arXiv subjects

Roy A. Lacey

Publications and source records attributed to Roy A. Lacey.

At least 19 recordsLinked to original sources

arXiv2026

Comment on "Non-Monotonicity of Transverse-Momentum Correlations in Au+Au Collisions at RHIC"

Recent measurements of two-particle transverse-momentum correlations, $C_{p_T}$, in Au+Au collisions have revealed a statistically significant non-monotonic beam-energy dependence over the range $\sqrt{s_{NN}}=3.0$--$7.7$~GeV that has been interpreted as potentially indicative of critical phenomena associated with a critical end point (CEP) in the QCD phase diagram. This interpretation is assessed in light of the absence of a controlled framework establishing $C_{p_T}$ as a quantitative proxy for the underlying critical response. The implications of this limitation are examined by considering the expected finite-size and finite-time modification of critical fluctuations, substantial non-critical dynamical contributions, the lack of demonstrated critical scaling, and the sizeable discrepancy between the CEP region inferred from the observed $C_{p_T}$ non-monotonicity and that constrained by susceptibility-based measurements and finite-size scaling analyses. Taken together, these considerations indicate that the observed $C_{p_T}$ non-monotonicity does not, by itself, establish a critical origin nor provide reliable quantitative constraints on the existence or location of a possible CEP.

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arXiv2026

Constraining the $f_0(980)$ Structure and Formation Dynamics via Anisotropy-Response Scaling

A species-resolved anisotropy-response scaling framework is used to investigate the structure and formation dynamics of the $f_0(980)$ in relativistic nuclear collisions. Published measurements in high-multiplicity p+Pb collisions at $\sqrt{s_{NN}}=8.16$~TeV exhibit broad scaling closure under a single-meson response construction, with a small effective final-state response, $ζ_{f_0}\simeq0.04$, whereas a symmetric ${\rm K\bar K}$ constituent-response construction gives substantially poorer closure. An explicitly molecular ${\rm K\bar K}$ calculation also exhibits single-meson-like scaling but gives a substantially larger effective final-state response, $ζ_{f_0}\simeq0.16$. Thus, single-meson scaling alone is not a unique structural discriminator. The combined response construction and $ζ_{f_0}$ constraints characterize the measured $f_0(980)$ by a single-meson-like anisotropy response with weak final-state sensitivity, distinctly different from the molecular benchmark. These results establish anisotropy-response scaling as a new experimental probe of $f_0(980)$ formation dynamics and motivate systematic tests across collision systems and beam energies.

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arXiv2026

Scaling Properties of the $Δγ$ Correlator: Constraints on Background and CME-Sensitive Charge Separation in Heavy-Ion Collisions

The scaling properties of the $Δγ$ correlator are used to investigate charge separation associated with the chiral magnetic effect (CME) in $p$+Au, $d$+Au, Ru+Ru, Zr+Zr, and Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$~GeV, and in $p$+Pb and Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ and $2.76$~TeV. Anomalous Viscous Fluid Dynamics (AVFD) calculations provide complementary benchmarks for background and signal+background behavior. Scaling $Δγ$ by the elliptic-flow coefficient $v_2$ reduces the leading flow-coupled background dependence, while the resulting $Δγ/v_2$ exhibits a common approximate $1/N_{\rm ch}$ scaling behavior for $p$+Au, $d$+Au, $p$+Pb, and Pb+Pb collisions, consistent with multiplicity dilution of background-driven charge correlations across markedly different collision systems and environments. In contrast, Ru+Ru, Zr+Zr, and Au+Au collisions exhibit systematic positive deviations from their respective $1/N_{\rm ch}$ background-scaling trends. These scaling violations are qualitatively similar to those generated by AVFD calculations with an input CME signal and, relative to the experimentally constrained background scaling, provide evidence for a CME contribution to the measured charge separation. The inferred CME fraction is approximately $27\%$ for mid-central Au+Au collisions and roughly a factor of two smaller for Ru+Ru and Zr+Zr, whose values are consistent within uncertainties. The inferred CME fractions reduce the expected Ru+Ru--Zr+Zr difference to only $\sim1.3\%$ in the total charge-separation signal, explaining its limited experimental sensitivity. These results demonstrate that scaling violations relative to an experimentally constrained background provide a quantitative strategy for isolating a CME contribution to charge separation in heavy-ion collisions.

