arXiv subjects

Xin Zhen

Publications and source records attributed to Xin Zhen.

6 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.

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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.

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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.

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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.

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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.

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arXiv2012

CT to Cone-beam CT Deformable Registration With Simultaneous Intensity Correction

Computed tomography (CT) to cone-beam computed tomography (CBCT) deformable image registration (DIR) is a crucial step in adaptive radiation therapy. Current intensity-based registration algorithms, such as demons, may fail in the context of CT-CBCT DIR because of inconsistent intensities between the two modalities. In this paper, we propose a variant of demons, called Deformation with Intensity Simultaneously Corrected (DISC), to deal with CT-CBCT DIR. DISC distinguishes itself from the original demons algorithm by performing an adaptive intensity correction step on the CBCT image at every iteration step of the demons registration. Specifically, the intensity correction of a voxel in CBCT is achieved by matching the first and the second moments of the voxel intensities inside a patch around the voxel with those on the CT image. It is expected that such a strategy can remove artifacts in the CBCT image, as well as ensuring the intensity consistency between the two modalities. DISC is implemented on computer graphics processing units (GPUs) in compute unified device architecture (CUDA) programming environment. The performance of DISC is evaluated on a simulated patient case and six clinical head-and-neck cancer patient data. It is found that DISC is robust against the CBCT artifacts and intensity inconsistency and significantly improves the registration accuracy when compared with the original demons.

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