Speakers 2025

We investigate a geometric approach to determining the complete set of numerators giving rise to finite Feynman integrals. Our approach proceeds graph by graph, and makes use of the Newton polytope associated to the integral's Symanzik polynomials. It relies on a theorem by Berkesch, Forsgård, and Passare on the convergence of Euler--Mellin integrals, which include Feynman integrals. We conjecture that a necessary in addition to a sufficient condition is that all parameter-space monomials lie in the interior of the polytope. We present an algorithm for finding all finite numerators based on this conjecture.

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polytopes-wordlline-meetings.pdf

Martin Formánek

Title of the talk:

Amplifying strong field signatures in flying focus pulses

Abstract:

Strong field signatures in laser-particle interactions accumulate with interaction time. This can be leveraged with “Flying Focus” (FF) laser pulses where the moving focal point of a FF forms an intensity peak that can travel at any velocity, over distances much longer than a Rayleigh range. Specifically, particle beams can propagate with the laser focus, so that the particles stay in the region of peak field intensity for prolonged interaction times. In this talk, we will introduce generation methods and analytical description of FF pulses with arbitrary focal velocities and discuss experimental configurations in which the long laser-particle interaction aids high-intensity applications. Using FF pulses, we demonstrated that radiation energy loss accumulation, and accumulation of phase shift induced by vacuum birefringence enable experimental access to strong field effects. FF pulses accomplish that at orders of magnitude lower laser powers and intensities than in conventional fixed-focus setups, while still achieving comparable response. Even more importantly, in the quantum regime of the laser-electron interaction the energy loss and photon yield scale more favorably with the interaction time than the laser intensity. In this regime, FF pulses provide an outright advantage over fixed-focus pulses, with simulations of 10 GeV electrons colliding with 10 J pulses predicting 5× increase in the yield of 1-20 MeV photons.

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Ginevra Braga

Title of the talk:

Proper time path integrals for gravitational waves: an improved wave optics framework.

Abstract:

One of the most intriguing aspects of gravitational wave lensing is the emergence of wave effects, interference and diffraction patterns in the waveforms caused by finite-size effects, arising when the wavelength of the wave is comparable to the Schwarzschild radius of the lens.

Current approaches to studying these phenomena are based on an analogy with quantum mechanics, treating wave effects similarly to quantum effects. This allows the use of tools such as the Schrödinger equation and its path integral formulation to describe the dynamics of lensed gravitational waves.

Despite their potential, these theoretical methods are constrained by two key assumptions: the eikonal and paraxial approximations, which limit their validity, and the neglect of spin effects, which restricts their applicability to realistic astrophysical scenarios.

In this talk, I will present our new formalism based on the established proper time technique in field theory. This approach provides a natural generalization of the diffraction integral, overcoming the limitations of existing methods and offering a more robust framework for studying wave-optics effects in gravitational wave lensing.

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Worldliners_Braga_pdf.pdf

Filippo Fecit

Title of the talk:

A worldline approach to resummation and (assisted) pair production

Abstract:

I will show how the Worldline Formalism can be adapted to explore non-perturbative phenomena that originate from the interaction of quantum fields with classical backgrounds. As a functional approach, it naturally lends itself to non-perturbative analyses, where obtaining suitably resummed expressions for effective actions and heat kernels plays a crucial role. Focusing on the specific case of a quantum scalar field coupled to a Yukawa background, I will present a systematic resummation of all the invariants constructed from powers, first derivatives, and second derivatives of the background field. These results allow for the computation of vacuum instability by evaluating the vacuum persistence amplitude and the associated Schwinger pair production probability. Furthermore, I will illustrate how the presence of an additional, rapidly varying background can significantly enhance pair production.

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A worldline approach to resummations and assisted pair production.pdf

Gustav Uhre Jakobsen

Title of the talk:

Spinning bodies in general relativity from bosonic worldline oscillators

Abstract:

The spin of black holes (or neutron stars) is an important parameter of binary black hole merger events and their observed gravitational wave signals on earth. In this talk, I will present recent progress on describing spinning black holes (or other compact objects) in the worldline quantum field theory (WQFT) framework designed for high-precision computations of general relativistic two-body scattering observables.

First, generic spinning compact objects are described as point particles in an effective field theory setting and a Hamiltonian is set up in terms of covariant phase space variables. The transition to the action-based WQFT framework is then achieved by the introduction of bosonic oscillators that parametrize the spinning variables in a surprisingly simple fashion. With the spinning WQFT in place, scattering observables are straightforwardly computed via Feynman diagrammatic expansions.

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WorldlinersSlidesGustav.pdf

Christian Kohlfürst

Title of the talk:

Relativistic Quantum Kinetic Theory: Higher order contributions in assisted Schwinger pair production

Abstract:

Quantum kinetic theory is an important tool for studying non-equilibrium, non-perturbative and non-linear interactions, and as such is able to provide an unprecedented view on particle production. In this talk, we show how a perturbative expansion can be integrated into the underlying equations of quantum kinetic theory of strong-field quantum electrodynamics. This expands the scope of kinetic theories to provide further context for particle production at the spectral level.

