Garcia-Consuegra (King's College London)

YTF24_LVGC.pdf

17:40 Recursion relations in FLRW spacetimes

How do we extend the power of recursion relations to curved spacetimes? In this talk, I will present a novel framework for computing Feynman diagrams on arbitrary FLRW spacetimes, breaking them into lower-order diagrams through causality-driven recursion relations. This approach generalizes the celebrated BCFW recursion relations, traditionally confined to Minkowski spacetime, to settings where new complexities arise.

Tree diagrams are pivotal in amplitude studies, as any loop integrand can ultimately be expressed as a sum of tree-level contributions. While Minkowski spacetime simplifies the calculation of tree diagrams to residue evaluations at poles, the de-Sitter case poses unique challenges with branch cuts appearing even at tree level. I will demonstrate how our recursion relations efficiently handle such singularities, showcasing explicit examples that align with established results in the literature.

Moreover, these novel relations go beyond the requirement of Bunch-Davy's initial state. I will present an explicit solution for a phenomenologically significant diagram involving an excited initial state, highlighting the versatility of this approach.

Speaker: Dhruv Pathak (King's College London)

RECURSION RELATIONS IN FLRW.pptx

18:10 → 18:40 Break

Cold drinks and biscuits

19:00 → 22:10 Poster Session & Dinner

Dinner is takeaway pizza

19:00 Modular Properties of Generalised Gibbs Ensembles

We investigate the modular properties of Generalised Gibbs Ensembles (GGEs) in two dimensional conformal field theories. These are obtained by inserting higher spin charges in the expressions for the partition function of the theory. We investigate the particular case where KdV charges are inserted in the GGE. We first determine an asymptotic expression for the transformed GGE. This expression is an expansion in terms of the zero modes of all the quasi-primary fields in the theory, not just the KdV charges. While these charges are non-commuting they can be re-exponentiated to give an asymptotic expression for the transformed GGE in terms of another GGE. As an explicit example we focus on the Lee-Yang model. We use the Thermodynamic Bethe Ansatz in the Lee-Yang model to first replicate the asymptotic results, and then find additional energies that need to be included in the transformed GGE in order to find the exact modular transformation.

Speaker: Faisal Karimi (King's College London)

19:20 Replica analysis of entanglement properties and conditions for islands

Entanglement entropy quantifies the degree of entanglement between two quantum systems or between two subregions in a QFT and hence is an important tool to understand the quantum system. However, its study in dimensions $> 2$ has been mostly limited to flat backgrounds and CFT vacuum states in specific subregions due to technical as well as conceptual difficulties. In this talk, I will present a systematic analysis of the properties of entanglement entropy in curved backgrounds using the replica approach. We will explore the analytic $(q−1)$ expansion of Rényi entropy $S_q$ and its variations; the setup applies to generic variations, from symmetry transformations to variations of the background metric or entangling region. Our methodology elegantly reproduces and generalises results from the literature on entanglement entropy in different dimensions, backgrounds, and states. We will then use this analytic expansion to explore the behaviour of entanglement entropy in static black hole backgrounds under specific scaling transformations. We will show that certain conditions on this quantity and hence the QFT spectrum have to be satisfied for the presence of islands of entanglement, which provide enough quantum corrections to restore unitarity in black hole evaporation.

Speaker: Arvind Shekar (University of Southampton)

19:40 Do we live on the End of the World?

