hot qcd matter
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Hot QCD Matter. Peter Jacobs Lawrence Berkeley National Laboratory/CERN. Lecture 1: Tools Lecture 2: Initial conditions: partonic structure and global observables Lecture 3: Collective flow and hydrodynamics Lecture 4: Jets and other hard probes. - PowerPoint PPT PresentationTRANSCRIPT
Hot QCD Matter - Lecture 2
Hot QCD Matter
Peter Jacobs
Lawrence Berkeley National Laboratory/CERN
6/14/12 1
Lecture 1: ToolsLecture 2: Initial conditions: partonic structure and global observablesLecture 3: Collective flow and hydrodynamicsLecture 4: Jets and other hard probes
Hot QCD Matter - Lecture 1 2
Nuclear geometry and hard processes: Glauber theory
d z d b b z , 1
Normalized nuclear density r(b,z):
T b d z b zA
,Nuclear thickness function
Inelastic cross section for p+A collisions:
Glauber scaling for hard processes with large momentum transfer• short coherence length successive NN collisions independent• p+A is incoherent superposition of N+N collisions
6/14/12
Hot QCD Matter - Lecture 1 3
sDrell-Yan/A in p+A at SPS
Experimental tests of Glauber scaling:hard cross sections in p(m)+A collisions
Glauber scaling expectation:
M.May et al, Phys Rev Lett 35, 407 (1975)
sinel for 7 GeV muons on nuclei
A1.00
A
Hard cross sections in p+A scale as A1.0
NA50 Phys Lett B553, 167
hardNN
hardpA A
6/14/12
Measuring collision geometry I
6/14/12 Hot QCD Matter - Lecture 1 4
Nuclei are “macroscopic”characterize collisions by impact parameter
Correlate particle yields from ~causally disconnected parts of phase space
correlation arises from common dependence on collision impact parameter
Measuring collision geometry II
6/14/12 Hot QCD Matter - Lecture 1 5
For
war
d ne
utro
ns
Charged hadrons h~3
• Order events by centrality metric• Classify into percentile bins of “centrality”
HI jargon: “0-5% central”
Connect to Glauber theory via particle production model:• Nbin: effective number of binary
nucleon collisions (~5-10% precision)
• Npart: number of (inelastically scattered) “participating” nucleons
Hot QCD Matter - Lecture 1 66/14/12
Scaling of cross sections using Glauber theory plays a central role in quantitative analysis of experimental measurements and connection to theory.
Let’s test it experimentally in A+A collisions…
7
Glauber scaling tests at LHC: Scaling of direct photon, Z, W yields in Pb+Pb vs p+p
6/14/12 Hot QCD Matter - Lecture 1Yields all scale with Nbin: Glauber scaling OK for hard processes
EW bosons do not interact with Quark-Gluon Plasma – should see perturbative production rates in Pb+Pb collisions
Hot QCD Matter - Lecture 2 8
Very simple question: can we understand the total number of particles generated in a heavy
ion collision (a.k.a. “multiplicity”)?
6/14/12
STAR
RHIC LHC
Hot QCD Matter - Lecture 2 9
Let’s start with the “initial state”: what is the role of the partonic structure of the projectiles?
