Speakers 2024

Kinga Gawrych

Title of the talk:

Circumventing Derrick's Theorem - constructing topological solitons in electroweak theory

Abstract:

Topological solitons arise in a variety of non-linear systems, including quantum field theory. Topological arguments are often used to argue for, but do not guarantee, the existence of solitonic solutions. For example, Derrick's theorem predicts that some of these are unstable.

In this talk, I will explain the significance of topological solitons in electroweak (EW) theory, with a focus on instantons (4D Euclidean solitons) and how they mediate baryon number violation in the Standard Model. I will describe how Derrick's theorem prevents the existence of EW instantons and subsequently show how to bypass it. I will also present results in a toy model for EW theory. To conclude, I will discuss experimental prospects and, if time permits, make connections to the worldline formalism in QFT.

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Harold Steinacker

Title of the talk:

Emergent 3+1-dimensional gravity from the IKKT matrix model

Abstract:

A mechanism for a 3+1-dimensional gravity on quantized branes in the IKKT matrix model is discussed. The Einstein-Hilbert action arises as a quantum effect on suitable backgrounds, as part of a higher-spin extended gauge theory. This can be seen as emergent gravity arising from the open string sector. The computation involves a novel technique for computing geometric traces based on string modes in matrix models.

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Maulik Parikh

Title of the talk:

Free-Falling in Quantum Spacetime

Abstract:

When the spacetime metric is regarded as a quantum field, the classical trajectories of freely falling objects are subject to random fluctuations, or “noise”. The noise can be regarded heuristically as arising from the bombardment of the falling object by gravitons. Remarkably, this fundamental noise might even be observable at gravitational wave detectors: if detected, it would provide experimental evidence for the quantization of gravity. The effect of the quantum-gravitational noise is to turn the classical geodesic deviation equation into a stochastic, Langevin-like equation. Moreover, when these results are extended to congruences of geodesics, one finds an additional term in the Raychaudhuri equation which suggests that, at least at the level of linearized quantum gravity, the quantum fluctuations of spacetime do not save general relativity from the menace of singularities.

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Filippo Fecit

Title of the talk:

Massive Gravity in the Worldline Formalism

Abstract:

Despite the considerable success of General Relativity, the search for alternatives has been an ongoing challenge since its formulation to address some of the most crucial open questions in physics, such as the cosmological constant problem and the origin of the late-time acceleration of the universe. An intriguing modification suggests that gravity could be propagated by a massive spin 2 particle: a massive graviton.

In this talk, I will present a novel discussion from a worldline perspective, employing the first-quantized models known as O(N) spinning particles. Specifically, our focus will be on the massive N=4 spinning particle. First, I will show that BRST quantization allows for the correct reproduction of the Fierz-Pauli theory on Minkowski. The extension to curved spacetime is trickier, and nilpotency of the BRST charge seems to require a Ricci-flat spacetime as the only consistent background. Once the BRST system is realized, it is possible to construct the worldline path integral on the circle, providing a worldline representation of the one-loop effective action of Linearized Massive Gravity. If time permits, I will discuss the calculation of the Seeley-DeWitt coefficients, including the a_3(D) coefficient previously not known in the literature.

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final - Massive Gravity in the Worldline Formalism.pdf

Antonia Micol Frassino

Title of the talk:

Gravitational waves and the quantum nature of weak gravitational fields

Abstract:

Potential experimental signatures of the quantum nature of gravity are becoming a thriving field with a multitude of approaches in different directions. In this talk I will present quantum optics techniques useful to probe the interaction between gravitational waves and an idealized detector. This idea is explored aiming at characterizing quantum signatures at gravitational wave interferometers.

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Matthias Carosi

Title of the talk:

Finite volume effects in the early universe: inducing a cosmological bounce with instantons

Abstract:

Inflation, the current paradigm of early universe cosmology, is known to present a number of conceptual challenges. The cosmological bounce is an alternative scenario which tries to address some of these issues. In order to induce a bounce, violation of the Null Energy Condition (NEC) is needed, which usually relies on exotic matter or modified gravity.

We consider a real scalar field in a double-well potential inside a finite flat universe, and show how purely quantum finite volume effects violate the NEC and could support a cosmological bounce. We derive analytically the full one-loop vacuum energy as the sum of Casimir energy and tunneling contribution. The latter is described by euclidean time instantons, and we show how to extract the contribution of the zero-mode and to compute the functional determinant around the instanton background.

The resulting energetic properties are non-trivial: both the Casimir effect and tunneling contribute to the NEC violation, arising from a non-extensive true vacuum energy.

