arXiv subjects

Zi-Wei Lin

Publications and source records attributed to Zi-Wei Lin.

At least 19 recordsLinked to original sources

arXiv2026

The role of strangeness in baryon and electric charge stoppings

Recently it has been proposed that comparing the net-baryon ($B$) stopping with net-electric charge ($Q$) stopping can help studies of the baryon stopping mechanism in nuclear collisions. Here we find the $B/Q\times Z/A$ ratio to be very sensitive to the difference between $s$ and $\bar s$ quark rapidity distributions. For mid-rapidity of isobar collisions at 200A GeV, a multi-phase transport (AMPT) model gives slightly more $\bar s$ than $s$, which leads to $B/Q\times Z/A<1$, while the model without the $s-\bar s$ asymmetry gives $B/Q\times Z/A \geq 1$. Comparing Ru+Ru and Zr+Zr isobar collisions, the AMPT (and UrQMD) model gives $B/\Delta Q\times \Delta Z/A<1$ at mid-rapidity at all centralities, which strongly contradicts the recent STAR data. We also find that the $B/\Delta Q\times \Delta Z/A$ ratio is very sensitive to the net-light quark ($u,d$) stoppings, but it is less sensitive to the $s-\bar s$ asymmetry than the $B/Q\times Z/A$ ratio by a factor of 3. These results are expected to help us resolve the stopping puzzle and better understand the baryon stopping mechanism in the future.

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arXiv2026

Investigating forward-backward asymmetry in D-meson production and anisotropic flow in p-Pb collisions at the LHC

We investigate the forward--backward asymmetry in the production and elliptic flow of prompt D0 mesons in proton--lead (p--Pb) collisions at$\sqrt{s_{\mathrm{NN}}}=8.16$ TeV using the heavy-flavor improved string-melting version of the AMPT model. The model calculations provide a simultaneous description of nuclear modification factor $R_{\mathrm{pPb}}$ and $v_2$ in forward and backward rapidities. We find that the observed asymmetry arises from the interplay of initial-state cold nuclear matter effects and final-state partonic interactions, with the competition between coalescence and fragmentation playing a critical role in shaping the transverse momentum and rapidity dependence of both observables. This work suggests that a partonic medium is formed in high-multiplicity p-Pb collisions at LHC energies.

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arXiv2026

The shear viscosity of quark-gluon matter calculated with parton transport and comparisons with the Chapman-Enskog results

We numerically calculate the shear viscosity of quark-gluon matter via the Green-Kubo relation with an improved ZPC model. We include all $2\leftrightarrow 2$ parton cross sections at finite temperature, which are based on perturbative QCD and screened with thermal masses, and consider massless quark-gluon systems with Boltzmann statistics in chemical equilibrium. We then compare the Green-Kubo results with the analytical results from the leading-order Chapman-Enskog method for the same parton cross sections over the temperature range $150-600$ MeV. We also examine the simpler case of isotropic and constant parton cross sections. Overall, we find that the two methods agree rather well. Specifically, the Green-Kubo results are greater than the Chapman-Enskog results by an average of $\sim 9\%$ for isotropic and constant cross sections and by an average of $\sim 3\%$ for finite-temperature pQCD cross sections, where the difference between the two methods is presumably due to higher-order corrections to the leading-order Chapman-Enskog results.

hep-ph↗

arXiv2026

VitaLLM: A Versatile and Tiny Accelerator for Mixed-Precision LLM Inference on Edge Devices

We present VitaLLM, a mixed precision accelerator that enables ternary weight large language models to run efficiently on edge devices. The design combines two compute cores, a multiplier free TINT core for ternary-INT projections and a BoothFlex core that reuses a radix-4 Booth datapath for both INT8$\times$INT8 attention and ternary-INT-sustaining utilization without duplicating arrays. A predictive sparse attention mechanism employs a leading-one (LO) surrogate with a comparison-free top-$K$ selector to prune key/value (KV) fetches by roughly $1-K/M$ for $M$ cached tokens, confining exact attention to $K$ candidates. System-level integration uses head-level pipelining and an absmax-based quantization barrier to standardize cross-core interfaces and overlap nonlinear reductions with linear tiles. A 16 nm silicon prototype at 1 GHz/0.8 V achieves 72.46 tokens/s in decode and 0.88 s prefill (64 tokens) within 0.214 mm^2 and 120 KB on-chip memory, while reducing KV traffic and improving utilization in ablations. These results demonstrate practical BitNet b1.58 (3B) inference on edge-class platforms and provide a compact blueprint for future mixed-precision LLM accelerators.

