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We study parton energy-momentum exchange with the quark gluon plasma (QGP) within a multistage approach composed of in-medium DGLAP evolution at high virtuality, and (linearized) Boltzmann Transport formalism at lower virtuality. This multistage simulation is then calibrated in comparison with high $p_T$ charged hadrons, D-mesons, and the inclusive jet nuclear modification factors, using Bayesian model-to-data comparison, to extract the virtuality-dependent transverse momentum broadening transport coefficient $\hat{q}$. To facilitate this undertaking, we develop a quantitative metric for validating the Bayesian workflow, which is used to analyze the sensitivity of various model parameters to individual observables. The usefulness of this new metric in improving Bayesian model emulation is shown to be highly beneficial for future such analyses., 24 pages, 16 figures

We utilize event-by-event Monte Carlo simulations within the JETSCAPE framework to examine scale-dependent jet-medium interactions in heavy-ion collisions. The reduction in jet-medium interaction during the early high-virtuality stage, where the medium is resolved at a short distance scale, is emphasized as a key element in explaining multiple jet observables, particularly substructures, simultaneously. By employing the MATTER+LBT setup, which incorporates this explicit reduction of medium effects at high virtuality, we investigate jet substructure observables, such as Soft Drop groomed observables. When contrasted with existing data, our findings spotlight the significant influence of the reduction at the early high-virtuality stages. Furthermore, we study the substructure of gamma-tagged jets, providing predictive insights for future experimental analyses. This broadens our understanding of the various contributing factors involved in modifying jet substructures., 6 pages, 4 figures, contribution to the Hard Probes 2023 proceedings

Hot QCD physics studies the nuclear strong force under extreme temperature and densities. Experimentally these conditions are achieved via high-energy collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). In the past decade, a unique and substantial suite of data was collected at RHIC and the LHC, probing hydrodynamics at the nucleon scale, the temperature dependence of the transport properties of quark-gluon plasma, the phase diagram of nuclear matter, the interaction of quarks and gluons at different scales and much more. This document, as part of the 2023 nuclear science long range planning process, was written to review the progress in hot QCD since the 2015 Long Range Plan for Nuclear Science, as well as highlight the realization of previous recommendations, and present opportunities for the next decade, building on the accomplishments and investments made in theoretical developments and the construction of new detectors. Furthermore, this document provides additional context to support the recommendations voted on at the Joint Hot and Cold QCD Town Hall Meeting, which are reported in a separate document., 190 pages, 69 figures

We present a new study of jet interactions in the quark-gluon plasma created in high-energy heavy-ion collisions, using a multistage event generator within the JETSCAPE framework. We focus on medium-induced modifications in the rate of inclusive jets and high transverse momentum (high-$p_{\mathrm{T}}$) hadrons. Scattering-induced jet energy loss is calculated in two stages: A high virtuality stage based on the MATTER model, in which scattering of highly virtual partons modifies the vacuum radiation pattern, and a second stage at lower jet virtuality based on the LBT model, in which leading partons gain and lose virtuality by scattering and radiation. Coherence effects that reduce the medium-induced emission rate in the MATTER phase are also included. The TRENTo model is used for initial conditions, and the (2+1)dimensional VISHNU model is used for viscous hydrodynamic evolution. Jet interactions with the medium are modeled via 2-to-2 scattering with Debye screened potentials, in which the recoiling partons are tracked, hadronized, and included in the jet clustering. Holes left in the medium are also tracked and subtracted to conserve transverse momentum. Calculations of the nuclear modification factor ($R_{\mathrm{AA}}$) for inclusive jets and high-$p_{\mathrm{T}}$ hadrons are compared to experimental measurements at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC). Within this framework, we find that with one extra parameter which codifies the transition between stages of jet modification -- along with the typical parameters such as the coupling in the medium, the start and stop criteria etc. -- we can describe these data at all energies for central and semicentral collisions without a rescaling of the jet transport coefficient $\hat{q}$., Comment: 34 pages, 23 figures

Shower development dynamics for a jet traveling through the quark-gluon plasma (QGP) is a multiscale process, where the heavy flavor mass is an important scale. During the high virtuality portion of the jet evolution in the QGP, emission of gluons from a heavy flavor is modified owing to heavy quark mass. Medium-induced radiation of heavy flavor is sensitive to microscopic processes (e.g. diffusion), whose virtuality dependence is phenomenologically explored in this study. In the lower virtuality part of shower evolution, i.e. when the mass is comparable to the virtuality of the parton, scattering and radiation processes of heavy quarks differ from light quarks. The effects of these mechanisms on shower development in heavy flavor tagged showers in the QGP is explored here. Furthermore, this multiscale study examines dynamical pair production of heavy flavor (via virtual gluon splittings) and their subsequent evolution in the QGP, which is not possible otherwise. A realistic event-by-event simulation is performed using the JETSCAPE framework. Energy-momentum exchange with the medium proceeds using a weak coupling recoil approach. Using leading hadron and open heavy flavor observables, differences in heavy versus light quark energy-loss mechanisms are explored, while the importance of heavy flavor pair production is highlighted along with future directions to study., Comment: 6 pages, 3 figures, contribution to the Hard Probes 2023 proceedings