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kohlfuerst_talk.pdf

Zeno Capatti

Title of the talk:

Systematics of the classical limit in the KMOC formalism

Abstract:

In the KMOC formalism, classical observables are obtained from quantum observables by a soft limit-taking procedure: the classical quantity is the leading-order term in an expansion that assumes all massless excitations carry small momentum. The leading-order term emerges only after delicate cancellations of “super-leading” contributions occur across the Feynman diagrams involved.

In this talk, I will present a general approach to organising the expansion in such a way that the cancellations take place locally and that the resulting integrand takes exactly the same form as in classical worldline approaches. This, in turn, provides an operational framework for demonstrating the equivalence between quantum and classical approaches. Key elements of the method include the combination of massive propagators into worldline objects and the treatment of certain “zero-measured” cuts that naturally arise when correctly accounting for the causal prescription of massless propagators. I will discuss applications to the impulse and angular momentum observables.

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Maria Kallimani

Title of the talk:

Worldline Geometries for Scattering Amplitudes

Abstract:

In this talk, I will discuss a framework for the computation of scattering amplitudes by using different geometries of the open worldline. To that end, I will discuss how a path integral can be defined for an infinite and semi-infinite worldline. I will argue that on the infinite worldline, asymptotic states are generated by inserting vertex operators at infinity, a process which implements LSZ reduction. Correlators on the semi-infinite worldline are treated as generalized vertex operators, which are tree sub-diagrams to be glued to another given worldline. All in all, we obtain a fully worldline based representation of scattering amplitudes, which in this talk we will apply in the case of scalar self-interacting theories.

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Kevin Falls

Title of the talk:

Gauge invariant effective actions for dressed fields

Abstract:

I will discuss the construction of effective actions which generate correlation functions of dressed, aka relational, fields in Yang-Mills theories and quantum gravity.

To this end I use a class of BRST invariant gauges which impose that the on-shell correlation functions are those of gauge invariant fields.

In addition to BRST symmetry, the effective actions are gauge and diffeomorphism invariant respectively, with appropriate transformations for the Faddeev-Popov ghosts and Nakanishi-Lautrup fields. I will contrast the opposing points of view that think of gauge choices as physical and unphysical respectively.

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Worldliners.pdf

Leonardo de la Cruz

Title of the talk:

Worldlines in a medium

Abstract:

In this talk, I will consider the deflection of light rays in a cold, non-magnetized plasma in the worldline framework. Starting from Synge’s Hamiltonian formalism, we construct a position-space action and use it perturbatively to calculate light bending angles. We will consider both the homogeneous and inhomogeneous cases. In the final part of the talk, I will introduce finite-temperature effects and demonstrate how the worldline framework provides a bridge to semi-classical kinetic theory.

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medium-wordline-meetings.pdf

Victor Banda Guzmán

Title of the talk:

The second order formalism of QED and QCD

Abstract:

The standard Feynman rules used for perturbative calculations in quantum chromodynamics (QCD) and quantum electrodynamics (QED) are derived from a Lagrangian that is first-order in derivatives. It includes a three-point vertex which obscures the precise disentangled manner in which spin and momentum are interchanged during the interactions. An unambiguous understanding of this interchange is insightful for efficiently extracting physically relevant information from various Green’s functions.

To separate the scalar and spin degrees of freedom and gain physical insight from the outset, we examine the quark-gluon and electron-photon vertex using the less commonly employed second-order formalism of QCD and QED.

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Second_order_rules_for_QCD_and_QED-1_VictorBanda.pdf

Rafael Aoude

Title of the talk:

Amplitudes for Hawking Radiation

Abstract:

In this talk, I will show an approach to compute Hawking radiation based on on-shell scattering amplitudes. The Hawking spectrum is obtained by exponentiating a series of Feynman diagrams describing a massless scalar field scattering through a collapse background. Using semiclassical methods, we obtain a generalized an in-in generalisation of an amplitude closely connected to the Bogoliubov coefficients. Finally, I will show how subdominant one-loop correction can be interpreted as finite-size corrections sensitive to the radius of the black hole.

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Aoude_WL_Series.pdf

Cristhiam Lopez Arcos

Title of the talk:

One-Loop N-Point Correlators in Pure Gravity

Abstract:

We propose a simple algebraic recursion for the complete one-loop integrands of N-graviton correlators. This formula automatically yields the correct symmetry factors of individual diagrams, taking into account both the graviton and the ghost loop, and seamlessly controlling the related combinatorics.

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Greger Torgrimsson

Title of the talk:

Worldline instantons for gravitational pair production

Abstract:

I will present our recent paper: “Worldline instantons for nonperturbative particle production by space and time dependent gravitational fields”, arXiv:2508.01901

We have shown how to study pair production using open worldline instantons. The middle of the instanton is complex and describes the creation region, while the end points describe the particles in the asymptotic future. As we follow different paths in the space of all time and space dependent metrics, g(t,x), we can see e.g. a continuous interpolation between a locally de-Sitter and a perturbative regime. We have found that branch points and singularities move around in the complex proper-time plane, so we have to carefully deform the proper-time contour so that it stays on the same side of those points. We have applied our methods to time-dependent metrics which have both a black-hole and a white-hole horizon, which appear in various analog black holes.

