Long Range Plan: Dense matter theory for heavy-ion collisions
and neutron stars
Alessandro Lovato
Physics Division, Argonne National Laboratory, Lemont, IL 60439, USA
Travis Dore
Fakult ̈at f ̈ur Physik, Universit ̈at Bielefeld, D-33615 Bielefeld, Germany
Robert D. Pisarski and Bjoern Schenke
Department of Physics, Brookhaven National Laboratory, Upton, NY 11793
Katerina Chatziioannou
Department of Physics, California Institute of Technology, Pasadena, California 91125, USA and
LIGO Laboratory, California Institute of Technology, Pasadena, California 91125, USA
Jocelyn S. Read
Nicholas and Lee Begovich Center for Gravitational Wave Physics and
Astronomy, California State University Fullerton, CA 92831, USA
Philippe Landry
Canadian Institute for Theoretical Astrophysics, University
of Toronto, Toronto, Ontario M5S 3H8, Canada
Pawel Danielewicz, Dean Lee, Scott Pratt
Facility for Rare Isotope Beams and Department of Physics and
Astronomy, Michigan State University, East Lansing, MI 48824, USA
Fabian Rennecke
Institute for Theoretical Physics, Justus Liebig University
Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany and
Helmholtz Research Academy Hesse for FAIR (HFHF), Campus Giessen, 35392 Giessen, Germany
Hannah Elfner
GSI Helmholtz Centre for Heavy-ion Research, Planckstr. 1, 64291 Darmstadt, Germany
Veronica Dexheimer, Rajesh Kumar, Michael Strickland
Department of Physics, Kent State University, Kent OH 44242 USA
arXiv:2211.02224v2 [nucl-th] 8 Nov 2022
2
Johannes Jahan, Claudia Ratti and Volodymyr Vovchenko
Department of Physics, University of Houston, Houston, TX 77204, USA
Mikhail Stephanov
University of Illinois at Chicago, Chicago, IL 60607
Dekrayat Almaalol, Gordon Baym, Mauricio Hippert, Jacquelyn
Noronha-Hostler, Jorge Noronha, Enrico Speranza, and Nicol ́as Yunes
University of Illinois at Urbana-Champaign, Urbana, IL 61801
Chuck J. Horowitz
Physics Department, Indiana University, Bloomington, IN 47405, USA
Steven P. Harris, Larry McLerran, Sanjay Reddy, Agnieszka Sorensen
Institute for Nuclear Theory, University of Washington, Seattle, WA 98195, USA
Srimoyee Sen
Department of Physics and Astronomy, Iowa State University, Ames IA 50011
Stefano Gandolfi and Ingo Tews
Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
M. Coleman Miller
University of Maryland, Department of Astronomy and Joint
Space-Science Institute, University of Maryland, College Park, MD 20742
Cecilia Chirenti
University of Maryland, Department of Astronomy, College Park, MD 20742
Astroparticle Physics Laboratory, NASA/GSFC, Greenbelt, MD 20771
Center for Research and Exploration in Space Science
and Technology, NASA/GSFC, Greenbelt, MD 20771 and
Center for Mathematics, Computation and Cognition, UFABC, Santo Andre, 09210-170, Brazil
Zohreh Davoudi
Department of Physics, Maryland Center for Fundamental Physics, and NSF Institute
for Robust Quantum Simulation, University of Maryland, College Park, MD 20742
Jamie M. Karthein and Krishna Rajagopal
Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, MA 02139
Salvatore Vitale
Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts
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Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA and
LIGO Laboratory, Massachusetts Institute of Technology,
185 Albany St, Cambridge, MA 02139, USA
Joseph Kapusta
School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455 USA
G ̈ok ̧ce Ba ̧sar
Department of Physics and Astronomy, The University of
North Carolina at Chapel Hill, Chapel Hill, NC, 27599
Thomas Schaefer and Vladimir Skokov
North Carolina State University, Raleigh, NC 27695
Ulrich Heinz
Department of Physics, The Ohio State University, Columbus, OH 43210-1117, USA
Christian Drischler, Daniel R. Phillips, Madappa Prakash
Department of Physics and Astronomy and Institute of Nuclear
and Particle Physics, Ohio University, Athens, OH 45701, USA
Zoltan Fodor
Pennsylvania State University, Department of Physics, University Park, Pennsylvania 16802, USA
Department of Physics, Wuppertal University, Gaussstr. 20, D-42119, Wuppertal, Germany
Juelich Supercomputing Centre, Forschungszentrum Juelich, D-52425 Juelich, Germany
Eotvos Lorand University, Institute for Theoretical Physics, H-1117, Budapest, Hungary and
Physics Department, UCSD, San Diego, CA 92093, USA
David Radice
Institute for Gravitation and the Cosmos, The Pennsylvania
State University, University Park, PA 16802, USA
Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA and
Department of Astronomy & Astrophysics, The Pennsylvania
State University, University Park, PA 16802, USA
Christopher Plumberg
Natural Science Division, Pepperdine University, Malibu, CA 90263, USA
Elias R. Most, Carolyn A. Raithel