We use a Bayesian-calibrated multistage viscous hydrodynamic model to explore deuteron yield, mean transverse momentum and flow observables in LHC Pb-Pb collisions. We explore theoretical uncertainty in the production of deuterons, including (i) the contribution of thermal deuterons, (ii) models for the subsequent formation of deuterons (hadronic transport vs coalescence) and (iii) the overall sensitivity of the results to the hydrodynamic model -- in particular to bulk viscosity, which is often neglected in studies of deuteron production. Using physical parameters set by a comparison to only light hadron observables, we find good agreement with measurements of the mean transverse momentum $\langle p_T \rangle$ and elliptic flow $v_2$ of deuterons; however, tension is observed with experimental data for the deuteron multiplicity in central collisions. The results are found to be sensitive to each of the mentioned theoretical uncertainties, with a particular sensitivity to bulk viscosity, indicating that the latter is an important ingredient for an accurate treatment of deuteron production., Comment: 11 pages, 8 figures

We report a new determination of $\hat{q}$, the jet transport coefficient of the Quark-Gluon Plasma. We use the JETSCAPE framework, which incorporates a novel multi-stage theoretical approach to in-medium jet evolution and Bayesian inference for parameter extraction. The calculations, based on the MATTER and LBT jet quenching models, are compared to experimental measurements of inclusive hadron suppression in Au+Au collisions at RHIC and Pb+Pb collisions at the LHC. The correlation of experimental systematic uncertainties is accounted for in the parameter extraction. The functional dependence of $\hat{q}$ on jet energy or virtuality and medium temperature is based on a perturbative picture of in-medium scattering, with components reflecting the different regimes of applicability of MATTER and LBT. In the multi-stage approach, the switch between MATTER and LBT is governed by a virtuality scale $Q_0$. Comparison of the posterior model predictions to the RHIC and LHC hadron suppression data shows reasonable agreement, with moderate tension in limited regions of phase space. The distribution of $\hat{q}/T^3$ extracted from the posterior distributions exhibits weak dependence on jet momentum and medium temperature $T$, with 90\% Credible Region (CR) depending on the specific choice of model configuration. The choice of MATTER+LBT, with switching at virtuality $Q_0$, has 90\% CR of $240$ GeV/c. The value of $Q_0$, determined here for the first time, is in the range 2.0-2.7 GeV., Comment: Published in Phys Rev C

We present a new determination of $\hat{q}$, the jet transport coefficient of the quark-gluon plasma. Using the JETSCAPE framework, we use Bayesian parameter estimation to constrain the dependence of $\hat{q}$ on the jet energy, virtuality, and medium temperature from experimental measurements of inclusive hadron suppression in Au-Au collisions at RHIC and Pb-Pb collisions at the LHC. These results are based on a multi-stage theoretical approach to in-medium jet evolution with the MATTER and LBT jet quenching models. The functional dependence of $\hat{q}$ on jet energy, virtuality, and medium temperature is based on a perturbative picture of in-medium scattering, with components reflecting the different regimes of applicability of MATTER and LBT. The correlation of experimental systematic uncertainties is accounted for in the parameter extraction. These results provide state-of-the-art constraints on $\hat{q}$ and lay the groundwork to extract additional properties of the quark-gluon plasma from jet measurements in heavy-ion collisions., Comment: contribution to the 2021 QCD session of the 55th Recontres de Moriond

It is now well established that jet modification is a multistage effect; hence a single model alone cannot describe all facets of jet modification. The JETSCAPE framework is a multistage framework that uses several modules to simulate different stages of jet propagation through the QGP medium. These simulations require a set of parameters to ensure a smooth transition between stages. We fine tune these parameters to successfully describe a variety of observables, such as the nuclear modification factors of leading hadrons and jets, jet shape, and jet fragmentation function. Photons can be produced in the hard scattering or as radiation from quarks inside jets. In this work, we study photon-jet transverse momentum imbalance and azimuthal correlation for both $p-p$ and $Pb-Pb$ collision systems. All the photons produced in each event, including the photons from hard scattering, radiation from the parton shower, and radiation from hadronization are considered with an isolation cut to directly compare with experimental data. The simulations are conducted using the same set of tuned parameters as used for the jet analysis. No new parameters are introduced or tuned. We demonstrate a significantly improved agreement with photons from $Pb-Pb$ collisions compared to prior efforts. This work provides an independent, parameter free verification of the multistage evolution framework., 4 pages, 7 figures

