arXiv subjects

B. King

Publications and source records attributed to B. King.

At least 19 recordsLinked to original sources

arXiv2025

Probing the vacuum as a chiral medium

We study the circular birefringence experienced by linearly polarised photons colliding with a circularly polarised background creating a vacuum of definite chirality (handedness). For this scenario the standard Heisenberg-Euler approach fails and must be supplemented by derivative corrections which we match to known Hilbert series. Choosing a plane wave background, we find equivalence between three approaches: (i) adding derivative corrections to the Heisenberg-Euler Lagrangian; (ii) improving the locally constant field approximation to the one-loop polarisation tensor; (iii) performing a low-energy expansion of the direct $2\to 2$ QED photon-photon scattering amplitude. Going beyond plane-wave backgrounds, we analyse an example of a circularly polarised standing wave sensitive to derivative corrections. We find a parameter regime where these corrections could be probed in experiments.

hep-ph↗

arXiv2025

Revealing signals of higher-order nonlinear showers in particle-laser collisions

Several high power laser facilities are reaching field strengths where leading order strong-field quantum electrodynamical (QED) processes can be measured in the non-perturbative regime for the first time. At very high, as yet unobtainable in the laboratory, field strengths, the contribution of higher-order processes is predicted to dominate, implying a breakdown of current calculational methods. Focusing on nonlinear showers and considering currently available experimental parameters, we find that if the momentum spectrum of the \emph{incident} particles is well known, asymmetries in the \emph{outgoing} particle spectrum may provide a useful signature of higher orders of nonlinear phototrident, trident and Compton scattering. These signatures could be used by experiment to test how accurate the current calculational framework is when applied to strong-field QED at higher orders.

hep-ph↗

arXiv2025

Vacuum muon decay and interaction with laser pulses

Muons decay in vacuum mainly via the leptonic channel to an electron, a muon neutrino and an electron antineutrino. Previous investigations have concluded that muon decay can only be significantly altered in a strong electromagnetic field when the muonic strong-field parameter is of order unity, which is far beyond the reach of lab-based experiments at current and planned facilities. In this letter, an alternative mechanism is presented in which a laser pulse affects the vacuum decay rate of a muon outside the pulse. Quantum interference between the muon decaying with or without interacting with the pulse generates fringes in the electron momentum spectra and can increase the muon lifetime by up to a factor 2. The required parameters to observe this effect are available in experiments today.

hep-ph↗

arXiv2025

Entanglement and pair production in intense electromagnetic fields

We investigate the spin correlations between electron-positron pairs created from a photon when it scatters in a high-intensity laser pulse via the nonlinear Breit-Wheeler process. We find that the spin states of the generated electron-positron pair can exhibit strong entanglement, with the degree being sensitive to the photon energy, laser intensity, and the relative polarisation of the photon and laser pulse. Photons with a high degree of polarisation can create strongly entangled pairs, and this entanglement can be maintained in the high-intensity (non-perturbative) regime. We find that if the photons are provided by a Compton source, strongly spin-entangled electron-positron pairs can be generated with technology available today.

hep-ph↗

arXiv2025

Input to the European Strategy for Particle Physics: Strong-Field Quantum Electrodynamics

This document sets out the intention of the strong-field QED community to carry out, both experimentally and numerically, high-statistics parametric studies of quantum electrodynamics in the non-perturbative regime, at fields approaching and exceeding the critical or `Schwinger' field of QED. In this regime, several exotic and fascinating phenomena are predicted to occur that have never been directly observed in the laboratory. These include Breit-Wheeler pair production, vacuum birefringence, and quantum radiation reaction. This experimental program will also serve as a stepping stone towards studies of elusive phenomena such as elastic scattering of real photons and the conjectured perturbative breakdown of QED at extreme fields. State-of-the-art high-power laser facilities in Europe and beyond are starting to offer unique opportunities to study this uncharted regime at the intensity frontier, which is highly relevant also for the design of future multi-TeV lepton colliders. However, a transition from qualitative observational experiments to quantitative and high-statistics measurements can only be performed with large-scale collaborations and with systematic experimental programs devoted to the optimisation of several aspects of these complex experiments, including detector developments, stability and tolerances studies, and laser technology.

hep-ph↗

arXiv2025

Input to the ESPPU: The LUXE Experiment

This document presents an overview of LUXE (Laser Und XFEL Experiment), an experiment that will combine the high-quality and high-energy electron beam of the European XFEL with a high-intensity laser, to explore the uncharted terrain of strong-field quantum electrodynamics. The scientific case, facility, and detector setup are presented together with an overview of the foreseen timeline and expected capital costs.

hep-ex↗

arXiv2025

On Measuring Stimulated Photon-photon Scattering using Multiple Ultraintense Lasers

