About

Prof. Steffen A. Bass is an Arts and Sciences Distinguished Professor of Physics at Duke University, with appointments in the Trinity College of Arts & Sciences since 2012. His research focuses on the intersection of theoretical nuclear and particle physics, particularly studying highly energetic collisions of heavy nuclei to create a primordial state of matter known as the Quark-Gluon-Plasma (QGP). This state of matter is characterized by extremely high temperatures and densities, resembling the conditions of the early Universe shortly after the Big Bang. His work involves understanding the properties of the QGP, including its nearly 'perfect liquid' behavior, through experimental data obtained from facilities such as the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory and the CERN Large Hadron Collider, as well as through computational modeling and phenomenology of ultra-relativistic heavy-ion collisions. Prof. Bass is recognized as a leading expert in the phenomenology of the QGP and in knowledge extraction from large-scale data sets via computational models. He has contributed significantly to the development of models describing these collisions and to the determination of the shear viscosity of the QGP. He is a member of the Divisions of Nuclear and Computational Physics of the American Physical Society, serves on the editorial board of the Journal of Physics G: Nuclear and Particle Physics, and has published over 160 peer-reviewed articles. In 2014, he was named Outstanding Referee for APS Journals and was elected a Fellow of the American Physical Society.

Research topics

• Physics
• Particle physics
• Computer Science
• Quantum mechanics
• Nuclear physics
• Statistical physics
• Thermodynamics
• Speech recognition
• Theoretical physics
• Chemistry
• Mathematical physics

Selected publications

• Effect of recoils on soft-drop-groomed observables in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -tagged jets in a multistage approach

  Physical review. C · 2026-02-06

  articleOpen access

  We investigate medium-induced modifications to jet substructure observables that characterize hard components in central Pb-Pb collisions at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msqrt> <a:msub> <a:mi>s</a:mi> <a:mrow> <a:mi>N</a:mi> <a:mi>N</a:mi> </a:mrow> </a:msub> </a:msqrt> <a:mo>=</a:mo> <a:mn>5.02</a:mn> </a:mrow> </a:math> TeV. Using a multistage Monte Carlo simulation of in-medium jet shower evolution, we explore flavor-dependent medium effects through simulations of inclusive and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>γ</b:mi> </b:math> -tagged jets. The results show that quark jets undergo a nonmonotonic modification compared with gluon jets in observables such as the Pb-Pb to <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>p</c:mi> <c:mtext>−</c:mtext> <c:mi>p</c:mi> </c:mrow> </c:math> ratio of the soft drop prong angle <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msub> <d:mi>r</d:mi> <d:mi>g</d:mi> </d:msub> </d:math> , the relative prong transverse momentum <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msub> <e:mi>k</e:mi> <e:mrow> <e:mi>T</e:mi> <e:mo>,</e:mo> <e:mi>g</e:mi> </e:mrow> </e:msub> </e:math> , and the groomed mass <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msub> <f:mi>m</f:mi> <f:mi>g</f:mi> </f:msub> </f:math> distributions. Due to this nonmonotonic modification, <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mi>γ</g:mi> </g:math> -tagged jets, enriched in quark jets, provide surprisingly clear signals of medium-induced structural modifications, distinct from effects dominated by selection bias. Further systematic studies demonstrate that these effects are dominated by recoil medium response. This work highlights the potential of hard substructures in <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mi>γ</h:mi> </h:math> -tagged jets as powerful tools for probing the jet-medium interactions in high-energy heavy-ion collisions, in particular by enabling detailed investigations of jet-medium parton scatterings via their associated medium response. All simulations for <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mi>γ</i:mi> </i:math> -tagged jet analyses carried out in this paper used triggered events containing at least one hard photon, which highlights the utility of these observables for future Bayesian analysis.

  PublisherDOI

• Open heavy-flavor transport and hadronization in heavy-ion collisions

  EPJ Web of Conferences · 2026-01-01

  articleOpen access

  We develop a comprehensive model for heavy-quark evolution in a realistic QGP, from their production in the initial collision to hadronic freeze-out. Heavy-quark transport is described by a Langevin approach including medium-induced radiation, coupled to a 2+1D viscous hydrodynamic bulk evolution. Transport coefficients are obtained from non-perturbative T -matrix calculations with resonant correlations near the transition temperature. Hadronization is implemented via two fragmentation+recombination schemes: an improved sudden coalescence model and a resonance recombination model. We present results for key open heavy-flavor observables, i.e., the nuclear modification factor and elliptic flow, and compare to LHC Pb-Pb data at √ s NN = 5.02 TeV.