The dynamics of shower development for a jet traveling through the QGP involves a variety of scales, one of them being the heavy quark mass. Even though the mass of the heavy quarks plays a subdominant role during the high virtuality portion of the jet evolution, it does affect longitudinal drag and diffusion, stimulating additional radiation from heavy quarks. These emissions partially compensate the reduction in radiation from the dead cone effect. In the lower virtuality part of the shower, when the mass is comparable to the transverse momenta of the partons, scattering and radiation processes off heavy quarks differ from those off light quarks. All these factors result in a different nuclear modification factor for heavy versus light flavors and thus for heavy-flavor tagged jets. In this study, the heavy quark shower evolution and the fluid dynamical medium are modeled on an event by event basis using the JETSCAPE Framework. We present a multi-stage calculation that explores the differences between various heavy quark energy-loss mechanisms within a realistically expanding quark-gluon plasma (QGP). Inside the QGP, the highly virtual and energetic portion of the shower is modeled using the MATTER generator, while the LBT generator models the showers induced by energetic and close-to-on-shell heavy quarks. Energy-momentum exchange with the medium, essential for the study of jet modification, proceeds using a weak coupling recoil approach. The JETSCAPE framework allows for transitions, on the level of individual partons, from one energy-loss prescription to the other depending on the parton's energy and virtuality and the local density. This allows us to explore the effect and interplay between the different regimes of energy loss on the propagation and radiation from hard heavy quarks in a dense medium., Comment: arXiv admin note: substantial text overlap with arXiv:2002.06643

Multistage models based on relativistic viscous hydrodynamics have proven successful in describing hadron measurements from relativistic nuclear collisions. These measurements are sensitive to the shear and the bulk viscosities of QCD and provide a unique opportunity to constrain these transport coefficients. Bayesian analyses can be used to obtain systematic constraints on the viscosities of QCD, through methodical model-to-data comparisons. In this manuscript, we discuss recent developments in Bayesian analyses of heavy ion collision data. We highlight the essential role of closure tests in validating a Bayesian analysis before comparison with measurements. We discuss the role of the emulator that is used as proxy for the multistage theoretical model. We use an ongoing Bayesian analysis of soft hadron measurements by the JETSCAPE Collaboration as context for the discussion., Comment: 4 pages, 1 figure, contribution to the Quark Matter 2019 proceedings

We study the medium response to jet evolution in the quark-gluon plasma within the JETSCAPE framework. Recoil partons' medium response in the weakly coupled description is implemented in the multi-stage jet energy-loss model in the framework. As a further extension, the hydrodynamic description is rearranged to include in-medium jet transport based on a strong-coupling picture. To interface hydrodynamics with jet energy-loss models, the hydrodynamic source term is modeled by a causal formulation employing the relativistic diffusion equation. The jet shape and fragmentation function are studied via realistic simulations with weakly coupled recoils. We also demonstrate modifications in the medium caused by the hydrodynamic response., Comment: 4 pages, 2 figures, contribution to the Quark Matter 2019 proceedings

"Hybrid Hadronization" is a new Monte Carlo package to hadronize systems of partons. It smoothly combines quark recombination applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems, following the picture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization integrates with PYTHIA 8 and can be applied to a variety of systems from $e^++e^-$ to $A+A$ collisions. It takes systems of partons and their color flow information, for example from a Monte Carlo parton shower generator, as input. In addition, if for $A+A$ collisions a thermal background medium is provided, the package allows sampling thermal partons that contribute to hadronization. Hybrid Hadronization is available for use as a standalone code and is also part of JETSCAPE since the 2.0 release. In these proceedings we review the physics concepts underlying Hybrid Hadronization and demonstrate how users can use the code with various parton shower Monte Carlos. We present calculations of hadron chemistry and fragmentation functions in small and large systems when Hybrid Hadronization is combined with parton shower Monte Carlos MATTER and LBT. In particular, we discuss observable effects of the recombination of shower partons with thermal partons., Comment: 4 pages, 3 figures, Proceedings of Hard Probes 2020, 1-6 June 2020, Austin, Texas; Updated Author list