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Lucas Manzo

Title of the talk:

Worldline approach for spinor fields in manifolds with boundaries

Abstract:

The worldline formalism is a technique with several applications in Quantum Field Theory. In this seminar we will focus on applying it to the much less studied case where boundaries play an important role. We will start by reviewing some aspects of the worldline formalism for scalar fields in manifolds without boundaries, to then review two generalizations: spinors fields in manifolds without boundaries, and scalar fields in manifolds with boundaries. From this, we will construct the heat kernel of the auxiliary point-particle for a spinor field with an Abelian background gauge field in a two-dimensional curved half-plane under MIT bag boundary conditions. As a verification of this construction, we will also highlight the computation of the heat-trace asymptotics by working out the first Seeley-DeWitt coefficients. This seminar is based on https://arxiv.org/abs/2403.00218.

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Worldline_Seminars_22_04_2024.pdf

Giulio Audagnotto

Title of the talk:

Dynamics, quantum states and Compton scattering in nonlinear gravitational waves

Abstract:

The classical dynamics and the construction of quantum states in a plane wave curved spacetime are examined, paying particular attention to the similarities with the case of an electromagnetic plane wave in flat spacetime. A natural map connecting the dynamics of a particle in the Rosen metric and the motion of a charged particle in an

electromagnetic plane wave is unveiled. We then discuss how this map can be translated into the quantum description by exploiting the large number of underlying symmetries. We examine the complete analogy between Volkov solutions and fermion states in the Rosen chart and properly extend this to massive vector bosons. We finally report the squared S-matrix element of Compton scattering in a sandwich plane wave spacetime in the form of a two-dimensional integral.

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worldliners2904.pdf

Robert Smith

Title of the talk:

From number theory to physics: Introducing η regularisation

Abstract:

In the mathematics of particle physics, it is not uncommon that a calculation should result in infinity. The long standing hope has been that the correct theory of quantum gravity will act as a sort of universal UV regulator in the low energy limit of QFT. As it stands, there is no one choice of regularisation that hints at any connection to quantum gravity, or which can be applied for all purposes and for all QFTs. Motivated by stringy behaviour in the UV, in this talk I will describe ongoing research that considers these questions. In particular, beginning at the nexus of number theory and physics, I will focus on [2401.10981] and describe a general class of $\eta$ regulator functions that give some naive hints at stringy structure.

I will show how this broad class of regulators can be extended as a potentially generalised method of regularisation (for all QFTs). In the important extension to gauge theories, I will then highlight the derivation of a set of generalised gauge consistency conditions and discuss the remarkable result that connects the sum of natural numbers being minus one twelfth to the preservation of gauge symmetries. To conclude this talk, I will comment about how $\eta$ regularisation connects to the Schwinger representation, before summarising current and future research efforts involving the study of string amplitudes, the worldline picture, among other related topics.

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From number theory to physics-Intro to eta regs (Worldliners).pdf

Karthik Rajeev

Title of the talk:

N-photon tree level amplitudes in plane wave and impulsive PP-wave backgrounds (Part 1: Scalar QED)

Abstract:

The presence of strong electromagnetic fields greatly increases the complexity of QED Feynman diagram calculations. However, in such cases, the worldline path-integral formalism has shown to be a useful tool for determining scattering amplitudes with arbitrary photon multiplicity. Given this, we start with a brief review of the application of this formalism to the computation of tree-level correlators with N photons and two scalar legs, in a class of external backgrounds.

Thereafter, we show that when specialized to an arbitrary plane wave background, a hidden Gaussianity of the worldline path integral enables exact computation of such correlators and the corresponding amplitudes. The plane wave background is treated without approximation throughout, meaning in particular that our formulae are valid in the strong-field regime of current theoretical and experimental interest. We shall also briefly discuss the implication of our result to the case of scattering on an arbitrary impulsive PP-wave background.

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Seminar_WL_meeting_2024_KR.pdf

Patrick Copinger

Title of the talk:

N-photon tree level amplitudes in plane wave and impulsive PP-waves backgrounds

(Part 2/2: Spinor QED)

Abstract:

Building on the scalar results presented last week by Karthik Rajeev, I further present

master formulae for spinor QED in a plane wave background, succinctly capturing an all-multiplicity description by virtue of the worldline formalism [1]. It is discussed how, for spinors as well, an exact evaluation of the path integral is possible by virtue of a hidden Gaussianity, leading to an implicit N-photon dependence in the background field. In the worldline formalism, at the level of the propagator's kernel, the extension from scalars to spinors is facilitated through an additional spin factor, which we likewise find permits an exact evaluation in a plane wave background.

