arXiv subjects

Shihang Shen

Publications and source records attributed to Shihang Shen.

At least 19 recordsLinked to original sources

arXiv2026

Nuclear lattice effective field theory as a testing ground for $\alpha$-cluster structures in ${}^{24}\mathrm{Mg}$

The framework of nuclear lattice effective field theory (NLEFT) is applied to $^{24}\mathrm{Mg}$, with the perspective of obtaining a model-independent density map of the geometry of a sample of excited states of the nucleus. The Hamiltonian incorporates Wigner SU(4)-symmetric nuclear forces as well as the Coulomb interaction. The coupling constants of the spin-isospin symmetric nucleon-nucleon potentials have been adjusted in order to reproduce the experimental ground-state (g.s.) energy of $^{24}\mathrm{Mg}$ as well as the experimental Tjon ratio between the binding energies of $^3\mathrm{H}$ and $^4\mathrm{He}$. The ensuing parameter set turns out to be capable of capturing the experimental trend of the binding energy per nucleon, reproducing simultaneously within 1% deviation the measured values for $^{18}\mathrm{F}$, $^{22}\mathrm{Na}$, $^{26}\mathrm{Al}$, $^{28}\mathrm{Si}$, $^{30}\mathrm{P}$ and $^{32}\mathrm{S}$. Considerations based on the convergence rate of Euclidean-time extrapolations for the two lowest energy eigenvalues highlight the dual nature of the $0_1^+$ and $2_1^+$ states, of hybrid mean-field and $\alpha$-cluster type. For the latter, triaxial $\alpha$-cluster configurations seem to be favoured over the axially-symmetric ones, whereas oblate superdeformation might characterize a rotational band at $20$ MeV excitation energy.

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arXiv2026

Reaction Cross Sections and $\alpha$-Cluster Geometry in $^{12}$C and Be Isotopes

Reaction cross sections $\sigma_{\rm R}$ are widely used to infer matter radii, yet their sensitivity to nuclear structure beyond radial one-body distributions is less well understood. We combine complete $A$-body nucleon configurations sampled from \textit{ab initio} nuclear lattice effective field theory (NLEFT) with event-by-event Monte Carlo Glauber calculations, thereby retaining the many-body correlations encoded in NLEFT. Using a fixed binary-collision prescription determined by the measured energy- and isospin-dependent total nucleon-nucleon cross sections, the calculations capture the overall magnitudes and energy dependence simultaneously for the available data on $^{12}$C and $^{9}$Be projectiles on carbon and hydrogen. Controlled randomization of angular correlations at fixed matter root-mean-square radius and spherically averaged one-body radial density produces only a weak change in $\sigma_{\rm R}$ for $^{12}$C but approximately a $10\%$ increase for $^{9}\mathrm{Be}+{}^{1}\mathrm{H}$. The calculations also capture the measured rise--plateau--sharp-rise--reduction trend across $^{7,9\text{--}12}$Be, a distinctive pattern reflecting the evolution of cluster and halo structures along the isotopic chain. These results show that $\sigma_{\rm R}$ retains sensitivity to intrinsic many-body geometry beyond a single inferred matter radius, opening a route to studies of exotic $\alpha$-cluster geometries and spatial nucleon correlations through reaction cross sections.

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arXiv2026

Deformation effects on reaction observables of beryllium nuclei from ab initio densities

We combine three-dimensional intrinsic densities from ab initio nuclear lattice effective field theory with a deformed Glauber model to study high-energy reactions of {7-12}Be. To connect the correlated many-body configurations to the core-plus-neutron reaction formalism without imposing a single-particle orbital, we introduce a configuration-resolved prescription that identifies the spatially outermost valence neutron after the two-cluster decomposition. For Be projectiles on 12C and 9Be targets at 790 MeV/A, explicit orientation averaging lowers the calculated reaction cross section of 11Be by up to approximately 50 mb relative to a calculation with the spherically averaged density. The deformed calculation reproduces the pronounced increase from 10Be to the established one-neutron halo nucleus 11Be for both targets. We further calculate the momentum distribution of the fragments after the one-neutron removal reaction of 11Be + 9Be , finding good agreement in shape with the measurement at 63 MeV/A and providing a prediction at 790 MeV/A. These results quantify how intrinsic deformation and weak binding are transmitted from microscopic many-body densities to reaction observables.