We present a jet quenching model within a unified multi-stage framework and demonstrate for the first time a simultaneous description of leading hadrons, inclusive jets, and elliptic flow observables which spans multiple centralities and collision energies. This highlights one of the major successes of the JETSCAPE framework in providing a tool for setting up an effective parton evolution that includes a high-virtuality radiation dominated energy loss phase (MATTER), followed by a low-virtuality scattering dominated (LBT) energy loss phase. Measurements of jet and charged-hadron $R_{AA}$ set strong constraints on the jet quenching model. Jet-medium response is also included through a weakly-coupled transport description., Comment: 4 pages, 4 figures, contribution to the Quark Matter 2019 proceedings

We present a multi-stage model for jet evolution through a quark-gluon plasma within the JETSCAPE framework. The multi-stage approach in JETSCAPE provides a unified description of distinct phases in jet shower contingent on the virtuality. We demonstrate a simultaneous description of leading hadron and integrated jet observables as well as jet $v_n$ using tuned parameters. Medium response to the jet quenching is implemented based on a weakly-coupled recoil prescription. We also explore the cone-size dependence of jet energy loss inside the plasma., Comment: 4 pages, 4 figures, Proceedings of Hard Probes 2020, 1-6 June 2020, Austin, Texas

The JETSCAPE framework is a modular and versatile Monte Carlo software package for the simulation of high energy nuclear collisions. In this work we present a new tune of JETSCAPE, called PP19, and validate it by comparison to jet-based measurements in $p+p$ collisions, including inclusive single jet cross sections, jet shape observables, fragmentation functions, charged hadron cross sections, and dijet mass cross sections. These observables in $p+p$ collisions provide the baseline for their counterparts in nuclear collisions. Quantifying the level of agreement of JETSCAPE results with $p+p$ data is thus necessary for meaningful applications of JETSCAPE to A+A collisions. The calculations use the JETSCAPE PP19 tune, defined in this paper, based on version 1.0 of the JETSCAPE framework. For the observables discussed in this work calculations using JETSCAPE PP19 agree with data over a wide range of collision energies at a level comparable to standard Monte Carlo codes. These results demonstrate the physics capabilities of the JETSCAPE framework and provide benchmarks for JETSCAPE users., 23 pages, 23 figures; v1.1: minor bug fixes in author information

Measurements indicate that the centrality dependence of the direct photon multiplicity scales approximately with the number of binary nucleon collisions. Importantly, these same measurements suggest that this scaling does not depend on the cutoff used to define the photon multiplicity. In this work we provide a theoretical perspective on these observations. We emphasize the importance of stronger experimental constraints, in particular for the cutoff independence of the centrality scaling. We highlight the importance of revisiting theoretical studies of jet-medium photons and of medium-modified prompt photons to clarify their contribution to the direct photon multiplicity, as well as the magnitude and direction of their momentum anisotropy., Contribution to Hard Probes 2018 proceedings. 4 pages

The JETSCAPE Collaboration has recently announced the first release of the JETSCAPE package that provides a modular, flexible, and extensible Monte Carlo event generator. This innovative framework makes it possible to perform a comprehensive study of multi-stage high-energy jet evolution in the Quark-Gluon Plasma. In this work, we illustrate the performance of the event generator for different algorithmic approaches to jet energy loss, and reproduce the measurements of several jet and hadron observables as well as correlations between the hard and soft sector. We also carry out direct comparisons between different approaches to energy loss to study their sensitivity to those observables.

The interaction of heavy flavor with the quark-gluon plasma (QGP) in relativistic heavy-ion collisions is studied using JETSCAPE, a publicly available software package containing a framework for Monte Carlo event generators. Multi-stage (and multi-model) evolution of heavy quarks within JETSCAPE provides a cohesive description of heavy flavor quenching inside the QGP. As the parton shower develops, a model becomes active as soon as its kinematic region of validity is reached. Two combinations of heavy-flavor energy-loss models are explored within a realistic QGP medium, using parameters which were tuned to describe {\it light-flavor} partonic energy-loss., 4 pages, 2 figures, contribution to the Quark Matter 2019 proceedings

The modification of jet substructure in relativistic heavy-ion collisions is studied using JETSCAPE, a publicly available software package containing a framework for Monte Carlo event generators. Multi-stage jet evolution in JETSCAPE provides an integrated description of jet quenching by combining multiple models, with each becoming active at a different stage of the parton shower evolution. Jet substructure modification due to different aspects of jet quenching is studied using jet shape and jet fragmentation observables. Various combinations of jet energy loss models are exploed, with medium background provided by (2 + 1)-D VISHNU with TRENTo+freestreaming initial conditions. Results reported here are from simulations performed within JETSCAPE framework., Comment: 4 pages, 3 figures, Proceedings of Hard Probes 2018, 30 September-5 October, Aix-Les-Bains, France

