Vol. 3 No. 4 (2022): Suplemento de la Revista Mexicana de Física. 37th Winter Workshop on Nuclear Dynamics

Preface 37th Winter Workshop on Nuclear Dynamics. February 27 - March 5,  2022. Marriott Puerto Vallarta Resort & Spa, Jalisco, México.

The ideas presented in this proceeding aims to be a first step towards a description of hadronic collisions where all soft processes are fundamentally strongly coupled and the same Universal strongly coupled physics drives both initial and final-state interactions. As it is not currently possible to derive such a picture from first principles, instead, an attempt to generalize the perfect liquid observation to a “perfect QCD” guiding principle is presented, focusing on implications for particle production in small systems. The first steps towards a microscopic model is taken by arguing that “perfect QCD” suggests that the screening in the initial state is so large that multi-parton interactions are of little or no importance. Instead, a target and projectile remnant is coherently excited and particle production is mainly driven by radiation in a qualitative similar manner as e⁺ e ̅  \rightarrow qq̅. Finally, some of the possible implications of this “excited remnant model” are presented. It is argued that the time ordering of soft and hard physics can explain the absence of jet quenching in small systems and that the coherence scale of the projectile and target provides insights into what small systems will exhibit flow.

We used model studies to investigate approaches to distin-guish signal and combinatorial jets.

The last few years have seen community-wide excitement in the study of jet substructure derived from the inner workings of clustering algorithms. Such efforts have resulted in the design of new observables which are related to partonic processes from final state hadrons. Since jets are multi-scale objects, they necessarily encode information about both the perturbative (pQCD) parton shower and non-perturbative (npQCD) physics including hadronization. Recent high precision measurements of jet substructure in proton-proton (pp) collisions have pushed the theoretical community into extending their predictions to higher orders resulting in the observation of large theoretical uncertainties from the non-perturbative regime of the calculations. We emphasize the importance of understanding a jet shower from a multidimensional point of view and highlight a recent measurement focused on distinguishing the pQCD vs. npQCD regimes within a jet shower. We introduce and discuss the utility of the formation time evaluated at varying stages of the jet shower in pp collisions. Finally, we present a monte-carlo study of the formation time of charged particles within the jet to gain a handle on hadronization mechanisms including stringbreaking, and outline a path forward for such observables in heavy ion collisions.

We discuss measurements of general balance functions recently reported by the ALICE collaboration in the context of a two-stage quark production model and towards the determination of light quark diffusivity.

The high collision energies reached at the LHC lead to significant production yields of light (anti)nuclei in proton-proton, p–Pb and Pb–Pb collisions. Light (anti)nuclei are identified using their specific energy loss (dE/dx), measured in the Time Projection Chamber, and their velocity using the Time-Of-Flight detector. The excellent tracking and particle identification capabilities of the ALICE experiment, as well as its low material budget, make this detector unique for measurements of these rarely produced particles. Results on (anti)deuteron, (anti)triton, (anti)3He and (anti) He production in Pb–Pb collisions at p  sNN = 5.02 TeV, including their transverse momentum (pT) spectra, production  yields and coalescence parameters BA, are presented. These results will be compared to the expectations of coalescence and statistical hadronization models to obtain information on the production mechanism of light (anti)nuclei in heavy-ion collisions. Furthermore, the first measurements of the d and ³He absorption cross section are shown.

We introduce a two-particle correlation observable that measures multiplicity-momentum correlations and may facilitate an estimate of the level of equilibration of the medium created in relativistic nuclear collisions. We calculate that multiplicity-momentum correlations should vanish in equilibrium in the Grand Canonical Ensemble, therefore non-zero measured values may indicate that the system has not reached local thermal equilibrium. Information about the level of equilibration of the system is important because many state-of-the-art models assume local equilibration either directly or through the use of an equation of state that makes this assumption. We make estimates of multiplicity-momentum correlations using PYTHIA/Angantyr and find positive values comparable in magnitude to well-measured correlations of transverse momentum fluctuations. We then outline a formalism that can use multiplicity-momentum correlations and correlations of transverse momentum fluctuations to quantify the level of partial thermalization of the system.

The equation of state (EoS) of QCD is a crucial input for the modeling of heavy-ion-collision (HIC) and neutron-star-merger systems. Calculations of the fundamental theory of QCD, which could yield the true EoS, are hindered by the infamous Fermi sign problem which only allows direct simulations at zero or imaginary baryonic chemical potential. As a direct consequence, the current coverage of the QCD phase diagram by lattice simulations is limited. In these proceedings, two different equations of state based on first-principle lattice QCD (LQCD) calculations are discussed. The first is solely informed by the fundamental theory by utilizing all available diagonal and non-diagonal susceptibilities up to O(µ 4 B) in order to reconstruct a full EoS at finite baryon number, electric charge and strangeness chemical potentials. For the second, we go beyond information from the lattice in order to explore the conjectured phase structure, not yet determined by LQCD methods, to assist the experimental HIC community in their search for the critical point. We incorporate critical behavior into this EoS by relying on the principle of universality classes, of which QCD belongs to the 3D Ising Model. This allows one to study the effects of a singularity on the thermodynamical quantities that make up the equation of state used for hydrodynamical simulations of HICs. Additionally, we ensure that these EoSs are valid for applications to HICs by enforcing conditions of strangeness neutrality and fixed charge-to-baryonnumber ratio.

