arXiv subjects

Diyu Shen

Publications and source records attributed to Diyu Shen.

11 recordsLinked to original sources

arXiv2026

A practical methodology for $Λ$ global polarization extraction in fixed-target experiments

Non-central heavy-ion collisions generate large orbital angular momentum in the created medium, which leads to polarization of final-state particles via spin-orbit coupling, known as global spin polarization. The observation of significant global polarization of $Λ$ hyperon in heavy-ion collisions indicates that the quark-gluon plasma is the most vortical fluid known in nature. Exploring $Λ$ global polarization at lower energies is important for understanding spin dynamics across different regions of the quantum chromodynamics (QCD) phase diagram. Low-energy nuclear experiments are typically conducted with asymmetric detector acceptance, as in fixed-target collisions at RHIC-STAR, and at facilities such as FAIR, NICA, HIAF and HIRFL-CSR. The asymmetric rapidity coverage in these experiments enhances the coupling between directed flow and detector inefficiencies, creating significant bias in $Λ$ global polarization measurements. In this paper, we propose a methodology to eliminate such bias arising from asymmetric detector acceptance. The method is validated using realistic detector simulations based on the STAR fixed-target configuration.

physics.data-an↗

arXiv2026

The High Level Trigger and Express Data Production at STAR

To meet the demands of the Beam Energy Scan phase-II (BES-II) program, the STAR experiment at RHIC developed a dual real-time framework consisting of a High Level Trigger (HLT) and an Express Data Production system (xProduction). The HLT operates online within the Data Acquisition (DAQ) chain on a multicore CPU cluster, with optional acceleration using Xeon Phi coprocessors. It employs parallelized algorithms, such as the Cellular Automaton track finder, for fast tracking, vertexing, and event filtering, enabling real-time event selection and detector monitoring. In parallel, xProduction runs independently of the DAQ loop and performs near offline-quality calibration and reconstruction within hours. Using the express data stream, enhanced by HLT selections, and the STAR calibration framework, it enables early physics analysis and provides collaboration-wide access to analysis-ready datasets. Together, HLT and xProduction form a complementary system combining real-time selection with rapid high-quality reconstruction. This framework has enabled prompt reconstruction of the ${}^5_Λ\mathrm{He}$ hypernucleus and efficient processing of large datasets, demonstrating scalability for future high-luminosity experiments.

physics.ins-det↗

arXiv2025

A Review on Intense Electromagnetic Fields in Heavy-Ion Collisions: Theoretical Predictions and Experimental Results

In heavy-ion collisions at relativistic energies, the incident nuclei travel at nearly the speed of light. These collisions deposit kinetic energy into the overlap region and create a high-temperature environment where hadrons ``melt'' into deconfined quarks and gluons. The spectator nucleons, which do not undergo scatterings, generate an ultra-intense electromagnetic field -- on the order of $10^{18}$ Gauss at Relativistic Heavy-Ion Collider, and $10^{19}$ Gauss at the Large Hadron Collider. These powerful electromagnetic fields have a significant impact on the produced particles, not only complicating the study of particle interactions but also inducing novel physical phenomena. To explore the nature of these fields and their interactions with deconfined quarks, we provide a detailed overview, encompassing theoretical estimations of their generation and evolution, as well as experimental efforts to detect them. We also provide physical interpretations of the discovered results and discuss potential directions for future investigations.

nucl-ex↗

arXiv2025

Comment on "Exploring Data-Driven Corrections for $ϕ$-Meson Global Spin Alignment Measurements" (arXiv:2508.18409)

The method in arXiv:2508.18409 constructs a ``data-driven correction'' from combinatorial (pseudo-$ϕ$) pairs and applies it to the signal. An explicit decomposition shows that the construction calibrates the background response rather than the signal: it is defined by the difference between an acceptance-free pseudo-$ϕ$ surrogate and its data-level realization. Promoting a background-derived correction to a signal correction requires a strong physics proof that signal and background share identical detector response at the pair level -- including acceptance-anisotropy couplings and dependencies on parent kinematics -- which the manuscript does not establish. Consequently, local numerical proximity in a restricted region of phase space is incidental rather than evidentiary; validation must rest on mechanism, not numerical coincidence. Moreover, the pseudo-$ϕ$ background is non-unique: with infinitely many admissible constructions, any apparent agreement for a few cases would not be dispositive -- no finite scan can substitute for a mechanism-level response equivalence. In the absence of such a demonstrated equivalence, the construction should be regarded as a background calibration rather than a signal correction.

