Jet modifications at RHIC Marco van Leeuwen, Utrecht University
Jan 18, 2018
Jet modifications at RHIC
Marco van Leeuwen, Utrecht University
2
QCD and quark parton model
S. Bethke, J Phys G 26, R27
Running coupling:s grows with decreasing Q2
Asy
mpt
otic
free
dom
At low energies, quarks are confined in hadrons
At high energies, quarks and gluons are manifest
gqqee
Running coupling: from confinement to asymptotic freedomQCD governs both extremes.
Can we study/conceptualise the evolution?
This is the basic theory, but what is the phenomenology?
QCD Lagrangian
3
Hard probes of QCD matter
Use the strength of pQCD to explore QCD matter
Use ‘quasi-free’ partons from hard scatterings
to probe ‘quasi-thermal’ QCD matterInteractions between parton and medium:-Radiative energy loss-Collisional energy loss-Hadronisation: fragmentation and coalescence
Sensitive to medium density, transport properties
Calculable with pQCD
Quasi-thermal matter: dominated by soft (few 100 MeV) partons
4
Radiative energy loss in QCD
2
ˆ q
2ˆ~ LqE Smed
Energy loss process characterized by a single constant
Transport coefficient
43~~ˆ glueq
Transport coefficient is a fundamental parameter of QCD matter
Energy loss
kT~
pQCD expectation
Transport coefficient sets medium properties
Non-perturbative: is a Wilson loopq̂
(Wiedemann)
369.26ˆ TNq cSYMSYM (Liu, Rajagopal, Wiedemann)e.g. N=4 SUSY: From AdS/CFT
(Baier et al)
5
STARSTAR
Relativistic Heavy Ion Collider
PHENIX STAR
Au+Au sNN= 200 GeV
RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+CuTwo large experiments: STAR and PHENIX
Smaller experiments: PHOBOS, BRAHMS decomissionedDedicated to study QCD: proton spin and Quark Gluon Plasma
6
High-pT hadron suppression
Size of medium
ppTbin
AuAuTAA dpdNN
dpdNR
/
/
Compare Au+Au spectra to properly scaled p+p spectra:
‘nuclear modification factor’
: no interactions
Hadrons: energy loss
RAA = 1
RAA < 1
Direct photons confirm volume scaling
Hadrons suppressed: energy loss
7
Energy loss in QCD matter
ppTbin
AuAuTAA dpdNN
dpdNR
/
/
: RAA = 1
0, h±: RAA ≈ 0.2
Au+Au 200 GeV, 0-5% centralCompare Au+Au spectra to properly scaled p+p spectra:
‘nuclear modification factor’
D. d’Enterria
Hard partons lose energy in the hot matter
Hadron suppression ~ independent of pT for pT>~4 GeV
: no interactions
Hadrons: energy loss
RAA = 1
RAA < 1
8
Di hadron correlations
associated
trigger
8 < pTtrig < 15 GeV
pTassoc > 3 GeV
Use di-hadron correlations to probe the jet-structure in p+p, d+Au
Near side Away side
and Au+Au
Combinatorialbackground
pT assoc > 3 G
eVp
T assoc > 6 GeV
d+Au Au+Au 20-40% Au+Au 0-5%
Suppression of away-side yield in Au+Au collisionsMeasures energy loss in di-jet events
No detectable broadening or change of peak shape: fragmentation after energy loss
High-pT hadron production in Au+Au dominated by (di-)jet fragmentation
Highest pT: focus on fragmentation
10
Di hadron yield suppression
No suppression Suppression byfactor 4-5 in central Au+Au
Away-side: Suppressed by factor 4-5 large energy loss
Near side Away side
STAR PRL 95, 152301
8 < pT,trig < 15 GeV
Yield of additional particles in the jet
Yield in balancing jet, after energy loss
Near side: No modification Fragmentation outside medium?
