arXiv subjects

V. G. Gueorguiev

Publications and source records attributed to V. G. Gueorguiev.

At least 19 recordsLinked to original sources

arXiv2023

Big-Bang Nucleosynthesis within the Scale Invariant Vacuum Paradigm

The Scale Invariant Vacuum (SIV) paradigm is applied to the Big-Bang Nucleosynthesis using the known analytic expressions for the expansion factor $a$ and the plasma temperature $T$ as functions of the SIV time $τ$ since the Big-Bang when $a(τ=0)=0$. The results are compared to the known standard BBNS model as calculated with the PRIMAT code. Potential SIV-guided deviations from the local statistical equilibrium are explored. Overall, we find that smaller than usual baryon and non-zero dark matter content, by a factor of three to five times reduction, result in compatible to the standard reproduction of the light elements abundances.

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arXiv2021

Geometric Justification of the Fundamental Interaction Fields for the Classical Long-Range Forces

Based on the principle of reparametrization invariance, the general structure of physically relevant classical matter systems is illuminated within the Lagrangian framework. In a straightforward way, the matter Lagrangian contains background interaction fields, such as a 1-form field analogous to the electromagnetic vector potential and symmetric tensor for gravity. The geometric justification of the interaction field Lagrangians for the electromagnetic and gravitational interactions are emphasized. The generalization to E-dimensional extended objects (p-branes) embedded in a bulk space M is also discussed within the light of some familiar examples. The concept of fictitious accelerations due to un-proper time parametrization is introduced, and its implications are discussed.The framework naturally suggests new classical interaction fields beyond electromagnetism and gravity. The simplest model with such fields is analyzed and its relevance to dark matter and dark energy phenomena on large/cosmological scales is inferred. Unusual pathological behavior in the Newtonian limit is suggested to be a precursor of quantum effects and of inflation-like processes at microscopic scales.

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arXiv2021

Reparametrization Invariance and Some of the Key Properties of Physical Systems

In this paper, we argue in favor of first-order homogeneous Lagrangians in the velocities. The relevant form of such Lagrangians is discussed and justified physically and geometrically. Such Lagrangian systems possess Reparametrization Invariance (RI) and explain the observed common Arrow of Time as related to the non-negative mass for physical particles. The extended Hamiltonian formulation, which is generally covariant and applicable to reparametrization-invariant systems, is emphasized. The connection between the explicit form of the extended Hamiltonian $\boldsymbol{H}$ and the meaning of the process parameter $λ$ is illustrated. The corresponding extended Hamiltonian $\boldsymbol{H}$ defines the classical phase space-time of the system via the Hamiltonian constraint $\boldsymbol{H}=0$ and guarantees that the Classical Hamiltonian $H$ corresponds to $p_{0}$ -- the energy of the particle when the coordinate time parametrization is chosen. The Schrödinger's equation and the principle of superposition of quantum states emerge naturally. A connection is demonstrated between the positivity of the energy $E=cp_{0}>0$ and the normalizability of the wave function by using the extended Hamiltonian that is relevant for the proper-time parametrization

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arXiv2019

Beyond the 2-Body Interaction Paradigm: The Case for Extended A-Body Pairing Interaction in Nuclei

We discuss modeling of nuclear structure beyond the 2-body interaction paradigm. Our first example is related to the need of three nucleon contact interaction terms suggested by chiral perturbation theory. The relationship of the two low-energy effective coupling parameters for the relevant three nucleon contact interaction terms $c_D$ and $c_E$ that reproduce the binding energy of $^3$H and $^3$He has been emphasized and the physically relevant parameter region has been illustrated using the binding energy of $^4$He. Further justification of A-body interaction terms is outlined based on the Okubo-Lee-Suzuki effective interaction method used in solving the nuclear many-body problem within a finite model space. The third example we use is an exactly solvable A-body extended pairing interaction applied to heavy nuclei with a long isotopic chains, in particular using $^{132}$Sn and $^{208}$Pb as closed core system illustrates a remarkable relationship between the extended pairing strength $G(A)$ and the size of the valence space $\dim(A)$ for the members of these two isotope chain: $G(A)=α\dim(A)^{-β}$ with $α=259.436$ for Sn and $α=366.77$ for Pb while the parameter $β$ is practically 1. These three cases present evidence for the need of better understanding of the three-nucleon (NNN), four-nucleon (NNNN), and A-body interactions in nuclei either derived from ChPT or from a phenomenological considerations.

