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Andrei Afanasev

Publications and source records attributed to Andrei Afanasev.

At least 55 records · Page 3Linked to original sources
arXiv2017

Circular Dichroism of Twisted Photons in the Non-Chiral Atomic Matter

We calculate the circular dichroism (CD) for absorption of the twisted photons, or optical vortices, by atoms, caused by atomic excitation into discrete energy levels. The effects of photon spin on the rates and cross sections of atomic photo-excitation are considered. It is demonstrated that although for electric dipole transitions the atomic excitation rates depend on the relative orientation of photon spin and orbital angular momentum (OAM), the resulting CD is zero. However, CD is nonzero for atomic transitions of higher multipolarity, peaking in the optical vortex center, resulting in preferred absorption of the photons with their spins aligned with OAM. The effects remain large in a paraxial limit, where analytic expressions are provided. The predicted spin asymmetries are equivalent to OAM dichroism for the fixed photon spin.

quant-ph↗
arXiv2016

High-Multipole Excitations of Hydrogen-Like Atoms by Twisted Photons near Phase Singularity

We calculate transition amplitudes and cross sections for excitation of hydrogen-like atoms by the twisted photon states, or photon states with angular momentum projection on the direction of propagation exceeding $\hbar$. If the target atom is located at distances of the order of atomic size near the phase singularity in the vortex center, the transitions rates into the states with orbital angular momentum $l_f>1$ become comparable with the rates for electric dipole transitions. It is shown that when the transition rates are normalized to the local photon flux, the resulting cross sections for $l_f>1$ are singular near the optical vortex center. Relation to the "quantum core" concept introduced by Berry and Dennis is discussed.

quant-ph↗
arXiv2014

Novel Properties of Twisted-Light Absorption

We discuss novel features of twisted-light absorption both by hydrogen-like atoms and by micro-particles. First, we extend the treatment of atomic photoexcitation by twisted photons to include atomic recoil, derive generalized quantum selection rules and consider phenomena of forbidden atomic transitions. Second, using the same electromagnetic potential for the twisted light beams, we analyze the radiation pressure from these beams on micro-sized particles, and verify that while the Poynting vector can in some circumstances point back toward the source, a complete analysis nonetheless gives a repulsive radiation pressure.

quant-ph↗
arXiv2014

A Proposal for an ALPs-Chameleon Experiments Station

It is generally accepted that certain astronomical and cosmological observations can be explained by invoking the concepts of Dark Matter and Dark Energy (DM/DE). Applying straightforward extensions of the Standard Model to DM/DE, results in scalar fields and predictions of particles generation via photo-magnetic coupling . Under the right conditions, these particles should be observable in earth-bound laboratory settings. Although many attempts have been made to observe these particles, none have succeeded. Heretofore, most searches have focused on detecting multi-GeV Dark Matter WIMPS. Recently, however, searches have been conducted in the lighter dark matter, sub-eV, WISP mass range. By comparison, little has been done to search for dark energy particles. The ALPs-Chameleon Experiments Stations (ACES) program, described herein, proposes a compact station that would search for both dark sector particles. Finally, it is noted that both "species" of particles - dark energy and dark matter - could be generated at the same time in the same magnetic field with the possibility of interaction between DM and DE particles. Thus, by using standard matter tools to produce particles from both dark sectors, ACES potentially could provide tri-sector discoveries with huge results for very little investment.

physics.ins-det↗
arXiv2013

Off-axis excitation of hydrogenlike atoms by twisted photons

We show that the twisted photon states, or photon states with large ($> \hbar$) angular momentum projection ($m_γ$) in the direction of motion, can photoexcite atomic levels for a hydrogen-like atom that are novel and distinct and are not restricted by $m_γ$, when the symmetry axis of the twisted-photon beam does not coincide with the center of the atomic target. Selection rules are given and interesting implications and observables for the above process are pointed out.

quant-ph↗
arXiv2013

Excitation of an Atom by Twisted Photons

Twisted photon states, or photon states with large ($> \hbar$) angular momentum projection in the direction of motion, can photoexcite atomic final states of differing quantum numbers. If the photon symmetry axis coincides with the center of an atom, there are known selection rules that require exact matching between the quantum numbers of the photon and the photoexcited states. The more general case of arbitrarily positioned beams relaxes the selection rules but produces a distribution of quantum numbers of the final atomic states that is novel and distinct from final states produced by plane-wave photons. Numerical calculations are presented using a hydrogen atom as an example.

quant-ph↗
arXiv2010

Positron production scenarios and the angular profile of the galactic center 511-keV line

The observed angular profile of the 511-keV photon excess from the Milky Way galactic center can allow us to select among combinations of various dark matter and other positron production mechanisms with various models for the dark matter distribution. We find that a relic decay scenario gives too flat an angular distribution for any dark matter distribution in our survey, but that a dark matter-dark matter collisional scenario, or a scenario that involves particles emitted from a localized central source producing positrons some distance out, can match the observed galactic center angular profile if the dark matter distribution is neither too flat nor too cuspy. Additionally, positron migration or diffusion before annihilation broadens the angular profile to an extent that an average migration of more than half a kiloparsec is not viable with most dark matter distributions. The observed angular profile is also consistent with the occurrence of transient events in the past, followed by isotropic positron diffusion.