11
Theory vs. data IPHENIX, arXiv:0801.1665,J. Nagle WWND08
PQM (Loizides, Dainese, Paic),Multiple soft-scatttering approx (Armesto, Salgado, Wiedemann)Realistic geometry
GLV (Gyulassy, Levai, Vitev), opacity expansion (L/), average path length
WHDG (Wicks, Horowitz, Djordjevic, Gyulassy)GLV + realistic geometry
ZOWW (Zhang, Owens, Wang, Wang) Medium-enhanced power corrections (higher twist) Hard sphere geometry
AMY (Arnold, Moore, Yaffe) Finite temperature effective field theory (Hard Thermal Loops)
For each model:
1. Vary parameter and predict RAA
2. Minimize 2 wrt data
Models have different but ~equivalent parameters:
• transport coeff. • gluon density dNg/dy• energy density 0
• coupling constant S
q̂
12
Medium density from RAA
PQM <q> = 13.2 GeV2/fm +2.1- 3.2
^
GLV dNg/dy = 1400 +270- 150
WHDG dNg/dy = 1400 +200- 375
ZOWW 0 = 1.9 GeV/fm +0.2- 0.5
AMY s = 0.280 +0.016- 0.012
Quantitative extraction gives medium density to 10-20% Method takes into account only exprimental uncertainties Theory uncertainties need to be further evaluted by comparing different formalisms and other model parameters
Different models approximately agree – except PQM, high density
Density 30-50x cold nuclear matter
13
d-Au
Au-Au
Medium density from di-hadron measurement
IAA constraintDAA constraintDAA + scale uncertainty
J. Nagle, WWND2008
associated
trigger
0=1.9 GeV/fm single hadrons
Medium density from away-side suppression and single hadron suppression agreeSome open questions: disagreement in d+Au?
14
Fundamental quantity P(E)
~15 GeV
Renk, Eskola, hep-ph/0610059
Salgado and Wiedemann, Phys. Rev. D68, 014008
Radiation spectrum Radiation in realistic medium
In realistic systems, energy loss is a broad distribution P(E)
Single-hadron and di-hadron observables fold production spectra with P(E)
Can we access P(E) experimentally?Need to fix parton energy: -jet events
Ejet = E
15
-jet in Au+Au
Use shower shape in EMCal to form 0 sample and -rich sample
Combinatorial subtraction to obtain direct- sample
A. Hamed, STAR, QM08
16
Away-side suppression with direct- triggers
A. Hamed et al QM08
First -jet results from heavy ion collisions
Measured suppression agrees with theory expectations
Next step: measure pTassoc dependence to probe E distribution
Model predictions tuned to hadronic
measurements
17
Lowering pT: gluon fragments/bulk response
3 < pt,trigger < 4 GeVpt,assoc. > 2 GeV
Au+Au 0-10%STAR preliminaryassociated
trigger
d+Au 40-100%Jet-like peak
`Ridge’: associated yield at large dN/d approx. independent of
Strong - asymmetry suggests coupling to longitudinal flowLong. flow Long. flow
J. Putschke, M. van Leeuwen, et al
18
More medium effects: away-side
3.0 < pTtrig < 4.0 GeV/c
1.3 < pTassoc < 1.8 GeV/c
A. Polosa, C. Salgado
Vitev, PLB630
Mach Cone/Shock wave
T. Renk, J. Ruppert
Stöcker, Casseldery-Solana et al
Gluon radiation+Sudakov
Au+Au 0-10%
d+Au
Near side:Enhanced yield in Au+Au consistent with ridge-effect Away-side:
Strong broadening in central Au+Au‘Dip’ at =
Medium response (shock wave)or gluon radiation with kinematic constraints?
(other proposals exist as well: kT-type effect or
Cherenkov radiation)
M. Horner, M. van Leeuwen, et al
Trigger particle
Note also: not shown is large background – some non-trivial may be hiding there?
19
pT evolution of correlations
Increase trigger pT
arXiv:0801.4545
A. Hanks et al, WWND08PHENIX arXiv 0801.4545
Low pT: recoil clear double-peak
Effect reduces with increasing pT?
20
Conclusion
• High-pT hadron production at RHIC probes jet structure and parton energy loss• Data/theory comparisons becoming quantitative
– Qhat ~ 10 GeV2/fm, dN/dy ~ 1500 medium density 30-50x nuclear matter
• ‘Golden probe’ -jet:– First results consistent with hadron measurements– Can we extract P(E)? Statistics needed …
• Intermediate pT 1 - 4 GeV, new phenomena:– Ridge: - assymetry, yield at large – Broad recoil distribution
Luminosity still increase… so is theoretical understandingMore to come!
21
0-12%
4.0 < pTtrig < 6.0 GeV/c 6.0 < pT
trig < 10.0 GeV/c3.0 < pTtrig < 4.0 GeV/c
Preliminary
Au+Au 0-12%1.3 < pT
assoc < 1.8 GeV/c
Low pTtrig: broad shape, two peaks High pT
trig: broad shape, single peak
Away-side shapes
Fragmentation becomes ‘cleaner’ as pTtrig goes up
Suggests kinematic effect?