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arXiv2026

Azimuthal Anisotropy Scaling Functions for Identified Particle and Anti-Particle Species across Beam Energies: Insights into Baryon Junction Effects

Azimuthal anisotropy scaling functions are constructed from species-resolved anisotropy measurements in Pb+Pb ($\sqrt{s_{NN}}$=2.76, 5.02~TeV) and Au+Au ($\sqrt{s_{NN}}$=7.7--200~GeV) collisions to probe baryon transport and medium response at finite baryon chemical potential ($μ_B$). Within this data-driven framework, meson and baryon anisotropies spanning the collective-flow and quenching regimes collapse onto common scaling curves, enabling quantitative separation of viscous attenuation, radial flow, and hadronic re-scattering. The attenuation scale $k_β$ exhibits a non-monotonic beam-energy dependence, coincident with the low-energy rise of hadronic re-scattering, consistent with a temperature-dependent specific shear viscosity featuring a near-minimum near the QCD critical region. A charge-odd baryon--antibaryon separation in the effective radial-flow response is negligible at LHC energies but grows toward lower $\sqrt{s_{NN}}$. This species-uniform, baryon-number-scaling separation across $p,Λ,Ξ,Ω$, and $d$ disfavors a purely hadronic origin and supports junction-driven net-baryon transport at finite $μ_B$, enhancing the experimental visibility of critical dynamics in finite, rapidly evolving systems. Together, these results establish species-resolved scaling functions as a compact and robust tool for constraining baryon stopping, medium opacity, and QGP transport properties.

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arXiv2026

Comment on "Lattice QCD constraints on the critical point from an improved precision equation of state"

A recent Letter~\cite{Borsanyi:2025dyp} employs lattice QCD calculations of the equation of state, combined with entropy-density contour analysis, to place a lower bound of $μ_B \gtrsim 450$~MeV on the location of the QCD critical endpoint (CEP). While the underlying lattice calculations represent an important advance in precision and systematic control, the method used to infer constraints on the CEP is not directly sensitive to critical behavior. In particular, the use of entropy contours does not directly probe the singular structure associated with the CEP, does not explicitly incorporate the relevant thermodynamic scaling fields, and relies on assumptions that are not strictly satisfied in finite systems. Consequently, the reported exclusion of a CEP below $μ_B \approx 450$~MeV cannot be regarded as model-independent, as model-independent constraints require observables that are directly sensitive to the singular scaling behavior associated with critical phenomena.

hep-lat↗

arXiv2026

Probing the QCD Critical End Point with Finite-Size Scaling of Net-Baryon Cumulant Ratios

Finite-size scaling (FSS) is applied to net-baryon cumulant ratios $C_2/C_1$, $C_3/C_2$, $C_4/C_2$, $C_3/C_1$, and $C_4/C_1$ measured in Au+Au collisions over the Beam Energy Scan Phase~I range $\sqrt{s_{NN}}=7.7$--$200$~GeV to constrain the location and universality class of the QCD critical end point (CEP). Although finite-size and finite-time effects suppress non-monotonic signatures in unscaled data, the FSS analysis reveals a collapse of measurements from different beam energies and centralities onto universal scaling functions. All cumulant ratios collapse under a single, common set of critical exponents and exhibit divergence patterns characteristic o 3D Ising critical behavior. The scaling results indicate a CEP at $\sqrt{s}_{\rm CEP}\approx33.0$~GeV, corresponding to $μ_{B,\rm CEP}\approx130$~MeV and $T_{\rm CEP}\approx158.5$~MeV. These findings demonstrate that finite-size scaling provides a robust, model-independent framework for accessing critical behavior in finite, dynamically evolving systems, where non-equilibrium baryon-number transport can enhance the experimental visibility of susceptibility-driven fluctuations without modifying the underlying universality class.

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arXiv2026

Finite-Size Scaling of Net-Proton Cumulants in Heavy-Ion Collisions: Remarks on the Interpretation of a Recent Analysis

Finite-size scaling (FSS) provides a framework for investigating the possible presence of a critical end point (CEP) in the QCD phase diagram using fluctuation observables measured in relativistic heavy-ion collisions. A recent analysis reported a finite-size scaling representation of a susceptibility constructed from net-proton cumulants and interpreted the resulting scaling behavior as evidence for a CEP near $μ_B \approx 625$ MeV \cite{FSSpaper}. This note examines several aspects of that scaling construction. These include identifying the pseudorapidity acceptance window with the physical system size used in the finite-size scaling relations, assessing the influence of acceptance-driven multiplicity scaling on the susceptibility used for the scaling analysis, and treating thermodynamic scaling fields in the scaling variable. These considerations clarify several issues relevant for the consistent implementation and interpretation of finite-size scaling analyses in heavy-ion collision experiments. Taken together, they indicate that the reported scaling behavior does not uniquely establish critical dynamics nor the presence of a CEP near $μ_B \approx 625$ MeV.

