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Register a new userInfra-red divergences in plane wave backgrounds
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We show that the emission of soft photons via nonlinear Compton scattering in a pulsed plane wave (laser field) is in general infra-red divergent. We give examples of both soft and soft-collinear divergences, and we pay particular attention to the case of crossed fields in both classical and quantum theories.
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Gauge invariance and soft limits can be enough to determine the analytic structure of scattering amplitudes in certain theories. This prompts the question of how gauge invariance is connected to analytic structure in more general theories. Here we focus on QED in background plane waves. We show that imposing gauge invariance introduces new virtuality poles into internal momenta on which amplitudes factorise into a series of terms. Each term is gauge invariant, has a different analytic structure in external momenta, and exhibits a hard/soft factorisation. The introduced poles are dictated by infra-red behaviour, which allows us to extend our results to scalar Yukawa theory. The background is treated non-perturbatively throughout.
The influence of nonperturbative fields on instantons in quantum chromodynamics is studied. Nonperturbative vacuum is described in terms of nonlocal gauge invariant vacuum averages of gluon field strength. Effective action for instanton is derived in bilocal approximation and it is demonstrated that stochastic background gluon fields are responsible for infra-red (IR)stabilization of instantons. Comparison of obtained instanton size distribution with lattice data is made.
We investigate the infrared singularity structure of Feynman diagrams entering the next-to-leading-order (NLO) DGLAP kernel (non-singlet). We examine cancellations between diagrams for two gluon emission contributing to NLO kernels. We observe the crucial role of color coherence effects in cancellations of infra-red singularities. Numerical calculations are explained using analytical formulas for the singular contributions.
4-derivative gravity provides a renormalizable theory of quantum gravity at the price of introducing a physical ghost, which could admit a sensible positive-energy quantization. To understand its physics, we compute ghost-mediated scatterings among matter particles at tree-level, finding a new power-like infra-red enhancement typical of 4-derivative theories, that we dub $ghostrahlung$. Super-Planckian scatterings get downgraded to Planckian by radiating hard gravitons and ghosts, which are weakly coupled and carry away the energy.
Recently the memory effect has been studied in plane gravitational waves and, in particular, in impulsive plane waves. Based on an analysis of the particle motion (mainly in Baldwin-Jeffery-Rosen coordinates) a velocity memory effect is claimed to be found in [P.-M. Zhang, C. Duval, and P. A. Horvathy. Memory effect for impulsive gravitational waves. Classical Quantum Gravity, 35(6):065011, 20, 2018]. Here we point out a conceptual mistake in this account and employ earlier works to explain how to correctly derive the particle motion and how to correctly deal with the notorious distributional Brinkmann form of the metric and its relation to the continuous Rosen form.
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