Some Remarks on pPb
Urs Achim Wiedemann
CERN PH-TH
LPCC Workshop
CERN, 9 May 2016
Heavy Ion Physics – Future Directions
sNN = 8.16 TeV
Luminosity*
Higher luminosity
pA physics*
sNN = 5.02 TeV
Higher luminosity
pPb and PbPb data
Status May 2016
Higher
Energy
(FCC)
Adependence*
(Ar, Xe, …)
• Recognized complementarity btw. different directions.
• Different opinions about opportunities of different research directions, but
not about the principal physics interest of exploring all of them.
Heavy Ions
1.
2.
3.
4.
Quarkonium dissociation (vs. centrality, pt, flow …)
Jet quenching studies
Electromagnetic probes
…
• Higher Luminosity in pA to study rarer probes
1.
2.
3.
4.
Constraining nuclear pdfs
Searching for non-linear QCD evolution (saturation)
Search for onset of quenching in small systems
…
1. Flow-like phenomena in smaller systems (pA, pp)
2. Flow of rarer probes (quarkonium, open HF)
3. …
Why is it important to understand “flow”? pto
Logic different from
high-lumi pp …
• Higher Luminosity to in pA to study soft probes, e.g.
Logic as in high-lumi pp …
• Higher Luminosity in A+A to study rarer probes
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) 71-107
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
Heavy Ion “Hydro” Phenomenology - scetch
Fundamental QCD
Lagrangian
QCD thermodynamics
Data on flow in A+A
Model
comparisons
QCD hydrodynamics
predicts long wavelength
behavior of hot QCD
P. Romatschke
arXiv.0902.3663
Assessing
uncertainties
Testing fundamental
properties of hot QCD
(equation of state, velocity of
sound, transport coefficients, …)
Flow-like phenomena in pPb (2013 run)
• Flow-like features of “similar strength as in PbPb, seen in pPb
CMS, PRL 115 (2015) 012301
• Arguably, these flow-like signatures (seen by ALICE, ATLAS and CMS)
are the most important novel issue raised by the 1st LHC pPb run:
Do small systems (=pPb) show fluid dynamic behavior?
(or: to what extent does one have to reassess the (accuracy of) the
the fluid dynamic analyses of PbPb collisions)
=> only one issue that can be addressed in 2016 pPb run,
but possibly the most central to be addressed
Basic considerations on event multiplicities and
“comparisons data”
• Given lack of knowledge of soft physics,
comparing physics at same multiplicity
(and same sqrt s) across system size is of
interest.
=> argument for run at 5 TeV reference
• Increased event multiplicity helps to
pursue cumulant flow analysis and
EbyE correlations
=> argument for run at 8 TeV reference
Heavy Flavor and Quarkonium Flow
• Given pQCD production, vn expected to result from heavy flavored partons being
“dragged along” with the flow field => qualitatively novel probe of flow paradigm
• There are strong reasons to push 8 TeV pPb data to
-> improve reach / reference data in hard probes
-> get better access to npdfs and possible signatures of “CGC”
-> make advances in the understanding of the soft sector
• There are strong reasons to push for 5 TeV pPb data, mainly
since reference data for PbPb and pp exist – provides interesting
information mainly for better understanding the soft physics sector.
• There is no a priori reason to prefer one of the two options in 2016.
There are reasons of practicality, not more.
•In an ideal world, both data sets will be taken (even if this would take more
than 4 weeks running time at the LHC – the arguments are strong).
Back-up
Initial conditions are not in thermal equilibrium
The small-x gluon distributions relevant for
particle production at large sNN ,
grow strongly in perturbation theory.
At high sNN , one expects a qualitatively
novel kinematical regime of QCD:
•Coupling constant is small
2
Q2 < Qsat
~ (2 - 3GeV)2
2
a s(Qsat
(x) >> LQCD )
• Maximal parton densities at small x
are over-occupied
r ~1 a
s
(thermal distributions are r ~ 1)
• Initial momentum distribution is
anisotropic
Venugopalan McLerran; JalilianMarian,Kovner,Leonidov,Weigert;
Balitsky; Kovchegov;…
How does such an
overoccupied anisotropic
system “hydrodynamize”?
The route of weakly coupled QCD to equilibrium
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 based effective Boltzmann eq
l = 4p Ncas
applies to leading order in
l f for p > m º l
2
2
òf
p
p and f <<1/ l
p
A. Kurkela, arXiv:1601.03283
Berges, Moore, Schlichting,
Eppelbaum, Venugopalan, …
Hydrodynamization of kinetic evolution
For sufficiently large coupling constant, dissipative hydrodynamics
describes long-time behavior of QCD effective kinetic theory
as = 0.03
Kurkela&Zhu, PRL115 (2015)182301
Dynamical understanding of “thermalization time”.
as = 0.3
Open heavy flavor at low pt
•
At low pt, Langevin dynamics determines how charm & beauty quarks move:
The perfect liquid is source of random forces
dpL
= xL (t) - m( pL ) pL ,
dt
dpT
= xL (t)
dt
xL (t)xL (t') º k L ( pL ) d (t - t')
xTi (t)xTj (t') º kT ( pL ) d ij d (t - t')
calculable from 1st principles in quantum field
theory, e.g. in strong coupling limit:
kT = p l T 3 g
•
k L = p lT 3g 5 / 2
This hard probe is unique in that we have
first experimental indications of flow.
Much more differential characterization
needed to constrain Langevin dynamics.
(High luminosity requirement!).