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arXiv2026

VitaLLM: A Versatile, Ultra-Compact Ternary LLM Accelerator with Dependency-Aware Scheduling

Deploying Large Language Models (LLMs) on resource-constrained edge devices faces critical bottlenecks in memory bandwidth and power consumption. While ternary quantization (e.g., BitNet b1.58) significantly reduces model size, its direct deployment on general-purpose hardware is hindered by workload imbalance, bandwidth-bound decoding, and strict data dependencies. To address these challenges, we propose \textbf{VitaLLM}, a hardware-software co-designed accelerator tailored for efficient ternary LLM inference. We introduce a heterogeneous \textbf{Dual-Core Compute Strategy} that synergizes specialized TINT-Cores for massive ternary projections with a unified BoothFlex-Core for mixed-precision attention, ensuring high utilization across both compute-bound prefill and bandwidth-bound decode stages. Furthermore, we develop a \textbf{Leading One Prediction (LOP)} mechanism to prune redundant Key-Value (KV) cache fetches and a \textbf{Dependency-Aware Scheduling} framework to hide the latency of nonlinear operations. Implemented in TSMC 16nm technology, VitaLLM achieves a decoding throughput of 70.70 tokens/s within an ultra-compact area of 0.223 mm$^2$ and a power consumption of 65.97 mW. The design delivers a superior Figure of Merit (FOM) of 17.4 TOPS/mm$^2$/W, significantly outperforming state-of-the-art accelerators. Finally, we explore an extended bit-serial design (BoothFlex-BS) to demonstrate the architecture's adaptability for precision-agile inference.

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arXiv2026

Chapman-Enskog calculation of the shear viscosity of quark-gluon plasma including all $2\leftrightarrow 2$ scatterings at finite temperature

We use the Chapman-Enskog method to investigate the shear viscosity of the quark-gluon plasma with a focus on its relation to parton cross sections. We use the recently obtained analytical expression for the shear viscosity $η$ of a massless quark-gluon gas at chemical equilibrium with Boltzmann statistics and all $2\leftrightarrow 2$ scatterings with arbitrary cross sections. Here we apply this general expression to cross sections at finite temperature that are based on perturbative-QCD and screened with scaled thermal masses $\sqrtκ\,m_D$ and $\sqrtκ\,m_F$. We find that the Chapman-Enskog results on $η\, g^4/T^3$ versus $m_D/T$ at $κ=1$ are qualitatively similar to but higher than the corresponding leading-order results from the AMY framework. We then find that using $κ=0.4$ allows the Chapman-Enskog results to match well the corresponding AMY results as it includes the effect of using thermal masses (instead of self-energies) to screen the cross sections. In addition, we show that the shear viscosity-to-entropy density ratio $η/s$ is very sensitive to the choice of momentum scale $Q$ in the strong coupling, where the choice of $Q=3T$ leads to $η/s \sim 0.15$ for $N_f=0$ or 3 at the QCD phase transition temperature $T_c$. These results lay the foundation for mapping parton cross sections to given shear viscosity in parton transport models and QCD effective kinetic theory.

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arXiv2026

Nuclear cluster structure effect in $^{16}$O+$^{16}$O collisions at the top RHIC energy

Using the improved string-melting version of a Multi-Phase Transport model, we investigated the impact of nuclear geometry of $^{16}$O on anisotropic flows in O+O collisions at $\sqrt{s_{\rm NN}} = 200$ GeV. To evaluate the influence of nuclear structure and potential alpha clustering, we implemented four candidate configurations: Woods-Saxon, tetrahedron, square, and Nuclear Lattice Effective Field Theory. Initial-state geometry is quantified via the eccentricity cumulant ratio $\varepsilon_{2}\{4\}/\varepsilon_{2}\{2\}$, which provides a robust and evolution-independent measure sensitive to configuration differences. The model reproduces $v_{2}(p_{\rm T})$ at low $p_{\rm T}$ and $v_{3}(p_{\rm T})$ across the full $p_{\rm T}$ range, with integrated $v_{2}\{2\}$ and $v_{3}\{2\}$ matching the STAR data, demonstrating that transport dynamics captures the essential collectivity in this intermediate-size system. These findings establish a baseline for extending nuclear-structure studies in O+O collisions to other energies and differential observables within a unified transport model framework.