End-of-the-World branes are codimension-one hypersurfaces that mark the ending of spacetime. Interestingly, these have been argued to be necessary ingredients in quantum gravity to prevent the presence of global symmetries. Can they also serve as braneworlds? (Spoiler: Yes.) Motivated by the Swampland Programme and in particular the Cobordism Conjecture, we propose a scenario in which a compact region of AdS$_5$ nucleates from nothing, with a dS$_4$ spacetime living on its boundary. We show that it can equivalently be interpreted as an up-tunnelling from AdS$_5$ with cosmological constant $\Lambda\to -\infty$, following Brown and Dahlen’s proposal for ‘nothing’. Their picture naïvely suggests that the brane has infinite negative tension. However, we show that it becomes finite and positive once we employ holographic renormalization, recovering the Bubble of Something, where the domain wall becomes a boundary of spacetime. The same holds true in any number of dimensions and, moreover, at the level of metric perturbations. This provides motivation for alternative routes of obtaining cosmology from quantum gravity or string theory using domain walls, departing from conventional vacuum approaches.

Speaker: Benjamin Muntz (University of Nottingham)

20:00 A novel probe of graviton dispersion relations at nano-Hertz frequencies

We generalise Phinney's 'practical theorem' to account for modified graviton dispersion relations motivated by certain cosmological scenarios. Focusing on specific examples, we show how such modifications can induce characteristic localised distortions, bumps, in the frequency profile of the stochastic gravitational wave background emitted from distant binary sources. We concentrate on gravitational waves at nano-Hertz frequencies probed by pulsar timing arrays, and we forecast the capabilities of future experiments to accurately probe parameters controlling modified dispersion relations.

Speaker: Bill Atkins (Swansea University)

20:20 The Double Copy: A Duality for Particles and Gravity

An open problem in theoretical physics is to combine all four of the fundamental forces of nature into one single theory. Problematically, gravity has proven difficult to reconcile with the other forces. Recently, relationships between scattering amplitudes (the quantity related to the probability for an interaction to occur between two or more particles) in non-abelian gauge theories (such as the theory of quarks and gluons) and theories of quantum gravity have led to the discovery of a relation known as the double copy. First observed in string theory, the double copy relates scattering amplitudes in quantum gravity to their counterparts for those in non-abelian gauge theories. This property has been extended to relate solutions in classical electromagnetism with those in general relativity, via a theory known as the classical double copy. As a tool, the double copy has been invaluable for deriving results in gravity, that otherwise would be harder or impossible to do from first principles.

Speaker: Kymani Armstrong-Williams (Queen Mary University of London)

20:40 Towards Classical Bulk Reconstruction of Asymptotically Flat Spacetimes

Abstract: The classical bulk reconstruction of Locally Asymptotically Anti-de Sitter (LAAdS) spacetimes and the associated computation of boundary correlators and a boundary stress-energy tensor constitute some of the first non-trivial checks of the AdS/CFT correspondence. I will begin this talk with a brief nod to this remarkable story, following which I will discuss setups that can be, and are being, considered for spacetimes that are asymptotically flat at null infinity. In particular, I will describe why a conformal Carrollian structure at null infinity could serve as a putative setting for a boundary dual. I will then outline a setup for a corresponding bulk reconstruction problem and comment on similarities and differences with the AdS scenario, with emphasis on the role played by gravitational radiation.

Comments: Based on ongoing work with Jelle Hartong, Emil Have and Gerben Oling.

Speaker: Vijay Nenmeli (University of Edinburgh)

21:00 Searching for Dark Matter and Astrophysical Signals at the LUX-ZEPLIN Experiment

In this poster, I will present the LUX-ZEPLIN experiment and how its world-leading sensitivity is being used to search for a variety of dark matter candidates and other astrophysical signals, such as coherent elastic neutrino nuclear scattering from Boron 8 solar neutrinos. Having already set the best limit on WIMP cross section, $\sigma_{SI} = 2.2\times 10^{-48}$cm$^{2}$ at $43$ GeV/c$^{2}$, we look at how LUX-ZEPLIN’s results can help constrain the parameter space of theories and support our understanding of the Standard Model.