Further, it will be discussed how only the kernel enters a worldline description of the N-photon dressed tree level amplitude, providing a major simplification.

I also present master formulae for spinor QED in an implusive PP-wave background,

whereby for e.g. multiple nonlinear Compton scattering, it is shown that off-shell currents, and scattering amplitudes, dressed with N-photons may be expressible as off-shell currents in 1) N+1 photons in vacuum, or 2) an implusive plane wave background [2].

[1] P.C., J.P. Edwards, A. Ilderton, K. Rajeev, [hep-th] 2311.14638 (2023), Phys. Rev. D.

[2] P.C., J.P. Edwards, A. Ilderton, K. Rajeev, [hep-th] 2405.07385 (2024).

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Copinger_Worldline_May_2024.pdf

Riccardo Gonzo

Title of the talk:

Classical scattering and bound observables from the worldline approach to the two-body problem

Abstract:

Motivated by the current and future gravitational wave experiments, I will consider the binary Kerr black hole dynamics using a point particle description. It was recently shown how data for scattering orbits can inform bound-orbit models, allowing to harness the power of the S-matrix and worldline tools to construct gravitational waveforms. First, I will derive the so-called radial action from the worldline formalism, focusing on the case of a spinning probe particle in Kerr spacetime for simplicity. I will then show that such radial action (and the S-matrix) is a natural generating functional of classical observables, uncovering a novel analytic continuation between the basis of scattering (coordinate time delay, elapsed proper time and deflection angle) and bound (radial frequency, averaged redshift and periastron advance) observables in the conservative aligned-spin case. Finally, I will then discuss the extension of the scatter-to-bound map to the misaligned spin case (i.e., with precession) and to the gravitational waveform itself, emphasizing the need of studying non-local-in-time effects to push this correspondence to all orders in perturbation theory.

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Pierre Vanhove

Title of the talk:

Amplitude and worldline EFT approaches to observables in classical gravity

Abstract:

In this talk we will present new formulations for computing classical observables for the gravitational interaction between two massive bodies. One approach is based on a reformulation of the standard S-matrix, but adapted to extract classical observables relevant for gravitational wave physics. A second approach is based on an effective worldline action of a massive point particle.

We will present the advantages for the calculation of either post-Minkowskian or self-force contributions, and the equivalence between the two approaches for the calculation of classical observables.

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Ravisankar Rajagopal

Title of the talk:

Classical Limit of the Worldline and the Eikonal Method

Abstract:

In this talk, we discuss the equivalence between the eikonal method and the WQFT method by systematically deriving the WQFT rules starting from the worldline formalism, which is the first quantized version of QFT. We will work with the operator version of the worldline formalism, which makes the eikonal expansion of the amplitude easy and straightforward. We will carefully analyse the \hbar counting (that allows us to extract the classical piece of the amplitude) and radiative corrections on the worldline.

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Yuchen Du

Title of the talk:

Worldline proof of eikonal exponentiation

Abstract:

In this talk, we will explore the concept of eikonal exponentiation, which lies at the core of the eikonal method. We will demonstrate that the worldline formalism, in the classical limit, naturally reproduces the structure predicted by the idea of exponentiation. This allows a direct investigation of the conjecture and provides insights into which types of theories are well-suited for taking the classical limit, in which case the exponentiation holds. For scalar QED and scalars interacting gravitationally, we prove the exponentiation of the 2-body eikonal phase to all orders in the eikonal expansion. We will also present an alternative approach to generate/calculate WQFT diagrams by using the scattering amplitude as a generating function.

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Misha Arturo Lopez-Lopez

Title of the talk:

Low-energy N-Photon amplitudes in constant fields

Abstract:

In this seminar, I will discuss the calculation of the low-energy limit of the N-photon amplitude for scalar and spinor QED in the presence of constant background fields using the Worldline formalism. Taking the low-energy limit for all photons allows us to perform almost all the integrals involved in the Worldline master formulas for N-photon amplitudes. I will demonstrate how to compute these integrals and obtain compact representations for specific cases of the N-photon amplitudes.

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Sebastian Grieninger

Title of the talk:

Entanglement entropy and holographic Schwinger pair creation in (time-dependent) electric fields.