hep-ph↗
arXiv2009

Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation

We propose measurements of the deeply virtual Compton amplitude (DVCS), gamma* to H H-bar gamma, in the timelike t = (p_{H} + p_{H-bar})^2 > 0 kinematic domain which is accessible at electron-positron colliders via the radiative annihilation process e+ e- to H H-bar gamma. These processes allow the measurement of timelike deeply virtual Compton scattering for a variety of H H-bar hadron pairs such as pi+ pi-, K+ K-, and D D-bar as well as p p-bar. As in the conventional spacelike DVCS, there are interfering coherent amplitudes contributing to the timelike processes involving C= - form factors. The interference between the amplitudes measures the phase of the C=+ timelike DVCS amplitude relative to the phase of the timelike form factors and can be isolated by considering the forward-backward e+ \leftrightarrow e- asymmetry. The J=0 fixed pole contribution which arises from the local coupling of the two photons to the quark current plays a special role. As an example we present a simple model.

hep-ph↗
arXiv2005

Collinear Photon Exchange in the Beam Normal Polarization Asymmetry of Elastic Electron-Proton Scattering

The parity-conserving single-spin beam asymmetry of elastic electron-proton scattering is induced by an absorptive part of the two-photon exchange amplitude. We demonstrate that this asymmetry has logarithmic and double-logarithmic enhancement due to contributions of hard collinear quasi-real photons. An optical theorem is used to evaluate the asymmetry in terms of the total photoproduction cross section on the proton, predicting its magnitude at a few parts per million for high electon beam energies and small scattering angles. At fixed 4-momentum transfers, the asymmetry is rising logarithmically with increasing electron beam energy, following the high-energy diffractive behavior of total photoproduction cross section on the proton.

hep-ph↗
arXiv2004

Large Logarithms in the Beam Normal Spin Asymmetry of Elastic Electron--Proton Scattering

We study a parity-conserving single-spin beam asymmetry of elastic electron-proton scattering induced by an absorptive part of the two-photon exchange amplitude. It is demonstrated that excitation of inelastic hadronic intermediate states by the consecutive exchange of two photons leads to logarithmic and double-logarithmic enhancement due to contributions of hard collinear quasi-real photons. The asymmetry at small electron scattering angles is expressed in terms of the total photoproduction cross section on the proton, and is predicted to reach the magnitude of 20-30 parts per million. At these conditions and fixed 4-momentum transfers, the asymmetry is rising logarithmically with increasing electron beam energy, following the high-energy diffractive behavior of total photoproduction cross section on the proton.

hep-ph↗
arXiv2003

Single-Spin Beam Asymmetry in Semi-Exclusive Deep-Inelastic Electroproduction

Recent measurements from Jefferson Lab show significant beam single spin asymmetries in deep inelastic scattering. The asymmetry is due to interference of longitudinal and transverse photoabsorption amplitudes which have different phases induced by the final-state interaction between the struck quark and the target spectators. We developed a dynamical model for a single-spin beam asymmetry in deep-inelastic scattering. Our results are consistent with the experimentally observed magnitude of this effect. We conclude that similar mechanisms involving quark orbital angular momentum (`Sivers effect') are responsible for both target and beam single-spin asymmetries.

hep-ph↗
arXiv2002

Interplay of Hard and Soft Processes at JLab Energies

Even at moderate energy machines, there is a regime where hard pion electroproduction proceeds by a perturbatively calculable process. The process, we claim, is not the leading twist fragmentation one but rather a higher twist process that produces kinematically isolated pions. Semiexclusive data may teach us more about parton distribution functions of the target and the pion distribution amplitude. In addition, there is a connection to generalized parton distribution calculations of exclusive processes in that the perturbative kernel is the same.

hep-ph↗
arXiv2002

Nucleon Compton Scattering with Two Space--Like Photons

We calculated two--photon exchange effects for elastic electron--proton scattering at high momentum transfers. The corresponding nucleon Compton amplitude is defined by two space--like virtual photons that appear to have significant virtualities. We make predictions for a) a single--spin beam asymmetry, and b) a single--spin target asymmetry or recoil proton polarization caused by an unpolarized electron beam.

hep-ph↗
arXiv2000

Scaling and Duality in Semi-exclusive Processes

We discuss extending scaling and duality studies to semi-exclusive processes. We show that semi-exclusive hard pion photoproduction should exhibit scaling behavior in kinematic regions where the photon and pion both interact directly with the same quark. We show that such kinematic regions exist. We also show that the constancy with changing momentum transfer of the resonance peak/scaling curve ratio, familiar for many resonances in deep inelastic scattering, is also expected in the semi-exclusive case.

hep-ph↗
arXiv1999

Soft Contributions to Hard Pion Photoproduction

Hard, or high transverse momentum, pion photoproduction can be a tool for probing the parton structure of the beam and target. We estimate the soft contributions to this process, with an eye toward delineating the region where perturbatively calculable processes dominate. Our soft process estimate is based on vector meson dominance and data based parameterizations of semiexclusive hadronic cross sections. We find that soft processes dominate in single pion photoproduction somewhat past 2 GeV transverse momentum at a few times 10 GeV incoming energy. The recent polarization asymmetry data is consistent with the perturbative asymmetry being diluted by polarization insensitive soft processes. Determining the polarized gluon distribution using hard pion photoproduction appears feasible with a few hundred GeV incoming energy (in the target rest frame).

hep-ph↗