Speaker: Ms Trinity Stenhouse (UCL)

Trinity_Poster_YTF.pdf

21:20 Dress for dS: Elevating Flat-Space Feynman Diagrams to Cosmological Correlators

Inflation is a period of the very early universe proposed to explain, among other experimental observations, the fact that the Cosmic Microwave Background is incredibly isotropic. The characteristic energy scale of such a period may have been anywhere up to $10^{15}$ GeV, and therefore observables from this period can give us information about physics well beyond the reach of any conceivable particle collider. Such observables are related to correlation functions on the future infinity boundary of de Sitter space. In this poster I will present our work on scalar $\varphi^4$ and $\varphi^3$ interactions, where we develop a way of obtaining correlation functions on the boundary of de Sitter by “dressing” flat-space Feynman diagrams with additional propagators.

Speaker: Joe Marshall (Durham University)

21:40 Algebraic methods for matrix quantum mechanics with orthogonal polynomials

Poster Abstract:
When using matrix methods in quantum mechanics it is common to use orthogonal polynomials as an expansion basis. The eigenvalues and eigenvectors of such provide the energies and wavefunctions for bound states in the Schrodinger equation, and when considering a complex scaling method we can extract the lifetimes of resonance states. Investigating the properties of orthogonal polynomials can provide insights into more computationally efficient methods for constructing the necessary matrices. We present results on the application of these methods to a number of simple test cases of $1D$ problems where we can compare to standard theory.

Speaker: Mackenzie Gibbons (Swansea University)

22:00 $B^{(*)}, D^{(*)}, B_s^{(*)}, D_s^{(*)}$ Decay Constants and Hyperfine Splittings from Lattice QCD using the Heavy-HISQ Method

Decay constants parametrise non-perturbative strong interactions in leptonic weak decays of mesons. Here, we present new lattice QCD results for the vector-to-pseudoscalar and tensor-to-vector ratios of decay constants of the $B^{(*)}, B_s^{(*)}, D^{(*)}$ and $D_s^{(*)}$ mesons. Many correlated uncertainties cancel in these ratios, yielding high-precision results. We use the Highly Improved Staggered Quark (HISQ) action for all valence quarks, second-generation MILC $n_f = 2+1+1$ HISQ gauge configurations, lattice spacings ranging from $0.15 \text{fm}$ down to $0.045 \text{fm},$ and pion masses ranging from $\approx 300 \text{MeV}$ down to the physical value. The mass dependence of these ratios is determined using the heavy-HISQ method: heavy-quark masses range from the physical charm up to the physical bottom on the finest lattices we use. We also calculate hyperfine splittings — mass differences between pseudoscalar mesons and their associated vectors. All values presented are preliminary. These quantities provide stringent tests of Standard Model flavour phenomenology, sensitivity to new physics, and important inputs to future theoretical calculations.

Speaker: Kerr Miller (University of Glasgow)

Thursday 19 December

09:00 → 09:10 General Address: Thursday Welcome

Welcome & Introduction: Elliot Fox & Ery McPartland
Thursday Welcome: Elliot Fox
Farewell Address: Elliot Fox & Malina Rosca

YTF 2024 Welcome Slides.pptx

09:10 → 10:40 Amplitudes and Double Copy

Chair: Hector Puerta Ramisa

09:10 Minimal Cuts and Genealogical Constraints on Feynman Integrals

We introduce an efficient method for deriving hierarchical constraints on the discontinuities of individual Feynman integrals. This method can be applied at any loop order and particle multiplicity, and to any configuration of massive or massless virtual particles. The resulting constraints hold to all orders in dimensional regularization, and complement the extended Steinmann relations -- which restrict adjacent sequential discontinuities -- by disallowing ordered pairs of discontinuities from appearing even when separated by (any number of) other discontinuities. We focus on a preferred class of hierarchical constraints, which we refer to as genealogical constraints, that govern what singularities can follow from certain minimal cuts that act as the primogenitors of the discontinuities that appear in Feynman integrals. While deriving the full set of hierarchical constraints on a given Feynman integral generally requires identifying all solutions to the (blown up) Landau equations, these genealogical constraints can be worked out with only minimal information about what singularities may appear. We illustrate the power of this new method in examples at one, two, and three loops, and provide evidence that genealogical constraints restrict the analytic structure of Feynman integrals significantly more than the extended Steinmann relations.

