Heavy Ion Physics – A Brief Theory Overview

Aleksi VuorinenUniversity of Helsinki

Lepton-Photon 2015, Ljubljana, August 20, 2015

Tremendously successful theory with very nontrivial properties:• Confinement → Nuclear

physics• Asymptotic freedom →

Collider physics• Collective behavior →

Heavy ion physics

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)

Theorists’ (naïve) hope:• Do first principles calculations using lattice, pQCD,…• Make predictions and compare to data• Confirm expectations and claim victory

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

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

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

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

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

1. Initial state and thermalization

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

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

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?

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

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

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]

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

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]

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

2. Quark gluon plasma in equilibrium

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!

Equilibrium properties of QGP:• Phase structure / transitions

at and • bulk thermodynamics• bulk thermodynamics• Transport properties and

spectral functions

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

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,…

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

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

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]

Minkowskian spectral function, needed for transport

With transport properties, run into problem:

Euclidean correlator, measurable on the lattice

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]

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

3. Hydrodynamic evolution

Nontrivial lesson from RHIC collisions: Hydrodynamic modeling of heavy ion collisions (predictions for particle spectra) works extremely well

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?

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

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

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

Main effect of hydrodynamic flow in HICs: Conversion of spatial anisotropy to momentum space

H. Niemi

Main effect of hydrodynamic flow in HICs: Conversion of spatial anisotropy to momentum space

[Heinz, Chen, Song]

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, …]

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;…]

4. Hard probes

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

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

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

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

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

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

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…

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

In fact, large excess of direct photons and their elliptic flow observed in AA collisions → “Direct photon puzzle”

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]

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]

Conclusions

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

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!