arXiv subjects

Junchen Pei

Publications and source records attributed to Junchen Pei.

At least 19 recordsLinked to original sources

arXiv2026

Stochastic Similarity Renormalization Group

By integrating the quantum Monte Carlo technique into the similarity renormalization group (SRG), we have developed a stochastic SRG framework (SRGQMC) capable of both free-space two-body and in-medium many-body evolutions. This approach circumvents the combinatorial tensor-space explosion of many-body flow equations by mapping continuous unitary transformations onto an ensemble of signed random walkers. We benchmark the SRGQMC against deterministic free-space SRG evolutions of realistic nucleon-nucleon (NN) interactions, as well as against in-medium SRG (IMSRG) many-body calculations with the Richardson pairing model at two- and three-body levels [IMSRG(2)/(3)]. While a deterministic extension to the four-body level [IMSRG(4)] remains unfeasible due to prohibitive computational costs, we have achieved the first IMSRG(4) calculation by using the stochastic technique, demonstrating a substantial improvement toward the full configuration-interaction limit. This stochastic framework provides a practical pathway to higher-order IMSRG calculations.

nucl-th↗

arXiv2026

Particle number projected energies at finite temperature

In this work, the particle number projection at finite temperature is incorporated into self-consistent Skyrme density functional calculations. In particular, the energies of compound nuclei as a function of deformations are calculated rigorously based on projected densities. Results show that the even-odd staggering effect in partition function gradually diminishes as the system approaches the critical temperature. The obtained fission barriers are similar to that without projection at finite temperature, although projected energies are different. The nuclear level density at the ground state and the barrier are also studied using the projection method and the discrete Gaussian method.

nucl-th↗

arXiv2026

Stochastic many-body perturbation theory for high-order calculations

High-order perturbative $\textit{ab initio}$ calculations are challenging due to the rapidly growing configuration space and the difficulty of assessing convergence. In this letter, we introduce perturbation theory quantum Monte Carlo (PTQMC), a stochastic approach designed to compute high-order many-body perturbative corrections. By representing the perturbative wave function with random walkers in configuration space, PTQMC avoids the exponential scaling inherent to conventional constructions of high-rank excitation operators. Benchmark calculations for the Richardson pairing model demonstrate that PTQMC accurately reproduces exact many-body perturbation theory (MBPT) coefficients up to 16th order, even in strongly divergent regimes. We further show that combining PTQMC with series resummation techniques yields stable and precise energy estimates in cases where the straightforward perturbative series fails. Finally, we propose the effective number of configurations, $e^{S}$, as a global measure of perturbative wave-function complexity that can be directly extracted within PTQMC. We demonstrate that the saturation behavior of $e^{S}$ provides a more reliable indicator of the validity of perturbative expansions than energy convergence alone.

nucl-th↗

arXiv2026

Microscopic Description of Rotational Nuclear Fission Elucidates Fragment Spin Generation and Scission Mechanism

The generation of fission fragment spin as a probe of scission mechanism remains a question of considerable interests. We present here microscopic calculations of rapidly rotating fission of the compound nucleus 240Pu under varying initial conditions within the time-dependent density functional theory framework. The obtained spin ratio of light to heavy nascent fragments is unchanged up to high spins but diminished as excitation energies increase, indicating sawtooth structures in fragment spins would fade away at high excitations rather than high angular momentum. Further analysis elucidates that the bending scission mode is predominated at low excitation energies, which has been under intense debates. Results also show thicker and elongated neck configurations, along with scission nucleons emitted perpendicular to the fission axis, owing to rapid rotations. These findings offer insights into the scission mechanism of rotating compound nuclei that have usually been overlooked in earlier studies.

nucl-th↗

arXiv2026

Toward $\textit{Ab Initio}$ Quantum Simulations of Atomic Nuclei Using Noisy Qubits

Quantum computers are expected to provide a ultimate solver for quantum many-body systems, although it is a tremendous challenge to achieve that goal on current noisy quantum devices. This work illustrated quantum simulations of ab initio no-core shell model calculations of $^3$H with chiral two-nucleon and three-nucleon forces. The measurement costs are remarkably reduced by using the general commutativity measurement together with the asymptotic optimization. In addition, the noise causes serious contaminations of configurations with undesired particle numbers, and the accuracies are much improved by applying the particle number projected measurement. By tackling the efficiency and noise issues, this work demonstrated a substantial step toward ab initio quantum computing of atomic nuclei.