6/14/12
Hadrons and nuclei arecompound objects with complex partonic structure
Multiple interactions drive the collision dynamics
we need to understand the initial (incoming) state…
Hot QCD Matter - Lecture 2 10
2223
3
,ˆ
,, QzDdt
dQxfQxf
dp
dE cch
cdab
bBbaAa
Perturbative QCD factorization in hadronic collisions
pQCD factorization:
+ fragmentation fn Dh/c
+ partonic cross section s
parton distribution fn fa/A
Hard process scale Q2>>L2QCD
6/14/12
x=momentum fraction of hadron carried by parton (infinite momentum frame)
Hot QCD Matter - Lecture 2 11
2223
3
,ˆ
,, QzDdt
dQxfQxf
dp
dE cch
cdab
bBbaAa
Q2 evolution of Parton Distribution and Fragmentation Functions
Parton Distribution Fucntions (PDFs) and fragmentation functions are not calculable ab initio in pQCD
They are essentially non-perturbative in origin (soft, long distance physics) and must be extracted from data at some scale Q0
2
pQCD then specifies how PDFs and fragmentation functions evolve from Q0
2 to any other scale Q2 (DGLAP evolution equations)
Q2 evolution
small Q2
large Q2
0.1 1.0 x6/14/12
Hot QCD Matter - Lecture 2 12
Precision measurements of proton structure:Deep Inelastic Scattering (DIS) of e+p
6/14/12
Hot QCD Matter - Lecture 2 13
Probing the structure of the proton with DIS
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Sum over quark flavors
Define a new quantity F2:parton density for flavor i
charge for flavor i
If a proton were made up of 3 quarks, each carrying 1/3 of proton’s momentum:
•If partons are point-like and incoherent then Q2 shouldn’t matter•Bjorken scaling: F2 has no Q2 dependence
with some smearing
F2
x
Hot QCD Matter - Lecture 2 14
Measurement of proton F2
6/14/12
Tour de force for perturbative QCD:
Q2 does matter!
• Partons are not point-like and incoherent. • Hadronic structure depends
on the scale at which you probe it!
Spectacular agreement with DGLAP evolution
Hot QCD Matter - Lecture 2 15
Parton Distribution Function in the proton
Low Q2: valence structure
Soft gluons
Q2 evolution (gluons)
6/14/12
2223
3
,ˆ
,, QzDdt
dQxfQxf
dp
dE cch
cdab
bBbaAa
Gluon density decreases towards lower Q2
Valence quarks (p = uud)x ~ 1/3
Hot QCD Matter - Lecture 2 16
Gluon saturation at low x
6/14/12
Fix Q2 and consider what happens as x is decreased…
Problem: low x gluon density cannot increase without limit (unitarity bound)Solution:• gluons carry color charge• if packed at high enough density they will recombine
gluon density is self-limiting gluon saturation !
Hot QCD Matter - Lecture 2 17
Gluon recombination in nuclei
6/14/12
Uncertainty principle: wave fn. for very low momentum (low x) gluons extends over entire depth of nucleus
Define gluon density per unit area in nucleus of mass A:
Gluon recombination cross section:Recombination occurs if:
Saturation momentum scale Qsat2 satisfies
self-consistent condition:
Gluon recombination for Q2 < Qsat2
Nuclear enhancement of Qsat
Hot QCD Matter - Lecture 2 18
Saturation scale vs nuclear mass
6/14/12
RHICHeavy ions
LHCHeavy ions
What’s that?
Hot QCD Matter - Lecture 2 19
Color Glass Condensate (CGC)
6/14/12
Semi-classical effective theory of saturation
Translation: number of (final-state) charged particles seen in
the detector in a heavy ion collision is proportional to number
of gluons scattered early in the collision (K~1), which is
suppressed due to saturation effects
Predicts specific scaling with collision centrality and √s
Hot QCD Matter - Lecture 2
Can we see Saturation experimentally?
6/14/12 20
d
Au
p0
Triggered hadron
xqpNxgpN
qp
Ep
Associated hadron
PT is balanced by many gluons
Dilute parton system
(deuteron)
Dense gluonfield (Au)
Mono-jet
Asymmetric deuteron+Au collisions at RHIC: • Look at forward 2-particle correlations• Back-scatter off Au-nucleus: low x in Au
Perturbative picture:Back-to-back jets
Saturation picture:“mono-jets”
What are we plotting? 2-particle correlations in azimuthal angle
21
Transverse plane
trigger
6/14/12 Hot QCD Matter - Lecture 2
d
Au
p0
Triggered hadron
xqpNxgpN
qp
Ep
Associated hadron
Trigger
Hot QCD Matter - Lecture 2 22
STAR: d+Au forward azimuthal correlations
6/14/12
p+p d+Au
Peripheral Central
CGC Model : Albacete+Marquet
(arXiv: 1005.4065)
Centrality Averaged
Mono-jet/saturation
perturbative
Hot QCD Matter - Lecture 2 236/14/12
But maybe not:
Conventional pQCD mechanisms plus conventional nuclear effects work as well…
J. Albacete, Hard Probes 2012
Hot QCD Matter - Lecture 2 24
Next step: p+A at LHC (November 2012 run)
6/14/12
C. Salgado, Hard Probes 2012
Region of greatest interest: low x and low Q2
Hot QCD Matter - Lecture 2 25
Summary thus far
6/14/12
QCD is remarkably successful in describing the partonic stucture of the proton over a vast kinematic range
There are good reasons to expect signficant modification of this structure in heavy nuclei saturation
• Some experimental evidence in favor of saturation in forward d+Au correlations at RHIC
• LHC p+A run this November will provide a wealth of new data to address the issue in more detail (crucially: much smaller x)
Does any of this play a role in high energy nuclear collisions?Let’s go back to our original question: what generates all the particles?