Stimulated photon-photon scattering is a predicted consequence of quantum electrodynamics that has yet to be measured directly. Measuring the cross-section for stimulated photon-photon scattering is the aim of a flagship experiment for NSF OPAL, a proposed laser user facility with two, 25-PW beamlines. We present optimized experimental designs for achieving this challenging and canonical measurement. A family of experimental geometries is identified that satisfies the momentum- and energy-matching conditions for two selected laser frequency options. Numerical models predict a maximum signal exceeding 1000 scattered photons per shot at the experimental conditions envisaged at NSF OPAL. Experimental requirements on collision geometry, polarization, cotiming and copointing, background suppression, and diagnostic technologies are investigated numerically. These results confirm that a beam cotiming shorter than the pulse duration and control of the copointing on a scale smaller than the shortest laser wavelength are needed to robustly scatter photons on a per-shot basis. Finally, we assess the bounds that a successful execution of this experiment may place on the mass scale of Born-Infeld nonlinear electrodynamics beyond the Standard Model of physics.

hep-ex↗

arXiv2024

Coherent enhancement of QED cross-sections in electromagnetic backgrounds

We introduce form factors that relate the amplitude of a QED process in vacuum to its corresponding background-field process. The latter is characterised by a reduced S-matrix element where one or more photon field operators are replaced by classical background fields. In the associated Feynman diagram, external photon lines are supplanted with lines representing the c-number field. This modifies the cross section by factors proportional to powers of the Fourier amplitude of the classical field (and its complex conjugate). We demonstrate this explicitly by comparing different reaction channels of low-energy photon-photon scattering in a classical background. We find that background field cross sections typically undergo coherent enhancement and for some reaction channels display a more favourable scaling with centre-of-mass energy compared to the vacuum process. Similar coherent enhancement may be found for leading-order pair annihilation to one photon, but this competes with kinematic suppression. This suppression can be minimised by using an x-ray free electron laser as the classical background.

hep-ph↗

arXiv2024

Bounding elastic photon-photon scattering at $\sqrt s \approx 1\,$MeV using a laser-plasma platform

We report on a direct search for elastic photon-photon scattering using x-ray and $\gamma$ photons from a laser-plasma based experiment. A gamma photon beam produced by a laser wakefield accelerator provided a broadband gamma spectrum extending to above $E_\gamma = 200$ MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at $E_x \approx 1.4$ keV, corresponding to an invariant mass of $\sqrt{s} = 1.22 \pm 0.22$ MeV. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy $\gamma$ photon and outputs two lower energy $\gamma$ photons. No changes in the $\gamma$ photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of $1.5 \times 10^{15}\,\mu$b. Although far from the QED prediction, this represents the lowest upper limit obtained so far for $\sqrt{s} \lesssim 1$ MeV.

hep-ex↗

arXiv2024

Letter of Intent: Towards a Vacuum Birefringence Experiment at the Helmholtz International Beamline for Extreme Fields

Quantum field theory predicts a nonlinear response of the vacuum to strong electromagnetic fields of macroscopic extent. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. A particularly distinct signature of the resulting optical activity of the quantum vacuum is vacuum birefringence. This offers an excellent opportunity for a precision test of nonlinear quantum electrodynamics in an uncharted parameter regime. Recently, the operation of the high-intensity laser ReLaX provided by the Helmholtz International Beamline for Extreme Fields (HIBEF) has been inaugurated at the High Energy Density (HED) scientific instrument of the European XFEL. We make the case that this worldwide unique combination of an x-ray free-electron laser and an ultra-intense near-infrared laser together with recent advances in high-precision x-ray polarimetry, refinements of prospective discovery scenarios, and progress in their accurate theoretical modelling have set the stage for performing an actual discovery experiment of quantum vacuum nonlinearity.

physics.ins-det↗

arXiv2024

Feasibility of measuring non-analytic QED coupling from pair creation in strong fields

In the quasistatic regimes of nonlinear Breit-Wheeler and trident pair creation, the rates can exhibit a non-analytic dependency on the fundamental coupling of quantum electrodynamics (QED), in a form similar to Schwinger vacuum pair creation. To reach this tunneling regime requires satisfying competing requirements: high intensity but low strong-field parameter with sufficient pair creation to be observed. Using a locally monochromatic approach, we identify the parameter regime where tunneling pair-creation could be measured for the first time in experiment. Studying several scenarios of collisions with focussed Gaussian pulses, including a bremsstrahlung and an inverse Compton source for nonlinear Breit-Wheeler and a Gaussian electron beam for nonlinear trident, we find the position of the tunneling parameter regime to be well-defined and robust.

hep-ph↗

arXiv2023

Technical Design Report for the LUXE Experiment

This Technical Design Report presents a detailed description of all aspects of the LUXE (Laser Und XFEL Experiment), an experiment that will combine the high-quality and high-energy electron beam of the European XFEL with a high-intensity laser, to explore the uncharted terrain of strong-field quantum electrodynamics characterised by both high energy and high intensity, reaching the Schwinger field and beyond. The further implications for the search of physics beyond the Standard Model are also discussed.

