arXiv subjects

B. C. He

Publications and source records attributed to B. C. He.

6 recordsLinked to original sources

arXiv2026

$l$-forbidden $\mathbf{M1}$ strengths near $^{100}$Sn from knockout reactions in Cd and Sn

Neutron knockout reactions on beams of $^{104,102}$Cd, and $^{104}$Sn are presented. States in the residual $^{103,101}$Cd and $^{103}$Sn nuclei are populated, including low-lying $7/2^+$ states of $\nu g_{7/2}$ character. These states have half-lives $\approx 400$ ps due to their low energy and hindered $B(M1; 7/2^+ \rightarrow 5/2^+)$ strengths. The excited-state half-lives were measured using their Doppler-shifted lineshapes, and the resulting $B(M1)$ strengths are compared to Valence Space In Medium Similarity Renormalization Group (VS-IMSRG) calculations. The VS-IMSRG calculations under-predict the $l$-forbidden $M1$ strengths in the $^{100}$Sn region, as well as in other regions of the nuclear chart near $^{40}$Ca and $^{208}$Pb.

nucl-ex↗

arXiv2026

Ab initio calculation of symmetry-breaking observables

Symmetry-violating observables such as the nuclear anapole and Schiff moments provide sensitive probes of the fundamental symmetries of nature and physics beyond the Standard Model. Their interpretation has been hindered, however, by the lack of ab initio nuclear structure calculations in the medium-mass and heavy nuclei of interest to experimentalists. To provide them, we introduce a new version of the in-medium similarity renormalization group (IMSRG) designed to target parity-violating operators. By generalizing the IMSRG flow equations to evolve the weak symmetry-breaking Hamiltonian - and the anapole or Schiff operators - alongside the strong nuclear Hamiltonian, we construct a systematically improvable framework for computing these parity-violating moments. We benchmark the method against the no-core shell model in light nuclei and obtain the first ab initio predictions of the anapole moment in $^{29}$Si and the Schiff moments in $^{129}$Xe. These heavier systems are of direct experimental interest.

nucl-th↗

arXiv2025

In-medium similarity renormalization group for a pairing-plus-particle-hole model

We benchmark two implementations of the in-medium similarity renormalization group (IMSRG) method, IMSRG(2) and IMSRG(2*), for the low-lying states of a pairing-plus-particle-hole model with varying numbers of fermions. In IMSRG(2), all operators are truncated up to the normal-ordered two-body terms, whereas IMSRG(2*) includes an additional term to partially account for higher-body contributions. The results are compared against exact solutions. We find that IMSRG(2*) consistently outperforms IMSRG(2) for both ground and excited states, although achieving convergence for excited states remains more challenging in strongly correlated systems than for the ground state.

nucl-th↗

arXiv2024

Improving the predictive power of empirical shell-model Hamiltonians

We present two developments which enhance the predictive power of empirical shell-model Hamiltonians for cases in which calibration data are sparse. A recent improvement in the ab initio derivation of effective Hamiltonians leads to a much better starting point for the optimization procedure. In addition, we introduce a protocol to avoid overfitting, enabling a more reliable extrapolation beyond available data. These developments will enable more robust predictions for exotic isotopes produced at rare isotope beam facilities and in astrophysical environments.

nucl-th↗

arXiv2024

IMSRG with flowing 3 body operators, and approximations thereof

We explore the impact of retaining three-body operators within the in-medium similarity renormalization group (IMSRG), as well as various approximations schemes. After studying two toy problems, idential fermions with a contact interaction and the Lipkin-Meshkov-Glick model, we employ the valence-space formulation of the IMSRG to investigate the even-$A$ carbon isotopes with a chiral two-body potential. We find that retaining only those commutators expressions that scale as $N^7$ provides an excellent approximation of the full three-body treatment.

nucl-th↗

arXiv2024

Factorized Approximation to the IMSRG(3)

We describe an approximation to the in-medium similarity renormalization group (IMSRG) method in which we include the effects of intermediate three-body operators arising within nested commutators. As an initial step, we present the relevant equations for two nested commutators, all of which can be factorized so that the method scales like the standard IMSRG(2) approximation, enabling large-scale calculations. We test the accuracy of this approximation scheme, and apply it to the isotopic chains of carbon, sulfur and nickel isotopic chains. We obtain an improved description of spectroscopy, and a reduced dependence on the choice of the valence space. In addition, we provide an explanation of the relative importance of the diagram topologies included, with an eye toward assessing the impact of remaining omitted terms.

nucl-th↗