arXiv subjects

Paul Sorensen

Publications and source records attributed to Paul Sorensen.

At least 19 recordsLinked to original sources

arXiv2026

Locating the QCD critical point with finite-size scaling of proton cumulants

We perform a finite-size scaling analysis of net-proton number cumulants in Au+Au collisions at center-of-mass energies between $\sqrt{s_{\rm{NN}}} = 2.4$ and $200~\rm{GeV}$ to search for evidence of a critical point in the QCD phase diagram. We use the second-order susceptibility and Binder cumulant, whose scaling with the system size is studied by using their dependence on the rapidity bin width $W$. We find that for collisions at $\sqrt{s_{\rm{NN}}} \geq 7.7~\rm{GeV}$, the susceptibility depends only weakly on $W$ but increases as a power law in the baryon chemical potential $μ_B$, $(μ_B - μ_{B,c})^{-γ}$ with $μ_{B,c}=700\pm130~{\rm MeV}$ and $γ=1.6\pm0.5$, consistent with several recent theory estimates for the location of the QCD critical point. We also find that the Binder cumulant appears to show a crossing within a similar range of $μ_B$ values. A comparable scaling behavior, however, is also found in dynamical simulations without critical fluctuations. Moreover, we find that a Skellam distribution leads to apparent scaling of the second-order susceptibility when evaluated along the freeze-out line. Simulations also show that the crossing of the Binder cumulant may be sensitive to changes in experimental acceptance at different beam energies. We conclude that while the observed scaling and crossing are consistent with the presence of a critical point, they cannot yet be taken as definitive evidence.

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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.

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arXiv2016

Chiral Magnetic Effect Task Force Report

In this report, we briefly examine the current status of the study of the chiral magnetic effect including theory and experimental progress. We recommend future strategies for resolving uncertainties in interpretation including recommendations for theoretical work, recommendations for measurements based on data collected in the past five years, and recommendations for beam use in the coming years of RHIC. We have specifically investigated the case for colliding nuclear isobars (nuclei with the same mass but different charge) and find the case compelling. We recommend that a program of nuclear isobar collisions to isolate the chiral magnetic effect from background sources be placed as a high priority item in the strategy for completing the RHIC mission.

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arXiv2015

The Hot QCD White Paper: Exploring the Phases of QCD at RHIC and the LHC

The past decade has seen huge advances in experimental measurements made in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and more recently at the Large Hadron Collider (LHC). These new data, in combination with theoretical advances from calculations made in a variety of frameworks, have led to a broad and deep knowledge of the properties of thermal QCD matter. Increasingly quantitative descriptions of the quark-gluon plasma (QGP) created in these collisions have established that the QGP is a strongly coupled liquid with the lowest value of specific viscosity ever measured. However, much remains to be learned about the precise nature of the initial state from which this liquid forms, how its properties vary across its phase diagram and how, at a microscopic level, the collective properties of this liquid emerge from the interactions among the individual quarks and gluons that must be visible if the liquid is probed with sufficiently high resolution. This white paper, prepared by the Hot QCD Writing Group as part of the U.S. Long Range Plan for Nuclear Physics, reviews the recent progress in the field of hot QCD and outlines the scientific opportunities in the next decade for resolving the outstanding issues in the field.

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arXiv2015

Constraining the Eq. of State of Super-Hadronic Matter from Heavy-Ion Collisions

The equation of state of QCD matter for temperatures near and above the quark-hadron transition (165 MeV) is inferred within a Bayesian framework through the comparison of data from the Relativistic Heavy Ion Collider and from the Large Hadron Collider to theoretical models. State-of- the-art statistical techniques are applied to simultaneously analyze multiple classes of observables while varying 14 independent model parameters. The resulting posterior distribution over possible equations of state is consistent with results from lattice gauge theory.

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arXiv2014

Azimuthal anisotropy in U+U collisions at STAR

The azimuthal anisotropy of particle production is commonly used in high-energy nuclear collisions to study the early evolution of the expanding system. The prolate shape of uranium nuclei makes it possible to study how the geometry of the colliding nuclei affects final state anisotropies. It also provides a unique opportunity to understand how entropy is produced in heavy ion collisions. In this paper, the two- and four- particle cumulant $v_2$ ($v_{2}\{2\}$ and $v_{2}\{4\}$) from U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV for inclusive charged hadrons will be presented. The STAR Zero Degree Calorimeters are used to select very central collisions. Differences were observed between the multiplicity dependence of $v_{2}\{2\}$ for most central Au+Au and U+U collisions. The multiplicity dependence of $v_{2}\{2\}$ in central collisions were compared to Monte Carlo Glauber model predictions and it was seen that this model cannot explain the present results.