Hot QCD Matter - Lecture 2 266/14/12
STAR
RHIC LHC
Multiplicity measurementsCount the number of charged particles per unit pseudo-rapidity
Simplest “bulk” observable that characterizes the collision
Charged particle multiplicityALICE PRL, 105, 252301 (2010), arXiv:1011.3916
√sNN=2.76 TeV Pb+Pb, 0-5% central, |η|<0.5
6/14/12 27Hot QCD Matter - Lecture 2
Normalize “per pair of participant
nucleons” to compare to p+p
(Glauber modeling)
LHC: 2 dNch/dη / <Npart> = 8.3 ± 0.4 (sys.)
dNch/dη: model comparisons
pp extrapolation
pQCD-based MC
Saturation
PRL, 105, 252301 (2010), arXiv:1011.3916
dNch/dη = 1584 ± 76 (sys.)
√sNN=2.76 TeV Pb+Pb, 0-5% central, |η|<0.5
Energy density estimate (Bjorken):
6/14/12 28Hot QCD Matter - Lecture 2
WAGs
dNch/dη: Centrality dependencePRL, 106, 032301 (2011), arXiv:1012.1657
Interpolation between 2.36 and 7 TeV pp
Pb+Pb, √sNN=2.76 TeV
2.5% bins
|η|<0.5
ALICELHC
scal
e RHIC scaleRHIC
peripheral central
6/14/12 29Hot QCD Matter - Lecture 2Striking centrality-independent scaling RHICLHC
Hot QCD Matter - Lecture 2 30
Does saturation play a role?
6/14/12
RHICLHC
peripheral central
Expectation from saturation models: factorization of centrality and energy dependence:
dNch/dη vs. centrality: modelsPRL, 106, 032301 (2011), arXiv:1012.1657
Two-component perturbative models Soft (~Npart) and hard
(~Nbin) processes
Saturation-type models Parametrization of the saturation
scale with centrality
Pb+Pb, √sNN=2.76 TeV
Albacete and Dumitru (arXiV:1011.5161):• Most sophisticated saturation model:
evolution, running coupling• Captures full centrality dependence…?
6/14/12 31Hot QCD Matter - Lecture 2
Hot QCD Matter - Lecture 2 32
Summary of Lecture 2
6/14/12
Initial state: approaching quantitative control
Final charged multiplicity closely related to initial gluon multiplicity:
Smooth evolution of multiplicity with collision energy and system size
Good evidence that gluon saturation in nuclei plays a role
Hot QCD Matter - Lecture 2 33
Why is any of this surprising? How could it be different?
6/14/12
Thermalized system: massive re-interactions, generation of large numbers of particles and softening of momentum spectra
expect stronger dependence on energy and system size…?
Apparently not the case
Next lecture: additional news about equilibration.
?
Hot QCD Matter - Lecture 2 34
Backup
6/14/12
Hot QCD Matter - Lecture 2 35
XpeXepe ee )( :CC , :NC
Simpler case: deep inelastic scattering (DIS) of e+p
6/14/12
Hot QCD Matter - Lecture 1 36
Glauber Theory for A+B Collisions
6/14/12
Nuclear overlap function:
Average number of binary NN collisions for B nucleon at coordinate sB:
Average number of binary NN collisions for A+B collision with impact parameter b: