Abstract:We study the processes of photon-photon scattering and photon splitting in a magnetic field in Born-Infeld theory. In both cases we combine the terms from the tree-level Born-Infeld Lagrangian with the usual one-loop QED contributions, where those are approximated by the Euler-Heisenberg Lagrangian, including also the interference terms. For photon-photon scattering we obtain the total cross section in the low-energy approximation. For photon splitting we compute the total absorption coefficient in the hexagon (weak field) approximation, and also show that, due to the non-birefringence property of Born-Infeld theory, the selection rules found by Adler for the QED case continue to hold in this more general setting. We discuss the bounds on the free parameter of Born-Infeld theory that may be obtained from this type of processes.

We study the ground state energy and the critical screening parameter of the Yukawa potential in non-relativistic quantum mechanics. After a short review of the existing literature on these quantities, we apply fifth-order perturbation theory to the calculation of the ground state energy, using the exact solutions of the Coulomb potential together with a cutoff on the principal number summations. We also perform a variational calculation of the ground state energy using a Coulomb-like radial wave function and the exact solution of the corresponding minimization condition. For not too large values of the screening parameter, close agreement is found between the perturbative and variational results. For the critical screening parameter, we devise a novel method that permits us to determine it to ten digits. This is the most precise calculation of this quantity to date, and allows us to resolve some discrepancies between previous results. † All authors contributed equally to this work 1 typeset using P T P T E X.cls arXiv:1706.09979v1 [physics.atom-ph]

In recent years efficient algorithms have been developed for the numerical computation of relativistic single-particle path integrals in quantum field theory. Here, we adapt this "worldline Monte Carlo" approach to the standard problem of the numerical approximation of the non-relativistic path integral, resulting in a formalism whose characteristic feature is the fast, non-recursive generation of an ensemble of trajectories that is independent of the potential, and thus universally applicable. The numerical implementation discretises the trajectories with respect to their time parametrisation but maintains a continuous spatial domain. In the case of singular potentials, the discretised action gets adapted to the singularity through a "smoothing" procedure. We show for a variety of examples (the harmonic oscillator in various dimensions, the modified Pöschl-Teller potential, delta-function potentials, the Coulomb and Yukawa potentials) that the method allows one to obtain fast and reliable estimates for the Euclidean propagator and use them in a certain time window suitable for extracting the ground state energy. As an aside, we apply it for studying the classical limit where nearly classical trajectories are expected to dominate in the path Email addresses: jedwards@ifm.umich.mx (James P. Edwards), gerberu@itp.unibe.ch (Urs Gerber), schubert@ifm.umich.mx (Christian Schubert), mtrejo@ifm.umich.mx (Maria Anabel Trejo), ttsiftsi@matmor.unam.mx (Thomai Tsiftsi), axel@ifm.umich.mx (Axel Weber)integral. We expect the advances made here to be useful also in the relativistic case.

We introduce two integral transforms of the quantum mechanical transition kernel that represent physical information about the path integral. These transforms can be interpreted as probability distributions on particle trajectories measuring respectively the relative contribution to the path integral from paths crossing a given spatial point (the hit function) and the likelihood of values of the line integral of the potential along a path in the ensemble (the path-averaged potential).