1. 2nd ECFA HL-LHC Workshop
“Heavy Ion Theory”
Urs Achim Wiedemann
CERN PH-TH
2nd ECFA HL-LHC Workshop
Aix-les-Bains, 21 Oct 2014

2. Heavy Ion Physics – Future Directions
Luminosity*
Higher Energy
(FHC)
pPb and PbPb data
Status Oct 2014
pA physics*
A-dependence*
(Ar, Xe, …)
If HL-LHC = LHC after LS3
Then three important directions * to go
Machine preparation & beam use should allow for exploration of these three directions.

3. Focus of this talk is on soft probes
Heavy Ions in the 2020’s
Higher Luminosity in A+A to study rarer probes
Quarkonium dissociation (vs. centrality, pt, flow …)
Jet quenching studies
Electromagnetic probes …
Logic as in high-lumi pp …
Higher Luminosity in pA to study rarer probes
Constraining nuclear pdfs
Searching for non-linear QCD evolution (saturation) …
Higher Luminosity to characterize soft probes, e.g.
Correlations of EbyE flow distributions
Flow of rarer probes (quarkonium, open HF, …
Logic different from high-lumi pp …
15 minutes = 1 argument,
Focus of this talk is on soft probes

4. The early years of heavy ion physics
Early Main Perspective:
QGP reveals itself via kinks in data.
e.g. J. Harris, B. Muller, Ann.Rev.Nucl.Part.Sci. 46 (1996)
Today:
no kinks in data unambiguously related to QGP, irrespective of whether plotted against cms energy, system size (p-Pb, Pb-Pb, centrality), ...
Early Critique/Doubt:
Thermal QGP equilibrium state not accessible in short-lived, mesoscopic, rapidly expanding system.
Today:
It is exactly via the rapid collective expansion that QGP properties become experimentally accessible.
Central role of Hydrodynamics in modern QGP research

5. Hydro = QGP-physics + conservation laws
Evolution equations of dissipative hydrodynamics are:
based on: E-p conservation:
2nd law of thermodynamics:
sensitive to properties of matter that are
calculable from first principles in quantum field theory
- EOS: and sound velocity
- transport coefficients: shear , bulk viscosity, conductivities …
- relaxation times: , , …
Lattice QCD =>
Finite Temp pQCD =>
AdS/CFT =>
Establishing hydro behavior is the door to the QGP.

6. Elliptic Flow: hallmark of a collective phenomenon
Compilation
ALICE, PRL 105, (2010)

7. Hydro-like phenomena at LHC
Idea: Matter flows along spatial density gradients (that can vary Event by Event).
Fig from M.Luzum, arXiv:
Flow in LHC
… and in p+Pb?
B. Schenke, MUSIC
Hydro simulations can account for flow data.
Applicability of fluid dynamics to small systems?

8. How can hydro apply to the smallest systems?
The strong coupling paradigm:
Strong coupling limit of N=4 SYM
Kovtun, Son, Starinets, hep-th/
Arnold, Moore, Yaffe, JHEP 11 (2000) 001
Fluid dynamics applies at
In perturbative scenario: hydro valid if
but
=> non-perturbative thermalization
Heller, Janik Witaszczyk,
PRL 108 (2012)
Chesler, Yaffe,
PRL 102 (2009)
No quasi-particles in medium
This interpretation lends itself to further experimental tests!
Tertium datur: applicability of strong coupling expansion in
does not invalidate perturbative expansion in
=> Quasi-particles in medium
Short thermalization times can be obtained in
perturbative equilibration scenarios, too.
A. Kurkela & E. Lu, arXiv:

9. From averages to EbyE statistics of flow measurements
The current frontier in EX and TH: relating statistics of initial distributions to statistics of measured flow distributions.
Distributions follow approximately Bessel-Gaussian distributions
ATLAS JHEP 1311 (2013) 183
One of the next frontiers: Cross-correlating these distributions
(can we test hydro-interactions between different azimuthal modes?)
Are these flow correlations in smaller systems (pA) also hydro-like?

10. Why open heavy flavor at low pt is interesting
Within the strong coupling paradigm,‘no-quasiparticle conjecture’implies that light low-momentum dressed quarks do not exist (i.e. do not propagate beyond )
In contrast, charm & bottom propagate (consequence of flavor conservation).
How? Dragged along as external test particles of flow.
At low pt, Langevin dynamics determines how charm & beauty quarks move: The perfect liquid is source of random forces
PRL 98, (2007)
calculable from 1st principles in quantum field theory, e.g. in strong coupling limit:
Can we separate mass and T-dependence
of ?
From Langevin dynamics, expect that heavy Q is dragged along with flow but does not necessarily show the same flow as medium.
=> motivates precise determination.

11. How can LHC experiments continue to test hydro?
All flow measurements are correlation measurements. Correlations can originate from initial conditions, from hydro evolution and/or from hadronization. Correlations that allow to disentangle these sources will be most useful.
Example: initial conditions do not specify flow dependence on particle species.
That makes particle identified flow (and its mass-ordering) a strong pillar of
the hydrodynamic interpretation of data
Does charm (beauty) flow?
Both TH and EXP are still far from
an answer of textbook quality!
What is missing?
=> Statistics
There are important problems in the
soft physics sector that warrant high
luminosity running.
=> Correlation tests
Is for event distributions?
=> Systematics
Does triangular, quadrangular … charm flow exist? Does it scale as hydro
flow? And p-Pb ..

12. Instead of a summary
What I have presented is a personal view. It is neither a summary of the future opportunities of heavy ion collisions, nor of the current understanding of flow.
Rather, it is an example to illustrate a very general point:
Also in the HL-LHC era, the improved characterization of soft physics phenomena will remain a driver of the experimental heavy ion programme.
What is at stake (amongst others) is to further exploit the possibility of establishing matter properties via fluctuation analysis.