  PublisherOA PDFDOI

• Transport-based initial conditions for heavy-ion collisions at finite densities

  Physical review. C · 2026-01-27 · 1 citations

  articleOpen access

  We employ the SMASH transport model to provide event-by-event initial conditions for the energy-momentum tensor and conserved charge currents in hydrodynamic simulations of relativistic heavy-ion collisions. We study the fluctuations and dynamical evolution of three conserved charge currents (net baryon, net electric charges, and net strangeness) with a 4D lattice-QCD-based equation of state, NEOS-4D, in the hydrodynamic phase. Out-of-equilibrium corrections at the particlization are generalized to finite densities to ensure the conservation of energy, momentum, and the three types of charges. These theoretical developments are integrated within the X-SCAPE code as a unified framework for studying the nuclear matter properties in the Beam Energy Scan program.

  PublisherOA PDFDOI

• Hybrid Hadronization -- A Study of In-Medium Hadronization of Jets

  ArXiv.org · 2025-01-27

  preprintOpen access

  QCD jets are considered important probes for quark gluon plasma created in collisions of nuclei at high energies. Their parton showers are significantly altered if they develop inside of a deconfined medium. Hadronization of jets is also thought to be affected by the presence of quarks and gluons. We present a systematic study of the effects of a thermal bath of partons on the hadronization of parton showers. We use the JETSCAPE framework to create parton showers both in vacuum and in a brick of quark gluon plasma. The brick setup allows important parameters, like the size of the plasma as well as the collective flow of partons, to be varied systematically. We hadronize the parton showers using Hybrid Hadronization, which permits shower partons to form strings with thermal partons, or to recombine directly with thermal partons as well as with each other. We find a sizeable amount of interaction of shower partons with thermal partons during hadronization, indicating a natural continuation of the interaction of jet and medium during this stage. The observed effects grow with the size of the medium. Collective flow easily transfers from the thermal partons onto the emerging jet hadrons. We also see a significant change in hadron chemistry as expected in the presence of quark recombination processes.

  PublisherOA PDFDOI

• Additive Multi-Index Gaussian Process Modeling, with Application to Multi-Physics Surrogate Modeling of the Quark-Gluon Plasma

  Journal of the American Statistical Association · 2025-07-23 · 1 citations

  articleSenior author
  PublisherDOI

• Signals for fluctuating constituent numbers in small systems

  ArXiv.org · 2025-08-28

  preprintOpen accessSenior author

  We propose an extension of the initial condition model TRENTo for sampling the number of partons inside the nucleons that participate in a heavy-ion collision. This sampling method is based on parton distribution functions (PDFs) and therefore has a natural dependence on the momentum transferred in the collision and the scale being probed during the collision. We examine the resulting distributions and their dependence on the momentum transfer. Additionally, we explore the sensitivity of different observables on the number of partons using the TRENTo framework and the estimators available therein for final-state observables.

  PublisherOA PDFDOI

• Bayesian inference analysis of jet quenching using inclusive jet and hadron suppression measurements

  Physical review. C · 2025-05-15 · 14 citations

  articleOpen access

  The Collaboration reports a new determination of the jet transport parameter <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mover accent="true"><a:mi>q</a:mi><a:mo>̂</a:mo></a:mover></a:math> in the quark-gluon plasma (QGP) using Bayesian inference, incorporating all available inclusive hadron and jet yield suppression data measured in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC). This multi-observable analysis extends the previously published Bayesian inference determination of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mover accent="true"><c:mi>q</c:mi><c:mo>̂</c:mo></c:mover></c:math>, which was based solely on a selection of inclusive hadron suppression data. is a modular framework incorporating detailed dynamical models of QGP formation and evolution, and jet propagation and interaction in the QGP. Virtuality-dependent partonic energy loss in the QGP is modeled as a thermalized weakly coupled plasma, with parameters determined from Bayesian calibration using soft-sector observables. This Bayesian calibration of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mover accent="true"><e:mi>q</e:mi><e:mo>̂</e:mo></e:mover></e:math> utilizes active learning, a machine-learning approach, for efficient exploitation of computing resources. The experimental data included in this analysis span a broad range in collision energy and centrality, and in transverse momentum. In order to explore the systematic dependence of the extracted parameter posterior distributions, several different calibrations are reported, based on combined jet and hadron data; on jet or hadron data separately; and on restricted kinematic or centrality ranges of the jet and hadron data. Tension is observed in comparison of these variations, providing new insights into the physics of jet transport in the QGP and its theoretical formulation.