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arXiv2026

Nuclear Charge Radius of $^9$Be from Muonic Atom Spectroscopy Using a Microcalorimeter

The $2p\to1s$ transition energy in muonic $^9$Be was measured using a metallic magnetic calorimeter, resulting in $E_{2p\to 1s}=33\,391.48(34)\,$eV. The result is 30 times more precise than the previous best measurement and enables the extraction of the corresponding nuclear charge radius $r_c($$^9$Be$)=2.5506(51)\,$fm. It is $2.4$ times more precise than the commonly used value based on electron scattering and differs from it by $2.3$ times the combined uncertainties. This measurement represents the first determination of a nuclear charge radius using muonic x-ray spectroscopy with microcalorimeters.

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arXiv2026

Ab initio study of the halo structure in $^{11}$Be

We present an ab initio study of the one-neutron halo nucleus $^{11}$Be using nuclear lattice effective field theory with high-fidelity chiral interactions at N3LO. By employing the wavefunction matching method to mitigate the sign problem and the pinhole algorithm to sample many-body correlations, we successfully reproduce the ground-state parity inversion and the extended matter radius characteristic of the halo structure. We analyze the intrinsic density distributions and geometric shapes of $^{11}$Be in comparison with the core nucleus $^{10}$Be. Our results reveal a prominent two-cluster structure in both nuclei and the occupation of the $\sigma$ molecular orbital by the valence neutron in $^{11}$Be. It enhances the prolate deformation as well as the diffuse neutron tail, distinct from the $\pi$-orbital occupation observed in the $^{10}$Be ground state.

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arXiv2026

Recent developments and applications of the relativistic chiral nuclear force

The nuclear force is central to our understanding of complex nuclear phenomena and to the applications of nuclear techniques. The nonperturbative nature of the low-energy strong interaction and the color confinement have made an ab initio understanding of the nuclear force a challenge for almost a century since the pioneering work of Yukawa. Since 1990, chiral effective field theory (ChEFT) has become the de facto standard for describing nuclear interactions--most prior studies employed heavy-baryon chiral perturbation theory. Only recently, there have been successful attempts to construct a chiral nuclear force employing covariant baryon chiral perturbation theory. In this work, we review recent developments and applications of relativistic chiral nuclear forces. We first elaborate on the necessity of relativistic/covariant theories, then present the construction of the first high-precision relativistic chiral nuclear force up to next-to-next-to-leading order (NNLO), and discuss the ongoing progress in higher-order nucleon-nucleon (NN) and $nd$ scattering, as well as their applications in nuclear matter, finite nuclei, and hypernuclear systems. Finally, we summarize the achievements and outline the future outlook of this research field.

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arXiv2026

Searching for the Tetraneutron Resonance on the Lattice

The nature of the tetraneutron ($4n$) system remains a pivotal question in nuclear physics. We investigate the $4n$ system using nuclear lattice effective field theory in finite volumes with a lattice size up to $L=30$~fm, employing both a high-precision N$^3$LO interaction and a simplified SU(4) symmetric one. The ground-state energy is found to decrease smoothly with increasing box size, showing no plateau characteristic of a resonance. We further compute the dineutron-dineutron scattering phase shift using L\"uscher's finite-volume method. At the smallest relative momenta, the extracted $2n$--$2n$ $S$-wave phase shift is small, consistent with a weak interaction in the dilute limit. At intermediate momenta, it exhibits a weak attraction with a peak of approximately $10^\circ$ at relative momentum of 60--84~MeV. While this structure does not constitute a resonance, the corresponding confined $4n$ energy of 1.7--3.3~MeV lies close to the experimentally observed low-energy peak.