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Princeton Center for Theoretical Science, Princeton University, Princeton, NJ 08544, USA
Princeton Gravity Initiative, Princeton University, Princeton, NJ 08544, USA and
School of Natural Sciences, Institute for Advanced Study, Princeton, NJ 08540, USA
Eduardo S. Fraga
Instituto de F ́ısica, Universidade Federal do Rio de Janeiro,
Caixa Postal 68528, 21941-972, Rio de Janeiro, RJ, Brazil
Aleksi Kurkela
Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway
James M. Lattimer
Stony Brook University, Stony Brook, NY 11794
Andrew W. Steiner
Department of Physics and Astronomy and University of
Tennessee, Knoxville, Knoxville, TN 37996, USA and
Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Jeremy W. Holt
Department of Physics and Astronomy and Cyclotron Institute,
Texas A&M University, College Station, TX 77843, USA
Bao-An Li
Texas A& M University-Commerce, Commerce, TX 75429, USA
Chun Shen
Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201, USA
Mark Alford, Alexander Haber, Saori Pastore, Maria Piarulli
Washington University in St. Louis, St. Louis, MO 63130, USA
(Dated: November 9, 2022)
Executive Summary
Since the release of the 2015 Long Range Plan in Nuclear Physics, major events
have occurred that reshaped our understanding of quantum chromodynamics (QCD)
and nuclear matter at large densities, in and out of equilibrium. The US nuclear
community has an opportunity to capitalize on advances in astrophysical observa-
tions and nuclear experiments and engage in an interdisciplinary effort in the theory
5
of dense baryonic matter that connects low- and high-energy nuclear physics, astro-
physics, gravitational waves physics, and data science.
•
Now is the time to pursue dense matter studies:
Over the past decade we
have seen the first detection of a gravitational wave signal from a binary neutron
star merger, together with its electromagnetic counterparts [1], and the radius mea-
surement of a two-solar-mass neutron star [2–4] from NASA’s NICER mission. New
experimental results possibly indicative of a QCD critical point [5–9] from heavy
ion collisions at STAR and HADES and new inference of the neutron-skin thickness
has led to (although with significant uncertainties) the extraction of the slope of the
symmetry energy, from PREX-II [10] and CREX data [11]. These results cover a
wide range of scales, from single nuclei to neutron stars, and physical probes, from
gravitational waves to high-energy particles, and call for an interdisciplinary effort to
unravel the properties of strongly interacting matter. Soon we expect to see new ex-
perimental information on neutron-rich atomic nuclei and the equation of state EOS
(including new kinds of neutron skin thickness measurements using mirror nuclei [12])
from FRIB, as well as from heavy-ion collisions at RHIC, SPS, and FAIR, the possi-
ble detection of multiple neutron star mergers per year by the LIGO-Virgo-KAGRA
detectors with electromagnetic counterparts [13, 14], and new data by the NICER
mission for PSR J0437. Interpreting this new experimental and observational data
fully will require significant coordinated efforts in dense matter theory beyond current
levels and improved coordination among theorists, experimentalists, and observers.
•
Interpreting Beam Energy Scan II data:
Results from phase II of the RHIC
Beam Energy Scan program (BESII) are anticipated over the coming year or two,
since data-taking concluded in 2021. A central goal of BESII is to measure the beam
energy dependence of fluctuation observables and identify a possible critical point in
the QCD phase diagram, addressing one of the big open questions in the field [15].
Significant theoretical efforts are needed to reliably interpret the results. This includes
tools to study the non-equilibrium evolution of non-Gaussian fluctuations, inclusion of
strangeness neutrality and electric charge diffusion in hydrodynamic models, and an
EOS and dynamic tools to study the meta-stable regime. All of these tools, together
6
with the already developed Beam Energy Scan Theory (BEST) Collaboration [16]
framework, have to be integrated in a Bayesian analysis framework. Only then will
the BESII data allow us to either confirm the discovery of a critical point in the phase
diagram and pinpoint its location, or place constraints on the location of any critical
point by excluding its presence in the regime explored in BESII. Either outcome is
important for understanding the QCD phase diagram. A critical point discovery
would imply that the transition at high baryon density,
n
B
, is discontinuous and
would motivate a program for exploring consequences of a coexistence region.