nucl-ex↗

arXiv2025

Studies of Directed Flow with Event Plane Method in the HIRFL-CSR External-target Experiment

The Cooling-Storage-Ring External-target Experiment (CEE) at Heavy Ion Research Facility in Lanzhou (HIRFL) is designed to study the properties of nuclear matter created in heavy-ion collisions at a few hundred MeV/$u$ to 1 GeV/$u$ beam energies, facilitating the research of quantum chromodynamics phase structure in the high-baryon-density region. Collective flow is one of the most important observables in heavy-ion collision experiments to study the bulk behavior of the created matter. Even though the standard event plane method has been widely used for collective flow measurements, it remains crucial to validate and optimize this method for the CEE spectrometer. In this paper, we study the experimental procedures of measuring directed flow in $^{238}$U+$^{238}$U collisions at 500 MeV/$u$ using event planes reconstructed by Multi Wire Drift Chamber and Zero Degree Calorimeter, respectively. Jet AA Microscopic (JAM) transport generator is used to generate events, and the detector response is simulated by the CEE Fast Simulation (CFS) package. Finally, the optimal kinetic region for proton directed flow measurements is discussed for the future CEE experiment.

physics.ins-det↗

arXiv2024

Effect of vector meson spin coherence on the measurements of chiral magnetic effect in heavy-ion collisions

The chiral magnetic effect (CME) in heavy-ion collisions reflects the local violation of ${\cal P}$ and ${\cal CP}$ symmetries in strong interactions and manifests as electric charge separation along the direction of the magnetic field created by the wounded nuclei. The experimental observables for the CME, such as the $γ_{112}$ correlator, the $R_{Ψ_2}(ΔS)$ correlator, and the signed balance functions, however, are also subject to non-CME backgrounds, including those from resonance decays. A previous study showed that the CME observables are affected by the diagonal component of the spin density matrix, the $ρ_{00}$ for vector mesons. In this work, we study the contributions from the other elements of the spin density matrix using a toy model and a multiphase transport model. We find that the real part of the $ρ_{1-1}$ component, $\mathrm{Re}\,ρ_{1-1}$, affects the CME observables in a manner opposite to that of the $ρ_{00}$. All three aforementioned CME observables show a linear dependence on $\mathrm{Re}\,ρ_{1-1}$ in the model calculations, supporting our analytical derivations. The rest elements of the spin density matrix do not contribute to the CME observables. The off-diagonal terms in the spin density matrix indicate spin coherence and may be nonzero in heavy-ion collisions due to local spin polarization or spin-spin correlations. Thus, $\mathrm{Re}\,ρ_{1-1}$, along with $ρ_{00}$, could play a significant role in interpreting measurements in search of the CME.

nucl-th↗

arXiv2024

Considerations on measuring spin-spin correlation of hyeprons in heavy-ion experiments

The significant global spin alignment observed for $ϕ$ mesons in heavy-ion collisions has sparked intense discussions about its origin and implications. One explanation suggests that fluctuations in the strong force field may introduce strong spin correlations between strange ($s$) and anti-strange ($\bar{s}$) quarks, leading to the global spin alignment of $ϕ$ mesons. Extending this line of research, the theoretical community has proposed studying the spin correlation between $Λ$ and $\barΛ$ hyperons. In this paper, we construct experimental observables and make connections between them and the theoretical proposed quantities.

nucl-th↗

arXiv2022

Impact of globally spin-aligned vector mesons on the search for the chiral magnetic effect in heavy-ion collisions