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arXiv2018

Exactly Solvable Pairing Models

Some results for two distinct but complementary exactly solvable algebraic models for pairing in atomic nuclei are presented: 1) binding energy predictions for isotopic chains of nuclei based on an extended pairing model that includes multi-pair excitations; and 2) fine structure effects among excited $0^+$ states in $N \approx Z$ nuclei that track with the proton-neutron ($pn$) and like-particle isovector pairing interactions as realized within an algebraic $sp(4)$ shell model. The results show that these models can be used to reproduce significant ranges of known experimental data, and in so doing, confirm their power to predict pairing-dominated phenomena in domains where data is unavailable.

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arXiv2017

Neutron Transfer Reactions for Deformed Nuclei Using Sturmian Basis States

We study the spin-parity distribution P(J$^π$,E) of $^{156}$Gd excited states above the neutron separation energy $S_{n}=8.536$ MeV \cite{156Gd_list_of_levels} that are expected to be populated via the 1-step neutron pickup reaction $^{157}$Gd($^{3}$He,$^{4}$He)$^{156}$Gd. In analogy with the rotor plus particle model \cite{Bohr&Mottelson-II}, we view excited states in $^{156}$Gd as rotational excitations built on intrinsic states consisting of a neutron hole in the $^{157}$Gd core, that is, a neutron removal from a deformed Woods-Saxon type single-particle state \cite{Woods-Saxon:1954} in $^{157}$Gd. The particle-core interaction usually dominated by a Coriolis coupling are accounted via first order perturbation theory \cite{VGGueorguiev:07062002}. The reaction cross section to each excited state in $^{156}$Gd is calculated as coherent contribution using a standard reaction code \cite{CHUCK} based on spherical basis states. The spectroscopic factor associated with each state is the expansion coefficient of the deformed neutron state in a spherical Sturmian basis along with the spherical form factors \cite{VGGueorguiev:07062002}. The total cross section, as a function of the excitation energy, is generated using Lorentzian smearing distribution function. Our calculations show that, within the assumptions and computational modeling, the reaction $^{3}$He+$^{157}$Gd $\rightarrow$ $^{4}$He+$^{156}$Gd$^{\star}$ has a smooth formation probability P(J$^π$,E) within the energy range relevant to the desired reaction $^{155}$Gd+n $\rightarrow$ $^{156}$Gd$^{\star}$. The formation probability P(J$^π$,E) resembles a Gaussian distribution with centroids and widths that differ for positive and negative parity states.

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arXiv2012

The Oblique Basis Method from an Engineering Point of View

The oblique basis method is reviewed from engineering point of view related to vibration and control theory. Examples are used to demonstrate and relate the oblique basis in nuclear physics to the equivalent mathematical problems in vibration theory. The mathematical techniques, such as principal coordinates and root locus, used by vibration and control theory engineers are shown to be relevant to the Richardson - Gaudin pairing-like problems in nuclear physics.

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arXiv2010

Richardson-Gaudin Algebras and the Exact Solutions of the Proton-Neutron Pairing

Many exactly solvable models are based on Lie algebras. The pairing interaction is important in nuclear physics and its exact solution for identical particles in non-degenerate single-particle levels was first given by Richardson in 1963. His solution and its generalization to Richardson-Gaudin quasi-exactly solvable models have attracted the attention of many contemporary researchers and resulted in the exact solution of the isovector pn-pairing within the so(5) RG-model and the equal strength spin-isospin pn-pairing within the so(8) RG-model. Basic properties of the RG-models are summarized and possible applications to nuclear physics are emphasized. MSC2010 Classification: 81U15 Exactly and quasi-solvable systems, 17B81 Applications to physics, 81V35 Nuclear physics, 81R40 Symmetry breaking.

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arXiv2010

Challenges for Modeling Nuclear Structure: Are the Proton and Neutron Masses and A-body Interactions Relevant?