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arXiv2026

Shear viscosity of a massless quark-gluon gas in chemical equilibrium in terms of all $2\leftrightarrow 2$ cross sections

The analytical expressions of the shear viscosity of both one and two particle species with Boltzmann statistics and $2 \rightarrow 2$ elastic scatterings are known from the Chapman-Enskog method and have been shown to be quite accurate. The expression for a multi-species hadronic gas under $2 \rightarrow 2$ elastic scatterings is also known. Here we use the Chapman-Enskog method to derive the explicit expression of shear viscosity of a massless quark-gluon gas of $N_f$ quark flavors in chemical equilibrium subjected to all $2 \rightarrow 2$ parton scatterings including for the first time inelastic scatterings. We then verify the expression in a general single-species limit, where the shear viscosity of the quark-gluon gas should reduce to the result for a single particle species. In addition, we show the explicit analytical result in terms of the seven independent cross sections for the special case of isotropic and energy-independent cross sections. The analytical expressions derived here can be useful for determining the shear viscosity of parton transport models with any $2 \rightarrow 2$ scattering cross sections. They can also be coupled with finite temperature QCD cross sections to help study the shear viscosity of the quark gluon plasma.

hep-ph↗

arXiv2025

Baryon and electric charge stoppings in nuclear collisions and the role of strangeness

It has been challenging to quantitatively understand the stopping of incoming nucleons in nuclear collisions, and recently it has been proposed that comparing the baryon stopping with electric charge stopping can help address the question. Here we focus on the $B/Q\times Z/A$ ratio, which can strongly depend on rapidity although its value is one for the full phase space. We find that this ratio is very sensitive to the difference between strange and anti-strange rapidity distributions (the $s-\bar s$ asymmetry), and slightly more anti-strange quarks at mid-rapidity would lead to a ratio well below one. This is the case for Zr+Zr and Ru+Ru isobar collisions at $200A$ GeV from a multi-phase transport (AMPT) model. Without the $s-\bar s$ asymmetry, the AMPT model would give a mid-rapidity $B/Q\times Z/A$ ratio at or above one. In addition, the AMPT model gives $B/ΔQ\times ΔZ/A<1$ at mid-rapidity for isobar collisions at all centralities, which strongly contradicts the recent data from the STAR Collaboration. We further find that the $B/ΔQ\times ΔZ/A$ ratio is very sensitive to the net-light quark ($u,d$) stoppings, but it is less sensitive to the $s-\bar s$ asymmetry than the $B/Q\times Z/A$ ratio by a factor of 3.

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arXiv2025

Effectiveness of parton cascade in solving the relativistic Boltzmann equation in a box

We benchmark the ZPC parton cascade with an exact analytical solution of the relativistic Boltzmann equation for a homogeneous and massless gas with a constant and isotropic elastic cross section. We measure the accuracy of ZPC with the relative mean deviation between its momentum distribution and the exact solution. We use two generalized collision schemes to further improve the accuracy of ZPC over the recent $t$-minimum collision scheme. We find that ZPC can reproduce very well the time evolution of the single-particle distribution function for the exact solution's initial condition, with one generalized collision scheme giving an accuracy better than $1\%$ for the momentum distribution at any time in all studied cases, including very high opacities where naively the parton cascade approach is expected to fail.