Using bifurcation theory, we study the secular resonances induced by the Sun and Moon on space debris orbits around the Earth. In particular, we concentrate on a special class of secular resonances, which depend only on the debris' orbital inclination. This class is typically subdivided into three distinct types of secular resonances: those occurring at the critical inclination, those corresponding to polar orbits, and a third type resulting from a linear combination of the rates of variation of the argument of perigee and the longitude of the ascending node. The model describing the dynamics of space debris includes the effects of the geopotential, as well as the Sun's and Moon's attractions, and it is defined in terms of suitable action-angle variables. We consider the system averaged over both the mean anomaly of the debris and those of the Sun and Moon. Such multiply-averaged Hamiltonian is used to study the lunisolar resonances which depend just on the inclination. Borrowing the technique from the theory of bifurcations of Hamiltonian normal forms, we study the birth of periodic orbits and we determine the energy thresholds at which the bifurcations of lunisolar secular resonances take place. This approach gives us physically relevant information on the existence and location of the equilibria, which help us to identify stable and unstable regions in the phase space. Besides their physical interest, the study of inclination dependent resonances offers interesting insights from the dynamical point of view, since it sheds light on different phenomena related to bifurcation theory., 39 pages, 10 figures

We consider the thermal production of dileptons and photons at temperatures above the critical temperature in QCD. We use a model where color excitations are suppressed by a small value of the Polyakov loop, the semi Quark-Gluon Plasma (QGP). Comparing the semi-QGP to the perturbative QGP, we find a mild enhancement of thermal dileptons. In contrast, to leading logarithmic order in weak coupling there are far fewer hard photons from the semi-QGP than the usual QGP. To illustrate the possible effects on photon and dileptons production in heavy ion collisions, we integrate the rate with a realistic hydrodynamic simulation. Dileptons uniformly exhibit a small flow, but the strong suppression of photons in the semi-QGP tends to bias the elliptical flow of photons to that generated in the hadronic phase., Comment: 5 pages, 5 figures

We use the framework of the HIJING/B\=B v2.0 model to simulate high-multiplicity (HM) $p+p$ collision events at the Large Hadron Collider (LHC) to study observables sensitive to possible collective phenomena, such as strong longitudinal color fields (SLCF) modeled by an enhanced string tension ($\kappa$). We focus on the hyperon/meson yield ratios at center-of-mass (c.m.) energy $\sqrt{s}$ = 7 TeV, in the transverse momentum region, $1 < p_T < 4 $ GeV/{\it c}. For minimum bias events these ratios are well described assuming an energy dependence $\kappa = \kappa(s)= \kappa_{0} (s/s_{0})^{0.04} {\rm GeV/fm}$ ($\kappa_{0}$= 1 GeV/fm), giving a value $\kappa = 2$ GeV/fm at $\sqrt{s}$ = 7 TeV. We compare minimum bias (MB) events to simulated HM events assuming that $\kappa(MB)=2$ GeV/fm could grow to an extreme value of $\kappa(HM)=5$ GeV/fm that saturates the strangeness suppression factor. With this assumption the model predicts a very strong enhancement of (multi)strange baryon/meson ratios in HM events. If observed, such an enhancement could be also interpreted as a possible signature for formation in HM $p+p$ collision events of a deconfined but out of local thermal equilibrium {\em mini quark-gluon plasma} (mQGP)., Comment: version2, text modifications,new figures and references added; 22 pages, 8 figures. Accepted for publication, Phys. Rev. C

We study the effect of strong longitudinal color fields (SCF) in p+p reactions up to Large Hadron Collider energies in the framework of the HIJING/BBbar v2.0 model that combines (collinear factorized) pQCD multiple minijet production with soft longitudinal string excitation and hadronization. The default vacuum string tension, kappa0 = 1 GeV/fm, is replaced by an effective power law energy dependent string tension, that increases monotonically with center-of-mass energy. The exponent 0.06 is found sufficient to reproduce well the energy dependence of multiparticle observables in RHIC, Tevatron, as well as recent LHC data. This exponent is found to be only half of that predicted by the Color Glass Saturation model, lambda(CGC)=0.115, where gluon fusion multiparticle production mechanisms are assumed. In HIJING/BBbar v2.0, the rapid growth of central-rapidity density with energy is due to the interplay of copious minijet production and increasing SCF contributions. The large (strange)baryon-to-meson ratios measured at Tevatron energies are well described. A significant enhancement of these ratios is predicted up to the highest LHC energy (14 TeV). The effect of JJbar loops and SCF on baryon-anti-baryon asymmetry and its relation to baryon number transport is also discussed., revtex4.1, 11 pages, 11 figures, updated references, slightly text modifications, accepted for publication in Phys. ReV. C (2011)

