arXiv subjects

Dmitry V. Karlovets

Publications and source records attributed to Dmitry V. Karlovets.

8 recordsLinked to original sources

arXiv2021

Wave function of a photon produced in the resonant scattering of twisted light by relativistic ions

We present a theoretical investigation of the resonant elastic scattering of twisted light, carrying angular momentum, by partially stripped ions. Special emphasis is placed on a question of whether the scattered radiation is also twisted. In order to investigate such an "angular momentum transfer", we develop an approach that allows us to find a quantum state of the final photon without projecting it onto a detector state. A general expression for this so--called \textit{evolved} state of outgoing radiation is derived and it can be used to analyze the resonant scattering by any ion, independently of its shell structure. Here, we illustrate our approach with the strong electric dipole $n S_{0} \to n' P_{1} \to n S_{0}$ transitions, which play an important role for the Gamma Factory project at CERN. For the incident Bessel light, the scattered radiation is shown to be in a superposition of twisted modes with the projections of the total angular momentum $m_f = 0, \pm 1$. The larger values of $m_f$ can be efficiently generated by inducing transitions of higher multiplicity. This angular momentum transfer, together with a remarkable cross section that is many orders of magnitude larger than that of the backward Compton scattering, makes the resonant photon scattering an effective tool for the production of twisted x-- and even gamma--rays at the Gamma Factory facility.

hep-ph↗

arXiv2020

Effects of the transverse coherence length in relativistic collisions

Effects of the quantum interference in collisions of particles have a twofold nature: they arise because of the auto-correlation of a complex scattering amplitude and due to spatial coherence of the incoming wave packets. Both these effects are neglected in a conventional scattering theory dealing with the delocalized plane waves, although they sometimes must be taken into account in particle and atomic physics. Here, we study the role of a transverse coherence length of the packets, putting special emphasis on the case in which one of the particles is twisted, that is, it carries an orbital angular momentum $\ell\hbar$. In $ee, ep$, and $pp$ collisions the interference results in corrections to the plane-wave cross sections, usually negligible at the energies $\sqrt{s} \gg 1$ GeV but noticeable for smaller ones, especially if there is a twisted hadron with $|\ell| > 10^3$ in initial state. Beyond the perturbative QCD, these corrections become only moderately attenuated allowing one to probe a phase of the hadronic amplitude as a function of $s$ and $t$. In this regime, the coherence effects can compete with the loop corrections in QED and facilitate testing the phenomenological models of the strong interaction at intermediate and low energies.

hep-ph↗

arXiv2017

Possibility to probe negative values of a Wigner function in scattering of a coherent superposition of electronic wave packets by atoms

Within a plane-wave approximation in scattering, an incoming wave packet's Wigner function stays everywhere positive, which obscures such purely quantum phenomena as non-locality and entanglement. With the advent of the electron microscopes with subnanometer-sized beams one can enter a genuinely quantum regime where the latter effects become only moderately attenuated. Here we show how to probe negative values of the Wigner function in scattering of a coherent superposition of two Gaussian packets with a non-vanishing impact-parameter between them (a Schrödinger's cat state) by atomic targets. For hydrogen in the ground $1s$ state, a small parameter of the problem, a ratio $a/σ_{\perp}$ of the Bohr radius $a$ to the beam width $σ_{\perp}$, is no longer vanishing. We predict an azimuthal asymmetry of the scattered electrons, which is found to be up to $10 \%$ and argue that it can be reliably detected. Production of beams with the not-everywhere positive Wigner functions and probing such quantum effects can open new perspectives for non-invasive electron microscopy, quantum tomography, particle physics, etc.

quant-ph↗

arXiv2016

Probing phase of a scattering amplitude beyond the plane-wave approximation

Within a plane-wave approach, a number of scattering events in a collision is insensitive to a general phase of a transition amplitude, although this phase is extremely important for a number of problems, especially in hadronic physics. In reality the particles are better described as wave packets, and here we show that the observables grow dependent upon this phase if one lays aside the simplified plane-wave model. We discuss two methods for probing how the Coulomb- and hadronic phases change with a transferred momentum $t$, either by colliding two beams at a non-vanishing impact-parameter or by employing such novel states as the vortex particles carrying orbital angular momentum or the Airy beams. For electron-electron collision, the phase contribution to a cross section can reach the values higher than $10^{-4} -- 10^{-3}$ for well-focused beams with energies of hundreds of keV.

hep-ph↗

arXiv2015

Gaussian and Airy wave packets of massive particles with orbital angular momentum

