NPL groups present many new results at Quark Matter Conference in Houston. Two Graduate Students win conference awards.


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Last week, seventeen Illinois Nuclear Physics scientists (2 Professors, 1 Research Scientist, 13 Graduate Students, and 1 Undergraduate Student) traveled to Houston to present their results at the XXXth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions, aka Quark Matter 2023. 

Prof. Jorge Noronha chairing a parallel Theory session at QM2023
Photo Credit: Angel Nava, @lavendergluon, QM2023 official photographer
Prof. Jorge Noronha chairing a parallel Theory session at QM2023

The NPL members presented several advancements on both the experimental and theoretical side of Nuclear Physics. 
New results from the heavy ion program of the ATLAS experiment at the CERN LHC, the latest news from the commissioning of the sPHENIX experiment at RHIC, latest new from the MUSES theoretical collaboration led by UIUC, as well as several other frontier advancements in Nuclear Theory were discussed in various talks and posters presented by UIUC NPL members. Two Illinois graduate students, Débora Mroczek and Anabel Romero Hernandez, received conference awards for being one of the best talks and posters, respectively. A total of 10 new papers, and 3 new preliminary results were discussed in the NPL contributions. Below you can find a short summary of each of them, with links to the relevant slideshow


Using multi-particle correlations to estimate fluctuations in jet and rare probe azimuthal anisotropies, by undergrad student Abraham Holtermann 

Jets, heavy flavor, and many more “rare probes” are useful for understanding QCD processes that occur within the Quark Gluon Plasma produced in heavy ion collisions. The azimuthal anisotropies of these rare probes are often measured by correlating their angles with soft particles using differential cumulants. We introduce a more comprehensive differential multiparticle correlation framework that allows for the study of fluctuations in rare probe azimuthal anisotropy. To test this method, we show the sensitivity of three new observables to a toy model, and show their ability to isolate both fluctuations in rare probe anisotropies, and the correlations between rare probe anisotropies and the collective flow of soft, low pT particles.

arXiv: 2307.16796

Structure in the speed of sound: from neutron stars to heavy-ion collisions, by grad student Nanxi Xiao

Neutron star equations of state that can sustain heavy neutron stars over 2 Msun necessitate a large, rapid rise in the speed of sound.  In our studies, we convert equations of state with a bump in the speed of sound that are compatible with massive neutron stars to nearly symmetric nuclear matter using the nuclear symmetry energy expansion with 4 coefficients. With a range of different coefficients, we are able to obtain upper and lower bounds for converted symmetric nuclear matter with causality and stability constraints. We compare our converted equation of state with heavy-ion collision data by hadronic transport method SMASH.

Exploring neutron stars with three conserved charges in a newly optimized C++ Chiral Mean Field code, by grad student Nikolás Cruz Camacho
The Chiral Mean Field model (CMF) has been successful in describing the equation of state at large baryon densities, such as those found in neutron stars, neutron star mergers, and heavy-ion collisions. The MUSES collaboration has rewritten the zero-temperature CMF model from Fortran 77 into a parallelized modern C++20 using OpenMP, which has resulted in at least an order of magnitude improvement in runtime. We obtained equations of state across \mu_B, \mu_S and \mu_Q, and within the metastable regime around the quark deconfinement phase transition. The improved numerical resolution allows for the accurate computation of higher-order derivatives such as susceptibilities.

Ph credits: Angel Nava, @lavendergluon, QM2023 official photographer

Investigation of initial state effects in p+Pb collisions at ATLAS via measurement of the centrality dependence of the dijet yield, by Research Scientist Riccardo Longo 

Proton-nucleus collisions at LHC energies represent an incredible source of information to further understand the building blocks of matter. The proton internal structure fluctuates, but when it collides with the nucleus, because of the very short transit time, it does it in a practically frozen configuration. By measuring dijet events using the full acceptance of the ATLAS calorimeter, we have explored how the proton configuration affects its own size and interaction strength. Our results, included in a recent ATLAS paper just submitted for publication (see arXiv 2309.00033), represent an unprecedented input towards the understanding of the proton’s size fluctuations and the associated changes in its interaction strength. 