The spectral density of the phi meson in a hot bath of nucleons and pions is calculated by relating the vector meson self-energy to the forward scattering amplitude, which is constrained by experimental data. Dispersion techniques are used to verify the relationship between real and the imaginary parts of the in-medium self energy. The position of the spectral peak of the phi meson is found to be shifted from its vacuum position by only a small amount, but its width is considerably increased., Comment: 12 pages, 8 figures

The nuclear modification factor R_AA for charged hadron production at the LHC is predicted from jet energy loss induced by gluon bremsstrahlung. The Arnold, Moore, and Yaffe formalism is used, together with an ideal hydrodynamical model., 2 pages, 1 figure, contributed to Workshop on Heavy Ion Collisions at the LHC: Last Call for Predictions, Geneva, Switzerland, 14 May - 8 Jun 2007

Strangeness and multi-strangeness particles production can be used to explore the initial transient field fluctuations in heavy ion collisions. We emphasize the role played by Junction anti-Junction (JJbar) loops and strong color electric fields (SCF) in these collisions. Transient field fluctuations of SCF on the baryon production in central (0-5%) Pb+Pb collisions at nucleon-nucleon centre of mass energy of 5.5 TeV will be discussed in the framework of HIJING/BBbar v2.0 model, looking in particular to the predicted evolution of nuclear modification factors R_AA from RHIC to LHC energies. Our results indicate the importance of a good description of the baseline elementary p+p collisions at this energy., Comment: Comments: 3 pages, 2 figures, submitted to the CERN Theory Institute Workshop, Heavy Ion Collisions at the LHC. Last Call Predictions, May 14 - June 8, 2007

We investigate the energy dependence of hadron production and of stopping power based on HIJING/BBbar v2.0 model calculations. Pseudorapidity spectra and transverse momentum distributions for produced charged particles as well as net baryons (per pair of partcipants) and their rapidity loss are compared to data at RHIC and predictions for LHC energies are discussed., Comment: 3 pages, 2 figures, submitted to the CERN Theory Institute Workshop, Heavy Ion Collisions at the LHC. Last Call Predictions, May 14 - June 8, 2007

We discuss real and virtual photon sources in heavy ion collisions and present results for dilepton yields in Pb+Pb collisions at the LHC at intermediate and large transverse momentum p_T., Comment: Contribution to "Heavy Ion Collisions at the LHC: Last Call for Predictions", to appear in J. Phys. G; 2 pages, 2 figures

The effect of fluctuations of SCF on the baryon production in d+Au and Au+Au collisions at 200A GeV is studied in the framework of HIJING/BB v2.0 model. An increase of the string tension from kappa_0= 1 GeV/fm, to in medium mean values of 1.5 to 2.0 GeV/fm and 2.0 to 3.0 GeV/fm, for d+Au and Au+Au respectively, results in a consistent description of the observed nuclear modification factors R_dAu and R_AuAu(that relates (d)Au+Au and p+p colllisions) and point to the relevance of fluctuations of transient color fields. The differences between nuclear modification factors R_AuAu and R_CP (that relates central and peripheral collisions) are also discussed. The measurement of multi-strange (anti)hyperons (\Xi, \Omega) yields would provide a crucial test of the importance of SCF fluctuations at RHIC energies., Comment: 13 pages,Latex (revtex4), 11 figures, 2 tables; abstract slightly modified, text modification, revised version accepted for publication in Phys. Rev. C

A new version, v2.0, of the HIJING/BB Monte Carlo nuclear collision event generator is introduced in order to explore further the possible role of baryon junctions loops in the baryon/meson anomaly (2 $< p_{T}, 27 pages, Latex(revtex), 8 figures

The original default mini-jet scale p_0=2 GeV/c and energy loss, dE/dx=2 GeV/fm in HIJING lead to a too rapid growth of the multiplicity with energy. RQMD model without mini-jet leads on the other hand to a too slowly increasing multiplicity with energy. We show that a slight increase of p_0 from the default 2.0 GeV/c at sqrt(s)=130A GeV to 2.2 GeV/c at sqrt{s}=200A GeV is sufficient to account for the bulk observables. Jet quenching of pi^0 above p_T>4 GeV/c at 200A GeV is found to be well reproduced in central Au+Au by the original HIJING default prediction. However, the effective surface emission in the HIJING model formulation of energy loss over-predicts the quenching in more peripheral collisions. Neither HIJING nor HIJING/B{bar}B can account for the observed anomalous excess of moderate p_T, 36 pages,Latex(revtex), 13 figures, submitted to Phys. Rev. C

We study the possibility of $��-��$ mixing via N-N excitations in dense nuclear matter. This mixing induces a peak in the dilepton spectra at an invariant mass equal to that of the $��$. We calculate the cross section for dilepton production through the mixing process and we compare its size with that of $��-��$ annihilation. In-medium masses and mixing angles are also calculated., 6 pages, 5 figures

