arXiv subjects

Petar Žugec

Publications and source records attributed to Petar Žugec.

10 recordsLinked to original sources

arXiv2026

A stop to field line misconceptions

For two hundred years - ever since Faraday's first conception of the field lines - hearts and minds of students have been pervaded by erroneous and unsubstantiated claims about these well defined mathematical objects. The most prominent misconceptions include: (1) a notion of the field lines as of 'lines of force'; (2) a notion that the field lines coincide with particle trajectories; (3) a notion that a field line density measures a magnitude of a vector field; (4) a notion that the field lines of a divergence-free field always form closed loops. Even the modern day literature systematically perpetuates some of these claims. We compile here the trivial counterexamples to these claims, providing physics instructors with an efficient and effective way of dispelling these misconceptions.

physics.class-ph↗

arXiv2026

A reconciliation of the Pryce-Ward and Klein-Nishina statistics for semi-classical simulations of annihilation photons correlations

Two photons from the ground state para-positronium annihilation are emitted in a maximally entangled singlet state of orthogonal polarizations. In case of the Compton scattering of both photons the phenomenon of quantum entanglement leads to a measurable increase in the azimuthal correlations of scattered photons, as opposed to a classical description treating the two scattering events as independent. The probability of the scattering of the system of the entangled photons is described by the Pryce-Ward cross section dependent on a difference of the azimuthal scattering angles in the fixed coordinate frame, while the independent scattering of single photons is described by the Klein-Nishina cross section dependent on the azimuthal angle relative to each photon's initial polarization. Since the singlet state of orthogonal polarizations is rotationally invariant, it does not carry any physical information on the initial polarizations of the single annihilation photons. In such bipartite state the angular origin for the Klein-Nishina cross section is undefined, making the Pryce-Ward and Klein-Nishina descriptions mutually exclusive. However, semi-classical simulations of the joint Compton scattering of entangled photons - implementing the Pryce-Ward cross section, but still treating the two photons as separate entities - can reconcile the Pryce-Ward correlations with the Klein-Nishina statistics for single photons by implementing a modified version of a scattering cross section presented in this work.

quant-ph↗

arXiv2025

Machine learning based parametrization of the resolution function for the first experimental area (EAR1) of the n_TOF facility at CERN

This study addresses a challenge of parametrizing a resolution function of the neutron beam from the neutron time of flight facility n_TOF at CERN. A difficulty stems from a fact that a resolution function exhibits rather strong variations in shape, over approximately 10 orders of magnitude in neutron energy. In order to avoid a need for a manual identification of the appropriate analytical forms - hindering past attempts at its parametrization - we take advantage of the versatile machine learning techniques. In particular, we parametrize it by training a multilayer feedforward neural network, relying on a key idea that such networks act as the universal approximators. The proof of concept is presented for a resolution function for the first experimental area of the n_TOF facility, from the third phase of its operation. We propose an optimal network structure for a resolution function in question, which is also expected to be optimal or near-optimal for other experimental areas and for different phases of n_TOF operation. In order to reconstruct several resolution function forms in common use from a single parametrized form, we provide a practical tool in the form of a specialized C++ class encapsulating the computationally efficient procedures suited to the task. Specifically, the class allows an application of a user-specified temporal spread of a primary proton beam (from a neutron production process at n_TOF) to a desired resolution function form.

physics.comp-ph↗

arXiv2025

Lorentz contraction of electric field lines for a point charge in uniform motion

We examine a logical foundation of depicting a Lorentz contraction of a Coulomb field (an electric field of a point charge in uniform motion) by means of the 'Lorentz contracted' field lines. Two existing arguments for a contraction of field lines sound appealing and lead to very simple calculations yielding the correct results. However, one of them is a victim to subtle logical weaknesses, as it relies on ascribing a degree of physical reality to the electric field lines. The other one correctly proves what it sets out to prove. But it does not provide a proof, or even a suggestion, of an additional result that can be obtained by a new poof that we present here. Though our idea is very simple, the calculations used to prove it - based on a little known, half a century old result by Tsien - are somewhat more involved than those from past arguments.

physics.gen-ph↗

arXiv2023

Students' confusions about the electric field of a uniformly moving charge

In light of a recent direct experimental confirmation of a Lorentz contraction of Coulomb field (an electric field of a point charge in a uniform motion), we revisit some common confusions related to it, to be mindful of in teaching the subject. These include the questions about a radial nature of the field, a role of the retardation effect due to a finite speed of information transfer and some issues related to a depiction of Coulomb field by means of the Lorentz contracted field lines.

physics.ed-ph↗

arXiv2022

A shadow of the repulsive Rutherford scattering in the laboratory frame

The paper explores the Rutherford scattering shadow in an entire class of comoving frames -- inertial frames moving along the initial projectile direction -- of which the laboratory frame, where the target is initially at rest, is a representative example. The paper is a continuation of the previous work addressing the scattering shadow in the fixed-target and the center-of-mass frame. It is shown that the transition from these frames is technically quite involved, due to the scattering shadow forming at an infinite distance from an initial target position. The central procedure involves solving the $5^\mathrm{th}$ degree polynomial as a part of an associated extremization procedure. The shadow existence itself is subject to certain conditions, dependent on a given comoving frame. A new and unexpected phenomenon is found within a certain set of comoving frames, including the laboratory frame itself. It consists of a phase transition between an entirely smooth type of shadow and the one characterized by a formation of a sharp edge.