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arXiv2013

Beam energy dependence of Elliptic and Triangular flow with the AMPT model

A beam energy scan has been carried out at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory to search for the onset of deconfinement and a possible critical point where the transition from a Quark Gluon Plasma to a hadronic phase changes from a rapid cross-over to a first order phase transition. Anisotropy in the azimuthal distribution of produced particles such as the second and third harmonics $v_2$ and $v_3$ are expected to be sensitive to the existence of a Quark Gluon Plasma phase and the Equation of State of the system. For this reason, they are of great experimental interests. In this Letter we report on calculations of $v_2$ and $v_3$ from the AMPT model in the Default(Def.) and String Melting(SM) mode to provide a reference for the energy dependence of $v_2$ and $v_3$ for $\sqrt{s_{_{NN}}}$ from 7.7 GeV to 2.76 TeV. We expect that in the case that collisions cease to produce QGP at lower colliding energies, data will deviate from the AMPT String Melting calculations and come in better agreement with the Default calculations.

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arXiv2012

Effects of initial state fluctuations in the final state elliptic flow measurements using the NeXSPheRIO model

We present a systematic study of the effects due to initial condition fluctuations in systems formed by heavy-ion collisions using the hydrodynamical simulation code NeXSPheRIO. The study was based on a sample of events generated simulating Au+Au collisions at center of mass energy of 200 GeV per nucleon pair with impact parameter ranging from most central to peripheral collisions. The capability of the NeXSPheRIO code to control and save the initial condition (IC) as well as the final state particles after the 3D hydrodynamical evolution allows for the investigation of the sensitivity of the experimental observables to the characteristics of the early IC. Comparisons of results from simulated events generated using fluctuating initial conditions and smooth initial condition are presented for the experimental observable elliptic flow parameter ($v_2$) as a function of the transverse momentum, $p_t$, and centrality. We compare $v_2$ values estimated using different methods, and how each method responds to effects of fluctuations in the initial condition. Finally, we quantify the flow fluctuations and compare to the fluctuations of the initial eccentricity of the energy density distribution in the transverse plane.

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arXiv2012

Highlights from Heavy Ion Collisions at RHIC and the Acoustics of the Little Bangs

At the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, heavy nuclei are collided at high energies to create matter that is hot enough and dense enough to dissolve hadrons into a quark-gluon-plasma (QGP). In this lecture, dedicated to the memory of Aditya Sambamurti, I present an introduction to heavy-ion collisions and highlights from the first decade of RHIC results.

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arXiv2011

Higher Flow Harmonics in Heavy Ion Collisions from STAR

We report STAR measurements relating to higher flow harmonics including the centrality dependence of two- and four-particle cumulants for harmonics 1 to 6. Two-particle correlation functions vs. Δηand Δϕare presented for pT and number correlations. We find the power spectra (Fourier Transforms of the correlation functions) for central collisions drop quickly for higher harmonics. The Δηdependence of v3{2}2 and the pT and centrality dependence of v2 and v3 are studied. Trends are conistent with expectations from models including hot-spots in the initial energy density and an expansion phase. We also present v3 and v2{2}2 - v2{4}2 vs. \surdsNN .

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arXiv2011

Statistical Origin of Constituent-Quark Scaling in the QGP hadronization

Nonextensive statistics in a Blast-Wave model (TBW) is implemented to describe the identified hadron production in relativistic p+p and nucleus-nucleus collisions. Incorporating the core and corona components within the TBW formalism allows us to describe simultaneously some of the major observations in hadronic observables at the Relativistic Heavy-Ion Collider (RHIC): the Number of Constituent Quark Scaling (NCQ), the large radial and elliptic flow, the effect of gluon saturation and the suppression of hadron production at high transverse momentum (pT) due to jet quenching. In this formalism, the NCQ scaling at RHIC appears as a consequence of non-equilibrium process. Our study also provides concise reference distributions with a least chi2 fit of the available experimental data for future experiments and models.