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arXiv2024

Disentangling the development of collective flow in high energy proton proton collisions with a multiphase transport model

In this work, we investigate the collective flow development in high energy proton proton (pp) collisions with a multiphase transport model (AMPT) based on PYTHIA8 initial conditions with a sub-nucleon structure. It is found that the PYTHIA8 based AMPT model can reasonably describe both the charged hadron productions and elliptic flow experimental data measured in pp collisions at $\sqrt{s}=13$ TeV. By turning on the parton and hadron rescatterings in AMPT separately, we find that the observed collective flow in pp collisions is largely developed during the parton evolution, while no significant flow effect can be generated with the pure hadronic rescatterings. It is also shown that the parton escape mechanism is important for describing both the magnitude of the two-particle cumulant and the sign of the four-particle cumulants. We emphasize that the strong mass ordering of the elliptic flow results from the coalescence process in the transport model and can thus be regarded as unique evidence related to the creation of deconfined parton matter in high energy pp collisions.

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arXiv2024

Search for baryon junctions in photonuclear processes and isobar collisions at RHIC

During the early development of Quantum Chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the midrapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near midrapidity indeed follows an exponential distribution with a slope of $-0.61\pm0.03$ as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes ($γ+p$/A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and $e+p$/A collisions at the EIC.

hep-ph↗

arXiv2024

Investigating $D^0$ meson production in $p-$Pb collisions at 5.02 TeV with a multi-phase transport model

We study the production of $D^0$ meson in $p$+$p$ and $p-$Pb collisions using the improved AMPT model considering both coalescence and independent fragmentation of charm quarks after the Cronin broadening are included. After a detailed discussion of the improvements implemented in the AMPT model for heavy quark production, we show that the modified AMPT model can provide good description of $D^0$ meson spectra in $p-$Pb collisions, the $Q_{\rm pPb}$ data at different centrality and $R_{\rm pPb}$ data in both mid- and forward (backward) rapidities. We also studied the effects of nuclear shadowing and parton cascade on the rapidity dependence of $D^{0}$ meson production and $R_{\rm pPb}$. Our results indicate that having the same strength of the Cronin (i.e $δ$ value) obtained from the mid-rapidity data leads to a considerable overestimation of the $D^0$ meson spectra and $R_{\rm pPb}$ data at high $p_{T}$ in the backward rapidity. As a result, the $δ$ is determined via a $χ^2$ fitting of the $R_{\rm pPb}$ data across various rapidities. This work lays the foundation for a better understanding of cold-nuclear-matter (CNM) effects in relativistic heavy-ion collisions.

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arXiv2024

The influence of hadronic rescatterings on the net-baryon number fluctuations

Fluctuations of conserved charges, such as the net-baryon number fluctuations, are influenced by different dynamical evolution processes. In this paper, we investigate the influence of hadronic rescatterings on different orders of cumulants of the net-baryon number distribution. At the start of hadronic rescatterings, we introduce net-baryon number distributions reconstructed based on net-baryon cumulants of different orders obtained from computation in functional renormalization group (FRG), where the distributions were constructed using the maximum entropy method. This way we introduce the critical fluctuations of Quantum Chromodynamics (QCD) into the AMPT model. Firstly, we find that hadronic rescatterings have distinct effects on cumulant ratios of different orders for the net-baryon number. Secondly, we observe that the effect of hadronic rescatterings is more significant for critical fluctuations than dynamical fluctuations, because the two-, three- and four-particle correlation functions due to critical fluctuations are weakened more significantly by hadronic rescatterings.

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arXiv2024

Collectivity inside high-multiplicity jets in high-energy proton-proton collisions

We present the first study of collectivity inside jets with high charged multiplicity $N^j_{\rm ch}$ in proton-proton collisions at the Large Hadron Collider. By incorporating final-state partonic and hadronic interactions through cascade models among jet shower partons and final hadrons, we investigate and compare to the CMS experimental data on multiplicity distribution, pseudorapidity distribution, and elliptic anisotropy coefficient $v^{j}_2$ of two-particle correlations within the jet. We show that final-state partonic interactions are essential for producing the flow-like long-range correlation, which leads to the enhanced tail in the $N^j_{\rm ch}$ dependence of $v^{j}_2$ above the non-flow correlation from jet parton showering at high multiplicities ($N^j_{\rm ch}\gtrsim70$) as observed in the CMS experimental data. In addition, we provide predictions for the pseudorapidity-gap dependence of $v^{j}_2$ that can be tested in future experimental measurements.

