arXiv subjects

Patrick McGlynn

Publications and source records attributed to Patrick McGlynn.

8 recordsLinked to original sources

arXiv2026

Uncertainty quantified three-body model applied to the two-neutron halo $^{22}$C

Two-neutron halo nuclei offer a fascinating probe into the behaviour of quantum few-body systems at the limits of binding. Although few nuclei have already been clearly identified, many of their properties remain poorly constrained. $^{22}$C, one of the heaviest, still lacks a precise identification of its static and dynamic properties, such as its mass and dipole strength in the continuum. One main difficulty is that properties of two-neutron halo nuclei are inferred from experimental data using a theoretical model. Therefore, accurately determining the characteristics of two-neutron halo nuclei requires an accurate theoretical model and careful quantification of the uncertainties. In this work, we examine $^{22}$C with a three-body model, seeing $^{22}$C as a $^{20}$C core and two halo neutrons, and quantify for the first time the uncertainties associated with the $^{20}$C-$n$ interaction using a Bayesian approach. We propagate these uncertainties to properties of bound and scattering states of $^{22}$C, as well as its dipole strength. The comparison of our prediction for the matter radius to experimentally-derived values suggests that $^{22}$C is bound by less than 0.35~MeV and is dominated by a $(s_{1/2})^2$ configuration. Our analysis of the dipole strength shows that final-state interaction needs to be included for an accurate description, the uncertainties on the strength function are about 50\% and are mostly influenced by uncertainties on the ground-state properties, and partial-wave occupation of $^{22}$C depends on the scattering length and the $d_{3/2}$ resonance energy of the $^{20}$C-$n$ unbound system. Such sensitivity of the dipole strength to the properties of both $^{21}$C and $^{22}$C properties motivates a precise measurement of the $^{22}$C dipole strength function, that will allow to precisely and accurately resolve the spectroscopy of these nuclei.

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arXiv2026

Development of an accurate formalism to predict properties of two-neutron halo nuclei: case study of $^{22}$C

When moving away from stability or in loosely-bound systems, few-body clusterized structures like two-neutron halo nuclei appear. These emerge from the interplay between the many- and few-body degrees of freedom, and/or strong coupling between bound and continuum states. This motivates the development of models that can accurately describe few-body dynamics while enforcing shell effects. This work has two goals: understanding how to accurately enforce the Pauli principle in few-body models, as well as presenting new technical developments that allow for more robust and cheaper three-body calculations. We focus on properties of the two-neutron halo 22C, but expect the conclusions to apply to other few-body systems. We use a three-body, hyperspherical harmonics formalism combined with the R-matrix method. We compare predictions for properties of 22C starting from phenomenological interactions and using two methods to remove Pauli-forbidden states, the projection and supersymmetric methods. We also present the algorithms and derivations used. Additionally, we explore model space truncations that allow for reduced computational time. We show convergence of the calculation of both bound and scattering states for $K_{max}\sim 40$. The two methods to enforce the Pauli-exclusion principle lead to different predictions of 22C properties; the projection method is more accurate. We find one efficient channel truncation that reduces the computational cost of our calculations by 20%. Our study clarifies that the projection method is more accurate than the supersymmetric one to enforce the Pauli-exclusion principle. Technical and algorithmic developments enable accurate and efficient computation of two-neutron halo properties. This development paves the way to robust uncertainty quantification in three-body predictions, and is a useful starting point to tackle more complex systems and observables.

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arXiv2025

On the extraction of fission mode properties from fragment mass distributions

Background: Fission modes are typically characterised by fragment mass and total kinetic energy centroids, around which a distribution of these variables is observed. These distributions are usually fitted with Gaussian functions. Purpose: To investigate how the properties of these ``Gaussian fission modes'' compare with underlying ``theoretical fission modes'' defined from the potential energy surface of the fissioning nuclei. Methods: A simple approach, inspired by the scission point model, is introduced to investigate the impact of anharmonicity of the potential along the scission line on Gaussian mode properties. This approach is also used to evaluated an ``effective potential'' from the yields. Results: Several Gaussian functions are usually required to fit yields from non-harmonic potentials associated with a unique theoretical mode. Similarly, ``effective fission modes'', defined from the wells of the effective potentials, are sometime very different to the Gaussian modes. For instance, the S1 and S2 Brosa modes contribute to the same effective potential well at scission. Conclusions: Gaussian fits are useful to identify the role of shell effects in fission. However, comparisons with theoretical potential energy surfaces are better carried with effective potentials extracted from the yields, assuming that a broad range of excitation energies is available.

