# Publikationen von

Dr. Greger Torgrimsson

Alle Publikationen des HI Jena

## 2021

**Conceptual design report for the LUXE experiment**

**230**, 2445 (2021)

## 2020

**Nonlinear photon trident versus double Compton scattering and resummation of one-step terms**

**102**, 116008 (2020)

**Abstract:** We study the photon trident process, where an initial photon turns into an electron-positron pair and a final photon under a nonlinear interaction with a strong plane-wave background field. We show that this process is very similar to double Compton scattering, where an electron interacts with the background field and emits two photons. We also show how the one-step terms can be obtained by resumming the small- and large -x expansions. We consider a couple of different resummation methods and also propose new resummations (involving Meijer-G functions) which have the correct type of expansions at both small and large x . These new resummations require relatively few terms to give good precision.

**Nonlinear trident in the high-energy limit: Nonlocality, Coulomb field, and resummations**

**102**, 096008 (2020)

**Abstract:** We study nonlinear trident in laser pulses in the high-energy limit, where the initial electron experiences, in its rest frame, an electromagnetic field strength above Schwinger s critical field. At lower energies the dominant contribution comes from the two-step' part, but in the high-energy limit the dominant contribution comes instead from the one-step term. We obtain new approximations that explain the relation between the high-energy limit of trident and pair production by a Coulomb field, as well as the role of the Weizsacker-Williams approximation and why it does not agree with the high-chi limit of the locally-constant-field approximation. We also show that the next-to-leading order in the large-a(0) expansion is, in the high-energy limit, nonlocal and is numerically very important even for quite large a(0). We show that the small-a(0) perturbation series has a finite radius of convergence, but using Pade-conformal methods we obtain resummations that go beyond the radius of convergence and have a large numerical overlap with the large-a(0) approximation. We use Borel-Pade-conformal methods to resum the small-chi expansion and obtain a high precision up to very large chi. We also use newer resummation methods based on hypergeometric/Meijer-G and confluent hypergeometric functions.

**Approximating higher-order nonlinear QED processes with first-order building blocks**

**102**, 016018 (2020)

**Trident process in laser pulses**

**101**, 056017 (2020)

**Abstract:** We study the trident process in laser pulses. We provide exact numerical results for all contributions, including the difficult exchange term. We show that all terms are in general important for a short pulse. For a long pulse, we identify a term that gives the dominant contribution even if the intensity is only moderately high, a0≳1, which is an experimentally important regime where the standard locally constant field (LCF) approximation cannot be used. We show that the spectrum has a richer structure at a0∼1, compared to the LCF regime a0≫1. We study the convergence to LCF as a0 increases and how this convergence depends on the momentum of the initial electron. We also identify the terms that dominate at high energy.

## 2019

**Perturbative methods for assisted nonperturbative pair production**

**99**, 096002 (2019)

**Abstract:** In the dynamically assisted Schwinger mechanism, the pair production probability is significantly enhanced by including a weak, rapidly varying field in addition to a strong, slowly varying field. In a previous paper we showed that several features of dynamical assistance can be understood by a perturbative treatment of the weak field. Here we show how to calculate the prefactors of the higher-orders terms, which is important because the dominant contribution can come from higher orders. We give a new and independent derivation of the momentum spectrum using the worldline formalism, and extend our WKB approach to calculate the amplitude to higher orders. We show that these methods are also applicable to doubly assisted pair production.

**Thermally versus dynamically assisted Schwinger pair production**

**99**, 096007 (2019)

**Abstract:** We study electron-positron pair production by the combination of a strong, constant electric field and a thermal background. We show that this process is similar to dynamically assisted Schwinger pair production, where the strong field is instead assisted by another coherent field, which is weaker but faster. We treat the interaction with the photons from the thermal background perturbatively, while the interaction with the electric field is nonperturbative (i.e., a Furry picture expansion in α). At O(α2) we have ordinary perturbative Breit-Wheeler pair production assisted nonperturbatively by the electric field. Already at this order we recover the same exponential part of the probability as previous studies, which did not expand in α. This means that we do not have to consider higher orders, so our approach allows us to calculate the preexponential part of the probability, which has not been obtained before in this regime. Although the prefactor is in general subdominant compared to the exponential part, in this case it can be important because it scales as α2≪1 and is therefore much smaller than the prefactor at O(α0) (pure Schwinger pair production). We show that, because of the exponential enhancement, O(α2) still gives the dominant contribution for temperatures above a certain threshold, but, because of the small prefactor, the threshold is higher than what the exponential alone would suggest.

## 2018

**Discrete worldline instantons**

**98**, 085009 (2018)

**Abstract:** The semiclassical approximation of the worldline path integral is a powerful tool to study nonperturbative electron-positron pair creation in spacetime-dependent background fields. Finding solutions of the classical equations of motion, i.e., worldline instantons, is possible analytically only in special cases, and a numerical treatment is nontrivial as well. We introduce a completely general numerical approach based on an approximate evaluation of the discretized path integral that easily and robustly gives the full semiclassical pair production rate in nontrivial multidimensional fields, and apply it to some example cases.

**Sauter-Schwinger pair creation dynamically assisted by a plane wave**

**97**, 096004 (2018)

**Abstract:** We study electron-positron pair creation by a strong and constant electric field superimposed with a weaker transversal plane wave which is incident perpendicularly (or under some angle). Comparing the fully nonperturbative approach based on the world-line instanton method with a perturbative expansion into powers of the strength of the weaker plane wave, we find good agreement—provided that the latter is carried out to sufficiently high orders. As usual for the dynamically assisted Sauter-Schwinger effect, the additional plane wave induces an exponential enhancement of the pair-creation probability if the combined Keldysh parameter exceeds a certain threshold.

**Trident pair production in plane waves: Coherence, exchange, and spacetime inhomogeneity**

**97**, 036021 (2018)

**Abstract:** We study the trident process in inhomogeneous plane-wave background fields. We obtain compact analytical expressions for all terms in the probability, including the exchange part, for an arbitrarily shaped plane wave. We evaluate the probability numerically using complex deformation of light-front time integrals and derive various analytical approximations. Our results provide insights into the importance of the one-step and exchange parts of the probability relative to the two-step process, and into the convergence to the locally constant field approximation.