Speaker: Maria Polackova (University of Edinburgh)

Genealogical Constraints - YTF 2024.pdf

09:40 The Double Copy: A Duality for Particles and Gravity

An open problem in theoretical physics is to combine all four of the fundamental forces of nature into one single theory. Problematically, gravity has proven difficult to reconcile with the other forces. Recently, relationships between scattering amplitudes (the quantity related to the probability for an interaction to occur between two or more particles) in non-abelian gauge theories (such as the theory of quarks and gluons) and theories of quantum gravity have led to the discovery of a relation known as the double copy. First observed in string theory, the double copy relates scattering amplitudes in quantum gravity to their counterparts for those in non-abelian gauge theories. This property has been extended to relate solutions in classical electromagnetism with those in general relativity, via a theory known as the classical double copy. As a tool, the double copy has been invaluable for deriving results in gravity, that otherwise would be harder or impossible to do from first principles.

Speaker: Kymani Armstrong-Williams (Queen Mary University of London)

Presentation_YTF_KAW.pdf

10:10 The Double Copy in AdS3 from Minitwistor Space

One of the main questions in the classical double copy is its precise realisation around curved spacetime. One can make progress using a recent twistor formulation of the double copy. In the case of topologically massive theories in curved spacetime, it can give rise to the previously known double copy of waves. However the curved spacetime alters the straightforward "equal mass" prescription to an unequal one, which has the advantage simplifying everything. Finally, it also enables to extend the double copy of these theories to warped black holes.

Speaker: Theo Keseman (Imperial College London)

Double_copy_in_AdS3_presentation_Theo_Keseman.pdf

09:10 → 10:40 Precision and Flavour Physics

Chair: Livia Maskos

09:10 Data Driven Calculation of Hadronic Vacuum Polarisation Contributions to Muon g-2

The 5.2$\sigma$ discrepancy between the 2020 theoretical evaluation of the muon anomalous magnetic moment $a_\mu$ and the most recent measurement made by the Fermilab muon $g-2$ collaboration has the potential to be one of the most significant results in modern particle physics. However, tensions between experimental inputs to the dispersive calculation of the hadronic vacuum polarisation (HVP) contribution to $a_\mu$, and further tensions with lattice calculations, suggest matters may not be so clear-cut and raise important and interesting questions.
I will present my ongoing work with the KNTW collaboration to produce a new calculation of $a_\mu^{HVP}$, with the ultimate aim of answering these questions and contributing significantly to an extremely precise test of the Standard Model.

Speaker: Aidan Wright (University of Liverpool)

YTF24 (3).pdf

09:40 The flavour structure of the LEFT

The growing array of precision measurements below the electroweak scale holds substantial significance for models of heavy new physics. To fully utilise these measurements, we must understand the running of the many operators of the Low Energy Effective Field Theory (LEFT aka WET). In this talk, I will introduce the LEFT and outline how decomposing operators according to their flavour and parity symmetries can simplify the running, making the map from electroweak scale to $b$-mass scale semi-analytically solvable. I will also discuss the applications of this work to flavour phenomenology.

Speaker: Ben Smith (University of Glasgow)

YTF_2024.pdf

10:10 Exploring Dimension-8 Operators in $W^+ W^-$ Production via Gluon Fusion at the LHC