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arXiv2011

Finite-size effects and the search for the critical endpoint of QCD

Taking into account the finiteness of the system created in heavy ion collisions, we show sizable results for the modifications of the chiral phase diagram at volume scales typically encountered in current experiments and demonstrate the applicability of finite-size scaling as a tool in the experimental search for the critical endpoint. Using data from RHIC and SPS and assuming finite-size scaling, we find that RHIC data from 200 GeV down to 19.6 GeV is only consistent with a critical point at μ\gtrsim 510 MeV. We also present predictions for the fluctuations at lower energies currently being investigated in the Beam Energy Scan program.

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arXiv2011

Finite-size scaling as a tool in the search for the QCD critical point in heavy ion data

Given the short lifetime and the reduced volume of the quark-gluon plasma (QGP) formed in high-energy heavy ion collisions, a possible critical endpoint (CEP) will be blurred in a region and the effects from criticality severely smoothened. Nevertheless, the non-monotonic behavior of correlation functions near criticality for systems of different sizes, given by different centralities in heavy ion collisions, must obey finite-size scaling. We apply the predicting power of scaling plots to the search for the CEP of strong interactions in heavy ion collisions using data from RHIC and SPS. The results of our data analysis exclude a critical point below chemical potentials $μ\sim 450 $MeV. Extrapolating the analysis, we speculate that criticality could appear slightly above $μ\sim 500 $MeV. Using available data we extrapolate our scaling curves to predict the behavior of new data at lower center-of-mass energy, currently being investigated in the Beam Energy Scan program at RHIC. If it turns out that the QGP phase is no longer achievable in heavy ion experiments before the CEP is reached, FSS might be the only way to experimentally estimate its position in the phase diagram.

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arXiv2011

The Rise and Fall of the Ridge in Heavy Ion Collisions

Recent data from heavy ion collisions at RHIC show unexpectedly large near-angle correlations that broaden longitudinally with increasing centrality. The amplitude of this ridge-like correlation rises rapidly, reaches a maximum, and then falls in the most central collisions. In this letter we explain how this behavior can be explained as initial-state coordinate-space anisotropies converted into final-state momentum-space correlations. We propose $v_n^2/ε_{n,\mathrm{part}}^{2}$ as a useful way to study length scales and provide a prediction for the ridge in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=$ 2.76 TeV.

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arXiv2011

The Rise and Fall of the Ridge

Recent data from heavy ion collisions at RHIC show unexpectedly large near-angle correlations that broaden longitudinally with centrality. The amplitude of this ridge-like correlation rises rapidly with centrality, reaches a maximum, and then falls in the most central collisions. In this talk we explain how this behavior can be easily understood in a picture where final momentum-space correlations are driven by initial coordinate space density fluctuations. We propose $v_n^2/ε_{n,part}^{2}$ as a useful way to study these effects and explain what it tells us about the collision dynamics.

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arXiv2011

Analyzing the Power Spectrum of the Little Bangs

In this talk we discuss the analogy between data from heavy-ion collisions and the Cosmic Microwave Background. We identify p_T correlations data as the heavy-ion analogy to the CMB and extract a power-spectrum from the heavy-ion data. We define the ratio of the final state power-spectrum to the initial coordinate-space eccentricity as the transfer-function. From the transfer-function we find that higher $n$ terms are suppressed and we argue that the suppression provides information on length scales like the mean-free-path. We make a rough estimate of the mean-free-path and find that it is larger than estimates based on the centrality dependence of v_2.

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arXiv2010

The Sound of the Little Bangs

Measurements of particles emitted from collisions of heavy ions at relativistic energies show evidence for temperature-fluctuations on the freeze-out surface of the expanding fireball. These can be understood as remnants of the density inhomogeneities in the initial collision overlap region. In this paper we present an analysis of heavy-ion collisions analogous to the analysis of the cosmic microwave background radiation. We use data on transverse momentum correlations to extract a power-spectrum from heavy-ion collisions and use a Glauber model for eccentricity to extract the transfer function needed to produce the observed correlations. We also discuss the acoustic signal, the sound that an observer inside the matter would hear.

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arXiv2010

Detecting a First-Order Transition in the QCD Phase Diagram with Baryon-Baryon Correlations

We suggest baryon-baryon correlations as an experimentally accessible signature for a first-order phase transition between a baryon-rich phase, like quarkyonic, and a baryon-suppressed hadronic phase in the QCD phase diagram. We examine the consequences of baryon-rich bubble formation in an expanding medium and show how the two-particle correlations vary in the transverse and longitudinal direction depending on the strength of the radial flow, the bubble temperature, and the time when the baryons are emitted.

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