We discuss some of the challenges that future nuclear modeling may face in order to improve the description of the nuclear structure. One challenge is related to the need for A-body nuclear interactions justified by various contemporary nuclear physics studies. Another challenge is related to the discrepancy in the NNN contact interaction parameters for 3He and 3H that suggests the need for accurate proton and neutron masses in the future precision calculations. MSC2010 Classification: 17B81 Applications to physics, 17B80 Applications to integrable systems, 81R12 Relations with integrable systems, 81V70 Many-body theory, 81V35 Nuclear physics, 81U15 Exactly and quasi-solvable systems, 82B23 Exactly solvable models; Bethe ansatz.

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arXiv2010

Application of the extended pairing model to heavy isotopes

Relative binding energies (RBEs) within three isotopic chains ($^{100-130}$Sn, $^{152-181}$Yb, and $^{181-202 }$Pb) have been studied using the exactly solvable extended pairing model (EPM) \cite{FPan04}. The unique pairing strength $G$, which reproduces the experimental RBEs, has been determined. Within EPM, $\log(G)$ is a smooth function of the model space dimension $\dim(A)$, as expected for an effective coupling strength. In particular, for the Pb and Sn isotopes $G$ can be described by a two parameter expression that is inversely proportional to the dimensionality of the model space, $G=α\dim(A)^{-β}$ with $β\approx 1$. PACS Classification: 21.10.Dr Binding energies, 71.10.Li Pairing interactions in model systems, and 21.60.Cs Shell model.

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arXiv2010

Mixed-Symmetry Shell-Model Calculations in Nuclear Physics

We consider a novel approach to the nuclear shell model. The one-dimensional harmonic oscillator in a box is used to introduce the concept of an oblique-basis shell-model theory. By implementing the Lanczos method for diagonalization of large matrices, and the Cholesky algorithm for solving generalized eigenvalue problems, the method is applied to nuclei. The mixed-symmetry basis combines traditional spherical shell-model states with SU(3) collective configurations. We test the validity of this mixed-symmetry scheme on 24Mg and 44Ti. Results for 24Mg, obtained using the Wilthental USD intersection in a space that spans less than 10% of the full-space, reproduce the binding energy within 2% as well as an accurate reproduction of the low-energy spectrum and the structure of the states - 90% overlap with the exact eigenstates. In contrast, for an m-scheme calculation, one needs about 60% of the full space to obtain compatible results. Calculations for 44Ti support the mixed-mode scheme although the pure SU(3) calculations with few irreps are not as good as the standard m-scheme calculations. The strong breaking of the SU(3) symmetry results in relatively small enhancements within the combined basis. However, an oblique-basis calculation in 50% of the full pf-shell space is as good as a usual m-scheme calculation in 80% of the space. Results for the lower pf-shell nuclei 44-48Ti and 48Cr, using the Kuo-Brown-3 interaction, show that SU(3) symmetry breaking in this region is driven by the single-particle spin-orbit splitting. In our study we observe some interesting coherent structures, such as coherent mixing of basis states, quasi-perturbative behavior in the toy model, and enhanced B(E2) strengths close to the SU(3) limit even though SU(3) appears to be rather badly broken.

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arXiv2008

Equation of State for a van der Waals Universe during Reissner-Nordstrom Expansion

In a previous work [E.M. Prodanov, R.I. Ivanov, and V.G. Gueorguiev, Reissner-Nordstrom Expansion, Astroparticle Physics 27 (150-154) 2007], we proposed a classical model for the expansion of the Universe during the radiation-dominated epoch based on the gravitational repulsion of the Reissner-Nordstrom geometry - naked singularity description of particles that "grow" with the drop of the temperature. In this work we model the Universe during the Reissner-Nordstrom expansion as a van der Waals gas and determine the equation of state.

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arXiv2008

Towards an Extended Microscopic Theory for the Upper fp-shell nuclei

An extended SU(3) shell model that for the first time explicitly includes unique-parity levels is introduced. Shell-model calculations for the isotopes of $^{64}$Ge and $^{68}$Se are performed where valence nucleons beyond the N=28=Z core occupy levels of the normal parity upper-$fp$ shell ($f_{5/2},p_{3/2},p_{1/2}$) and the unique parity $g_{9/2}$ intruder configuration. The levels of the upper-$fp$ shell are handled within the framework of an m-scheme basis as well as its pseudo-SU(3) counterpart, and respectively, the $g_{9/2}$ as a single level and as a member for the complete $gds$ shell. It is demonstrated that the extended SU(3) approach allows one to better probe the effects of deformation and to account for many key properties of the system by using a highly truncated model space.