nucl-th↗

arXiv2025

$\textit{Ab Initio}$ Exact Calculation of Strongly Correlated Nucleonic Matter

Dense nucleonic matter is of vital importance for understanding compact stars and inferring the transition into deconfined quark phase. We present $\textit{ab initio}$ exact calculations of infinite nucleonic matter with the state-of-the-art full configuration-interaction quantum Monte Carlo method, enabling us to rigorously benchmark many-body methods and assess the degree to which the nucleonic matter is correlated. Our method has been numerically validated against exact diagonalization within a small model space. Calculations of nucleonic matter using chiral nuclear forces reveal that symmetric nuclear matter is strikingly strongly correlated, raising questions on previous $\textit{ab initio}$ calculations of nuclear matter with many-body expansion truncations and offering insights into simultaneous descriptions of finite nuclei and infinite nucleonic matter from first principles.

nucl-th↗

arXiv2025

Many-Body Effects on Nuclear Short Range Correlations

We reveal nuclear many-body effects on short range correlations by ab initio no-core shell model calculations of the scaling factor a2. The factor a2 characterizes the abundance of SRC pairs and is linearly related to the EMC effect. Our study employs the fifth-order N4LO chiral nuclear force without softening, enabling to distinguish the influences of nuclear states with different quantum numbers on SRC. It is striking to find that a2 is reduced and close in triplet isobaric analog states of neighboring nuclei, indicating that it is insufficient to estimate SRC abundances by considering only mean-field shell structures. This is explained as specific nuclear states suppress the formation of deuteron-like component, impacting our understandings of the link between high-energy partonic properties and low-energy nuclear physics.

nucl-th↗

arXiv2025

Core screening effect in knockout reactions

The systematic quenching of spectroscopic factors in terms of separation energy asymmetry in single-nucleon knockout reactions remains a puzzle. We propose a core screening effect to consider the hindrance when strongly bound nucleons in the projectile nucleus are removed by the heavy-ion target. The core screening effect is simulated as a density dependent suppression of single-particle wave functions inside the core of projectile. Our study shows that the parameterized core screening effect can significantly reduce the isospin dependence of quenching factors, offering insights into nuclear reaction mechanisms.

nucl-th↗

arXiv2025

Energy partition between splitting fission fragments

From the microscopic view, the energy partition between two fission fragments are associated with the splitting of wave functions of an entangled fissioning system, in contrast to most fission models using an explicit statistical partition of excitation energies by invoking level densities of fragments. The dynamical fission evolution is described within the time-dependent Hartree-Fock+BCS framework. Excitation energies of isotopic fission fragments are obtained with the particle-number projection method after the dynamical splitting of $^{238}$U. The resulting excitation energies of light and heavy fragments illustrate the appearance of sawtooth structures. We find that the paring strengths have significant influences on the partition of excitation energies. Furthermore, excitation energies of isotopic fragments increase with increasing neutron numbers, suppressing the production of neutron-rich beams in rare-isotope beam facilities.

nucl-th↗

arXiv2025

Non-perturbative calculations of nuclear matter using in-medium similarity renormalization group

The non-perturbative {\it ab initio} calculations of infinite nuclear matter using In-Medium Similarity Renormalization Group (IMSRG) method is developed in this work, which enables calculations with chiral two and three-nucleon forces at N$^2$LO and N$^3$LO. Results from the many-body perturbation theory at different orders and coupled-cluster theory are also presented for comparison. It is shown that different many-body approaches lead to obvious discrepancies with a harder nuclear interaction for both pure neutron matter and symmetric nuclear matter. This work provides a novel alternative infrastructure for future studies of dense nuclear matter and strongly-correlated many-body systems.

nucl-th↗

arXiv2024

Survival Probabilities of Compound Superheavy Nuclei Towards Element 119

To synthesize superheavy element 119 is becoming highly concerned as several experimental projects in major laboratories are being pursued. This work studied the survival probabilities of compound superheavy nuclei after multiple neutron emissions based on microscopic energy dependent fission barriers, demonstrating a significant role of triaxial deformation in decreasing the first fission barriers in the heaviest region. Together with the fusion cross sections by the dinuclear system model, the optimal energy and the residual cross section of $^{243}$Am($^{48}$Ca, 3$n$)$^{288}$Mc are reproduced. Finally the cross sections and optimal beam energies of $^{243}$Am+$^{54}$Cr and $^{249}$Bk+$^{50}$Ti reactions are estimated, providing clues for the synthesis of new elements.

nucl-th↗

arXiv2024

Quantum Entanglement in Nuclear Fission

Nuclear fission presents a unique example of quantum entanglement in strongly interacting many-body systems. A heavy nucleus can split into hundreds of combinations of two complementary fragments in the fission process. The entanglement of fragment wave functions is persistent even after separation and impacts the partition of particles and energies between fragments. Based on microscopic dynamical calculations of the fission of $^{240}$Pu, this work finds that quantum entanglement is indispensable in the appearance of sawtooth distributions of average excitation energies of fragments and thus neutron multiplicities, but not in average neuron excess of fragments. Both sawtooth slopes from particle-number projections are found to be steep -- a feature which can be alleviated by random fluctuations. These findings may impact the understanding of quantum entanglement more broadly in mesoscopic systems.