M. Horner, M. van Leeuwen, et al
22
Background level for di-hadrons
Signal is few per centSo is v2-modulation
Δ12
2-Part Correlation
Flow background
“Jetty”signal
C. Pruneau, Q
M06
23
Energy loss in a QCD medium
+ alternative scenarios, e.g. shock wave
Energy loss and fragmentation
Unmodified fragmentation after energy loss
Fragmentation in the medium completely modified
A more complete picture
FragmentationIn-medium energy loss
Or in-medium fragmentation
Or
Time-scales matterh
Th m
p~Hadron formation time
Lower pTassoc: measure radiation fragments
Lower pTtrig: explore timescale
24
Baryon enhancement
Large baryon/meson ratio in Au+Au ‘intermediate pT‘
Hadronisation by coalescence?3-quark pT-sum wins over fragmentation
M. Konno, QM06
High pT: Au+Au similar to p+p Fragmentation dominates
p/ ~ 1, /K ~ 2
25
Hadronisation through coalescence
fragmenting parton:ph = z p, z<1
recombining partons:p1+p2=ph
Fries, Muller et alHwa, Yang et al
Baryon pT=3pT,parton
MesonpT=2pT,parton
‘Shower-thermal’ recombinationwill result in larger associated
B/M at intermediate pT
Recombination of thermal (‘bulk’) partons
produces baryons at larger pT No associated yield
(Hwa, Yang)
Recombination enhancesbaryon/meson ratios
Hard parton
Hot matter
26
Associated yields from coalescence
Baryon pT=3pT,parton
MesonpT=2pT,parton
Expect large baryon/meson ratio associated with high-pT
trigger
No associated yield with baryons from coalescence:
Expect reduced assoc yield with baryon triggers 3<pT<4 GeV
(Hwa, Yang)Hard parton
Hot matter
Baryon pT=3pT,parton
MesonpT=2pT,parton
Hard parton
Hot matter
Recombination of thermal (‘bulk’) partons
‘Shower-thermal’ recombination
27
Baryon/meson ratios in ‘jets’
• Shape similar for mesons and baryons– provides constraint on
models describing modification of away-side
• Baryon to Meson ratio similar to the bulk– inconsistent with vacuum
fragmentation– consistent with jet induced
medium excitation
PHENIX arXiv, A. Hanks WWND 08
28
STAR Preliminary
Associated baryon/meson ratios
STAR Preliminary
p/ ratio in jet-peak < inclusive p/ ratio in ridge > inclusive
Ridge and jet-peak have different hadro-chemistry, different production mechanism
Jet-peak Ridge regionpT
trig > 4.0 GeV/c
2.0 < pTAssoc
< pTtrig
29
Jet-like peak: (Λ+Λ)/2K0
S ≈0.5
STAR Preliminary
Associated baryon/meson ratios
STAR Preliminary
Ridge: (Λ+Λ)/2K0S ≈ 1
Note: systematic error due to v2 not shown
Similar to p+p inclusive ratio
Baryon/meson enhancement in the ridge?
L. Gaillard, J. Bielcikova, C. Nattras et al.
No shower-thermal contribution?
30
Separating jet and ridge: pT-spectraJet spectra
Yie
ld (p
t,ass
oc >
pt,a
ssoc
,cut
)
Ridge spectra
Yie
ld (p
t,ass
oc >
pt,a
ssoc
,cut
)
pt,assoc,cutpt,assoc,cut
Jet (peak) spectra harden with pT,trig
Peak dominated by jet fragmentation Radiated gluons ‘thermalise’ in the medium?