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arXiv2007

Light nuclei from chiral EFT interactions

Recent developments in nuclear theory allow us to make a connection between quantum chromodynamics (QCD) and low-energy nuclear physics. First, chiral effective field theory (chiEFT) provides a natural hierarchy to define two-nucleon (NN), three-nucleon (NNN), and even four-nucleon interactions. Second, ab initio methods have been developed capable to test these interactions for light nuclei. In this contribution, we discuss ab initio no-core shell model (NCSM) calculations for s-shell and p-shell nuclei with NN and NNN interactions derived within chiEFT.

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arXiv2007

Neutron Transfer Reactions for Deformed Nuclei Using Sturmian Basis

We study the spin-parity distribution P(J$^π$,E) of $^{156}$Gd excited states above the neutron separation energy that are expected to be populated via the neutron pickup reaction $^{157}$Gd($^{3}$He,$^{4}$He)$^{156}$Gd. In general, modeling of the spin-parity distribution is important for the applicability of the surrogate reaction technique as a method of deducing reaction cross sections. We model excited states in $^{156}$Gd as rotational states built on intrinsic states consisting of a hole in the core where the hole represents neutron removal form a deformed single particle state. The reaction cross section to each excited state is calculated using standard reaction code that uses spherical reaction form-factor input. The spectroscopic factor associated with each form-factor is the expansion coefficient of the deformed neutron state in a spherical Sturmian basis consisting of the spherical reaction form-factors.

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arXiv2007

Reissner-Nordstrom Expansion

We propose a classical mechanism for the cosmic expansion during the radiation-dominated era. This mechanism assumes that the Universe is a two-component gas. The first component is a gas of ultra-relativistic "normal" particles described by an equation of state of an ideal quantum gas of massless particles. The second component consist of "unusual" charged particles (namely, either with ultra-high charge or with ultra-high mass) that provide the important mechanism of expansion due to their interaction with the "normal" component of the gas. This interaction is described by the Reissner--Nordström metric purely geometrically -- the ``unusual'' particles are modeled as zero-dimensional naked singularities inside spheres of gravitational repulsion. The radius of a repulsive sphere is inversely proportional to the energy of an incoming particle or the temperature. The expansion mechanism is based on the inflating of the "unusual" particles (of charge $Q$) with the drop of the temperature -- this drives apart all neutral particles and particles of specific charge $q/m$ such that ${sign}(Q) q/m \ge - 1$. The Reissner--Nordström expansion naturally ends at recombination. We discuss the range of model parameters within which the proposed expansion mechanism is consistent with the restrictions regarding quantum effects.

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arXiv2007

Structure of A=10-13 nuclei with two- plus three-nucleon interactions from chiral effective field theory

Properties of finite nuclei are evaluated with two-nucleon (NN) and three-nucleon (NNN) interactions derived within chiral effective field theory (EFT). The nuclear Hamiltonian is fixed by properties of the A=2 system, except for two low-energy constants (LECs) that parameterize the short range NNN interaction. We constrain those two LECs by a fit to the A=3 system binding energy and investigate sensitivity of 4He, 6Li, 10,11B and 12,13C properties to the variation of the constrained LECs. We identify a preferred choice that gives globally the best description. We demonstrate that the NNN interaction terms significantly improve the binding energies and spectra of mid-p-shell nuclei not just with the preferred choice of the LECs but even within a wide range of the constrained LECs. At the same time, we find that a very high quality description of these nuclei requires further improvements to the chiral Hamiltonian.

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arXiv2006

Exact Solution of the Isovector Proton Neutron Pairing Hamiltonian

The complete exact solution of the T=1 neutron-proton pairing Hamiltonian is presented in the context of the SO(5) Richardson-Gaudin model with non-degenerate single-particle levels and including isospin-symmetry breaking terms. The power of the method is illustrated with a numerical calculation for $^{64}$Ge for a $pf+g_{9/2}$ model space which is out of reach of modern shell-model codes.

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