Power Week pQCD Energy Loss Introduction

1
Power Week pQCDEnergy LossIntroduction
Marco van Leeuwen, Utrecht University
2
Hard probes of QCD matter
Heavy-ion collisions producequasi-thermal QCD
matter Dominated by soft partons p T 100-300
MeV
Hard-scatterings produce quasi-free partons ?
Initial-state production known from pQCD ? Probe
medium through energy loss
Use the strength of pQCD to explore QCD matter
Sensitive to medium density, transport properties
3
Plan of the next few days

• Goals
• Provide hands-on experience with all ingredients
  of a simple energy loss model
• Increase understanding of the models, the
  assumptions and uncertainties
• Provide a basic knowledge and experience that
  allows you to answer your own questions

• Perturbative QCD tools
• PDFs, matrix elements, Fragmentation
• DGLAP evolution and Monte-Carlo showers
• Geometry
• Woods-Saxon geometry, tools
• Energy loss models
• Using Quenching weights multiple gluon radiation

Plus introduction to MC techniques
4
Hard processes in QCD

• Hard process scale Q gtgt LQCD
• Hard scattering High-pT parton(photon) QpT
• Heavy flavour production m gtgt LQCD

Factorization

• Cross section calculation can be split into
• Hard part perturbative matrix element
• Soft part parton density (PDF), fragmentation
  (FF)

parton density
matrix element
FF
QM interference between hard and soft suppressed
(by Q2/L2 Higher Twist)
Soft parts, PDF, FF are universal independent of
hard process
5
Jet Quenching

• How is does the medium modify parton
  fragmentation?
• Energy-loss reduced energy of leading hadron
  enhancement of yield at low pT?
• Broadening of shower?
• Path-length dependence
• Quark-gluon differences
• Final stage of fragmentation outside medium?
• 2) What does this tell us about the medium ?
• Density
• Nature of scattering centers? (elastic vs
  radiative mass of scatt. centers)
• Time-evolution?

6
p0 RAA high-pT suppression
? RAA 1
?0 RAA 0.2
Hard partons lose energy in the hot matter
7
A simple model
Parton spectrum
Energy loss distribution
Fragmentation (function)
known pQCDxPDF
extract
known from ee-
This is where the information about the medium is
P(DE) combines geometry with the intrinsic
process Unavoidable for many observables

• Notes
• This formula is the simplest ansatz Independent
  fragmentationafter E-loss assumed
• Jet, g-jet measurements fix E, removing one of
  the convolutions

We will explore this model during the week was
state of the art 3-5 years ago
8
Two extreme scenarios
(or how P(DE) says it all)
Scenario I P(DE) d(DE0)
Scenario II P(DE) a d(0) b d(E)
1/Nbin d2N/d2pT
Energy loss
Absorption
pp
Downward shift
AuAu
Shifts spectrum to left
pT
P(DE) encodes the full energy loss process
RAA not sensitive to energy loss distribution,
details of mechanism
9
Energy loss distribution
Typical examples with fixed L
ltDE/Egt 0.2
R8 RAA 0.2
Brick L 2 fm, DE/E 0.2 E 10 GeV
Significant probability to lose no energy (P(0))
Broad distribution, large E-loss (several GeV,
up to DE/E 1)
Theory expectation mix of partial
transmissioncontinuous energy loss Can we see
this in experiment?
10
Geometry
Density along parton path
Density profile
Profile at t tform known
Longitudinal expansion dilutes medium ?
Important effect
(Glauber geometry)
Space-time evolution is taken into account in
modeling
11
Some existing calculations
Bass et al, PRC79, 024901
Large density AMY T 400 MeV Transverse kick
qL 10-20 GeV
All formalisms can match RAA, but large
differences in medium density
This week looking behind the scenes for such
calculations
After long discussions, it turns out that these
differences are mostly due to uncontrolled
approximations in the calculations ? Best guess
the truth is somewhere in-between
At RHIC DE large compared to E, differential
measurements difficult
12
RAA at LHC
Nuclear modificationfactor
RAA at LHC increase with pT ? first sign of
sensitivity to P(DE)
13
Comparing to theory
Many theory calculations available

• Ingredients
• pQCD production
• Medium density profiletuned to RHIC data, scaled
• Energy loss model

• Large spread of predictions
• Will be narrowed down by discussion/thought
• Need to understand models/calculations to sort
  it out

All calculations show increase with pT
14
Path length dependence RAA vs L
RAA as function of angle with reaction plane
PHENIX, PRC 76, 034904
Out of Plane
In Plane
3ltpTlt5 GeV/c
Relation between RAA(j) and v2
Suppression depends on angle, path length
15
Path length dependence and v2
PHENIX PRL105, 142301
v2 at high pT due to energy loss
Most calculations give too small effect Path
length dependence stronger than expected? Depends
strongly on geometry stay tuned
16
Dihadron correlations
8 lt pTtrig lt 15 GeV
associated
pTassoc gt 3 GeV
??
trigger
Near side
Away side
Use di-hadron correlations to probe the
jet-structure in pp, dAu
and AuAu
17
Di-hadrons at high-pT recoil suppression
dAu
AuAu 20-40
AuAu 0-5
pTassoc gt 3 GeV
pTassoc gt 6 GeV
High-pT hadron production in AuAu dominated by
(di-)jet fragmentation
Suppression of away-side yield in AuAu
collisions energy loss
18
Dihadron yield suppression
Away side
8 lt pT,trig lt 15 GeV
Near side
Yield in balancing jet, after energy loss
Yield of additional particles in the jet
trigger
STAR PRL 95, 152301
Away side associated
Away-side Suppressed by factor 4-5 ? large
energy loss
Near side No modification ? Fragmentation
outside medium?
19
Path length II surface bias
Near side trigger, biases to small E-loss
Away-side large L
Away-side suppression IAA samples longer
path-lengths than inclusives RAA
20
L scaling elastic vs radiative
T. Renk, PRC76, 064905
Radiative scenario fits data elastic scenarios
underestimate suppression
RAA input to fix density
Indirect measure of path-length dependence
single hadrons and di-hadrons probe different
path length distributions
Confirms L2 dependence ? radiative loss dominates
21
Factorisation in perturbative QCD
Parton density function Non-perturbative
distribution of partons in proton Extracted from
fits to DIS (ep) data
Matrix element Perturbative component
Fragmentation function Non-perturbative Measured/e
xtracted from ee-
Factorisation non-perturbative parts
(long-distance physics) can be factored out in
universal distributions (PDF, FF)
22
Subprocesses and quark vs gluon
ppbar dominantly from gluon fragmentation?
23
Comparing quark and gluon suppression
Baryon meson NMF
PRL 97, 152301 (2006) STAR Preliminary, QM08
STAR Preliminary
Curves X-N. Wang et al PRC70(2004) 031901
Protons less suppressed than pions, not more
No sign of large gluon energy loss
24
Quark vs gluon suppression
GLV formalism
BDMPS formalism
WHDG
renk plot
Renk and Eskola, PRC76,027901
Quark/gluon difference larger in GLV than
BDMPS (because of cut-off effects DE lt Ejet?)
10 baryons from quarks, so baryon/meson effect
smaller than gluon/quark
Are baryon fragmentation functions under control?
Conclusion for now some homework to do... Day 1,
3 of this week
25
Equalibration of rare probes

• Rare probes not chemically equilibrated in the
  jet spectrum.
• Example 1 flavor not contained in the medium,
  but can be produced off the medium (e.g. photons)
• Need enough yield to outshine other sources of
  Nrare.
• Example 2 flavor chemically equilibrated in the
  medium
• E.g. strangeness at RHIC
• Coupling of jets (flavor not equilibrated) to the
  equilibrated medium should drive jets towards
  chemical equilibrium.

R. Fries, QM09
26
Determining the initial energy
Parton spectrum
Energy loss distribution
Fragmentation (function)
known pQCDxPDF
extract
known from ee-
This is where the information about the medium is
P(DE) combines geometry with the intrinsic
process
Jet, g-jet measurements fix E, removing one of
the convolutions
Allows to study energy loss as function of E (at
least in principle)
27
Generic expectations from energy loss
Ejet
kTm
l
fragmentation after energy loss?