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arXiv2024

Shell Effects in Quasi-fission in Reactions Forming 226Th Compound Nucleus

Quasi-fission (QF) reactions occur in fully damped heavy-ion collisions without the formation of an equilibrated compound nucleus, leading to the formation of fragments with similar properties as in fission reactions. Similar shell effects are expected to affect fragment formation in both fission and QF. Our purpose is to investigate QF dynamics in different reactions forming the same compound nucleus and search for possible signatures of shell effects in fragment formation. 50Ca+176Yb and 96Zr+130Sn QF reactions are simulated with the time-dependent Hartree-Fock code Sky3D near the Coulomb barrier. Evolutions of the quadrupole (Q20) and octupole (Q30) moments are interpreted in terms of features of the potential energy surface (PES) of the 226Th compound nucleus. Both reactions encounter QF. In 50Ca+176Yb, those only occur at finite angular momenta. In the more symmetric 96Zr+130Sn reaction with stronger Coulomb repulsion in the entrance channel, QF also occurs in central collisions. In agreement with earlier predictions, 50Ca+176Yb encounters partial mass equilibration that is stopped when the heavy fragment reaches Z~54 protons, as in the asymmetric fission mode of 226Th. 96Zr+130Sn encounters an inverse QF also leading to similar fragments as in asymmetric fission. In both systems, QF trajectories in the Q20-Q30 plane follow the asymmetric fission valley of 226Th PES. The observation of an inverse QF is a clear prediction that shell effects have a strong influence in QF. The similarity between fragments formed in asymmetric fission and QF supports the idea that the same shell effects are at play in both mechanisms. Interpreting QF dynamics with PES used in fission is naturally limited by the fact that these PES are usually computed with axial symmetry, no angular momentum and no excitation energy, thus motivating future developments of PES for QF.

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arXiv2024

Quantum corrections to tunnelling amplitudes of neutral scalar fields

Though theoretical treatments of quantum tunnelling within single-particle quantum mechanics are well-established, at present, there is no quantum field-theoretic description (QFT) of tunnelling. Due to the single-particle nature of quantum mechanics, many-particle effects arising from quantum field theory are not accounted for. Such many-particle effects, including pair-production, have proved to be essential in resolving the Klein-paradox. This work seeks to determine how quantum corrections affect the tunnelling probability through an external field. We investigate a massive neutral scalar field, which interacts with an external field in accordance with relativistic quantum mechanics. To consider QFT corrections, we include another massive quantised neutral scalar field coupling to the original via a cubic interaction. This study formulates an all-order recursive expression for the loop-corrected scalar propagator, which contains only the class of vertex-corrected Feynman diagrams. This equation applies for general external potentials. Though there is no closed-form analytic solution, we also demonstrate how to approximate the QFT corrections if a perturbative coupling to the quantised field is assumed.

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arXiv2024

Tunnelling amplitudes through localised external potentials from Feynman diagram summation

Currently there is no general theory of quantum tunnelling of a particle through a potential barrier which is compatible with QFT. We present a complete calculation of tunnelling amplitudes for a scalar field for some simple potentials using quantum field-theoretic methods. Using the perturbative S-matrix formalism, starting with the Klein-Gordon Lagrangian, we show that an infinite summation of Feynman diagrams can recover tunnelling amplitudes consistent with relativistic quantum mechanics. While this work does not include many-particle effects arising from a fully quantised QFT, it is necessary to investigate QFT corrections to tunnelling amplitudes.

hep-th↗

arXiv2023

Time-dependent Hartree-Fock study of quasifission trajectories in reactions forming $^{294}$Og

Background: Fission modes in superheavy nuclei are expected to be impacted by quantum shell effects. Similar shell effects may be present in quasifission reactions, acting to hinder the mass equilibration process in heavy-ion collisions. Purpose: To investigate quasifission mechanisms in five different reactions forming $^{294}$Og as a compound nucleus and compare quasifission trajectories with predicted fission modes. Methods: The potential energy surface (PES) of $^{294}$Og is calculated using the static Hartree-Fock approach with BCS pairing correlations. Quasifission trajectories for central collisions at various energies are studied with the time-dependent Hartree-Fock theory. Results: The exit channel strongly depends on initial mass asymmetry and orientation, but it only exhibits small dependences in the reaction energy. The $^{48}$Ca$+^{246}$Cf reaction is affected by the PES topology, leading to either fusion or asymmetric fission. Spherical shell effects associated with the $Z=50$ magic gap hinder charge and mass equilibrations in $^{126}$Sn$+^{168}$Er, resulting in large total kinetic energies and compact scission configurations. Conclusions: Quasifission trajectories can be interpreted in terms of the underlying PES for low excitation energies. Future investigations of quasifission with temperature and angular momentum dependent PES could be considered.

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arXiv2020

Imaginary Time Mean-Field Method for Collective Tunneling

Background: Quantum tunneling in many-body systems is the subject of many experimental and theoretical studies in fields ranging from cold atoms to nuclear physics. However, theoretical description of quantum tunneling with strongly interacting particles, such as nucleons in atomic nuclei, remains a major challenge in quantum physics. Purpose: An initial-value approach to tunneling accounting for the degrees of freedom of each interacting particle is highly desirable. Methods: Inspired by existing methods to describe instantons with periodic solutions in imaginary time, we investigate the possibility to use an initial value approach to describe tunneling at the mean-field level. Real-time and imaginary-time Hartree dynamics are compared to the exact solution in the case of two particles in a two-well potential. Results: Whereas real-time evolutions exhibit a spurious self-trapping effect preventing tunneling in strongly interacting systems, the imaginary-time-dependent mean-field method predicts tunneling rates in excellent agreement with the exact solution. Conclusions: Being an initial-value method, it could be more suitable than approaches requiring periodic solutions to describe realistic systems such as heavy-ion fusion.

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