Triple-differential cross sections for neutrons from high-multiplicity La-La collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per nucleon were measured at several polar angles as a function of the azimuthal angle with respect to the reaction plane of the collision. The reaction plane was determined by a transverse-velocity method with the capability of identifying charged-particles with Z=1, Z=2, and Z > 2. The flow of neutrons was extracted from the slope at mid-rapidity of the curve of the average in-plane momentum vs the center-of-mass rapidity. The squeeze-out of the participant neutrons was observed in a direction normal to the reaction plane in the normalized momentum coordinates in the center-of-mass system. Experimental results of the neutron squeeze-out were compared with BUU calculations. The polar-angle dependence of the maximum azimuthal anisotropy ratio $r(\theta)$ was found to be insensitive to the mass of the colliding nuclei and the beam energy. Comparison of the observed polar-angle dependence of the maximum azimuthal anisotropy ratio $r(\theta)$ with BUU calculations for free neutrons revealed that $r(\theta)$ is insensitive also to the incompressibility modulus in the nuclear equation of state., Comment: ReVTeX, 16 pages, 17 figures. To be published in Physical Review C

Dilepton and photon production in heavy-ion collisions at SPS energies is studied in the relativistic transport model that incorporates self-consistently the change of hadron masses in dense matter. It is found that the dilepton spectra in proton-nucleus reactions can be well described by the conventional mechanisms of Dalitz decay, primary vector meson decay, decay of charmed mesons, and the initial Drell-Yan processes. However, to provide a quantitative explanation of the observed dilepton spectra in central heavy-ion collisions requires contributions other than these direct decays and also various medium effects. Introducing a decrease of vector meson masses in hot dense medium, we find that the low-mass dilepton enhancement can be satisfactorily explained. Furthermore, to explain the intermediate-mass dilepton enhancement in heavy-ion collisions, secondary processes such as $\pi a_1\to l{\bar l}$ are found to be very important. Finally, the single photon spectra in our calculations with either free or in-medium meson masses do not exceed the upper limit measured by the WA80 Collaboration., Comment: 19 pages, including 7 ps figures. Based on talk presented at the APCTP (Asia Pacifit Center for Theoretical Physics) Workshop on Hadrons in Medium, Seoul, Oct. 1997

We establish links between three different models for multifragmentation: the percolation model, the lattice gas model and the statistical multifragmentation model. There are remarkable similarities between the lattice gas model and the statistical multifragmentation model. For completeness, we also compare with a model based upon classical molecular dynamics which gives rather different results., 11 pages, ReVTeX type. 10 Postscript figures (included)

The traditional calculation of dilepton yields from bremsstrahlung relies on the assumption that electromagnetic and strong processes factorize. We argue that this assumption, embodied by the soft photon approximation, cannot hold true for invariant mass spectra on very general grounds. Deriving a formula for the dilepton cross section for pion-pion scattering that does not rely on such factorization, we formulate the problem exactly in terms of three-particle phase space invariants. Using a simple one boson exchange model for comparison, we find that dilepton cross sections and yields are generally overestimated by the soft photon approximation by factors 2--8. In extreme cases, overestimation up to a factor 40 is possible., Comment: Conference version for Busy Executives with no interest in detail. Latex, 10 pages, 11 .eps figures included via epsf

We have made a detailed and quantitative study of dilepton production via bremsstrahlung of a virtual photon during pion-pion collisions. Most calculations of electromagnetic radiation from strong interaction processes rely on the soft photon approximation (SPA). The conditions underlying this approximation are generally violated when dilepton spectra are calculated in terms of their invariant mass, so that an approach going beyond the SPA becomes necessary. Superseding previous derivations, we derive an exact formula for the bremsstrahlung cross section. The resulting formulation is compared to various forms based on the SPA, the two-particle phase space approximation and R\"uckl's formula using a relativistic One Boson Exchange (OBE) model. Within the OBE approach, we show that approximations to the bremsstrahlung dilepton cross sections often differ greatly from the exact result; discrepancies become greater both with rising temperature and with invariant mass. Integrated dilepton production rates are overestimated by R\"uckl-based approximations by factors 1.5--8.0. The largest discrepancies occur for the reaction $\pi^+\pi^+ \to \pi^+\pi^+\ell^+\ell^-$, where such approximations overestimate the exact rate by factors ranging from 2 to 30 for invariant masses between 10 and 500 MeV. Our findings, combined with recent estimates of the Landau-Pomeranchuk effect, indicate that bremsstrahlung dileptons rates in ultrarelativistic heavy ion collisions should be even more suppressed than had been thought before., Comment: REVTeX 3.0, 26 pages plus 15 figures in uuencoded PostScript