Understanding the spin structure of the proton is of large interest to the nuclear physics community and it is one of the main goals of the spin physics program at the Relativistic Heavy Ion Collider (RHIC). Measurements from data taken by the PHENIX detector with transverse (p ↑ + p, p ↑ + Al, p ↑ + Au) and longitudinal (⃗p + ⃗p) proton polarization play an important role in this, in particular due to the leading order access to gluons in polarized protons. This is a crucially important counterpart to measurements made in lepton-hadron scattering experiments where gluons are not accessible at leading order. Transverse spin asymmetries provide insight into initial and final-state spinmomentum and spin-spin parton-hadron correlations, while longitudinal spin asymmetries provide access to parton polarization. A number of important recent PHENIX results and ongoing analyses will be presented, along with a discussion of how these measurements contribute to our understanding of proton spin structure.

The comparison of experimental data and theoretical predictions is important for our understanding of the mechanisms for interactions and particle production in hadron collisions, both at the Large Hadron Collider and at the Relativistic Heavy-Ion Collider experiments. Several tools were ideated to help with that. Rivet (Robust Independent Validation of Experiment and Theory) is a framework that facilitates the comparison between measurements from high-energy physics experiments and Monte Carlo event generators able to produce outputs using the HepMC package. Rivet contains a repository with analysis algorithms developed by experiments, providing analysis documentation and preservation. The recent developments for the implementation of centrality and multiplicity classes in Rivet are presented in this contribution.

By using gravity/gauge correspondence, we employ an Einstein-Maxwell-Dilaton model to compute the equilibrium and out-of-equilibrium properties of a hot and baryon rich strongly coupled quark-gluon plasma. The family of 5-dimensional holographic black holes, which are constrained to mimic the lattice QCD equation of state at zero density, is used to investigate the temperature and baryon chemical potential dependence of the equation of state. We also obtained the baryon charge conductivity, and the bulk and shear viscosities with a particular focus on the behavior of these observables on top of the critical end point and the line of first order phase transition predicted by the model.

The UA1 and CDF collaborations have reported the presence of events with a very extended structure of low momentum tracks filling in a uniform way the η − ϕ phase space. However, these events have not caused interest with respect to the details of the interaction that provoke them. We have undertaken to study whether such events are predicted by present event generators like PYTHIA 8. We report the existence of events without any discernible jetty structure in high-multiplicity pp collisions. In the simulations those events originate from several parton-parton scatterings within the same pp collision. We introduce the event shape ρ (f latenicity) as a tool to tag those events in experiments.

While it is well known that there is a significant amount of conserved charges in the initial state of nuclear collisions, the production of these due to gluon splitting has yet to be thoroughly investigated. The ICCING (Initial Conserved Charges in Nuclear Geometry) algorithm reconstructs these quark distributions, providing conserved strange, baryon, and electric charges, by sampling a given model for the g → qq¯ splitting function over the initial energy density, which is valid at top collider energies, even when µB = 0. The ICCING algorithm includes fluctuations in the gluon longitudinal momenta, a structure that supports the implementation of dynamical processes, and the c++ version is now open-source. A full analysis of parameter choices on the model has been done to quantify the effect these have on the underlying physics. We find there is a sustained difference across the different charges that indicates sensitivity to hot spot geometry.

The nuclear modification of J/ψ production has been studied at forward and backward rapidity in the collision systems p+Al, p+Au, and ³He+Au. A comparison of results for p+Au and ³He+Au is presented, with a focus on possible differences caused by the different particle multiplicity in the final state. The modification of ψ(2S) production in p+Au collisions has also been studied at forward and backward rapidity as a function of centrality. The ψ(2S) results complement earlier results at mid-rapidity at RHIC energies and results obtained at LHC energies, revealing a strong rapidity dependence of the ψ(2S) nuclear modification at RHIC energies that is indicative of final state effects.

We report on work aimed to describe the Λ and Λ¯ global polarizations in a heavy-ion collision environment using the core-corona model, where the source of these hyperons is a high-density core and a less dense corona. We show that the overall properties of the polarization excitation functions can be linked to the relative abundance of Λs and Λ¯s coming from the core versus those coming from the corona. Both global polarizations peak at collision energies √ sNN . 10 GeV. The exact positions and heights of these peaks depend not only on a reversal of relative abundances with collision energy, but also on the centrality class, both related to the QGP volume and lifetime.

These proceedings give a br.ief overview of the measurements of the effective temperature of the quark-gluon plasma.