physics.class-ph↗

arXiv2022

Optimization of detector modules for measuring gamma-ray polarization in Positron Emission Tomography

Detection of $γ$-ray polarization in Positron Emission Tomography (PET) is as yet an unexploited feature that could be used as an additional handle to improve signal-to-background ratio in this imaging modality. The $γ$ polarization is related to the azimuthal angle in the Compton scattering process, so the initial correlation of polarizations of the annihilation quanta translates to the correlation of the azimuthal angles in events where both annihilation photons undergo Compton scattering. This results in a modulated distribution of the azimuthal angle difference for true events, while this modulation is lacking for the background events. We present a comprehensive experimental study of five detector configurations based on scintillator matrices and silicon photomultipliers, suitable for measuring the azimuthal modulation. The modules consist of either GaGG:Ce or LYSO:Ce pixels with sizes varying from 1.9x1.9x20 $\mathrm{mm^3}$ to 3x3x20 $\mathrm{mm^3}$. The distinctive feature of the modules is that they can reconstruct the Compton scattering by detecting the recoil electron and the scattered gamma in a single detector layer, which simplifies extension to larger systems. The amplitude modulation of the azimuthal angles' difference is clearly observable in all configurations ranging from $0.26\pm0.01$ to $0.34\pm 0.02$ depending on the event selection criteria. The results suggest that finer detector segmentation plays a leading role in achieving higher modulation factors.

physics.ins-det↗

arXiv2020

A shadow of the repulsive Rutherford scattering in the fixed-target and the center-of-mass frame

The paper explores the shadow of the repulsive Rutherford scattering - the portion of space entirely shielded from admitting any particle trajectory. The geometric properties of the projectile shadow are analyzed in detail in the fixed-target frame as well as in the center-of-mass frame, where both the charged projectile and the charged target cast their own respective shadows. In both frames the shadow is found to take an extremely simple, paraboloidal shape. In the fixed-target frame the target is precisely at the focus of this paraboloidal shape, while the focal points of the projectile and target shadows in the center-of-mass frame coincide. In the fixed-target frame the shadow takes on a universal form, independent of the underlying physical parameters, when expressed in properly scaled coordinates, thus revealing a natural length scale to the Rutherford scattering.

physics.class-ph↗

arXiv2020

Single-layer Compton detectors for measurement of polarization correlations of annihilation quanta

Measurement of gamma-ray polarization can provide valuable insight in different areas of physics. A possible application is in Positron Emission Tomography, where annihilation quanta with orthogonal polarizations are emitted. Since polarization can be measured via Compton scattering, the initial orthogonality of polarizations can be translated to correlation of azimuthal scattering angles, and this correlation may be exploited as an additional handle to identify the true coincidence events. To examine the concept of using the polarization correlations in PET, we have used a system of two compact, position and energy-sensitive Compton detectors in coincidence mode. Each consists of a single matrix of scintillation pixels, read-out by an array of Silicon photomultipliers. The Compton events in each module are clearly identified and the scattering angles are reconstructed from the energy deposition and event topology. We have extracted the polarimetric modulation factors from the distributions of the difference of the azimuthal angles of the two Compton-scattered gammas and studied their dependence on Compton scattering angles $θ$ and on azimuthal resolution $Δϕ$. For scattering angles around $θ_{1,2}=82^\circ$, where the maximum modulation is expected, the modulation factors from $μ=0.15\pm 0.01$ to $μ=0.27\pm 0.02$ have been measured, depending on the azimuthal resolution, which is governed by event topology in the detectors. Analogously, for scattering around $θ_{1,2}=70^\circ$, modulation factors from $μ=0.12\pm 0.01$ to $μ=0.21\pm 0.02$ have been obtained. The results show that the measurement of the polarization correlations of annihilation quanta are feasible with compact single-layer, single-side read-out detectors, which may be used to build cost-efficient systems for various applications where gamma-ray polarization information is of interest.

physics.ins-det↗

arXiv2011

A coincidental timing model for the scintillating fibers

A model describing the coincidental timing of scintillating fibers is developed. Fiber geometry, the rate of scintillation decay together with the mean number, spatial dispersion and attenuation of emitted photons is considered. For a specific selection of probability distributions and parameters involved, the entire coincidental timing distributions, corresponding FWHM values and the photon detection efficiencies are extracted. The significance of the number of photons from the scintillation process is specially emphasized. Additionally, the model is extended to include a triggering feature, experimentally realized by coupling fibers to any photon resolving device. Finally, the measurements of a coincidental timing distribution were performed, with an excellent agreement found between the experimental and predicted theoretical results.

physics.ins-det↗