A new approach to stochastic relativistic fluid dynamics from information flow, by Graduate Student Nicki Mullins 

The matter formed in heavy-ion collisions is well described as a viscous relativistic fluid. The fluctuation-dissipation theorem guarantees that such a theory will also experience thermal fluctuations, but these are often ignored in the modeling of the quark-gluon plasma. To better understand how these fluctuations enter in relativistic contexts, we constructed the first theory of stochastic hydrodynamics that is guaranteed to be causal, stable against fluctuations, and foliation independent. Using this theory, we constructed an effective action for fluctuating relativistic hydrodynamics and derived a symmetry of this action that can be used to implement the fluctuation-dissipation theorem. 


Neutral Pion and Eta Meson Reconstruction with the sPHENIX Detector, by Post-Doctoral Research Associate Anthony Hodges 

sPHENIX is a new detector at the Relativistic Heavy-Ion Collider (RHIC) designed to make precision jet and upsilon measurements in 200 GeV p + p, p + Au, and Au + Au collisions and will begin taking data in 2023. In addition to having the first hadronic calorimeter (HCal) at mid-rapidity at RHIC, sPHENIX also contains a tungsten-scintillator based Electromagnetic Calorimeter (EMCal) for measuring the energy of photons and electrons. The EMCal absorber blocks were largely constructed by the UIUC NPL (see this Condensate article for more details). Anthony presented in his poster the performance of the calorimeter as evaluated in test beam data, as well as the first calibration data available from the first year of sPHENIX running. 

Ph. Credits: Peter Steinberg, Xiaoning Wang 

ATLAS measurements of b-jet suppression and heavy-flavor azimuthal correlations in 5.02 TeV Pb+Pb collisions, by Prof. Anne Sickles 

This talk focused on recent ATLAS measurements on how jets probe the QGP.  We showed the first direct measurement of a different suppression of jets originating from bottom quarks compared to inclusive jets (Eur. Phys. J. C (2023) 83:438).  We also showed the first measurement of the modification of the momentum balance between jet pairs depending on the size of the jet (ATLAS-CONF-2023-060, analysis led by Anabel Romero and also shown in her flash talk). Both these ATLAS analyses were led by Sickles group. 

Measurements of azimuthal anisotropy of charged particles in Pb+Pb collisions with the ATLAS detector, by Grad Student Xiaoning Wang

Xiaoning Wang presented her new measurement [ATLAS-CONF-2023-007] using ATLAS Pb+Pb data (see contribution here). This result investigates properties of the quark-gluon plasma, a special state of matter in which elementary particles dissolve into a dense, hot soup of quarks and gluons, produced at extreme temperatures and densities of the Large Hadron Collider at CERN. By measuring the angular modulation of particle productions, Xiaoning’s work provides new insights into the geometry and fluctuations in the QGP. With improved statistics, this work has pushed the measurement to an unprecedented kinematic range and attracted the attention of theorists at the conference. 

Ph. credits: Riccardo Longo & Angel Nava, @lavendergluon, QM2023 official photographer

QGP vortex rings as a new probe for jet-induced medium response and longitudinal dynamics, by Post-Doctoral Research Associate Willian Matioli Serenone

The quark-gluon plasma is created in relativistic heavy-ion collisions in colliders such as the LHC and RHIC. It is a strongly-interacting medium that often is described as a low-viscosity fluid. When a fast moving particle goes through this fluid, it will interact with depositing momentum along its path, in a similar fashion as a bullet goes through water. We predict that this should create a vortex ring during the evolution of the system. We performed simulations using state-of-the-art viscous hydrodynamic simulations of this phenomena and proposed an observable that may eventually be measured in experiments.

Phys.Lett.B 820 (2021) 136500

A new stable and causal theory of viscous chiral hydrodynamics, by Graduate Student Nick Abboud

Viscous relativistic hydrodynamics successfully describes the large-scale behavior of hot, dense, and fast-moving fluids like the quark-gluon plasma produced in heavy-ion collisions. Most microscopic details are qualitatively unimportant to the hydrodynamic behavior of the fluid, but not all. In particular, the chirality (handedness) of the particles in a spinning fluid can cause macroscopic currents to form. Hydrodynamic theories that were previously developed to describe such effects are plagued by fundamental issues: some allow faster-than-light signal propagation, some predict spontaneous explosion of equilibrium states, and some aren't even solvable! In this work, we developed the first viscous theory of chiral relativistic hydrodynamics rigorously proved to avoid these issues.


Influence of baryon number, strangeness, and electric charge fluctuations on spectra and collective flow at the LHC, by Post-Doctoral Research Associate Dekrayat Almaalol

At LHC energies it is possible to generate local baryon, strangeness, and electric charge density fluctuations from $q\overline{q}$ pair production. This creates an opportunity to implement and quantify charge dynamics in the very well controlled regime of heavy ion collisions simulations. We implemented BSQ charge dynamics in a fully integrated framework. The local charge density fluctuations are generated using the ICCING model, then propagated within an upgraded version of the hydrodynamic model, v-USPhydro, that conserves the BSQ charge densities exactly. Our hydrodynamics simulation is informed by the full 4-D equation of state ${T,\mu_B,\mu_S,\mu_Q}$ from Lattice Quantum Chromodynamics and includes decays from the latest Particle Data Group 2016+. We find relatively large fluctuations in the chemical potentials in local fluid cells at the freeze-out hypersurface even at LHC energies and  discuss possible new experimental observables which will be sensitive to these fluctuations.


Ph. Credits: Angel Nava, @lavendergluon, QM2023 official photographer

Influence of the latest hadronic resonances from the Particle Data Group on thermal models, lattice QCD comparisons, and SMASH, by Graduate Student Jordi Salinas San Martin
Quarks and gluons are the fundamental building blocks of the matter that surrounds us. Nevertheless, they never wander alone, they group together to form the particles –called hadrons– that we measure in experiments all over the world. A century after the discovery of the proton, we have evidence to believe that not all hadrons that exist in nature have been observed yet. Here, we gathered the latest experimental information of particle properties, checked its consistency by comparing to lattice QCD simulations, and observed the consequence of including more of these particles on models that can describe different stages of a heavy-ion collision, such as the freeze-out and hadronic scattering phases. The conclusions from this study will guide researchers to extract information from a real heavy-ion collision more reliably and ultimately, to understand the nature of the strong nuclear force.

arXiv: 2309.01737

Far-from-equilibrium relativistic hydrodynamics in neutron-star mergers, by Graduate Student Yumu Yang
For realistic equations of state of neutron stars, pressure can be very nonlinear and motivates a far-from-beta-equilibrium description of bulk-viscous transport in neutron star mergers. This description is the Israel-Stewart formulation, which describes a hydrodynamical system. We calculated the transport coefficients for the Israel-Stewart formulation based on realistic equations of state, and we showed that these transported coefficients are significantly affected by nuclear properties of the equations of state, such as the nuclear symmetry energy. 

arXiv: 2309.01864 

Location of the QCD critical point predicted by holographic Bayesian analysis, by Mauricio Hippert Texeira
 It is conjectured that QCD, the fundamental theory of strong interactions, undergoes a phase transition under very high densities of particles minus antiparticles. However, methods to determine QCD thermodynamics from first principles are only available at lower densities, where no phase transition is found. In this work, we used a model of QCD thermodynamics based on the gauge-gravity duality to predict the point at which the QCD phase transition begins, known as the QCD critical point. Using tools of Bayesian statistical inference, we constrained the parameters of this model using first-principles results from lattice QCD calculations to obtain confidence regions for the location of the QCD critical point



Angel Nava, @lavendergluon, QM2023 official photographer

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This story was published September 9, 2023.