•
Uncertainty quantification in chiral effective field theory:
At low to moder-
ate
n
B
and temperatures
T
, the dense-matter EOS can be computed using nuclear
many-body methods that use as input nuclear interactions derived from chiral ef-
fective field theory (
χ
EFT) [17, 18]. These microscopic calculations of dense mat-
ter have become more sophisticated, with different computational methods agreeing
to good accuracy [19–27]. Present theoretical uncertainties are dominated by the
nuclear interactions employed in these calculations.
χ
EFT enables us to estimate
these uncertainties as it is based on a systematic momentum expansion. The use
of Bayesian tools has enabled tremendous progress in the rigorous quantification of
EFT uncertainties over the past 7 years [28–35]. However, there remain several prob-
lems pertinent to the theory of dense QCD matter where model uncertainties need
to be assessed. Further methodological developments and software tools are needed
to achieve this goal [36]. Several studies have indicated that
χ
EFT calculations of
neutron-rich dense matter might be valid up to twice the nuclear saturation density
(
n
sat
= 0
.
16 fm
−
3
) [33, 34, 37], but it is still unclear where and how
χ
EFT breaks
down. For studies of neutron-star mergers, it is crucial to access dense matter at finite
temperatures [38–40]. Finite temperatures and the addition of protons might influ-
ence the breakdown of the theory in dense matter. The possibility of a large neutron
skin in
208
Pb [41] might lead to tension with
χ
EFT calculations [42] should future
experiments confirm its central value [10] with improved precision [43]. However,
the relatively small neutron skin in
48
Ca inferred by the CREX measurements [11] is
more in line with
χ
EFT calculations [44]. Grounding the EOS at low
n
B
in reliable
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nuclear-theory calculations, including lattice QCD [45], is extremely important, given
the upcoming data from observations and experiments.
•
Connecting laboratory experiments to astrophysics:
With the upcoming BE-
SII data, low-energy FRIB data, observational data of neutron stars and their merg-
ers, the advances of lattice QCD, perturbative QCD (pQCD), and
χ
EFT, and previ-
ous knowledge of the liquid-gas phase transition, we will have disconnected regions in
the QCD phase diagram that will need to be connected through effective models. In
equilibrium, multidimensional EOS, in terms of
T
and different chemical potentials
μ
, must be flexible enough to allow studies of parameter space to quantify uncer-
tainties and reproduce all known constraints. They will need to provide particle
composition, necessary for calculations of in and out of equilibrium properties, such
as strangeness production, neutrino emissivity, and bulk viscosity. Additionally, dy-
namical models are needed to make direct connection to experimental heavy-ion data
and post-merger signals that will require stable and causal equations of motion that
can handle multiple conserved charges, microscopic models of transport coefficients,
state-of-the-art hadronic transport codes, and proper handling of neutrino interac-
tions. These challenges require interdisciplinary collaborations to share knowledge
and develop open-source tools that can be applied within and across the different
communities.
•
Leveraging novel simulation and computation technologies:
Hamiltonian
simulations performed on quantum simulators and computers has the promise of en-
abling efficient first-principles simulation of dense matter, in and out of equilibrium,
by avoiding the sign problem in current Monte Carlo based methods. Quantum in-
formation tools provide novel probes of quantum state of matter, phases and phase
transitions, and equilibration and thermalization processes. Over the next decade, the
community will identify computational problems that can benefit from quantum ad-
vantage, develop efficient algorithms to access them in a quantum-simulating devise,
and engage in implementation and co-design efforts involving quantum technologists
to gain a deeper understanding of gauge-theory dynamics even on near-term noisy
quantum simulators and computers.
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CONTENTS
I. Overview
9
II. EOS at large densities
11
A. Lattice QCD
12
B. Many-body theory and chiral effective field theory
13
C. pQCD
15
D. Effective models
16
E. Transport coefficients
17
F. New phases of matter
18
III. Heavy-ion collisions
19
A. Relativistic viscous fluid dynamics
21
B. Hadronic transport
22
C. Far-from-equilibrium relativistic fluids
24
D. Bayesian analyses
25
IV. Neutron stars
26
A. X-Ray observations
27
B. Inspiral of neutron stars
28
C. Potential ultraheavy neutron stars
29
D. Merging neutron stars and multi-messenger signals
30
E. Beyond the standard model
32
V. Merging heavy-ions and neutron stars
32
A. Connecting symmetric and asymmetric nuclear matter
33
B. Signatures of phase transitions
34
C. Statistical analyses
35
D. Magnetic fields
36
E. Chemical equilibration processes in neutron star mergers
36
VI. Exploratory directions: Quantum information science for dense-matter theory
37
References
40