arXiv subjects

Carl E. Carlson

Publications and source records attributed to Carl E. Carlson.

At least 19 recordsLinked to original sources

arXiv2026

Non-Diffractive Topological Spin Textures of Relativistic Twisted Fermion Beams

In optics and acoustics, in structured beams, non-diffracting polarization measures in clearly diffracting beams, and spin direction distributions in the core of these waves that have Skyrmionic behavior have been found. We here study the equivalents of these for fermions and show that in corresponding circumstances the non-diffractive spin textures persist independently of spin, statistics, or kinematics (or propagation speed of the structured wave). As a part of the study, we present LG solutions for Dirac particles valid for both relativistic and non-relativistic kinematics.

physics.optics↗

arXiv2025

Semi-inclusive pion electroproduction at the highest transverse momenta

At the energies of present and future electron accelerators designed to study the structure of hadrons, there is a regime where hard pion electroproduction proceeds by a perturbatively calculable process in QCD. The process is not the leading twist fragmentation one but rather a higher twist process that produces kinematically isolated pions. Semi-inclusive 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 electroproduction of mesons in that the perturbative kernel is the same.

hep-ph↗

arXiv2024

New Spin Structure Constraints on Hyperfine Splitting and Proton Size

The 1S hyperfine splitting in hydrogen is measured to an impressive ppt precision and will soon be measured to ppm precision in muonic hydrogen. The latter measurement will rely on theoretical predictions, which are limited by knowledge of the proton polarizability effect $Δ_\text{pol}$. Data-driven evaluations of $Δ_\text{pol}$ have long been in significant tension with baryon chiral perturbation theory. Here we present improved results for $Δ_\text{pol}$ driven by new spin structure data, reducing the long-standing tension between theory and experiment and halving the dominating uncertainty in hyperfine splitting calculations.

nucl-ex↗

arXiv2023

Comments and extensions of a suggestion for a relativistic charge density definition

A recent suggested definition of a relativistically correct three dimensional charge density of an extended hadron is shown to be physically and intuitively connected to an earlier relativistically correct two dimensional charge density studied in the context of light-front physics. Looking at spin-1/2 hadrons, such a connection is shown to exist for both the polarized and unpolarized cases.

hep-ph↗

arXiv2022

Superkicks and momentum density tests via micromanipulation

There is an unsettled problem in choosing the correct expressions for the local momentum density and angular momentum density of electromagnetic fields (or indeed, of any non-scalar field). If one only examines plane waves, the problem is moot, as the known possible expressions all give the same result. The momentum and angular momentum density expressions are generally obtained from the energy-momentum tensor, in turn obtained from a Lagrangian. The electrodynamic expressions obtained by the canonical procedure are not the same as the symmetric Belinfante reworking. For the interaction of matter with structured light, for example, twisted photons, this is important; there are drastically different predictions for forces and angular momenta induced on small test objects. We show situations where the two predictions can be checked, with numerical estimates of the size of the effects.

physics.optics↗

arXiv2022

Vorticity of Twisted Spinor Fields

Spinor fields with a vortex structure in free space that allow them to have arbitrary integer orbital angular momentum along the direction of motion have been studied for some time. Relatively new is the observation in a certain context that the vortex center of this field structure is, unlike a classical whirlpool, not singular. We point out that there are several ways to calculate the local velocity of the spinor field and that all but one show a singular vorticity at the vortex line. That one, using the Dirac bilinear current with no derivatives, is the only one so far (to our knowledge) studied in the literature in this context and we further show how to understand an apparent conflict in the existing results.

quant-ph↗

arXiv2022

Superkicks and the photon angular and linear momentum density

We address a problem of proper definition of momentum density for spatially structured electromagnetic fields. We show that the expressions for the momentum and angular momentum obtained locally are not the same when one uses the canonical energy-momentum tensor instead of the symmetric Belinfante energy-momentum tensor in electrodynamics. This has important consequences for interaction of matter with structured light, for example, twisted photons; and would give drastically different results for forces and angular momenta induced on small test objects. We show, with numerical estimates of the size of the effects, situations where the canonical and symmetrized forms induce very different torques or (superkick) recoil momenta on small objects or atomic rotors, over a broad range of circumstances.