The discovery potential of the Standard Model Effective Field Theory (SMEFT) is expanded by studying dimension-8 operators and their impact on $W^+W^−$ production in gluon-gluon fusion at the Large Hadron Collider (LHC). This channel, which may contain large Beyond the Standard Model (BSM) contributions in the high-energy tail of its di-lepton mass distribution, presents unique challenges due to the reliance on jet-vetoes to suppress $t\bar t$ backgrounds. We investigate the interplay between dimension-6 and dimension-8 operators, highlighting their respective energy-growing contributions and the implications for EFT fits. In this talk I will present current constraints and sensitivity projections for the High Luminosity LHC using state-of-the-art accuracy predictions for both signal and background. I will show the effect that the jet-veto and other higher order effects have on the ability to constrain these operators. Additionally, we explore scenarios in which stringent dimension-6 constraints justify the independent examination of dimension-8 operators. This talk presents an interesting case study on how one channel can be used to constrain a subset of EFT operators and how it fits into the bigger picture of global EFT fits using large numbers of collider observables.

Speaker: Daniel Gillies (University of Sussex)

Durham Slides.pdf

10:40 → 11:00 Break

Cold drinks and biscuits

11:00 → 12:30 Axions and Astroparticle Phenomenology

Chair: James Maxwell

11:00 Supernova bounds on new scalars from resonant and soft emission

The observations of supernovae can place strong bounds on the couplings of particles beyond the standard model, with masses all the way up to ~200MeV. We study new scalar particles and show that for masses less than the plasma frequency in the supernova core, they are predominantly produced by resonant mixing with an emergent in-medium degree of freedom known as the longitudinal photon. Unlike the nucleon-nucleon bremsstrahlung production mechanism which dominates at larger masses, this mechanism is free from nuclear physics uncertainties.

Speaker: Henry Stubbs (University of Oxford)

Supernova Bounds H Stubbs YTF.pptx

11:30 Relaxing Limits from Big Bang Nucleosynthesis on Heavy Neutral Leptons with Axion-like Particles

Heavy neutral leptons (HNLs) are constrained by requirements of Big Bang Nu- cleosynthesis (BBN) as their decays significantly impact the formation of the primordial elements. We propose here a model where the primary decay channel for the HNLs is to an axion-like particle (ALP) and a neutrino. Consequently, HNLs can decay earlier and evade the BBN bound for lower masses, provided the ALPs themselves decay considerably later. Further cosmological and astrophysical constraints limit severely the range of validity of the ALP properties. We find that a new parameter region opens up for HNLs with masses be- tween 1 MeV and 1 GeV, and active-sterile neutrino mixing strengths between 10−9 and 10−6 that is consistent with constraints and can be probed in future searches. In such a scenario, current bounds as well as sensitivities of future direct HNL searches such as at NA62 and DUNE will be affected.

Speaker: Zhong Zhang (UCL)

zhongzhang_YTF.pdf

12:00 Axion-photon conversion from Neutron star populations

The QCD axion, a favourable candidate for dark matter and a solution to the strong CP problem, can efficiently convert into photons in the presence of high magnetic fields. Neutron stars harbour high magnetic fields ($ \approx 10^{12}$ G) and serve as powerful probes to search for axion-photon conversion via observation of radio emission at the axion frequency $\hbar \omega = m_a c^2$, with $m_a$ the axion mass. The non-observation of signals currently places upper limits on the axion-photon coupling. Recently, much discussion has arisen about the efficient modelling of the neutron star population in the galaxy and a comparison between a ‘single star observation’ vs. a ‘population’ type observational approach. In our current work, I (with my collaborators) tackled these issues and used PsrPopPy, a Python-based package for modelling neutron stars in the galaxy and estimated the axion-photon signal from the galactic population. We provide the pros and cons of conducting a population analysis over a single-star analysis both for existing constraints and future MeerKAT and SKA observations.