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GT_Worldline_Meetings_2025.pdf

Marc Schneider

Title of the talk:

Radiative properties of non-singular black holes

Abstract:

We study the radiative properties of a spherical and singularity-free black-hole geometry recently proposed in the literature. Contrary to the Schwarzschild spacetime, this geometry is geodesically complete and regular, and, instead of the singularity, it presents a minimal surface that connects a trapped (black-hole) with an antitrapped (white-hole) region. The geometry is characterized by two parameters: the Schwarzschild radius and another parameter that measures the area of the minimal surface. This parameter is related to certain corrections expected in the context of loop quantum gravity to the classical general-relativistic dynamics. We explicitly compute the spectrum of the Hawking radiation and the gray-body factor. Since the gravitational potential is shallower than in Schwarzschild, the emission spectrum turns out to be colder and purer (less gray). From this, we sketch the evaporation history of this geometry and conclude that, under certain assumptions, instead of completely evaporating, the black hole naturally leads to a remnant, which provides a possible resolution to the information-loss issue.

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Karthik Rajeev

Title of the talk:

Tunnelling amplitudes and Hawking radiation from worldline formalism

Abstract:

We compare Hawking radiation in a collapse background with Schwinger pair creation in an electric field. The comparison is driven by the presence of an analogue horizon in the Schwinger case, which causally divides spacetime for classical particles, but through which quantum fields can tunnel. Amplitudes for tunnelling processes are encoded in the asymptotic behaviour of solutions to the appropriate background-coupled wave equation. We construct these solutions, in both gravity and QED, using the worldline approach, where tunnelling and particle creation manifest as complex saddle points of a real- time path integral. For the Schwinger effect, these saddles correspond to complex worldlines, while for Hawking radiation the corresponding worldlines are real, but appear complex when extended beyond a certain coordinate patch. (Based on 2508.00997)

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Joonhwi Kim

Title of the talk:

Worldline Formalisms in Phase Space

Abstract:

This talk provides an overview of worldline techniques for classical and quantum particles, formulated in phase spaces. We start by observing universal features of the Feynman rules in symplectic sigma models that are useful to keep in mind. We then explore the worldline techniques in two, distinct, categories: in-out and in-in. The former implements the original worldline formalism in phase space to compute scattering amplitudes, while automating LSZ reduction via worldline topologies. The latter uses the so-called worldline quantum field theory (WQFT) framework to compute the “S-symplectomorphism,” which is the classical counterpart of S-matrix whose generator is the classical eikonal. Applications are outlined for spin-resummed post-Minkowskian gravity and color-kinematics duality via twistor-theoretic ideas. Throughout, attention will be given to manifest gauge covariance, which creates a tension with symplectivity and necessitates employing non-canonical coordinates in phase space.

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Vincent He

Title of the talk:

Generalised Unitarity Method in Worldline Field Theory

Abstract:

In this talk, I present a generalized unitarity method for theories of point-particle worldlines coupled to gravity. I will show how perturbative observables in this worldline field theory can be fixed by basic principles such as locality, unitarity and symmetry, thus avoiding gauge redundancies and the use of Feynman diagrams, facilitating the construction of Feynman integrands and making explicit the presence of Wilson coefficients in the EFT. Along the way, I will touch upon the relevance of worldline formalism to gravitational wave physics and the distinction between in-in and in-out observables.

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Chul Min Kim

Title of the talk:

Nonlinear Compton Scattering Experiments with 𝜒~=1

Abstract:

Nonlinear Compton scattering (NCS), merging multiple photons into a single photon through collision with an electron, is a fundamental process in strong-field quantum electrodynamics (SFQED). Its degree of nonlinearity is quantified by the quantum nonlinearity parameter 𝜒, which is defined as the electromagnetic field strength observed by the electron in its rest frame relative to the intrinsic field strength of QED, 𝐸_S = 1.3 × 1018 V/m [1]. In this regard, the condition of 𝜒 = 1 is an important milestone marking the regime of SFQED, and thus, its realization has been intensely pursued in the ultra-intense laser community. In this talk, we present recent experimental results obtained under the conditions 𝜒 = 0.46 [2] and 𝜒 = 1.4 at the Center for Relativistic Laser Science (CoReLS), reporting an entry into the strongly nonlinear regime. We also discuss the physical implications of these results, anticipating that nonlinear Breit-Wheeler pair production and QED cascades may be realized in the near future.

[1] V. I. Ritus, “Quantum effects of the interaction of elementary particles with an intense

electromagnetic field,” Journal of Soviet Laser Research 6, 497 (1985).

[2] M. Mirzaie et al., “All-optical nonlinear Compton scattering performed with a multi-

petawatt laser,” Nature Photonics 18, 1212 (2024)

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2025_CMKim_Worldline_meeting_for_distribution.pdf

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