nucl-th↗

arXiv2024

High Quality Microscopic Nuclear Masses of Superheavy Nuclei

To synthesize new superheavy elements, the accurate prediction of nuclear masses of superheavy nuclei is essential for calculations of reaction $Q$ values, neutron separation energies and $\alpha$-decay energies, which are important for estimating beam energies, survival probabilities and also for identifications. In this work, we include existing $\alpha$-decay energies of superheavy nuclei in the fitting procedure of extended Skyrme density functionals as corresponding nuclear masses are not available. Systematic $\alpha$-decay energies are well reproduced with deviations smaller than 0.2 MeV. The high quality $\alpha$-decay energies make it feasible for direct identification of new elements and new isotopes. The resulting binding energies in the heaviest region are surprisingly close to the inferences by AME2020. Our work should be useful for guiding experimental synthesis of new elements 119 and 120.

nucl-th↗

arXiv2024

Properties of chiral nucleon-nucleon interaction at N$^3$LO with high cutoffs studied by local projection

The chiral nucleon-nucleon ($NN$) interaction at high cutoffs has been plagued by the presence of spurious bound states. In this work, the chiral $NN$ interaction at N$^3$LO is studied by the local projection method as the cutoff increases. The evolution of short-range behaviors of pion-exchange interactions and contact interactions is intuitively demonstrated. The $P$-channel potentials toward high cutoffs appear to be erratic at short ranges to compromise with phase shifts, while such erratic behaviors can be avoided in $S$ and $D$ channels. Furthermore, a chiral $NN$ interaction at N$^3$LO is studied at a cutoff of 700 MeV. The properties of deuteron and triton are testified with this interaction. Such a hard interaction is expected to provide an alternative choice for studies of short-range correlations and high density nuclear matter.

nucl-th↗

arXiv2023

Speed of Sound and Phase Transitions in Neutron Stars Indicated by the Thick Neutron Skin of $^{208}$Pb

The speed of sound is a novel probe of equation of state and phase transitions in dense cores of neutron stars. Recently nuclear experiments extracted a surprising thick neutron skin of $^{208}$Pb, causing tensions to reproduce the tidal deformability in gravitational-wave observations. This work finds that exotic structures in the speed of sound with a small softening slope followed by a steep-rising peak are required to reconcile the thick neutron skin of $^{208}$Pb with astronomical observations of neutron stars. Furthermore, the peak of speed of sound is narrowly constrained around two times the nuclear saturation density with the thick neutron skin. Consequently early and strong first-order phase transitions are comparatively more favorable.

nucl-th↗

arXiv2023

Machine learning in nuclear physics at low and intermediate energies

Machine learning is becoming a new paradigm for scientific research in various research fields due to its exciting and powerful capability of modeling tools used for big-data processing task. In this mini-review, we first briefly introduce different methodologies of the machine learning algorithms and techniques. As a snapshot of many applications by machine learning, some selected applications are presented especially for low and intermediate energy nuclear physics, which include topics on theoretical applications in nuclear structure, nuclear reactions, properties of nuclear matter as well as experimental applications in event identification/reconstruction, complex system control and firmware performance. Finally, we also give a brief summary and outlook on the possible directions of using machine learning in low-intermediate energy nuclear physics and possible improvements in ML algorithms.

nucl-th↗

arXiv2022

Quantum computing of the pairing Hamiltonian at finite temperatures

In this work, we study the pairing Hamiltonian with four particles at finite temperatures on a quantum simulator and a superconducting quantum computer. The excited states are obtained by the variational quantum deflation (VQD). The error-mitigation methods are applied to improve the noisy results. The simulation of thermal excitation states is performed using the same variational circuit as at zero temperature. The results from quantum computing become close to exact solutions at high temperatures, and demonstrate a smooth superfluid-normal phase transition as a function of temperatures as expected in finite systems.

nucl-th↗

arXiv2021

Energy and pairing dependence of dissipation in real-time fission dynamics

This work presents a microscopic study of dissipation in fission dynamical evolutions with the time-dependent Hartree-Fock+BCS method in terms of energy dependence and pairing dependence. The friction coefficients and dissipation energies are extracted by mapping the symmetric fission process of $^{258}$Fm into a classical equation of motion. Density-constrained calculations are used to obtain the dynamical potential. The obtained friction coefficients have a strong dependence of deformations, and averagely match the coefficients adopted in statistical models. The dissipation indeed increase with increasing initial excitation energies or with decreasing pairings. It is also shown that post-scission dissipations play a significant role. The demonstrated characteristics of dissipations will be valuable for calibrations of various fission models.

nucl-th↗