The proposed Electron-Ion Collider (EIC) will operate high-energy high-luminosity electron+proton and electron+nucleus collisions to solve several unresolved fundamental questions. Due to their large masses (mc,b > ΛQCD), heavy quarks and their hadron products are ideal probes to study the nucleon/nuclear parton distribution functions in the high Bjorken-x (xBJ > 0.1) region and explore the hadronization process within the unconstrained kinematic region. Recently, the Electron-Ion Collider Comprehensive Chromodynamics Experiment (ECCE) consortium detector conceptual design has been selected as the reference design for the EIC project detector. The precise momentum and spatial resolutions provided by the ECCE tracking detector enable a series of open heavy flavor and quarkonia measurements. The physics projections of these proposed heavy flavor measurements in simulation studies using the ECCE detector design will be presented.

In heavy-ion collisions, fluctuations of conserved charges are known to be sensitive observables to probe criticality for the QCD phase transition and to locate the position of the putative critical end point (CEP). In this work we seek to show that the Linear Sigma Model with quarks produces an effective description of the QCD phase diagram in which deviations from a Hadron Resonance Gas are due to plasma screening effects, encoded in the contribution of the ring diagrams. Accounting for these, it is possible to include in the description the effect of long-range correlations. To set the model parameters we use LQCD results for the crossover transition at vanishing chemical potential. Finally, studying baryon number fluctuations from the model, we show that the CEP can be located within the HADES and/or the lowest end of the NICA energy domain, √ sNN ∼ 2 GeV.

Light-nuclei production yields in heavy-ion collisions are well described in the framework of Statistical Hadronization Models (SHM) but a thorough understanding of the underlying dynamics is still missing. In a complementary approach, synthesis of light nuclei can be modeled in terms of final-state coalescence of nucleons. While yielding an equally good description in central heavy-ion collisions, coalescence predictions are substantially different to those from SHM in small collision systems, in particular for the loosely bound hypertriton. This should allow a firm distinction of the two production scenarios in small collision systems. Comprehensive data on light-nuclei and hypertriton production in pp and p–Pb collisions from the ALICE Collaboration are presented in this contribution. Complementary to the measurement of production yields, the dynamics of nuclear cluster formation can be inferred from the measurement of final-state correlations of nucleons and light nuclei. Preliminary p-d correlation results from high-multiplicity pp collisions at √ 11 s = 13 TeV are compared to calculations based on experimental scattering parameters and discussed in the context of nuclear cluster formation.

The sPHENIX detector, being constructed at BNL’s Relativistic Heavy Ion Collider (RHIC), will begin measuring a plethora of Heavy Flavor and Quarkonia observables with unprecedented statistics and kinematic reach at RHIC energies starting in 2023. This includes the largest recorded sample of b-flavored hadron decays from Heavy Ion collisions at RHIC, allowing for precise probes of the QGP using charm and beauty quarks. These measurements are enabled by the excellent vertexing of the MAPS-based micro-VerTeX detector (MVTX), timing of the INTermediate silicon strip Tracker (INTT), precision tracking by the Time Projection Chamber (TPC), and the ElectroMagnetic and Hadronic Calorimetry systems (EMCal and HCal, respectively), the latter of which is deployed for the first time at RHIC. The sPHENIX collaboration has created the reconstruction software stack as well as realistic data simulations, which allow for testing and optimization of the software and physics selections.

The finite nuclear thickness affects the energy density (t) and conserved-charge densities such as the net-baryon density nB(t) produced in heavy ion collisions. While the effect is small at high collision energies where the Bjorken energy density formula for the initial state is valid, the effect is large at low collision energies, where the nuclear crossing time is not small compared to the parton formation time. The temperature T(t) and chemical potentials µ(t) of the dense matter can be extracted from the densities for a given equation of state (EOS). Therefore, including the nuclear thickness is essential for the determination of the T-µB trajectory in the QCD phase diagram for relativistic nuclear collisions at low to moderate energies such as the RHIC-BES energies. In this proceeding, we will first discuss our semi-analytical method that includes the nuclear thickness effect and its results on the densities є(t), nB(t), nQ(t), and nS(t). Then, we will show the extracted T(t), µB(t), µQ(t), and µS(t) for a quark-gluon plasma using the ideal gas EOS with quantum or Boltzmann statistics. Finally, we will show the results on the T-µB trajectories in relation to the possible location of the QCD critical end point. This semi-analytical model provides a convenient tool for exploring the trajectories of nuclear collisions in the QCD phase diagram.

Recent lattice QCD results, comparing to a hadron resonance gas model, have shown the need for hundreds of particles in hadronic models. These extra particles influence both the equation of state and hadronic interactions within hadron transport models. Here, we introduce the PDG21+ particle list, which contains the most up-to-date database of particles and their properties. We then convert all particles decays into 2 body decays so that they are compatible with SMASH in order to produce a more consistent description of a heavy-ion collision.

We present a study of the demographics of major conferences in heavy ion physics. We look at the distribution of talks by gender for Quark Matter, Strangeness in Quark Matter, Initial Stages, and Hard Probes between 2011–2022. We find that women are often underrepresented among plenary speakers and usually underrepresented among parallel speakers. At Quark Matter, women are more likely to be given a poster presentation in lieu of an oral presentation. The Quark Matter summary talk has never been given by a woman. We discuss the collection of data and possible approaches to make the field more equitable and, therefore, more scientifically productive.