1. How to make progess on the “central picture”?
LHC and beyond:
How to make progess on the “central picture”?
A theory concept that LHC HI tests
and challenges in qualitatively novel ways
Jet quenching  final state interaction  hydrodynamization  medium properties

2. Quenching and fluidity are related in a perturbative picture
Bottom-up thermalization formalizes relation between fluid dynamics and jet quenching
R.Baier, A.H. Mueller, D. Schiff, D.T. Son, 2001
Kurkela&Zhu, PRL115 (2015)182301
Kurkela, arXiv:
Partonic distributions f(p) governed by Boltzmann equation.
We know:
Berges, Eppelbaum, Kurkela, Moore, Schlichting, Venugopalan, …
f(p) hydrodynamizes on sub-fermi time scale
“Hydro” & “jet quenching” arise from the same collision kernels
2->2 collision kernel
LPM splitting kernel
Jet quenching and fluid dynamics = two manifestations of the same physics

3. Quenching and perfect fluidity in a strongly coupled picture
Also in the (N=4 SYM) strong coupling limit, perfect fluidity and rapid hydrodynamization
Chesler, Yaffe,
PRL 102 (2009)
Heller, Janik Witaszczyk,
PRL 108 (2012)
Arnold, Moore, Yaffe, JHEP 11 (2000) 001
Free energy
Pressure anisotropy
Kovtun, Son, Starinets, hep-th/
are accompanied by the quenching of hard probes
Irrefutable logic: if there are final state interactions, then there are final state interactions. Hence:
That fluid dynamic behavior implies “some” jet quenching is a robust theory prediction!

4. In AA, quenching and signs of fluidity seem correlated
CMS Coll., JHEP 01 (2016) 006
Jet thermalization (aka quenching)
Softening and isotropization
of jet structure
 hallmarks of local equilibration
Back-up
CMS, PRC89 (2014)
 Dynamically consistent with
fluid dynamic interpretation of
momentum asymmetries vn
(i.e. it is conceivable that the same physics / same
collision kernels underlie jet quenching and flow)

5. Fluid dynamic interpretation of vn in pp and pA
is problematic (even if some hydro-models can account for it)
Back-up
CMS, PLB 765 (2017) 193
since vn are apparently not accompanied by jet quenching.
This indicates a lack of final interactions.
(that are needed to build up flow)
This is a qualitatively novel insight from LHC that should be central to further investigation at HL-LHC.

6. The “Flow w/o quenching” puzzle in pp / pA & LHC: implications, possible resolutions, exp. strategies
1. possibility: vn in small systems due to final state interactions
then there must be small jet quenching effects that must be seen in pp/pA
test this hypothesis by looking for jet quenching effects in pp / pA this motivates detailed studies of jets / dijets / heavy flavor in small systems. Effects small => precision needed => motivation for future pA (and pp comparison data)
TH improvements needed to relate jet quenching and vn signals.
Jet-medium interactions may also imply small changes in hadro-chemistry in pp at high multiplicity;
Mangano, Nachman arXiv:

7. Other possible resolutions …
2. possibility: fluid dynamics negligible, azimuthal correlations from escape mechanism
Liang He et al., Phys. Lett. B753 (2016) ; AMTP
mechanism to be understood outside a MC code like AMTP
small jet quenching effects must be seen in pp/pA
similar implications for further experimentation
3. High-density Scenario: strongly coupled fluid paradigm (à la AdS/CFT) for pp/pA
small jet quenching effects must be seen in pp/pA
UE model radically different from that in MC generators
Different rescattering properties in medium
UE (underlying event) physics in pp multi-purpose MC event generators based on dilute system of up to O(10) MPIs (multi parton interactions)
If saturated initial state needed to describe pp UE, then dramatic implications: Torbjorn go home.
One needs to understand whether initial density effects are necessary for azimuthal correlations.

8. Other possible resolutions …cont’d
4. High-density Scenario: azimuthal correlations from a saturated initial state (“CGC”)*
Altinoluk, Armesto, Beuf, Dumitru, Gotsman, Jalilian-Marian, Kovner, Lappi, Levin, Lublinsky, McLerran, Skokov, Schlichting, Venugopalan, ….
No Jet quenching required in pp / pA
If saturated initial state needed to describe pp UE, then dramatic implications. Tests …
5. Low-density Scenario: Collectivity from interference
B.Blok, C. Jäkel, M. Strikman, UAW, arXiv:
No initial density and no initial asymmetry, no final state interactions
Contribution to vn from QM interference & color correlations?
This discussion is not (yet) tailored to HL-LHC but it illustrates the importance of studying in the 2020s small systems (pA, smaller AA) to address the main conceptual issues (collectivity/ fluid dynamics / hadrochemistry) at LHC heavy ion physics.
Scrutinize need of pA (only one run planned in 2020s)
Scrutinize need of smaller AA (ArAr, XeXe), (nothing foreseen yet)