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Heavy Ion Physics – A Brief Theory Overview
Aleksi VuorinenUniversity of Helsinki
Lepton-Photon 2015, Ljubljana, August 20, 2015
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Tremendously successful theory with very nontrivial properties:• Confinement → Nuclear
physics• Asymptotic freedom →
Collider physics• Collective behavior →
Heavy ion physics
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Typical collision:• Valence quarks of participating nucleons source
color fields that lose energy and eventually create anisotropic yield of hadrons
• Using experimental data, try to infer creation of deconfined QGP with MeV (depending on collision energy)
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Theorists’ (naïve) hope:• Do first principles calculations using lattice, pQCD,…• Make predictions and compare to data• Confirm expectations and claim victory
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Theorists’ (naïve) hope:• Do first principles calculations using lattice, pQCD,…• Make predictions and compare to data • Confirm expectations and claim victory
First principles methods extremely tedious to apply to many interesting problems
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Theorists’ (naïve) hope:• Do first principles calculations using lattice, pQCD,…• Make predictions and compare to data • Confirm expectations and claim victory
Several early theory expectations turned out qualitatively wrong
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In practice:• Need to expand toolbox: Apply effective theories and
fundamentally new first principles machinery• When necessary, use phenomenological models to
make contact with experimental data
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In practice:• Need to expand toolbox: Apply effective theories and
fundamentally new first principles machinery• When necessary, use phenomenological models to
make contact with experimental data
This talk: Concentrate on first principles advances, even if it sometimes means making bold extrapolations or even modifying the theory
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Four main branches of heavy ion theory:
1. Description of initial state and system’s approach to local thermal equilibrium
2. Equilibrium properties of the quark gluon plasma3. Hydrodynamic expansion and hadronization4. Hard probes of the plasma
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1. Initial state and thermalization
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At high energies, systematic framework for description of initial state (at small x): Color Glass Condensate [McLerran, Venugopalan; …]
• Overoccupied () soft gluons described via classical YM fields
• Hard partons act as color sources• Sharp distinction between “soft” and “hard”:
saturation scale
A. Kurkela
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Recent successes/advances in small-x physics:• NLO perturbative
corrections to small-x evolution [Balitsky et al; Kovner et al; Iancu et al; Lappi et al; …]
• Quantitative description of the ridge correlation, also in pp and pA collis. [Dumitru et al; Kovner et al; …]
• New experimental idea: Do DIS at the LHC using ultraperipheral AA collisions
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Key theory questions for the description of HICs: • How to describe early dynamics and evolution
towards thermalization/hydrodynamization?• What are the correct initial conditions to be fed to
hydro codes?
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Key theory questions for the description of HICs: • How to describe early dynamics and evolution
towards thermalization/hydrodynamization?• What are the correct initial conditions to be fed to
hydro codes?
Quantitative answers available from first principles calculations in two opposite limits: 1. Asymptotically weak coupling: 2. Strong coupling via AdS/CFT: Strongly coupled
large- N = 4 Super Yang-Mills theory
Real world somewhere in between
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At weak coupling, power counting arguments → Bottom-up thermalization [Baier, Mueller, Schiff, Son], where• Expansion makes system underoccupied before
thermalization• Soft gluons first create thermal bath, then hard
modes undergo radiational breakup
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Tools for quantitative study at weak coupling:• Classical lattice simulations for [Gelis et al; …]
• Kinetic theory [Arnold, Moore, Yaffe] for
Recently impressive progress in kinetic theory simulations: When extrapolated to match with hydrodynamics at fm/c [Kurkela, Zhu]
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Opposite limit: Collision of planar shock waves in AdS space – “HICs” in strongly coupled N = 4 SYM• At high T, theory qualitatively similar to QCD:
deconfinement, Debye screening, SUSY broken,…• Very hard dynamical problems in field theory
turned into calculations in classical (super)gravity
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Lessons and milestones at strong coupling:• Hydrodynamization isotropization [Chesler, Yaffe]
• Naturally fast dynamics, fm • Successful testing ground for hydrodynamics [Heller,
Janik et al; Chesler, Yaffe; …]
• Universal features of black holes formation universality in thermalization at strong coupling
• Transverse structure (AA, pA, pp) now feasible [Chesler]
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Key challenge for future: How to approach physical situation of QCD at intermediate energy/coupling?• Derive and carry out simulations in NLO kinetic
theory [Ghiglieri et al]
• Compute finite coupling corrections to holographic thermalization [Steineder, Stricker, AV; …]
• Merge weak and strong coupling descriptions with semi-holography [Iancu, Mukhopadhyay]
From purely phenomenological point of view, not so clear if all of this important: Hydrodynamic simulations insensitive to many details of thermalization
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2. Quark gluon plasma in equilibrium
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Two takes on equilibrium properties of QGP:• Phenomenology: Need
only few inputs (EoS, transport coeffs.) for hydro
• Theory: Many fundamental properties of theory (phase diagram,EoS,…) equilibrium quantities
Historically very important problems; major motivator of heavy ion experiments!
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Equilibrium properties of QGP:• Phase structure / transitions
at and • bulk thermodynamics• bulk thermodynamics• Transport properties and
spectral functions
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Equilibrium properties of QGP: • Phase structure / transitions
at and • bulk thermodynamics• bulk thermodynamics• Transport properties and
spectral functions
Lattice QCD feasible → Reliable nonperturbative first principles results available
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Equilibrium properties of QGP:• Phase structure / transitions
at and • bulk thermodynamics• bulk thermodynamics• Transport properties and
spectral functions
Lattice simulations unfeasible/problematic due to the Sign Problem → Resort to pQCD, models, holography,…
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Lattice results for QCD thermodynamics: Brief review
1. Cross-over deconfinement and chiral transitions around 160 and 155 MeV [HotQCD; Wuppertal-Budapest groups]
• Typically determined from Wilson line and chiral susceptibilities, respectively
• Realistic quark masses no longer a problem
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Lattice results for QCD thermodynamics: Brief review
2. EoS and various susceptibilities at accurately determined [HotQCD; WB]
• Excellent agreement with Hadron Resonance Gas around and resummed pQCD [Kajantie et al; Andersen et
al; AV] from onwards
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Lattice results for QCD thermodynamics: Brief review
3. Several methods proposed to attack finite density; Taylor expansion in most prominent [HotQCD]
• Good agreement with resummed pQCD [Andersen et
al; AV] as long as convergence ( 1)• Ultimate goal: Find tricritical point – at the
moment, even existence uncertain [Stephanov et al]
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Minkowskian spectral function, needed for transport
With transport properties, run into problem:
Euclidean correlator, measurable on the lattice
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Analytic continuation from Euclidean lattice data possible for some quantities [Burnier, Laine]
• Unfortunately, shear viscosity notoriously difficult parameter; for it, only rough estimates () reliably available [Meyer]
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Alternative in large- limit: Holographic QCD
IHQCD [Kiritsis et al]: Bottom-up holographic model for QCD• Dilaton with phenomenol.
potential fitted to -fnc• Lattice results for bulk
thermodynamics repdoduced accurately;
• Quarks added in Veneziano limit with full backreaction [Järvinen, Kiritsis]; used to map ph. diag. in terms of
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3. Hydrodynamic evolution
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Nontrivial lesson from RHIC collisions: Hydrodynamic modeling of heavy ion collisions (predictions for particle spectra) works extremely well
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Nontrivial lesson from RHIC collisions: Hydrodynamic modeling of heavy ion collisions (predictions for particle spectra) works extremely well
• What is hydrodynamics? What goes in and what comes out?
• How do we know hydro works, and what does it teach us?
• Where do we stand at the moment?
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Hydrodynamics = Effective description of the evolution of conserved currents in a collective medium, valid at distances • Describes system in local thermodynamic equilibrium• Can be improved order by order in a derivative
expansion
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Hydrodynamics = Effective description of the evolution of conserved currents in a collective medium, valid at distances • Describes system in local thermodynamic equilibrium• Can be improved order by order in a derivative
expansion• LO: Ideal fluid, need only EoS as input
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Hydrodynamics = Effective description of the evolution of conserved currents in a collective medium, valid at distances • Describes system in local thermodynamic equilibrium• Can be improved order by order in a derivative
expansion• NLO: Dissipative effects from shear and bulk viscosity
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Main effect of hydrodynamic flow in HICs: Conversion of spatial anisotropy to momentum space
H. Niemi
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Main effect of hydrodynamic flow in HICs: Conversion of spatial anisotropy to momentum space
[Heinz, Chen, Song]
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Surprises from hydro results:• Very short initialization
times ( 0.5 fm), consistent w/ strong coup. [Heinz et al; Romatschke et al;...]
• Extremely small viscosity: for RHIC, for LHC energies [Romatschke et al; Kovtun, Son, Starinets]
• Indications that hydro works in surprisingly small systems (pA, pp) [Niemi et al, Bozek et al, …]
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New developments:• Attempts to read off
temperature dependence of shear viscosity from data [Eskola, Niemi, Paatelainen; …]
• Constraints on the EoS from comparison with data [Pratt et al,…]
• Incorporation of effects from magnetic fields and anomalies via Chiral MagnetoHydroDynamics [Kharzeev, Yee;…]
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4. Hard probes
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Hard probes = Use of high energy () observables to study the collision • Asymptotic freedom → pQCD applies at high
enough energies• Probe physics before thermalization/hydrodynamiz:
Information about initial state• Practical tools: Jet (quenching), heavy flavors and
EM probes
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Hard probes = Use of high energy () observables to study the collision • Asymptotic freedom → pQCD applies at high
enough energies• Probe physics before thermalization/hydrodynamiz:
Information about initial state• Practical tools: Jet (quenching), heavy flavors and
EM probes
Two examples
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Jet quenching and broadening:• Hard process → Back to back
partons → Symmetric pair of jets in vacuum
• In dense medium, jets lose energy (asymmetrically) → `Jet quenching’
• Related observation: Lots of soft hadrons at large angles
Challenge for theory: Explain findings from 1st principles!
NB: Expect interplay between weak and strong coupling
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Long history of energy loss calculations [Baier et al; Gyulassy et
al; Arnold, Moore, Yaffe]: Distinction between collisional (heavy flavors) and radiative (light quarks) energy loss
Nontrivial to turn this insight into quantitative jet structure calculations in HICs
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Long history of energy loss calculations [Baier et al; Gyulassy et
al; Arnold, Moore, Yaffe]: Distinction between collisional (heavy flavors) and radiative (light quarks) energy loss
Nontrivial to turn this insight into quantitative jet structure calculations in HICs
Two qualitative pictures:
Vacuum: Ordered branching leads to coherent cascade
Medium: Democr. branching, momentum broadening
Y. Mehtar-Tani
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Medium induced jet modification parameterized via one quantity: Jet quenching parameter • Momentum broadening • Collisional energy loss
Many ways to attempt evaluation of :• Weak coupling [Caron-Huot; Laine; Blaizot, Mehtar-Tani; …]
• Combination of lattice and effective theory [Panero, Rummukainen, Schäfer]
• AdS/CFT [Liu, Rajagopal, Wiedemann; Buchel; …]
Typical estimates for relevant temperatures:/fm
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EM probes (photons and dileptons) in HICs:• Probe all stages of the collision• Are sensitive to ICs, prethermal flow, as well as
EoS and viscosities• Interact weakly: Escape the plasma almost freely
In particular, thermal photons and dileptons should be a good thermometer of the equilibrium plasma…
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EM probes (photons and dileptons) in HICs:• Probe all stages of the collision• Are sensitive to ICs, prethermal flow, as well as
EoS and viscosities• Interact weakly: Escape the plasma almost freely
In particular, thermal photons and dileptons should be a good thermometer of the equilibrium plasma…
… if only we could separate them from prompt, jet-thermal, hadron gas thermal and decay photons
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In fact, large excess of direct photons and their elliptic flow observed in AA collisions → “Direct photon puzzle”
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In fact, large excess of direct photons and their elliptic flow observed in AA collisions → “Direct photon puzzle”
Promising recent progress involving inclusion of• Accurate thermal photon emission rates• Inclusion of viscosity in hydro and photon emission• Nonperturbative corrections close to
[Chen, Heinz, Paquet, Kozlov, Gale]
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Key development: Extension of thermal photon and dilept. production to NLO in pQCD [Ghiglieri et al; Ghisoiu, Laine; …]
Also, NLO results in finite coupling expansion within strongly coupled N = 4 SYM [Hassanain, Schvellinger]: Consistent interpolation between weak and strong coupling limits
In holography, even studies of off-equilibrium production possible [Baier, Stricker, Taanila, AV]
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Conclusions
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Quantitatively describing heavy ion collisions with first principles calculations is a daunting task…
…but appears to be feasible with a combination of • Hard work using old and fundamentally new tools• Taking full advantage of effective theories• Drawing insights from experimental data
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Major challenges remain, though:• Thermalization: From qualitative to quantitative• Nonperturbative (lattice) studies at finite density• Tackling transport with lattice and/or pQCD• More accurate first principles determination of • …
Plus many more in phenomenology!