Speaker: Utkarsh Bhura (King's College London)

YTF_Presentation.pdf

11:00 → 12:30 Mathematical Structures in Quantum Field Theory

Chair: Hector Puerta Ramisa

11:00 Varieties of four-dimensional gauge theories

Given a gauge Lie algebra, it is natural to seek representations for four-dimensional spacetime fermions that are anomaly-free and complex. Even for irreducible representations, where the problem reduces to studying $\mathfrak{su}_n$ for $n\geq3$, solutions seem to be few and far between: a trial-and-error scan by Eichten, Kang and Koh found only three for $\mathfrak{su}_5$, for example. In this talk, I will explain how concepts and constructs in algebraic geometry show that there are in fact infinitely many such representations and yield them all. In particular, I will show that the problem is nearly identical to finding chiral solutions to the $U(1)$ anomaly cancellation equations for $n$ Weyl fermions in four spacetime dimensions.

Speaker: Khoi Le Nguyen Nguyen (University of Cambridge)

YTF_2024_Varieties_Presentation.pdf

11:30 Effective field theories for conformal defects

In this talk I shall talk about insertion of defects into a conformal field theory. Specifically, I shall discuss a setup with two conformal defects, and an effective field theory description of this setup, and the symmetries that this setup preserves. This is based on work with Petr Kravchuk and Ritam Sinha (arXiv:2406.04561).

Speaker: Alex Radcliffe (King's College London)

YTF Final.pdf

12:00 Replica analysis of entanglement properties and conditions for islands

Entanglement entropy quantifies the degree of entanglement between two quantum systems or between two subregions in a QFT and hence is an important tool to understand the quantum system. However, its study in dimensions $> 2$ has been mostly limited to flat backgrounds and CFT vacuum states in specific subregions due to technical as well as conceptual difficulties. In this talk, I will present a systematic analysis of the properties of entanglement entropy in curved backgrounds using the replica approach. We will explore the analytic $(q−1)$ expansion of Rényi entropy $S_q$ and its variations; the setup applies to generic variations, from symmetry transformations to variations of the background metric or entangling region. Our methodology elegantly reproduces and generalises results from the literature on entanglement entropy in different dimensions, backgrounds, and states. We will then use this analytic expansion to explore the behaviour of entanglement entropy in static black hole backgrounds under specific scaling transformations. We will show that certain conditions on this quantity and hence the QFT spectrum have to be satisfied for the presence of islands of entanglement, which provide enough quantum corrections to restore unitarity in black hole evaporation.

Speaker: Arvind Shekar (University of Southampton)

Arvind_Shekar_YTF_Durham.pdf

12:30 → 13:30 Buffet Lunch

13:30 → 14:30 Baryogenesis and Leptogenesis

Chair: Ansh Bhatnagar

13:30 Baryogenesis in the 2HDM+a

There are a multitude of Standard Model extensions that can accommodate a first-order electroweak phase transition in the Early Universe, with the aim of providing an explanation for the observed baryon asymmetry. In this talk, I will introduce a particular extension: a Two Higgs Doublet Model plus a SM singlet pseudoscalar (2HDM+a). I will discuss how one starts from this model and ends with a viable parameter space that produces the observed baryon asymmetry.

Speaker: Tom Gent (University of Sussex)

YTF Presentation.pdf

14:00 A Natural Mechanism for Resonant Leptogenesis

I will present a natural mechanism to achieve resonant leptogenesis through a novel approach involving thermal corrections to sterile neutrino masses. In standard leptogenesis, right-handed neutrinos are introduced as an extension to the Standard Model, generating a lepton asymmetry via CP-violating decays. This asymmetry is then converted into a baryon asymmetry through sphaleron processes. Resonant leptogenesis, where the masses of two sterile neutrinos are nearly degenerate, enhances CP violation but requires fine-tuning.

In this work, I propose a model of non-thermal leptogenesis gauged under $U(1)_{B−L}$ symmetry, where the near-degeneracy of sterile neutrino masses is dynamically achieved through thermal corrections at early times. I will show that the dominance of thermal masses over zero-temperature masses across a large parameter space naturally fulfils the degeneracy condition, enabling resonant leptogenesis without fine-tuning. Preliminary benchmark results indicate that this mechanism allows for significantly lighter sterile neutrino masses than standard non-thermal leptogenesis, expanding the parameter space for viable models.

I will conclude by discussing extensions to this framework, including the exciting prospect of testing our predictions through the detection of a gravitational wave background linked to the $U(1)_{B−L}$ symmetry breaking.

Speaker: Mr Angus Spalding (University of Southampton)

A_Natural_Mechanism_for_Resonant_Leptogenesis.pdf

13:30 → 14:30 Branes and Higher Dimensions

Chair: Hector Puerta Ramisa

13:30 Kaluza-Klein theory with fuzzy sphere fibre

In the Kaluza-Klein theory with fuzzy sphere fiber, I will show how SU(2) Yang-Mills field and a real-symmetric-matrix valued Liouville-sigma model field arise from the dimensional reduction of Ricci scalar. Moreover, with this noncommutative fiber, we can derive the cylinder ansatz in usual KK theory.

Additionally, the KK tower of scalar and spinor field will obtain the natural truncation without using any approximation. Especially, for 0-radius fuzzy sphere, a massless singlet spinor will become a massive doublet spinor. This could provide new insights into Standard Model.

Speaker: Chengcheng Liu (Queen Mary University of London)

kk theory with fuzzy sphere fibre.pdf

14:00 Do we live on the End of the World?

End-of-the-World branes are codimension-one hypersurfaces that mark the ending of spacetime. Interestingly, these have been argued to be necessary ingredients in quantum gravity to prevent the presence of global symmetries. Can they also serve as braneworlds? (Spoiler: Yes.) Motivated by the Swampland Programme and in particular the Cobordism Conjecture, we propose a scenario in which a compact region of AdS$_5$ nucleates from nothing, with a dS$_4$ spacetime living on its boundary. We show that it can equivalently be interpreted as an up-tunnelling from AdS$_5$ with cosmological constant $\Lambda\to -\infty$, following Brown and Dahlen’s proposal for ‘nothing’. Their picture naïvely suggests that the brane has infinite negative tension. However, we show that it becomes finite and positive once we employ holographic renormalization, recovering the Bubble of Something, where the domain wall becomes a boundary of spacetime. The same holds true in any number of dimensions and, moreover, at the level of metric perturbations. This provides motivation for alternative routes of obtaining cosmology from quantum gravity or string theory using domain walls, departing from conventional vacuum approaches.

Speaker: Benjamin Muntz (University of Nottingham)

ETW - YTF 24 Durham.pdf

14:30 → 14:50 Break

Cold drinks and biscuits

14:50 → 16:20 Dark Matter and Gravitational Observables

Chair: Jack Franklin

14:50 Gravitational waves and de Sitter spacetime

Abstract:

Understanding and detecting gravitational waves is key to testing new theories.
Gravitational wave calculations from black holes all assume the background spacetime is flat.
Understanding these in a de Sitter background is imperative, and has a two-fold effect:
- It expands the reach of future gravity waves observations to the earliest universe (inflation) and also the largest distances (where our universe’s expansion becomes relevant).
- Develops our understanding of the asymptotic behaviour of gravitational waves in a de Sitter spacetime.

Speaker: David Waisman Andrade (Queen Mary University of London)

YTF Durham presentation.pptx

15:20 A novel probe of graviton dispersion relations at nano-Hertz frequencies

We generalise Phinney's 'practical theorem' to account for modified graviton dispersion relations motivated by certain cosmological scenarios. Focusing on specific examples, we show how such modifications can induce characteristic localised distortions, bumps, in the frequency profile of the stochastic gravitational wave background emitted from distant binary sources. We concentrate on gravitational waves at nano-Hertz frequencies probed by pulsar timing arrays, and we forecast the capabilities of future experiments to accurately probe parameters controlling modified dispersion relations.