quant-ph↗

arXiv2021

Delta baryon photoproduction with twisted photons

A future gamma factory at CERN or accelerator-based gamma sources elsewhere can include the possibility of energetic twisted photons, which are photons with a structured wave front that can allow a pre-defined large angular momentum along the beam direction. Twisted photons are potentially a new tool in hadronic physics, and we consider here one possibility, namely the photoproduction of $Δ$(1232) baryons using twisted photons. We show that particular polarization amplitudes isolate the smaller partial wave amplitudes and they are measurable without interference from the terms that are otherwise dominant.

hep-ph↗

arXiv2020

Recoil Momentum Effects in Quantum Processes Induced by Twisted Photons

We consider physical processes caused by the twisted photons for a wide range of energy scales, from optical (eV) to nuclear (MeV) to high-energy gamma-rays (TeV). We demonstrate that in order to satisfy angular momentum conservation, absorption of a twisted photon leads to a transverse recoil of the final particle or a system of particles leading to increased threshold energy required for the reaction to proceed. Modification of the threshold energy is predicted for (a) Photo-absorption on colds trapped ions of $^{40}$Ca, along with emerging new transverse-motion sidebands, (b) photo-disintegration of deuterium and (c) photo-production of electron-positron pairs in astrophysics environment.

quant-ph↗

arXiv2020

Polarization Transfer from the Twisted Light to an Atom

When polarized light is absorbed by an atom, the excited atomic system carries information about the initial polarization of light. For the light that carries an orbital angular momentum, or the twisted light, the polarization states are described by eight independent parameters, as opposed to three Stokes parameters for plane waves. We use a parameterization of the spin-density matrix of the twisted light in terms of vector and tensor polarization, in analogy with massive spin-1 particles, and derive formulae that define atom's response to specific polarization components of the twisted light. It is shown that for dipole ($S\to P$) atomic transitions, the atom's polarization is in one-to-one correspondence with polarization of the incident light; this relation is violated, however, for the transitions of higher multipolarity ($S\to D$, $S\to F$, etc.) We pay special attention to contributions of the longitudinal electric field into the matrix elements of atomic transitions.

quant-ph↗

arXiv2020

Low energy doubly-virtual Compton scattering from di-lepton electroproduction on a nucleon

We propose a new way to experimentally determine the subleading low-energy structure constant of doubly-virtual Compton scattering on a proton. Such empirical determination will reduce the theoretical model error in estimates of the hadronic correction to the muonic hydrogen Lamb shift. We demonstrate that the di-lepton forward-backward asymmetry in the $e^- p \to e^- p \, e^- e^+$ process, which can be accessed at electron scattering facilities, yields a large sensitivity to this so far unknown low-energy constant.

hep-ph↗

arXiv2019

Excitation of E1-forbidden Atomic Transitions with Electric, Magnetic or Mixed Multipolarity in Light Fields Carrying Orbital and Spin Angular Momentum

Photons carrying a well-defined orbital angular momentum have been proven to modify spectroscopic selection rules in atomic matter. Excitation profiles of electric quadrupole transitions have been measured with single trapped $^{40}$Ca$^+$ ions for varying polarizations. We further develop the photo-absorption formalism to study the case of arbitrary alignment of the beam's optical axis with respect to the ion's quantization axis and mixed multipolarity. Thus, predictions for M1-dominated $^{40}Ar^{13+}$, E3-driven $^{171}Yb^+$ and $^{172}Yb^+$, and B-like $^{20}Ne^{5+}$ are presented. The latter case displays novel effects, coming from the presence of a strong photon -- magnetic dipole coupling.