Jet and ridge separate dynamics
inclusive
Ridge yield and spectra independent of pT,trigSlope of spectra similar to inclusives
J. Putschke, M
. van Leeuwen, et al
inclusive
31
Properties of medium at RHIC
Broad agreement between different observables, and with theory
432ˆ qpQCD:
2.8 ± 0.3 GeV2/fmq̂
(Baier)
23 ± 4 GeV/fm3
T 400 MeV
Transport coefficient
Total ETViscosity
10.008.0ˆ
25.13
q
Ts
(model dependent)
= 0.3-1fm/c
~ 5 - 15 GeV/fm3 T ~ 250 - 350 MeV
(Bjorken)
sTS
34
From v2
dydE
RVE T
02
1
GeV580dy
dET1.0s
(Majumder, Muller, Wang)
Lattice QCD:/s < 0.1
A quantitative understanding of hot QCD matter is emerging
(Meyer)
32
Fixing the jet energy:-jet events
T. Renk, PRC74, 034906
-jet: monochromatic source sensitive to P(E)
Expectations for different P(E)
E = 15 GeV
-jet events are rare, need large luminosity
First results from 2007 RHIC run
p+p
33
RHIC summary
• Jets interact strongly with medium at RHIC
• High pT: yield suppression, but no change in shapes– Fragmentation after energy loss
• Lower pT: enhancement, strongly modified shapes– Gluon fragments, medium response, etc
• Large Baryon/Meson ratio suggests coalescence of ‘free’ quarks– Test shower-thermal contribution by di-hadron correlations
2.8 ± 0.3 GeV2/fmq̂Transport coefficient from high-pT results:
34
Extra slides
35
Extracting the transport coefficient
Zhang, H et al, nucl-th/0701045
Di-hadrons provide stronger constrain on density
Extracted transport coefficient from singles and di-hadrons consistent
2.8 ± 0.3 GeV2/fmqq ˆ~00
2-minimum narrower for di-hadrons
Di-hadron suppression
Inclusive hadron suppression
Di-hadronsInclusive hadrons
36
Theory vs. data II
HEDP/HEDLA meeting APS St. Louis Apr 08 Jet probes of the QGP 36
016.0012.0S
32.05.00
200375
g
270150
g
21.22.3
280.0AMY
GeV/fm9.1ZOWW
1400dy
dNWHDG
1400dy
dNGLV
fm/GeV2.13ˆPQM/ASW
q Model parameters are constrained within ~20%
Values are large: ~30-50 times cold nuclear matter density!
Additional assumptions → different models are broadly consistent (except PQM – much larger than others)
Strong conclusions: • initially generated medium is highly opaque to energetic partons • very dense, high temperature matter has been created
PHENIX ’08; J. Nagle WWND08
37
Two extreme scenarios
p+p
Au+Au
pT
1/N
bin d
2 N/d
2 pT
Scenario IP(E) = (E0)
‘typical energy loss’
Shifts spectrum to left
Scenario IIP(E) = a (0) + b (E)
‘partial transmission’
Downward shift
(or how P(E) says it all)
P(E) encodes the full energy loss process
RAA cannot distinguish those two extreme scenarios
… need more differential probes
38
Radiative energy loss: calculational frameworks
38
A. Majumder, nucl-th/0702066
GLV (Gyulassy, Levai, Vitev): systematic expansion in small number of scatterings (“opacity”) n=L/
ASW (Armesto, Salgado, Wiedemann): multiple soft interactions
ZOWW (Zhang, Owens, Wang Wang): medium-enhanced power corrections to vacuum fragmentation function (higher twist)
AMY (Arnold, Moore, Yaffe): finite temperature effective field theory (Hard Thermal Loops) at small coupling
39
The extremes of QCD
This is the basic theory, but what is the phenomenology?
Small coupling Quarks and gluons
are quasi-free
Calculable with pQCD
Two basic regimes in which QCD theory gives quantitative results:Hard scattering and bulk matter
QCD Lagrangian
Nuclear matter Quark Gluon Plasma
High densityQuarks and gluons
are quasi-free
Bulk QCD matter
Calculable with Lattice QCD
Hard scattering
40
Expectations for hot QCD matter
Bernard et al. hep-lat/0610017
Tc ~ 170 -190 MeV
Energy density from Lattice QCD
Deconfinement transition: sharp rise of energy density at Tc Measure energy density
does not reach Boltzmann limit
Signals remaining interactions, structure?