While wave-packet solutions for relativistic wave equations are oftentimes thought to be approximate (paraxial), we demonstrate that there is a family of such solutions, which are exact, by employing a null-plane (light-cone) variables formalism. A scalar Gaussian wave-packet in transverse plane is generalized so that it acquires a well-defined z-component of the orbital angular momentum (OAM), while may not acquire a typical "doughnut" spatial profile. Such quantum states and beams, in contrast to the Bessel ones, may have an azimuthal-angle-dependent probability density and finite quantum uncertainty of the OAM, which is determined by the packet's width. We construct a well-normalized Airy wave-packet, which can be interpreted as a one-particle state for relativistic massive boson, show that its center moves along the same quasi-classical straight path and, what is more important, spreads with time and distance exactly as a Gaussian wave-packet does, in accordance with the uncertainty principle. It is explained that this fact does not contradict the well-known "non-spreading" feature of the Airy beams. While the effective OAM for such states is zero, its quantum uncertainty (or the beam's OAM bandwidth) is found to be finite, and it depends on the packet's parameters. A link between exact solutions for the Klein-Gordon equation in the null-plane-variables formalism and the approximate ones in the usual approach is indicated, generalizations of these states for a boson in external field of a plane electromagnetic wave are also presented.

hep-ph↗

arXiv2013

Polarization radiation of vortex electrons with large orbital angular momentum

Vortex electrons, - freely propagating electrons whose wavefunction has helical wavefronts, - could become a novel tool in the physics of electromagnetic radiation. They carry a non-zero intrinsic orbital angular momentum (OAM) $\ell$ with respect to the propagation axis and, for \ell \gg 1, a large OAM-induced magnetic moment, μ~ \ell μ_B (μ_B is the Bohr magneton), which influences the radiation of electromagnetic waves. Here, we consider in detail the OAM-induced effects by such electrons in two forms of polarization radiation, namely in Cherenkov radiation and transition radiation. Thanks to the large \ell, we can neglect quantum or spin-induced effects, which are of the order of \hbar ω/E_e \ll 1, but retain the magnetic moment contribution \ell \hbar ω/E_e \lesssim 1, which makes the quasiclassical approach to polarization radiation applicable. We discuss the magnetic moment contribution to polarization radiation, which has never been experimentally observed, and study how its visibility depends on the kinematical parameters and the medium permittivity. In particular, it is shown that this contribution can, in principle, be detected in azimuthally non-symmetrical problems, for example when vortex electrons obliquely cross a metallic screen (transition radiation) or move nearby it (diffraction radiation). We predict a left-right angular asymmetry of the transition radiation (in the plane where the charge radiation distributions would stay symmetric), which appears due to an effective interference between the charge radiation field and the magnetic moment one. Numerical values of this asymmetry for vortex electrons with E_e = 300keV and \ell = O(100-1000) are O(0.1-1%), and we argue that this effect could be detected with existing technology. The finite conductivity of the target and frequency dispersion play the crucial roles in these predictions.

physics.optics↗

arXiv2013

Detecting transition radiation from a magnetic moment

Electromagnetic radiation can be emitted not only by particle charges but also by magnetic moments and higher electric and magnetic multipoles. However experimental proofs of this fundamental fact are extremely scarce. In particular, the magnetic moment contribution has never been observed in any form of polarization radiation. Here, we propose to detect it using vortex electrons carrying large orbital angular momentum (OAM) \ell. The relative contribution of the OAM-induced magnetic moment, \ell \hbar ω/E_e, becomes much larger than the spin-induced contribution \hbar ω/E_e, and it can be observed experimentally. As a particular example, we consider transition radiation from vortex electrons obliquely incident on an interface between a vacuum and a dispersive medium, in which the magnetic moment contribution manifests itself via a left-right angular asymmetry. For electrons with E_e = 300 keV and \ell = 100-1000, we predict an asymmetry of the order of 0.1%-1%, which could be measured with existing technology. Thus, vortex electrons emerge as a new tool in the physics of electromagnetic radiation.

physics.optics↗

arXiv2012

Twisted electron in a strong laser wave

Electrons carrying orbital angular momentum (OAM) have recently been discovered theoretically and obtained experimentally that opens up possibilities for using them in high-energy physics. We consider such a twisted electron moving in external field of a plane electromagnetic wave and study how this field influences the electron's OAM. Being motivated by the development of high-power lasers, we focus our attention on a classically strong field regime for which $-e^2 \bar {A^2}/m_e^2 c^4 \gtrsim 1$. It is shown that along with the well-known "plane-wave" Volkov solution, Dirac equation also has the "non-plane-wave" solutions, which possess OAM and a spin-orbit coupling, and generalize the free-electron's Bessel states. Motion of the electron with OAM in a circularly polarized laser wave reveals a twofold character: the wave-packet center moves along a classical helical trajectory with some quantum transverse broadening (due to OAM) existing even for a free electron. Using the twisted states, we calculate the electron's total angular momentum and predict its shift in the strong-field regime that is analogous to the well-known shifts of the electron's momentum and mass (and to a less known shift of its spin) in intense fields. Since the electron's effective angular momentum is conserved in a plane wave, as well as in some more general field configurations, we discuss several possibilities for accelerating non-relativistic twisted electrons by using the focused and combined electromagnetic fields.

hep-ph↗