Abstract:

We computed the entanglement of a Schwinger quark and anti-quark pair created by external (time dependent) electric fields of arbitrary strength, using a holographic dual description of QCD. In the semi-classical approximation, the pair creation by a pulse of external electric field is captured by a periodic worldline instanton. In the dual gravity description, the worldsheet instanton exhibits a falling worldsheet wormhole in Anti-de Sitter space. In particular, we clarify the effect of real radiation off the produced particles on quantum entanglement of the pair and the impact of color confinement. (Based on arXiv:2305.07121 and arXiv:2310.12042)

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Allic Sivaramakrishnan

Title of the talk:

Proper Time Correlators in Field Theory and Gravity

Abstract:

Proper time is a simple classical observable, but its correlations are less well understood. We define correlation functions of proper time for massive worldlines coupled to quantum field theory and quantum gravity, and we show how to compute perturbative corrections using Feynman diagrams.

When the worldline endpoints are held fixed, proper time correlators are derivatives of the on-shell action with respect to mass, or more generally, derivatives of the logarithm of local correlators. These proper time correlators encode correlated path fluctuations. When the worldline endpoints are dynamical and determined relationally in a gravitational system, a proper time delay operator is a smeared graviton operator to leading order. The two-point function computes the leading-order signature of quantum gravity in a toy model of a LIGO-type interferometer. This prediction agrees qualitatively with the experimental observable computed in a more realistic model.

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Richard Myers

Title of the talk:

Carrollian Partition Functions and the Flat Limit of AdS

Abstract:

In AdS/CFT, the generating function of boundary correlators plays a central role and is computed in the bulk by the AdS path integral as a functional of its boundary conditions. The analogous object in Minkowski space is the S-matrix path integral. I will define this object and show that it can also be interpreted as the generating functional of correlators in a putative dual Carrollian CFT supported, for massless particles, on null infinity. Should time allow, I will also describe how, in the flat space limit of AdS/CFT, the AdS path integral is related in a natural way to the S-matrix path integral of flat space.

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presentation_RichardM.pdf

Anton Sokolov

Title of the talk:

Worldline path integral approach to magnetic monopoles and the dual axion-photon coupling

Abstract:

One of the most general approaches to magnetic monopoles in the framework of quantum field theory is the first-quantized Schwinger-Zwanziger two-potential theory. I will introduce this theory, and explain how worldline path integral methods allow derivation of the Dirac-Schwinger-Zwanziger quantization condition for electric and magnetic charges.

Then, I will discuss a recent work where we used worldline methods in the context of the Schwinger-Zwanziger theory to calculate the effect of an intermediate heavy monopole state on the electromagnetic interactions of a well-motivated dark matter candidate called the axion. I will explain why the obtained result has important implications for axion detection experiments.

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Worldline_Seminar_Sokolov.pdf

Gustav Mogull

Title of the talk:

Black Holes Scattering from Worldline Quantum Field Theory

Abstract:

The Worldline Quantum Field Theory (WQFT) formalism has proven itself a powerful tool for perturbatively calculating the physical observables involved in two-body gravitational scattering events — change in momentum, scattering angles, radiated energy and angular momentum. In this talk I will review the WQFT formalism and summarise this progress. In particular, I will discuss recent work on scattering spinning black holes using bosonic worldline oscillators, and our recent state-of-the-art calculation of 5PM (4-loop) scattering observables, which includes functions deriving from a Calabi-Yau threefold geometry.

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Puxin Lin

Title of the talk:

Schwinger effect of extremal Reissner-Nordstrom black holes via worldline instantons

Abstract:

In this talk, a full computation of the spatial profile of the Schwinger production rate outside an extremal Reissner Nordstrom black hole will be presented. The Schwinger rate computed displays a switch-off threshold for charged particles with q < m and an exponential fall-off behavior when moving away from the horizon, which has not been captured in the existing literature. We will discuss the significance of understanding the Schwinger effect of charged black holes, its relation to the Cosmic Censorship Conjecture and Weak Gravity Conjecture and their implications.

We provide the details of the evaluation of the black hole Schwinger production rate via the worldline formalism and stationary point approximation, and demonstrate the success of a new method to diagonalize the second variation of the action. Our method could be numerically implemented for more generic setups. We will discuss the future directions of performing the Schwinger rate calculation for black holes in spacetimes with cosmological constants and comment on the gravitational backreactions of strong Schwinger effects.

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Maor Ben-Shahar

Title of the talk:

Massive spinning particles in background fields.

Abstract:

I will explain methods of coupling massive spinning particles to background fields, their quantization, as well as recent applications of the classical theories for gravitational wave physics. I will pay particular attention to understanding the degrees of freedom of the particles and how they describe spinning classical and quantum objects.

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