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arXiv2025

Azimuthal Anisotropy Scaling for Identified Mesons and Baryons: Insights into Medium Transport Properties, Equation of State and Hadronic Re-scattering

Scaling functions for the centrality and transverse momentum dependence of $v_2(p_T,\text{cent})$ and $v_3(p_T,\text{cent})$ are constructed for identified mesons and baryons in Pb+Pb ($\sqrt{s_{NN}}=2.76,\ 5.02$~TeV), Xe+Xe (5.44~TeV), and Au+Au (0.2~TeV) collisions. These species-resolved functions capture the interplay of initial geometry, viscous attenuation, radial flow, partonic energy loss, and hadronic re-scattering across both flow- and quenching-dominated regimes. The systematic growth of the radial-flow parameter $ζ_{\rm rf}$ with multiplicity and beam energy provides direct empirical constraints on the equation of state (EOS) of hot QCD matter. The extracted parameters also yield differential constraints on the specific shear viscosity $η/s$, the jet-quenching parameter $\hat{q}$, and late-stage hadronic dynamics. LHC systems exhibit low $η/s$, strong radial flow, and negligible re-scattering at high energy density, whereas Au+Au at RHIC energy shows even lower $η/s$, weaker radial flow, finite re-scattering, and reduced energy density. The coexistence of strong radial flow at low $p_T$ and significant jet suppression at high $p_T$ emerges as a defining hallmark of QGP formation, establishing the framework as a quantitative probe of QGP transport properties and EOS stiffness.

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arXiv2025

Do QGP Droplets Drive Anisotropy in Small Systems? Insights from RHIC and the LHC

Azimuthal anisotropy scaling functions for identified mesons and baryons are analyzed in large (Pb+Pb at $\sqrt{s_{NN}} = 2.76$ and 5.02 TeV, Au+Au at $\sqrt{s_{NN}} = 200$ GeV), intermediate (Cu+Cu at $\sqrt{s_{NN}} = 200$~GeV), and small (p+Pb at $\sqrt{s_{NN}} = 5.02$ and 8.16 TeV, p+Au, d+Au, and $^3$He+Au at $\sqrt{s_{NN}} = 200$ GeV) collision systems. The scaling functions' fidelity supports a hydrodynamic-like origin for anisotropies in the flow-dominated regime. Central Pb+Pb, Au+Au, and Cu+Cu reflect QGP-driven expansion with strong radial flow and significant jet quenching, while peripheral Pb+Pb and Cu+Cu exhibit hadronic-dominated dynamics. In contrast, central RHIC small systems show hadronic-dominated behavior, with strong re-scattering, negligible radial flow, and suppressed jet quenching, following the hierarchy p+Au $>$ d+Au $>$ $^3$He+Au. At the LHC, ultra-central p+Pb collisions display enhanced radial flow, reduced re-scattering, and small but nonzero jet quenching. Scaling violations at high $p_T$ reflect partial suppression of partonic energy loss. These findings demonstrate that QGP-like behavior in small systems depends sensitively on both system size and beam energy, and establish the scaling framework as a robust diagnostic of collectivity and medium properties across diverse collision conditions.

hep-ex↗

arXiv2024

A study of nuclear structure of light nuclei at the Electron-Ion Collider

Understanding the substructure of atomic nuclei, particularly the clustering of nucleons inside them, is essential for comprehending nuclear dynamics. Various cluster configurations can emerge depending on excitation energy, the number and types of core clusters, and the presence of excess neutrons. Despite the prevalence of tightly bound cluster formations in low-lying states, understanding the correlation between clusters and their formation mechanisms remains incomplete. This exploring study investigates nuclear clustering at the Electron-Ion Collider (EIC) using simulations based on the modified BeAGLE model. By simulating collisions involving $e$+$^{9}$Be, $e$+$^{12}$C, and $e$+$^{16}$O nuclei, we find that the average energy of particles $\langle E \rangle$ and the system size ratios of particles at forward rapidity exhibit sensitivity to alpha clustering and its various configurations. These findings offer valuable insights into the dynamics of nuclear clustering and its implications for future studies at the EIC.