  PublisherOA PDFDOI

• Soft-hard framework with exact four-momentum conservation for small systems

  Physical review. C · 2025-06-17 · 5 citations

  articleOpen access

  A new framework, called , for the combined study of both hard and soft transverse momentum sectors in high-energy proton-proton (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>p</a:mi><a:mtext>−</a:mtext><a:mi>p</a:mi></a:mrow></a:math>) and proton-nucleus (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>p</b:mi><b:mtext>−</b:mtext><b:mi>A</b:mi></b:mrow></b:math>) collisions is set up. A dynamical initial state is set up using the model with transverse locations of hotspots within each incoming nucleon. A hard scattering that emanates from two colliding hotspots is carried out using the Pythia generator. Initial state radiation from the incoming hard partons is carried out in a new module called , which includes the longitudinal location of initial splits. The energy-momentum of both the initial hard partons and their associated beam remnants is removed from the hot spots, depleting the energy-momentum available for the formation of the bulk medium. Outgoing showers are simulated using the generator, and results are presented for both cases, allowing for and not allowing for energy loss. First comparisons between this hard-soft model and single inclusive hadron and jet data from <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>p</c:mi><c:mtext>−</c:mtext><c:mi>p</c:mi></c:mrow></c:math> and minimum bias <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>p</d:mi><d:mtext>−</d:mtext><d:mtext>Pb</d:mtext></d:mrow></d:math> collisions are presented. Single hadron spectra in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>p</e:mi><e:mtext>−</e:mtext><e:mi>p</e:mi></e:mrow></e:math> are used to carry out a limited (in number of parameters) Bayesian calibration of the model. Fair comparisons with data are indicative of the utility of this new framework. Theoretical studies of the correlation between jet <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>p</f:mi><f:mi>T</f:mi></f:msub></f:math> and event activity at mid and forward rapidity are carried out.

  PublisherDOI

• Quark flavor equilibration of the quark-gluon plasma

  arXiv (Cornell University) · 2025-01-11

  preprintOpen access

  The early stage of a heavy-ion collision is marked by rapid entropy production and the transition from a gluon saturated initial condition to a plasma of quarks and gluons that evolves hydrodynamically. However, during the early times of the hydrodynamic evolution, the chemical composition of the QCD medium is still largely unknown. We present a study of quark chemical equilibration in the (Q)GP using a novel model of viscous hydrodynamic evolution in partial chemical equilibrium. Motivated by the success of gluon saturated initial condition models, we initialize the QCD medium as a completely gluon dominated state. Local quark production during the hydrodynamic phase is then simulated through the evolution of time-dependent fugacities for each independent quark flavor, with the timescales set as free parameters to compare different rates of equilibration. We present the results of complete heavy-ion collision simulations using this partial chemical equilibrium model, and show the effects on hadronic and electromagnetic observables. In particular, we show that the development of flow is sensitive to the equilibration timescale, providing an empirical way to probe the chemical equilibration of the QCD medium.

  PublisherOA PDFDOI

• Interplay of prompt and non-prompt photons in photon-triggered jet observables

  ArXiv.org · 2025-07-01

  preprintOpen access

  Prompt photons are important yet challenging to observe in relativistic heavy-ion collisions, as they are produced in the early stages and traverse almost the entire QGP medium without interaction. Experimental analyses typically employ isolation cuts, in the hope to identify prompt photons. Most theoretical studies consider only events with actual prompt photons, assuming no contribution from isolated non-prompt photons to reduce computational cost. For the first time, we present a study that compares simulation results generated using inclusive (bremsstrahlung) and prompt-photon events with multiple experimental observables for both $p-p$ and $Pb-Pb$ collisions at $5.02$ TeV. Simulations are carried out using the multi-stage JETSCAPE framework tuned to describe the quenching of jets and hadrons. Isolated non-prompt photons are generated in hard photon bremsstrahlung, where the photon is radiated at a sufficient angle to the jet. Several photon triggered jet and jet substructure observables show significant contributions from inclusive photons, yielding an improvement in comparison with experimental data. Novel photon triggered jet substructure observables are also expected to show new structures, yet to be detected in experiment. This effort examines the significance of isolated non-prompt photons using parameters tuned for a simultaneous description of the leading hadron and jet spectrum, and thus provides an independent verification of the multistage evolution framework.

  PublisherOA PDFDOI

Recent grants

• Probing the QCD Equation of State and Hadronization at RHIC

  NSF · $56k · 2003–2007

• SI2-SSI: Collaborative Research: Jet Energy-Loss Tomography with a Statistically and Computationally Advanced Program Envelope (JETSCAPE)

  NSF · $574k · 2016–2021

Frequent coauthors

• H. Stöcker

  Frankfurt Institute for Advanced Studies

  92 shared

• Marcus Bleicher

  Goethe University Frankfurt

  92 shared

• Walter Greiner

  86 shared

• Marlene Nahrgang

  IMT Atlantique

  74 shared

• Weiyao Ke

  Lawrence Berkeley National Laboratory

  71 shared

• Berndt Müller

  Duke University

  67 shared

• S. Soff

  Goethe University Frankfurt

  66 shared

• Shanshan Cao

  Shandong University

  60 shared

Education

• Ph.D., Physics

  Johann Wolfgang Goethe Universitaet Frankfurt

  1997

Awards & honors

• Outstanding Referee for APS Journals (2014)
• Fellow of the American Physical Society