hep-ex↗

arXiv2023

Strong field vacuum birefringence in plane wave pulses

By combining an adiabatic approach based on a `locally monochromatic' approximation with a local Hilbert transform, it is demonstrated how vacuum birefringence in the strong field regime can be calculated using a rate approach suitable for Monte Carlo simulation codes. Results for the flipping of the photon's polarisation (helicity) are benchmarked with evaluation of exact expressions in a circularly (linearly) polarised plane wave of finite extent. Example probabilities are given for typical experimental parameters.

hep-ph↗

arXiv2023

Classical Radiation Reaction in Red-Shifted Harmonics

The collision of a finite electromagnetic plane wave with an electron subject to the Landau-Lifshitz radiation reaction force is studied. A locally monochromatic approximation is derived and compared to numerical evaluation of the exact plane wave result. Energy and transverse momentum spectra are calculated, which clearly display the red-shifting of harmonic features due to radiation reaction effects. Simple formulas are presented to predict the shifting of harmonic edges, whose position can be used in experiments as evidence of radiation reaction.

hep-ph↗

arXiv2023

Simulations of laser-driven strong-field QED with Ptarmigan: Resolving wavelength-scale interference and $\gamma$-ray polarization

Accurate modelling is necessary to support precision experiments investigating strong-field QED phenomena. This modelling is particularly challenging in the transition between the perturbative and nonperturbative regimes, where the normalized laser amplitude $a_0$ is comparable to unity and wavelength-scale interference is significant. Here we describe how to simulate nonlinear Compton scattering, Breit-Wheeler pair creation, and trident pair creation in this regime, using the Monte Carlo particle-tracking code Ptarmigan. This code simulates collisions between high-intensity lasers and beams of electrons or $\gamma$ rays, primarily in the framework of the locally monochromatic approximation (LMA). We benchmark our simulation results against full QED calculations for pulsed plane waves and show that they are accurate at the level of a few per cent, across the full range of particle energies and laser intensities. This work extends our previous results to linearly polarized lasers and arbitrarily polarized $\gamma$ rays.

hep-ph↗

arXiv2023

Strong-field vacuum polarisation with high energy lasers

When photons propagate in vacuum they may fluctuate into matter pairs thus allowing the vacuum to be polarised. This linear effect leads to charge screening and renormalisation. When exposed to an intense background field a nonlinear effect can arise when the vacuum is polarised by higher powers of the background. This nonlinearity breaks the superposition principle of classical electrodynamics, allowing for light-by-light scattering of probe and background photons mediated through virtual pairs dressed by the background. Vacuum polarisation is a strong-field effect when all orders of interaction between the virtual pair and the background must be taken into account. In this investigation we show that multiple scattering processes of this type may be observed by utilising high-energy laser pulses with long pulse duration, such as are available at facilities like ELI Beamlines. In combination with appropriate sources of high-energy probe photons, multiple probe-background light-by-light scattering allows for testing the genuine nonlinear regime of strong-field QED. This provides access to the uncharted nonperturbative regime beyond the weak-field limit.

hep-ph↗

arXiv2022

Locally monochromatic two-step nonlinear trident process in a plane wave

In many-cycle plane waves at intermediate intensities, the nonlinear trident process can be well-approximated by the two sequential steps of nonlinear Compton scattering of a polarised real photon followed by its transformation into an electron-positron pair via nonlinear Breit-Wheeler pair creation. We investigate this two-step process in the intermediate intensity regime by employing the locally monochromatic approximation for each step and numerically evaluating resulting expressions. When photon polarisation is included, it is found to produce an order 10% decrease in the trident rate: the importance of polarisation increases at lower intensities, and decreases at higher intensities. Its importance persists at higher intensities in a linearly-polarised background, but disappears at high intensities in a circularly-polarised background. If the two steps are made to take place in two linearly-polarised plane wave pulses with perpendicular polarisations, the pair yield can be increased by approximately 30% compared to two plane waves with the same polarisation. It is also shown that harmonic structures in the Compton step can be passed to the pair step if the Compton edge is at an energy of the order of the threshold for linear Breit-Wheeler.

hep-ph↗

arXiv2022

Using nonlinear Breit-Wheeler to test nonlinear vacuum birefringence

Quantum electrodynamics predicts that the quantum vacuum is birefringent, but due to the very small cross-section this is yet to be confirmed by experiment. Vacuum birefringence arises as the elastic part of photon-photon scattering; the inelastic part is Breit-Wheeler pair-production. We outline how measurements of the photon-polarised nonlinear Breit-Wheeler process can be used to infer a measurement of nonlinear vacuum birefringence. As an example scenario, we calculate the accuracy of such a measurement for parameters anticipated at upcoming laser-particle experiments.

hep-ph↗