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arXiv2024

Anisotropy Scaling Functions in Heavy-Ion Collisions: Insights into the `Ultra-Central Flow Puzzle' and Constraints on Transport Coefficients and Nuclear Deformation

Anisotropy scaling functions derived from comprehensive measurements of transverse momentum- and centrality-dependent anisotropy coefficients $v_2(p_T,\text{cent})$ and $v_3(p_T,\text{cent})$ in Pb+Pb collisions at 5.02 and 2.76 TeV, Xe+Xe collisions at 5.44 TeV and Au+Au collisions at 0.2 TeV, offer new insights into the `ultra-central flow puzzle.' These functions integrate diverse measurements into a single curve, clarifying anisotropy attenuation throughout the entire $p_T$ and centrality range. They reveal the influence of initial-state eccentricities ($\varepsilon_{n}$), dimensionless size ($\mathbb{R}$), radial flow, viscous correction to the thermal distribution function ($δ_f$), the medium's stopping power ($\Hat{q}$), and specific shear viscosity ($η/s$) on the observed anisotropies. This analysis not only enhances understanding of transport coefficients but also provides crucial constraints on nuclear deformation.

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arXiv2022

Scaling properties of background- and chiral-magnetically-driven charge separation: evidence for detection of the chiral magnetic effect in heavy ion collisions

The scaling properties of the $R_{Ψ_2}(ΔS)$ correlator and the $Δγ$ correlator are used to investigate a possible chiral-magnetically-driven (CME) charge separation in $p$+Au, $d$+Au, Ru+Ru, Zr+Zr, and Au+Au collisions at $\sqrt s_{\mathrm{NN}}=200$~GeV, and in $p$+Pb ($\sqrt s_{\mathrm{NN}}=5.02$~TeV) and Pb+Pb collisions at $\sqrt s_{\mathrm{NN}}=5.02$ and $2.76$~TeV. The results for $p$+Au, $d$+Au, $p$+Pb, and Pb+Pb collisions, show the $1/{\rm N_{ch}}$ scaling for background-driven charge separation. However, the results for Au+Au, Ru+Ru, and Zr+Zr collisions show scaling violations which indicate a CME contribution in the presence of a large background. In mid-central collisions, the CME accounts for approximately 27\% of the signal + background in Au+Au and roughly a factor of two smaller for Ru+Ru and Zr+Zr, which show similar magnitudes.

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arXiv2022

Rebuttal to: "Comment on Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt{s}_{\rm NN}=200$ GeV"

Recently, F. Wang commented on our work "Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt{s}_{\rm NN} = 200$ GeV" and made several claims to support his conclusion that our results are fallacious. His conclusion and claims are not only incorrect; they show a fundamental disconnect with the rudiments of the $R_{Ψ_2}(ΔS)$ correlator. This rebuttal addresses the root misconception responsible for Wang's false claims.

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arXiv2022

Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt s_{\mathrm{NN}}=200$ GeV

The Anomalous Viscous Fluid Dynamics model, AVFD, is used in concert with the charge-sensitive correlator $R_{Ψ_2}(ΔS)$ to investigate the scaling properties of background- and chiral-magnetically-driven (CME) charge separation ($ΔS$), characterized by the inverse variance $\mathrm{σ^{-2}_{R_{Ψ_2}}}$ of the $R_{Ψ_{2}}(ΔS)$ distributions obtained in collisions at $\sqrt s_{\mathrm{NN}}=200$ GeV. The $\mathrm{σ^{-2}_{R_{Ψ_2}}}$ values for the background are observed to be event-shape-independent. However, they scale with the reciprocal charged-particle multiplicity $(1/\left< N_{\rm ch} \right>)$, indicating an essential constraint for discerning background from the signal and a robust estimate of the difference between the backgrounds in Ru+Ru and Zr+Zr collisions. By contrast, the $\mathrm{σ^{-2}_{R_{Ψ_2}}}$values for signal + background show characteristic $1/\left< N_{\rm ch} \right>$ scaling violations that characterize the CME-driven contributions. Corrections to recent ${R_{Ψ_2}(ΔS)}$ measurements \cite{STAR:2021mii} that account for the background difference in Ru+Ru and Zr+Zr collisions indicate a charge separation difference compatible with the CME. The results further suggest that $\mathrm{σ^{-2}_{R_{Ψ_2}}}$ measurements for peripheral and central collisions in concert with $1/\left< N_{\rm ch} \right>$ scaling, provides a robust constraint to quantify the background and aid characterization of the CME.