quant-ph↗

arXiv2018

Dilepton photoproduction on a deuteron target

We investigate the sensitivity of the cross section for lepton pair production off a deuteron target, $γd \to l^+ l^- d$, to the deuteron charge radius. We show that for small momentum transfers the Bethe-Heitler process dominates, and that it is sensitive to the charge radius such that a cross section ratio measurement of about $0.1 \%$ relative accuracy could give a deuteron charge radius more accurate that the current electron scattering value and sufficiently accurate to distinguish between the electronic and muonic atomic values.

hep-ph↗

arXiv2018

Atomic Spectroscopy with Twisted Photons: Separation of M1--E2 Mixed Multipoles

We analyze atomic photoexcitation into the discrete states by twisted photons, or photons carrying extra orbital angular momentum along their direction of propagation. From the angular momentum and parity considerations, we are able to relate twisted-photon photoexcitation amplitudes to their plane-wave analogues, independently of the details of the atomic wave functions. We analyzed the photo-absorption cross sections of mixed-multipolarity $E2-M1$ transitions in ionized atoms and found fundamental differences coming from the photon topology. Our theoretical analysis demonstrates that it is possible to extract the relative transition rates of different multipolar contributions by measuring the photo-excitation rate as a function of the atom's position (or the impact parameter) with respect to the optical vortex center. The proposed technique for separation of multipoles can be implemented if the target's atom position is resolved with sub-wavelength accuracy, for example, with Paul traps. Numerical examples are presented for Boron-like highly-charged ions (HCI).

physics.atom-ph↗

arXiv2017

Experimental Verification of Position-Dependent Angular-Momentum Selection Rules for Absorption of Twisted Light by a Bound Electron

We analyze the multipole excitation of atoms with twisted light, i.e., by a vortex light field that carries orbital angular momentum. A single trapped $^{40}$Ca$^+$ ion serves as a localized and positioned probe of the exciting field. We drive the $S_{1/2} \to D_{5/2}$ transition and observe the relative strengths of different transitions, depending on the ion's transversal position with respect to the center of the vortex light field. On the other hand, transition amplitudes are calculated for a twisted light field in form of a Bessel beam, a Bessel-Gauss and a Gauss-Laguerre mode. Analyzing experimental obtained transition amplitudes we find agreement with the theoretical predictions at a level of better than 3\%. Finally, we propose measurement schemes with two-ion crystals to enhance the sensing accuracy of vortex modes in future experiments.

quant-ph↗

arXiv2017

Beam normal spin asymmetry for the $e p \to e Δ(1232)$ process

We calculate the single spin asymmetry for the $e p \to e Δ(1232)$ process, for an electron beam polarized normal to the scattering plane. Such single spin asymmetries vanish in the one-photon exchange approximation, and are directly proportional to the absorptive part of a two-photon exchange amplitude. As the intermediate state in such two-photon exchange process is on its mass shell, the asymmetry allows one to access for the first time the on-shell $Δ\to Δ$ as well as $N^\ast \to Δ$ electromagnetic transitions. We present the general formalism to describe the $e p \to e Δ$ beam normal spin asymmetry, and provide a numerical estimate of its value using the nucleon, $Δ(1232)$, $S_{11}(1535)$, and $D_{13}(1520)$ intermediate states. We compare our results with the first data from the Qweak@JLab experiment and give predictions for the A4@MAMI experiment.

hep-ph↗

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↗

arXiv2017

Lattice corrections to the quark quasidistribution at one-loop

We calculate radiative corrections to the quark quasidistribution in lattice perturbation theory at one loop to leading orders in the lattice spacing. We also consider one-loop corrections in continuum Euclidean space. We find the infrared behavior of the corrections in Euclidean and Minkowski space are different. We explore features of momentum loop integrals and demonstrate why loop corrections from the lattice perturbation theory and Euclidean continuum do not correspond with their Minkowski brethren, and comment on a recent suggestion for transcending the differences in the results. Further, we examine the role of the lattice spacing $a$ and of the $r$ parameter in the Wilson action in these radiative corrections.

hep-ph↗