In high-energy heavy-ion collisions, the chiral magnetic effect (CME) is predicted to arise from the interplay between the chirality imbalance of quarks in the nuclear medium and the intense magnetic field, and will cause a charge separation along the magnetic field direction. While the search for the CME is still ongoing in experiments at Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), the CME-sensitive observables need to be scrutinized to exclude the non-CME contributions. In this work, we examine the influence of globally spin-aligned $ρ$ mesons on the $γ_{112}$ correlator, the $R_{Ψ_2}(ΔS)$ correlator, and the signed balance functions, via a toy model and a multiphase transport model (AMPT). The global spin alignment of vector mesons could originate from non-CME mechanisms in heavy-ion collisions, and is characterized by the 00-component of the spin density matrix, $ρ_{00}$. We find that the CME observables show similar dependence on $ρ_{00}$, and could receive a positive (negative) contribution from $ρ$-decay pions, if the $ρ_{00}$ of $ρ$ mesons is larger (smaller) than 1/3. Since pions are the most abundant particles in such collisions, the $ρ_{00}$ measurements for $ρ$ mesons become crucial to the interpretation of the CME data.

nucl-th↗

arXiv2021

Investigation of Experimental Observables in Search of the Chiral Magnetic Effect in Heavy-ion Collisions in the STAR experiment

The chiral magnetic effect (CME) is a novel transport phenomenon, arising from the interplay between quantum anomalies and strong magnetic fields in chiral systems. In high-energy nuclear collisions, the CME may survive the expansion of the quark-gluon plasma fireball and be detected in experiments. Over the past decade, the experimental searches for the CME have aroused extensive interest at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The main goal of this article is to investigate three pertinent experimental approaches: the $γ$ correlator, the $R$ correlator and the signed balance functions. We will exploit both simple Monte Carlo simulations and a realistic event generator (EBE-AVFD) to verify the equivalence in the kernel-component observables among these methods and to ascertain their sensitivities to the CME signal for the isobaric collisions at RHIC.

nucl-ex↗

arXiv2021

The effect of hadronic scatterings on the measurement of vector meson spin alignments in heavy-ion collisions

Spin alignments of vector mesons and hyperons in relativistic heavy-ion collisions have been proposed as signals of the global polarization. The STAR experiment first observed the $\rm Λ$ polarization. Recently, the ALICE collaboration measured the transverse momentum ($p_T$) and the collision centrality dependence of $K^*$ and $ϕ$ spin alignments in Pb-Pb collisions at $\rm \sqrt{s_{NN}}$ = 2.76 TeV. A large signal is observed in the low $p_T$ region of mid-central collisions for $K^*$ while the signal is much smaller for $ϕ$, and these have not been understood yet. Since vector mesons have different lifetimes and their decay products have different scattering cross sections, they suffer from different hadronic effects. In this paper, we study the effect of hadronic interactions on the spin alignment of $K^*$, $ϕ$ and $ρ$ mesons in relativistic heavy-ion collisions with a multi-phase transport model. We find that hadronic scatterings lead to a deviation of the observed spin alignment matrix element $ρ_{00}$ away from the true value for $ρ$ and $K^*$ mesons (with a bigger effect on $ρ$) while the effect is negligible for the $ϕ$ meson. The effect depends on the kinematic acceptance: the observed $ρ_{00}$ value is lower than the true value when the pseudorapidity ($η$) coverage is small while there is little effect when the $η$ coverage is big. Our study thus provides valuable information to understand the vector meson spin alignment signals observed in the experiments.

nucl-ex↗

arXiv2019

Charge asymmetry dependence of flow and a novel correlator to detect the chiral magnetic wave in a multiphase transport model

In a multiphase transport model with the initial electric quadrupole moment, we studied and discussed the charge asymmetry ($A_{\rm ch}$) dependence of flow at varied kinematic windows in semi-central Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. We then proposed a novel correlator $W$ which specially focuses on the difference of elliptic flow between positively and negatively charged hadrons induced by the chiral magnetic wave and, more importantly, is irrelevant to the ambiguous $A_{\rm ch}$. We found that the distribution of the second order correlator $W_2$ displays a convex structure in the absence of the quadrupole and a concave shape in the presence of the quadrupole. We also studied the response of $W_n$ for both signal and resonance background in a toy model and in analytical calculation. Such a method provides a new way to detect the chiral magnetic wave.

hep-ph↗