• Longitudinal modification
• out-of-cone ? energy lost, suppression of yield,
  di-jet energy imbalance
• in-cone ? softening of fragmentation
• Transverse modification
• out-of-cone ? increase acoplanarity kT
• in-cone ? broadening of jet-profile

28
Fragmentation functions
Qualitatively
Fragmentation functions sensitive to
P(DE) Distinguish GLV from BDMPS?
29
Modified fragmentation functions
Small-z enhancement from gluon fragments (only
included in HT, not important for RAA)
A. Majumder, MvL, arXiv1002.2206
Differences between formalisms large, both
magnitude of supresion and z-dependence
Can we measure this directly? Jet reconstruction
30
Jet shapes
q-Pythia, Eur Phys J C 63, 679
Energy distribution in sub-jets
Energy loss changes radial distribution of energy
Several new observables considered Discussion
sensitivity ? viability ongoing
31
Fixing the parton energy with g-jet events
T. Renk, PRC74, 034906
Input energy loss distribution
Away-side spectra in g-jet
Eg 15 GeV
Nuclear modification factor
Away-side spectra for g-jet are sensitive to
P(DE)
g-jet know jet energy ? sensitive to P(DE)
RAA insensitive to P(DE)
32
Direct-g recoil suppression
8 lt ET,g lt 16 GeV
STAR, arXiv0912.1871
DAA (zT)
IAA(zT)
Dpp (zT)
Large suppression for away-side factor 3-5
Reasonable agreement with model predictions
NB gamma pT jet pT still not very large
33
Jet reconstruction algorithms

• Two categories of jet algorithms
• Sequential recombination kT, anti-kT, Durham
• Define distance measure, e.g. dij
  min(pTi,pTj)Rij
• Cluster closest
• Cone
• Draw Cone radius R around starting point
• Iterate until stable h,jjet lth,jgtparticles

Sum particles inside jet Different prescriptions
exist, most natural E-scheme, sum 4-vectors
Jet is an object defined by jet algorithm If
parameters are right, may approximate parton
For a complete discussion, see
http//www.lpthe.jussieu.fr/salam/teaching/PhD-co
urses.html
34
Jets at LHC
LHC jet energies up to 200 GeV in PbPb from 1
short run Large energy asymmetry observed for
central events
35
Jets at LHC
Centrality
ATLAS, arXiv1011.6182 (PRL)
Large asymmetry seen for central events
Jet-energy asymmetry
Energy losses tens of GeV, expected from
BDMPS, GLV etc beyond kinematic reach at RHIC
N.B. only measures reconstructed di-jets
Does not show lost jets
Large effect on recoil qualitatively consistent
with RHIC jet IAA
36
Jets at LHC
CMS, arXiv1102.1957
CMS sees similar asymmetries
37
Jet RCP
R0.2
R0.4
RCP lt 1 jet production suppressed, even at high
pT ? Out-of-cone radiation with R0.4
significant
NB Jet-measurements are difficult important
experimental questions about (trigger) bias and
background fluctuations
38
Jet imbalance calculations
Coleman-Smith, Qin, Bass, Muller, arXiv1108.5662
Qin, Muller, arXiv1012.5280
Parton transport (brick)
Radiation plus evolution
Several calculations describemeasured imbalance
Young, Schenke, Jeon, Gale, arXiv1103.5769
Need to keep track of all fragments Various
approximations made
Most natural approach parton showers (qPYTHIA,
qHERWIG, JEWEL ?)
MARTINI AMYMC
39
Fragmentation and parton showers
MC event generators implement parton
showers Longitudinal and transverse dynamics
High-energy parton (from hard scattering)
Hadrons
Q mH LQCD
large Q2
Analytical calculations Fragmentation Function
D(z, m) zph/Ejet Only longitudinal dynamics
40
Getting ready

• Software set-up
• ROOT
• LHAPDF
• Fragmentation function libraries
• AliFastGlauber
• AliQuenchingWeights

Day 1
Day 2
Day 3
See also http//www.staf.science.uu.nl/leeuw179/p
owerweek/software
Make sure that you have the code and that the
test macros work
Questions/problems ? See me or Andreas
41
Extra slides
42
Seeing quarks and gluons
In high-energy collisions, observe traces of
quarks, gluons (jets)