1. LHC Days in Split: Heavy Ion Theory
+ LQCD
QCD precision
EWK precision
Higgs physics
BSM physics
Urs Achim Wiedemann
CERN PH-TH
Thu 22/09, afternoon
How do collective phenomena and properties of matter emerge from this fundamental Lagrangian?
Thu 22/09, morning
Mo 19/09, afternoon
+ LBSM
Tue 22/09, afternoon
Urs Wiedemann
CERN TH Department

2. Flow in PbPb and pPb Reaction plane collectivity
CMS, PRL 115 (2015)
Azimuthal asymmetries vm persist unattenuated in many-particle correlations
collectivity
pT-dependence of vm
PID-dependence of vm
rapidity (in)dependence of vm
centrality dependence
correlations of reaction
plane orientations
support fluid dynamic picture of heavy ion collisions

3. Fluid dynamic modeling of heavy ion collsions
Few model parameters characterize
Initial conditions
Hydro evolution
Particlization
Bayesian analysis
Leads to very good posteriors
Best parameters indicate a minimally dissipative fluid
prior
J.E. Bernhard et al. ,
arXiv: v2
posterior

4. How can hydrodynamic behavior arise?
Initial conditions
Pre-equilibrium stage
Hydrodynamic expansion
non-equilibrated
over-saturated
anisotropic
determined by effective kinetic theory
(if dynamics is perturbative)
qualitatively understood in models of gauge-gravity duality
(if dynamics is non-perturbative)
leads to rapid hydrodynamization
controlled matching on pre-equilibrium stage
determined by very few thermal equilibrium properties
(calculable from 1st principles in QCD)
in more detail …

5. 1. Initial conditions Gluon distributions grow rapidly at small x.
Small-x growth in incoming nuclei can reach maximal parton densities
Venugopalan McLerran; Jalilian-Marian,Kovner,Leonidov,Weigert; Balitsky; Kovchegov;…
Phase space density of produced partons
is over-occupied (thermal distribution fgluon ~ 1)
Initial momentum distribution of
is anisotropic

6. 2a. Pre-equilibrium evolution (perturbative)
Under longitudinal expansion, initially overoccupied systems become underoccupied before reaching local thermal equilibrium.
R.Baier, A.H. Mueller, D. Schiff, D.T. Son, 2001
QCD effective kinetic theory:
to order and for
modes with
satisfy Boltzmann equation
Berges, Eppelbaum, Kurkela, Moore, Schlichting, Venugopalan, …
2->2 collision kernel
LPM splitting term
Kurkela, arXiv:

7. 2a. Hydrodynamization of kinetic evolution
Dissipative hydrodynamics describes long-time behavior of QCD effective kinetic theory
Kurkela&Zhu, PRL115 (2015)182301
Sub-fermi hydrodynamization time scale

8. 2b. Pre-equilibrium evolution (non-perturbative)
Gauge-gravity duality gives access to pre-equilibrium dynamics of a class of non-abelian plasmas in the strong coupling limit
Strongly coupled non-abelian plasmas equilibrate fast
Chesler, Yaffe, PRL 102 (2009)
Heller, Janik Witaszczyk, PRL 108 (2012)
Free energy
Pressure anisotropy
Sub-fermi hydrodynamization time scale

9. Take-away message on fluid dynamics
Theory has reached recently an improved understanding of
how QCD fluid dynamics can emerge in pre-equilibrium dynamics.
Rapid hydrodynamization (on sub-fm time scale)
Hydrodynamization prior to thermalization
Anisotropic hydrodynamics
Anomalous hydrodynamics
Thermal fluctuations in relativistic hydrodynamics
This extends standard textbook treatments of relativistic fluid dynamics in various ways.
Data on soft particle production are well-described by fluid dynamic simulations.

10. CMS Coll. arXiv:
Flow is also seen in Are there alternatives to fluid dynamics that account for collectivity in small systems?

11. Alternatives to fluid dynamics?
Azimuthal anisotropy from interference?
Color field domains in incoming hadrons?
Kovner, Lublinsky, Dumitru,
Gelis,
Jalilian-Marian, Lappi, Venugopalan, Dusling, …
Still open questions
Scaling of higher order cumulants
System size dependence
Odd harmonics
Studies typically done in CGC saturation model, but physics conclusions may hold more generally.

12. Hard Probes: Jet fragmentation vs. Jet thermalization

13. Medium-induced gluon radiation: naïve estimate
Medium characterized by
BDMPS transport coefficient:
Brownian motion
Characteristic
gluon energy
Phase accumulated in medium:
Number of coherent scatterings:
, where
Gluon energy distribution:
Non-abelian LPM-effect
Average energy loss
Soft gluons emitted first.
Fromation time

14. Phenomenology of parton energy loss
Supplementing parton showers with LPM-type medium-induced splittings accounts for “jet quenching observables”:
RAA of single inclusive hadrons
RAA of heavy flavor
RAA of jets
Dijet asymmetries
Medium-modified jet fragmentation functions
TALK BY Y. PACHMAYER
G. Milhano, K. Zapp, arXiv:

15. Parametric understanding of jet quenching
Clarifies sensitivities to:
A. Kurkela & UAW, PLB740 (2015) 172
color field strength of medium,
i.e. jet quenching parameter
constituents of medium
accessible via large-angle
scattering
Longitudinal phase space =>
thermalization:
the LPM splitting function is
exactly the C1->2 kernel of the
Boltzmann equation underlying
hydrodynamization
Angular phase space =>

16. Measuring medium-modified SPLITting functions
Jet grooming: “Soft drop” procedure
Larkoski, Marzani, Soyez Thaler (2014,2015)
remove soft junk
to probe first perturbative splitting
Could medium-modified splitting functions be testable by jet grooming?
Studies of medium-mod color antenna:
Mehtar-Tani,Salgado, Tywoniuk, ….
First data on groomed jet splitting functions in heavy ion collisions
CMS-PAS-HIN
p.t.o.

17. PbPb vs pp pT,jet: 140-160 GeV Slide from CMS-PAS-HIN-16-006
Marta Verweij
CMS-PAS-HIN
PbPb vs pp pT,jet: GeV
Strong modification of splitting observed in central PbPb collisions
Branching more imbalanced in central PbPb
Marta Verweij

18. This was not an overview.
I did not try to tell everything, I tried to say something:
I gave an example of how the LHC heavy ion program
addresses the fundamental question:
How do collective phenomena and properties of matter emerge from the fundamental Lagrangian?