1. Introduction to the WG plans
WG5: Heavy Ions
Introduction to the WG plans
Urs Achim Wiedemann
CERN TH Department
Workshop on the physics of HL-LHC, 30 Oct-1 Nov 2017

2. Fundamental question:
How do collective phenomena and macroscropic properties of matter arise from the elementary interactions of a non-abelian quantum field theory?
Opportunities
Tools
Status
Constraining equilibrium properties of QCD matter (eos, )
Flow and fluctuation measurements in AA
advanced
Accessing microscropic structure of QCD matter in AA
Measuring medium properties with hard auto-generated probes ( )
Jet substructure, heavy flavor transport
Quarkonia, RAA’s , photons
in progress
in reach
Controlling initial conditions
pA (light AA) runs, npdf global fits, small-x
Testing hydrodynamization and thermalization
Combined jet and flow analyses
strategy t.b.d.
Understanding “heavy-ion like behavior” in small systems (pp, pA)
Flow, hadrochemistry, jets
recent surprises
List to be refined and extended

3. Directions during the HL-LHC era and beyond
Luminosity in AA
fundamental questions in QCD Collectivity
(i.e. AA, pA and soft pp)
pA runs
pp comparison ?
A-dependence*
(Ar, Xe, …)
Beyond expected advances due to detector upgrades and increased integrated luminosity, the WG plans to document the relevance of LHC’s versatility (in terms of nuclear beam species, pA runs, energy dependence) for a scientific programme aimed at understanding QCD collectivity.

4. Constraining QCD equilibrium properties - continued
Heavy Ions (Cosmic microwave background) are the smallest (largest) physical systems for which a fluctuation analysis gives access to material properties.
Accuracy limited mainly by knowledge of initial conditions. Need better control over
initial glue (motivates HL-LHC)
transverse density distributions (motivates pA and other beam species), more severe in smaller systems
Tu 15:15-16:30, session 2 (Floerchinger, Spousta)

5. Constraining QCD equilibrium properties
QCD transport properties are computable from 1st principles in quantum field theory. (tools: gauge gravity duality, lattice QCD) E.g. for shear viscosity
Status: data – theory comparison via fluctuation analysis (e.g. Bayesian analysis of fluid dynamic simulations)
prior
posterior
J.E. Bernhard et al. , arXiv: v2, fit to ALICE data.
T-dependence of transport coefficients, bulk viscosity, charge transport, … may come in reach due to precision.
Tu 15:15-16:30, session 2 (Floerchinger, Spousta)

6. Improving constraints on QCD transport / fluid behavior
Event shape engineering:
(may motivate collecting much larger event samples, to be quantified)
Selecting specific spatial topologies in event samples
Correlating tails of vn-distributions to explore scenarios of maximal flow
ATLAS JHEP 1311 (2013) 183
Flow of rare hadrons (measurements at low pT, motivates ALICE ITS upgrade)
Heavy flavor
Quarkonium
Tu 11:00-12:30, session 1 (Beraudo, Bruna, Chapon, Weber)

7. Transport properties imply microscopic properties of plasma
Value of shear viscosity minimal,
=> perfect liquid,
strongly coupled plasma
Strong coupling limit of N=4 SYM
Kovtun, Son, Starinets,
hep-th/
Arnold, Moore, Yaffe, JHEP 11 (2000) 001
A “strongly coupled” plasma is unique in that it does not carry quasi-particle excitations
perturbatively
but is O(1/T).
H-T. Ding et al, arXiv:
Fundamental question:
Quasi-particle peak melts
Up to which momentum scale is the plasma strongly coupled ?

8. Open heavy flavor at low pT
‘No-quasiparticle conjecture’ implies that light low-momentum
dressed quarks do not exist (i.e. do not propagate beyond )
Low-pT heavy charm and bottom quarks provide a unique test of QCD transport theory, test how system approaches equilibirum.
Langevin dynamics determines how charm & beauty quarks move: The liquid is then a source of random forces
calculable from 1st principles,
e.g. in strong coupling limit:
For p->0, Einstein relation
Y. Xu et al pdf
How precisely can one determine the heavy quark diffusion HL-LHC?
Tu 11:00-12:30, session 1 (Beraudo, Bruna, Chapon, Weber)

9. Hunting for the quasi-particle structure of hot QCD @ HL-LHC
Basic signature:
in a strongly coupled plasma, kT of hard partons broadens ~ L (Brownian motion).
Constituents in plasma induce large-angle Molière scattering and recoil.
Basic problem: signature dominates in small region of phase space only.
Kurkela & UAW, PLB740 (2015) 172
Jet substructure can characterize recoil:
Longitudinal phase space =>
Milhano, UAW, Zapp, pdf
Angular phase space =>
Tu 14:00-15:15, session 2 (Zapp, Verweij)

10. Hard HL-LHC
Run 3 & 4: 1.5 – 3/nb per run per experiment, 10-20/nb in total in AA
From slides of G Roland, CMS
Increased luminosity provides differential access to
Jet tagging (e.g. Z-jet provides jet energy calibration independent of energy loss mechanism)
Jet substructure (access to microscopic structure of medium)
Wider ET-reach (constrains dynamics of parton energy loss models)
Heavy Flavor at high pT
(Upper bounds on) jet quenching in pA? (tests relevance of final state interactions in small systems, see following slide).
Tu 14:00-15:15, session 2 (Zapp, Verweij)

11. Novel challenges in the sector of small systems (pp & pA)
Why do small systems flow?
Why does hadrochemistry show heavy-ion like behavior in high-multiplicity pp?
Tu 17:00-18:00, session 3 (Bierlich, Kalweit)
How can pPb show sizeable vn with negligible jet quenching? What is the role of FSI?
The observation of heavy-ion like behavior in pp collisions at the LHC suggests that more physics mechanisms are at play than traditionally assumed”
Fischer & T. Sjöstrand, JHEP01(2017)140

12. Nuclear parton distribution functions (npdfs)
Data from LHC used in recent global fits of npdfs.
Needed to constrain initial conditions for PbPb collisions (current constraints have large uncertainties).
In absence of an e-A collider, LHC provides the best access to npdfs.
Measurements at small-x test non-linear QCD evolution (“parton saturation”).
Future measurements will test the process-independence of npdfs.
We 9:30-10:30, session 4 (Armesto, Bossu)
Eskola, Paakinen, Paukunen, Salgado, Eur.Phys.J. C77 (2017) no.3, 163 

13. Towards a unified picture of quenching and fluid behavior
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
LHC and beyond, We 11:00-13:00, session 5 (Y-J. Lee, Wiedemann)

14. Theory during the HL-LHC era
Non-abelian plasma physics* via gauge-gravity duality
Finite temperature field theory* Lattice QCD, pert. QCD
AA & pA & soft pp phenomenology
@ HL-LHC and beyond
High Energy Physics Jet (sub)structure, heavy flavor, small-x, MPIs, etc
MC simulations Standards (Rivet), Resources, …
QCD transport theory* hydrodynamics (including anisotropic and anomalous fluids)
Cosmology,
Astrophysics
Hadron Physics QCD modeling
Theory connects the LHC heavy ion programme to the wider physics community.
contributes to lasting insights* well beyond a phenomenological understanding.
How to structure and support the theory needed for HL-LHC?

15. The scope of the working groups “Heavy Ions” goes beyond heavy ions,
it includes pA, soft pp, UPC…
Need to rename the WG?