Measure transport properties-Viscosity -Transport coefficient q̂
c ~ 1 GeV/fm3
41
Bulk QCD matter in heavy ion collisions
Azimuthal anisotropy:
v 2
Elliptic flow
pT (GeV)
We create bulk QCD matter at RHIC
))(2cos21( 2 RvddN
Initial state pressure accelerates matter
v2 = 0 free streaming
Au+Au event
dNch/dy 600
For central Au+Auat √sNN = 200 GeV
Low pT: Qualitative agreement with hydrodynamics:
viscosity, mean free path small
42
Experimental probes of energy loss
• Particle spectra– High statistics– Integrates over all production mechanisms
• Di-hadron correlations– Probe jet-structure– Some control over parton kinematics
• Identified particles– Probe hadronisation mechanisms– Heavy flavours: systematics of energy loss
Focus of this talk
43
Model dependence of
Different calculational frameworks
C. Loizides hep-ph/0608133v2
/fmGeV 24ˆ6 2 q2.8 ± 0.3 GeV2/fmq̂
Di-hadronsInclusive hadrons
Zhang, H et al, nucl-th/0701045
q̂
Multiple soft scattering (BDMPS, Wiedemann, Salgado,…)
Twist expansion (Wang, Wang,…)
Different approximations to the theory give significantly different results
Main uncertainties:- Formalism for QCD radiation- Geometry (density profile)
44
ALICE
2008: p+p collisions @ 14 TeV2009: Pb+Pb collisions @ 5.5 TeV
3 Large general purpose detectors
ALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV
Large Hadron Collider at CERN
ATLAS
CMS
45
From RHIC to LHC
RHICs=200 GeV Au+Au s=5.5 TeV Pb+Pb
LHC
- Larger pT-reach:typical parton energy > typical E
- New observablese.g. jet reconstruction fix parton energy
Larger initial density= 10-15 GeV/fm3 at RHIC ~ 100 GeV/fm3 at LHC
10k/year
Large cross sections for hard processes
Including heavy flavours
Validate understanding of RHIC data
Direct access to energy loss dynamics, P(E)
And others, e.g. gluon saturation
46
RAA at LHC
S. Wicks, W. Horowitz, QM2006
T. Renk, QM2006
Expected rise of RAA with pT depends on energy loss formalism
Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)
LHC: typical parton energy > typical E
GLV BDMPS
RHIC RHIC
47
Jet modifications at LHC
Radial profileFragmentation function
PQM with fragmentation of radiated gluons (A. Morsch)
Energy loss depletes high-zand populates low-z
Low-z fragments from gluon radiation at large R
In-medium energy loss redistributes momenta in jetsMeasure these modifications to extract P(E), medium properties
Expectations from QCD+jet quenchingJet reconstruction
Ejet = 125 GeV
=ln(EJet/phadron)z 0.37 0.14 0.05 0.02 0.007
48
ALICE EMCal
Lead-scintillator sampling calorimeter||<0.7, =110o
~13k towers (x~0.014x0.014)
ALICE-EMCal project: -Approved in 2007 -Full detector by 2011
US-France-Italy project
Testbeam:E
E %11%2)(
Support frame installed
EMCal module
Improves jet energy resolutionProvides jet triggers
49
Full jet reconstruction performanceSimulation input
referenceMedium modified (APQ)
Simulated result
Full jet reco in ALICE is sensitive to modification of fragmentation function
E > E, explore dynamics of energy loss process
50
Conclusion
• Large effects of medium on parton fragmentation– Lower pT: various effects
• Large baryon/meson ratio• Near-side ridge• Away-side broadening
– High pT: fragmentation after energy loss• Quantitative understanding
– Transport coefficient– High energy density ~ 10 - 30 GeV/fm3
– Low viscosity /s ~ 0.1
Clear picture of in-medium energy loss and medium properties at RHIC developing
Future at RHIC and LHC:Direct measurements of energy loss -jet- High-pT : E > E- Full jet reconstruction
Crucial tests of energy loss theory
2.8 ± 0.3 GeV2/fmq̂
51
Thank you for your attention
52
M. Lamont (STAR), J.Phys.G32:S105-S114,2006J. Bielcikova (STAR), v:0707.3100 [nucl-ex]
(Λ
+Λ
)/2K
0 S
Baryon/meson ratio in jets, ridge and inclusive
2 < pT,trig < 3 GeV
53
PreliminaryPreliminary
Near side yield||>0.9
Away side yield||<0.9
8 < pTtrig < 15 GeV 8 < pT < 15 GeV
zT=pTassoc/pT
trigzT=pTassoc/pT
trig
Energy loss in action
Both near- and away-side show yield enhancement at low pT
Possible interpretation:
di-jet → di-jet (lower Q2) + gluon fragments
‘primordial process’ High-pT fragmentsas in vacuum
Near side: ridgeAway-side: broadening
M. H
orner, M. van Leeuw
en, et al
Au+
Au
/ d+A
u
8 < pT < 15 GeV
Near side yield ratio
zT=pTassoc/pT
trig
0.2
1.0Lower pTtrig
Preliminary
Away side yield ratio
zT=pTassoc/pT
trig
Au+
Au
/ d+A
u
M. Horner, M. van Leeuwen, et al
Lower pTtrig
54
Results and interpretation
Extraction of direct away-side yields
R=Y-rich+h/Y0+hnear near
Y+h = (Y-rich+h - RY0+h )/1-Raway away
Assume no near-side yield for direct
then the away-side yields per trigger obey
55
Results and interpretationDirect away-side yields
The away-side yield of the associated particle per trigger in -jet is suppressed.