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arXiv2025

Quantifying alpha clustering in the ground states of 16-O and 20-Ne

Understanding the role of multi-nucleon correlations in the structure of light nuclei is at the forefront of modern nuclear science. In this letter, we present a quantitative benchmark study of alpha-cluster correlations in the ground states of 16-O and 20-Ne. Experimental data provide direct evidence that the wave functions of the ground states of 16-O and 20-Ne are dominated by alpha-cluster correlations, in agreement with the predictions of sophisticated nuclear structure models. We also provide a new model-independent constraint for the alpha asymptotic normalization coefficient of the 16-O ground state and discuss the implications of these findings on the 12-C(alpha,gamma)16-O reaction, which is of critical importance for nuclear astrophysics.

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arXiv2025

From bare two-nucleon interaction to nuclear matter and finite nuclei in a relativistic framework

Understanding nuclear forces, infinite nuclear matter, and finite nuclei within a unified framework has remained a central challenge in nuclear physics for decades. While most \textit{ab initio} studies employ nonrelativistic Schr\"odinger-equation frameworks, this work offers a relativistic perspective. Using a leading-order (LO) relativistic chiral interaction, we describe two-nucleon scattering via the Thompson equation, symmetric nuclear matter, and medium-mass nuclei (Ca, Ni, Zr, Sn) via the relativistic Brueckner-Hartree-Fock theory. Systematic uncertainties from regulator cutoffs and interaction parameters are analyzed. The empirical saturation region of nuclear matter is reproduced, and the binding energies and charge radii of medium-mass nuclei agree reasonably well with experimental data, significantly improving the ``Coester line". These results highlight that the relativistic approach, employing a leading-order chiral force with only four low-energy constants and no three-nucleon forces, can capture the most important dynamics and offer a complementary pathway to address longstanding challenges in nuclear \textit{ab initio} studies.

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arXiv2025

Nuclear matter in relativistic Brueckner-Hartree-Fock theory with local and nonlocal covariant chiral interactions at leading order

The simultaneous description for nuclear matter and finite nuclei has been a long-standing challenge in nuclear ab initio theory. With the success for nuclear matter, the relativistic Brueckner-Hartree-Fock (RBHF) theory with covariant chiral interactions is a promising ab initio approach to describe both nuclear matter and finite nuclei. In the description of the finite nuclei with the current RBHF theory, the covariant chiral interactions have to be localized to make calculations feasible. In order to examine the reliability and validity, in this letter, the RBHF theory with local and nonlocal covariant chiral interactions at leading order are applied for nuclear matter. The low-energy constants in the covariant chiral interactions determined with the local regularization are close to those with the nonlocal regularization. Moreover, the RBHF theory with local and nonlocal covariant chiral interactions provide equally well description of the saturation properties of nuclear matter. The present work paves the way for the implementation of covariant chiral interactions in RBHF theory for finite nuclei.

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arXiv2024

Lattice simulation of nucleon distribution and shell closure in the proton-rich nucleus $^{22}$Si

The proton-rich nucleus $^{22}$Si is studied using Nuclear Lattice Effective Field Theory with high-fidelity chiral forces. Our results indicate that $^{22}$Si is more tightly bound than $^{20}$Mg, thereby excluding the possibility of two-proton emission. The $Z = 14$ shell closure in $^{22}$Si is supported by the evolution of the $2^+$ state in the neighboring nuclei. We then focus on the charge radius and spatial distribution information of $^{22}$Si, considering the novel phenomena that may emerge due to the small two-proton separation energy and the shell closure. We present the distribution of the $14$ protons and $8$ neutrons obtained from our lattice simulation, revealing insights into the spatial arrangement of the nucleons. Moreover, the spatial localization of the outermost proton and neutron suggests that $^{22}$Si is a doubly magic nucleus. Furthermore, we develop the pinhole method based on the harmonic oscillator basis, which gives insight into the nuclear structure in terms of the shell model picture from lattice simulations. Our calculated occupation numbers support that $Z = 14$ and $N = 8$ are the shell closures and show that the $\pi 1s_{1/2}$ orbital component is minor in $^{22}$Si.

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arXiv2024

Ab initio study of the beryllium isotopes $^{7}$Be to $^{12}$Be

We present a systematic ab initio study of the low-lying states in beryllium isotopes from 7Be to 12Be using nuclear lattice effective field theory with the N3LO interaction. Our calculations achieve good agreement with experimental data for energies, radii, and electromagnetic properties. We introduce a novel, model-independent method to quantify nuclear shapes, uncovering a distinct pattern in the interplay between positive and negative parity states across the isotopic chain. By combining Monte Carlo sampling of the many-body density operator with a novel nucleon-grouping algorithm, the prominent two-center cluster structures, the emergence of one-neutron halo, complex nuclear molecular dynamics such as {\pi} orbital and {\sigma} orbital, emerge naturally.

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arXiv2024

Anisotropic flow in fixed-target $^{208}$Pb+$^{20}$Ne collisions as a probe of quark-gluon plasma

The System for Measuring Overlap with Gas (SMOG2) at the LHCb detector enables the study of fixed-target ion-ion collisions at relativistic energies ($\sqrt{s_{\rm NN}}\sim100$ GeV in the centre-of-mass). With input from \textit{ab initio} calculations of the structure of $^{16}$O and $^{20}$Ne, we compute 3+1D hydrodynamic predictions for the anisotropic flow of Pb+Ne and Pb+O collisions, to be tested with upcoming LHCb data. This will allow the detailed study of quark-gluon plasma (QGP) formation as well as experimental tests of the predicted nuclear shapes. Elliptic flow ($v_2$) in Pb+Ne collisions is greatly enhanced compared to the Pb+O baseline due to the shape of $^{20}$Ne, which is deformed in a bowling-pin geometry. Owing to the large $^{208}$Pb radius, this effect is seen in a broad centrality range, a unique feature of this collision configuration. Larger elliptic flow further enhances the quadrangular flow ($v_4$) of Pb+Ne collisions via non-linear coupling, and impacts the sign of the kurtosis of the elliptic flow vector distribution ($c_2\{4\}$). Exploiting the shape of $^{20}$Ne proves thus an ideal method to investigate the formation of QGP in fixed-target experiments at LHCb, and demonstrates the power of SMOG2 as a tool to image nuclear ground states.

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arXiv2024

The unexpected uses of a bowling pin: exploiting $^{20}$Ne isotopes for precision characterizations of collectivity in small systems

Whether or not femto-scale droplets of quark-gluon plasma (QGP) are formed in so-called small systems at high-energy colliders is a pressing question in the phenomenology of the strong interaction. For proton-proton or proton-nucleus collisions the answer is inconclusive due to the large theoretical uncertainties plaguing the description of these processes. While upcoming data on collisions of $^{16}$O nuclei may mitigate these uncertainties in the near future, here we demonstrate the unique possibilities offered by complementing $^{16}$O$^{16}$O data with collisions of $^{20}$Ne ions. We couple both NLEFT and PGCM ab initio descriptions of the structure of $^{20}$Ne and $^{16}$O to hydrodynamic simulations of $^{16}$O$^{16}$O and $^{20}$Ne$^{20}$Ne collisions at high energy. We isolate the imprints of the bowling-pin shape of $^{20}$Ne on the collective flow of hadrons, which can be used to perform quantitative tests of the hydrodynamic QGP paradigm. In particular, we predict that the elliptic flow of $^{20}$Ne$^{20}$Ne collisions is enhanced by as much as 1.170(8)$_{\rm stat.}$(30)$_{\rm syst.}$ for NLEFT and 1.139(6)$_{\rm stat.}$(39)$_{\rm syst.}$ for PGCM relative to $^{16}$O$^{16}$O collisions for the 1% most central events. At the same time, theoretical uncertainties largely cancel when studying relative variations of observables between two systems. This demonstrates a method based on experiments with two light-ion species for precision characterizations of the collective dynamics and its emergence in a small system.

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arXiv2023

Ab initio calculation of the alpha-particle monopole transition form factor

We present a parameter-free ab initio calculation of the $\alpha$-particle monopole transition form factor in the framework of nuclear lattice effective field theory. We use a minimal nuclear interaction that was previously used to reproduce the ground state properties of light nuclei, medium-mass nuclei, and neutron matter simultaneously with no more than a few percent error in the energies and charge radii. The results for the monopole transition form factor are in good agreement with recent precision data from Mainz.

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arXiv2022

Wavefunction matching for solving quantum many-body problems

Ab initio calculations play an essential role in our fundamental understanding of quantum many-body systems across many subfields, from strongly correlated fermions to quantum chemistry and from atomic and molecular systems to nuclear physics. One of the primary challenges is to perform accurate calculations for systems where the interactions may be complicated and difficult for the chosen computational method to handle. Here we address the problem by introducing a new approach called wavefunction matching. Wavefunction matching transforms the interaction between particles so that the wavefunctions up to some finite range match that of an easily computable interaction. This allows for calculations of systems that would otherwise be impossible due to problems such as Monte Carlo sign cancellations. We apply the method to lattice Monte Carlo simulations of light nuclei, medium-mass nuclei, neutron matter, and nuclear matter. We use high-fidelity chiral effective field theory interactions and find good agreement with empirical data. These results are accompanied by new insights on the nuclear interactions that may help to resolve long-standing challenges in accurately reproducing nuclear binding energies, charge radii, and nuclear matter saturation in ab initio calculations.

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arXiv2022

Emergent geometry and duality in the carbon nucleus

The carbon atom provides the backbone for the complex organic chemistry composing the building blocks of life. The physics of the carbon nucleus in its predominant isotope, $^{12}$C, is similarly full of multifaceted complexity. Some nuclear states of $^{12}$C can be preferentially treated as a collection of independent particles held by the mean field of the nucleus, while other states behave more as a collection of three alpha-particle clusters. But these two pictures are not mutually exclusive, and some states can be described in either fashion$^{1,2}$. In this work, we provide the first model-independent tomographic scan of the three-dimensional geometry of the nuclear states of $^{12}$C using the {\it ab initio} framework of nuclear lattice effective field theory. We find that the well-known but enigmatic Hoyle state is composed of a "bent-arm" or obtuse triangular arrangement of alpha clusters. We identify all of the low-lying nuclear states of $^{12}$C as having an intrinsic shape composed of three alpha clusters forming either an equilateral triangle or an obtuse triangle. From these basic structural formations, the various nuclear states correspond to different rotational and vibrational excitations as well as either distortions or large-amplitude displacements of the alpha clusters. The states with the equilateral triangle formation also have a dual description in terms of particle-hole excitations in the mean-field picture. We compare our theoretical calculations with experimental data for binding energies, quadrupole moments, electromagnetic transitions, charge densities, and form factors. The overall agreement is good, and further studies using higher-fidelity interactions are planned.

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arXiv2021

Wigner SU(4) symmetry, clustering, and the spectrum of $^{12}$C

We present lattice calculations of the low-lying spectrum of $^{12}$C using a simple nucleon-nucleon interaction that is independent of spin and isospin and therefore invariant under Wigner's SU(4) symmetry. We find strong signals for all excited states up to $\sim 15$~MeV above the ground state, and explore the structure of each state using a large variety of $α$ cluster and harmonic oscillator trial states, projected onto given irreducible representations of the cubic group. We are able to verify earlier findings for the $α$ clustering in the Hoyle state and the second $2^+$ state of $^{12}$C. The success of these calculations to describe the full low-lying energy spectrum using spin-independent interactions suggest that either the spin-orbit interactions are somewhat weak in the $^{12}$C system, or the effects of $α$ clustering are diminishing their influence. This is in agreement with previous findings from {\it ab initio} shell model calculations.

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