The JETSCAPE Collaboration reports a new determination of jet transport coefficients in the Quark-Gluon Plasma, using both reconstructed jet and hadron data measured at RHIC and the LHC. The JETSCAPE framework incorporates detailed modeling of the dynamical evolution of the QGP; a multi-stage theoretical approach to in-medium jet evolution and medium response; and Bayesian inference for quantitative comparison of model calculations and data. The multi-stage framework incorporates multiple models to cover a broad range in scale of the in-medium parton shower evolution, with dynamical choice of model that depends on the current virtuality or energy of the parton. We will discuss the physics of the multi-stage modeling, and then present a new Bayesian analysis incorporating it. This analysis extends the recently published JETSCAPE determination of the jet transport parameter $\hat{q}$ that was based solely on inclusive hadron suppression data, by incorporating reconstructed jet measurements of quenching. We explore the functional dependence of jet transport coefficients on QGP temperature and jet energy and virtuality, and report the consistency and tensions found for current jet quenching modeling with hadron and reconstructed jet data over a wide range in kinematics and $\sqrt{s_{\text{NN}}}$. This analysis represents the next step in the program of comprehensive analysis of jet quenching phenomenology and its constraint of properties of the QGP., Comment: 6 pages, 2 figures, contribution to the Quark Matter 2022 proceedings

We present predictions and postdictions for a wide variety of hard jet-substructure observables using a multi-stage model within the JETSCAPE framework. The details of the multi-stage model and the various parameter choices are described in [A. Kumar et al., arXiv:2204.01163]. A novel feature of this model is the presence of two stages of jet modification: a high virtuality phase (modeled using MATTER), where coherence effects diminish medium-induced radiation, and a lower virtuality phase (modeled using LBT), where parton splits are fully resolved by the medium as they endure multiple scattering induced energy loss. Energy loss calculations are carried out on event-by-event viscous fluid dynamic backgrounds constrained by experimental data. The uniformed and consistent descriptions of multiple experimental observables demonstrate the essential role of coherence effects and the multi-stage modeling of the jet evolution. Using the best choice of parameters from [A. Kumar et al., arXiv:2204.01163], and with no further tuning, we present calculations for the medium modified jet fragmentation function, the groomed jet momentum fraction $z_g$ and angular separation $r_g$ distributions, as well as the nuclear modification factor of groomed jets. These calculations provide accurate descriptions of published and preliminary data from experiments at RHIC and LHC. Furthermore, we provide predictions from the multi-stage model for future measurements at RHIC., 20 pages, 12 figures

We investigate the effect of nucleon-nucleon correlations on the initial condition of ultra-central heavy ion collisions at LHC energies. We calculate the eccentricities of the MC-Glauber and IP-Glasma models in the 0--1% centrality class and show that they are considerably affected by the inclusion of such type of correlations. For an IP-Glasma initial condition, we further demonstrate that this effect survives the fluid-dynamical evolution of the system and can be observed in its final state azimuthal momentum anisotropy., Comment: 6 pages, 4 figures

The JETSCAPE simulation framework is an overarching computational envelope for developing complete event generators for heavy-ion collisions. It allows for modular incorporation of a wide variety of existing and future software that simulates different aspects of a heavy-ion collision. The default JETSCAPE package contains both the framework, and an entire set of indigenous and third party routines that can be used to directly compare with experimental data. In this article, we outline the algorithmic design of the JETSCAPE framework, define the interfaces and describe the default modules required to carry out full simulations of heavy-ion collisions within this package. We begin with a description of the various physics elements required to simulate an entire event in a heavy-ion collision, and distribute these within a flowchart representing the event generator and statistical routines for comparison with data. This is followed by a description of the abstract class structure, with associated members and functions required for this flowchart to work. We then define the interface that will be required for external users of JETSCAPE to incorporate their code within this framework and to modify existing elements within the default distribution. We conclude with a discussion of some of the physics output for both $p$-$p$ and $A$-$A$ collisions from the default distribution, and an outlook towards future releases. In the appendix, we discuss various architectures on which this code can be run and outline our benchmarks on similar hardware., Comment: 93 pages, 13 figures

We explore jet-medium interactions at various scales in high-energy heavy-ion collisions using the JETSCAPE framework. The physics of the multi-stage modeling and the coherence effect at high virtuality is discussed through the results of multiple jet and high-$p_{\mathrm{T}}$ particle observables, compared with experimental data. Furthermore, we investigate the jet-medium interaction involved in the hadronization process., 6 pages, 5 figures, contribution to the Quark Matter 2022 proceedings