hep-ph↗

arXiv2023

Coalescence sum rule and the electric charge- and strangeness-dependences of directed flow in heavy ion collisions

The rapidity-odd directed flows ($v_{\rm 1}$) of identified hadrons are expected to follow the coalescence sum rule when the created matter is initially in parton degrees of freedom and then hadronizes through quark coalescence. A recent study has considered the $v_{\rm 1}$ of produced hadrons that do not contain $u$ or $d$ constituent quarks. It has constructed multiple hadron sets with a small mass difference but given difference in electric charge $Δq$ and strangeness $ΔS$ between the two sides, where a nonzero and increasing $Δv_{\rm 1}$ with $Δq$ has been proposed to be a consequence of electromagnetic fields. In this study, we examine the consequence of coalescence sum rule on the $Δv_{\rm 1}$ of the hadron sets in the absence of electromagnetic fields. We find that in general $Δv_{\rm 1} \neq 0$ for a hadron set with nonzero $Δq$ and/or $ΔS$ due to potential $v_{\rm 1}$ differences between $\bar u$ and $\bar d$ and between $s$ and $\bar s$ quarks. We further propose methods to extract the coefficients for the $Δq$- and $ΔS$-dependences of the direct flow difference, where a nonzero constant term would indicate the breaking of the coalescence sum rule. The extraction methods are then demonstrated with transport model results.

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arXiv2023

Study of Baryon Number Transport Dynamics and Strangeness Conservation Effects Using $Ω$-hadron Correlations

In nuclear collisions at RHIC energies, an excess of $Ω$ hyperons over $\barΩ$ is observed, indicating that $Ω$ carries a net baryon number despite $s$ and $\bar{s}$ quarks being produced in pairs. The baryon number in $Ω$ could have been transported from the incident nuclei and/or produced in baryon-pair production of $Ω$ with other types of anti-hyperons, such as $\barΞ$. To investigate these two scenarios, we propose to measure correlations between $Ω$ and $K$, as well as between $Ω$ and anti-hyperons. We will use two versions, the default and string-melting, of a multiphase transport (AMPT) model to illustrate the method to measure the correlation and to demonstrate the general shape of the correlation. We will present the $Ω$-hadron correlations from simulated $\mathrm{Au}$+$\mathrm{Au}$ collisions at $\sqrt{s_{NN}} = 7.7$ and $14.6 \ \mathrm{GeV}$, and discuss the dependence on collision energy and on the hadronization scheme in these two AMPT versions. These correlations can be used to explore the mechanism of baryon number transport and the effects of baryon number and strangeness conservation in nuclear collisions.

hep-ph↗

arXiv2023

Resolving the $R_{\rm pA}$ and $v_2$ puzzle of $D^0$ mesons in $p-$Pb collisions

It has been difficult to reconcile the experimental data on the $D^0$ meson nuclear modification factor and elliptic flow in $p-$Pb collisions at LHC energies. Here we study these observables with the string melting version of a multi-phase transport model, which has been improved with the implementation of the Cronin effect (or transverse momentum broadening) and independent fragmentation for charm quarks. Using a strong Cronin effect allows us to provide the first simultaneous description of the $D^0$ meson $R_{\rm pA}$ and $v_2$ data at $p_{\rm T} \leq$ 8 GeV$/c$. The model also provides a reasonable description of the $D^0$ meson $p_{\rm T}$ spectra and the low-$p_{\rm T}$ (below $\sim$ 2 GeV$/c$) charged hadron spectra in $p+p$ and $p-$Pb collisions as well as $R_{\rm pA}$ and $v_2$ in $p-$Pb collisions. We find that both parton scatterings and the Cronin effect are important for the $D^0$ meson $R_{\rm pA}$, while parton scatterings are mostly responsible for the $D^0$ meson $v_2$. Our results indicate that it is crucial to include the Cronin effect for the simultaneous description of the $D^0$ meson $R_{\rm pA}$ and $v_2$. Since the Cronin effect is expected to grow with the system size, this work implies that the Cronin effect could also be important for heavy hadrons in large systems.

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