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arXiv2021

Model study of the energy dependence of the correlation between anisotropic flow and the mean transverse momentum in Au+Au collisions

A hybrid model that employs the hadron-string transport model UrQMD and the (3+1)D relativistic viscous hydrodynamic code vHLLE, is used to investigate the beam energy dependence of the correlation coefficient $ρ(v^{2}_{2},[p_{T}])$ between the average transverse momentum $[p_{T}]$ of hadrons emitted in an event and the square of the anisotropic flow coefficient $v_2^2$. For Au+Au collisions, the model predicts characteristic patterns for the energy and event-shape dependence of the variances for $[p_{T}]$ and $v_n^2$ (${\rm Var}([p_T])$ and ${\rm Var}(v_2^2)$), and the covariance of $v_n^2$ and $[p_{T}]$ (${\rm cov}(v_{2}^{2},[p_T])$), consistent with the attenuation effects of the specific shear viscosity $η/s$. In contrast, $ρ(v^{2}_{2},[p_{T}])$ is predicted to be insensitive to the beam energy but sensitive to the initial-state geometry of the collisions. These observations suggest that a precise set of measurements for ${\rm Var}([p_T])$, ${\rm Var}(v_2^2)$, ${\rm cov}(v_{2}^{2},[p_T])$ and $ρ(v^{2}_{2},[p_{T}])$ as a function of beam-energy and event-shape, could aid precision extraction of the temperature and baryon chemical-potential dependence of $η/s$ from the wealth of Au+Au data obtained in the RHIC beam energy scan.

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arXiv2021

Model investigations of the correlation between the mean transverse momentum and anisotropic flow in shape-engineered events

The correlation between the event mean-transverse momentum $[p_{\mathrm{T}}]$, and the anisotropic flow magnitude $v_n$, $ρ(v^{2}_{n},[p_{T}])$, has been argued to be sensitive to the initial conditions in heavy-ion collisions. We use simulated events generated with the AMPT and EPOS models for Au+Au at $\sqrt{\textit{s}_{NN}}$ = 200 GeV, to investigate the model dependence and the response and sensitivity of the $ρ(v^{2}_{2},[p_{T}])$ correlator to collision-system size and shape, and the viscosity of the matter produced in the collisions. We find good qualitative agreement between the correlators for the string melting version of the AMPT model and the EPOS model. The model investigations for shape-engineered events as well as events with different viscosity ($η/s$), indicate that $ρ(v^{2}_{2},[p_{T}])$ is sensitive to the initial-state geometry of the collision system but is insensitive to sizable changes in $η/s$ for the medium produced in the collisions. These findings suggest that precise differential measurements of $ρ(v^{2}_{2},[p_{T}])$ as a function of system size, shape, and beam-energy could provide more stringent constraints to discern between initial-state models and hence, more reliable extractions of $η/s$.

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arXiv2021

A model investigation of the longitudinal broadening of the transverse momentum two-particle correlator

The Multi-Phase Transport model (AMPT) is used to investigate the longitudinal broadening of the transverse momentum two-particle correlator $C_{2}\left(Δη,Δφ\right)$, and its utility to extract the specific shear viscosity, $η/s$, of the quark-gluon plasma formed in ultra-relativistic heavy ion collisions. The results from these model studies indicate that the longitudinal broadening of $C_{2}\left(Δη,Δφ\right)$ is sensitive to the magnitude of $η/s$. However, reliable extraction of the longitudinal broadening of the correlator requires the suppression of possible self-correlations associated with the definition of the collision centrality.

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arXiv2021

Quantification of the Chiral Magnetic Effect in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV

The Multi-Phase Transport model, AMPT, and the Anomalous Viscous Fluid Dynamics model, AVFD, are used to assess a possible chiral-magnetically-driven charge separation ($ΔS$) recently measured with the ${R_{Ψ_2}(ΔS)}$ correlator in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV. The Comparison of the experimental and simulated ${R_{Ψ_2}(ΔS)}$ distributions indicates that background-driven charge separation is insufficient to account for the measurements. The AVFD model calculations, which explicitly account for CME-driven anomalous transport in the presence of background, indicate a CME signal quantified by the $P$-odd Fourier dipole coefficient ${a_1'}\approx 0.5\%$ in mid-central collisions. A similar evaluation for the $Δγ$ correlator suggests that only a small fraction of this signal ($f_{\rm CME}=Δγ_{\rm CME}/Δγ\approx 25\%$) is measurable with this correlator in the same collisions. The related prediction for signal detection in isobaric collisions of Ru+Ru and Zr+Zr are also presented.

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