Speaker: Bill Atkins (Swansea University)

dispersionytf.odp

15:50 Listening for ultra-heavy dark matter with underwater acoustic detectors

The possibility of detecting ultra-heavy dark matter often poses a significant challenge due to the incredibly suppressed flux. As a result, even models with high cross sections with respect to the weak scale WIMPs remain unconstrained. In this talk, we show the potential for future large scale acoustic detectors in the ocean initially proposed for ultra high energy neutrino detection could probe the ultra-heavy dark matter parameter space.

Speaker: Damon Cleaver (King's College London)

YTF24_DCleaver.pdf

14:50 → 16:20 Gravity and Black Holes

Chair: Hector Puerta Ramisa

14:50 Higgs Mechanism for the Ashtekar Self-Dual Connection

We introduce the Higgs mechanism for the self-dual spin connection (also known as the Ashtekar connection), using the Pleba\'{n}ski formulation of gravity. We develop our formalism within the framework of the chiral action and derive the equations of motion of the theory. One particular test model is explored: since anisotropy is an intrinsic property of the theory, a modified version of the spatially flat Bianchi I model with two different scale factors is considered. We apply our formalism and derive the Friedmann equations which regulate the scale factors and the Higgs field. We also present a Proca-like term for the connection, which when reduced to minisuperspace with a positive $\Lambda$ yields a De Sitter universe with an effective cosmological constant that depends on the mass of the gauge fields. We finally investigate the effect of these mass terms on gravitational waves and find that the wave equation remains unchanged relatively to GR; however the Weyl tensor is scaled by a constant which depends on the mass of the connection components.

Speaker: Bruno Alexandre (Imperial College London)

HM_BrunoAlexandre.pdf

15:20 Massive spinning fields in 3d quantum gravity

While understanding the consequences of quantising gravity remains an ongoing challenge, valuable insights have been gained by considering models in lower dimensions. In particular, I shall focus on Einstein gravity in 2+1d, which is known to be related to Chern-Simons theories, with a gauge group dependent on the sign of the cosmological constant. Unlike in 3+1d where gravitational waves are an important feature, Einstein gravity in 2+1d is a topological theory. Despite this, the theory retains a number of interesting features present in 3+1d gravity, such as the (potentially spinning) BTZ black hole. In this talk I will review the connection between 2+1d gravity and Chern-Simons theory, both in the classical and quantum cases. I will then discuss some recent work which focuses on how massive matter fields on the gravitational side may be included in the Chern-Simons construction.

Speaker: Robert Bourne (University of Cambridge)

YTF24 - Robert Bourne.pdf

15:50 Multi-metric black holes and the Gregory-Laflamme instability

Multi-metric gravity is the umbrella term for a class of modified gravitational theories, motivated by a number of problems at the interface between gravity and particle physics, that extend general relativity (GR) via the inclusion of additional interacting massive spin-2 fields beyond the single massless graviton of GR. Nonlinearly, the extra interactions manifest as a framework where multiple metric tensors interact with one another on the same spacetime manifold (hence the name). To date, black hole solutions of these multi-metric theories are only understood for the simplest case of bigravity (N=2 metrics) in 4 dimensions. It is, for example, known that one class of bigravity black holes is unstable for certain values of the graviton mass, with the instability taking the same form as the notorious Gregory-Laflamme (GL) instability plaguing higher dimensional black strings. In this talk, I will show how to generalise the 4d bigravity results to the full multi-metric theory in arbitrary dimension, constructing a wide class of black hole solutions of the general theory, and determine their linear stability. I will also elucidate the link between the instabilities of these multi-metric black holes and those of higher dimensional black strings. The result seems to suggest that the GL instability may be more fundamentally linked to the nature of massive spin-2 interactions.

Speaker: Kieran Wood (University Of Nottingham)

Black holes multigravity YTF talk.pdf

16:20 → 16:30 General Address: Farewell Address

Welcome & Introduction: Elliot Fox & Ery McPartland
Thursday Welcome: Elliot Fox
Farewell Address: Elliot Fox & Malina Rosca

YTF 2024 Welcome Slides.pptx