Hard Probes of the Quark Gluon Plasma
Lecture II: heavy flavour, geometry
Marco van Leeuwen, Nikhef and Utrecht University
Lectures at: Quark Gluon Plasma and Heavy Ion Collisions
Siena, 8-13 July 2013
2
So what are we trying to do...
• First: understand (parton) energy process– Magnitude, dominant mechanism(s)– Probability distribution
• Lost energy• Radiated gluons/fragments
– Path length dependence*– Flavour/mass dependence*– Large angle radiation?
• Goal: use this to learn about the medium
High-energy
parton(from hard scattering)
Hadrons
* Topics of today’s lecture
3
)/()( , jethadrTjetshadrT
EpDEPdEdN
dpdN
`known’ from e+e-knownpQCDxPDF
extract
Parton spectrum Fragmentation (function)Energy loss distribution
This is where the information about the medium isP(E) combines geometry with the intrinsic process
A simplified approachThis is the simplest ansatz – most calculation to date use it (except some MCs)
From yesterday’s lecture: Absolute calibration unknown Always need to compare 2 or more measurements
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Heavy quarks
Definition: heavy quarks, m >> QCD
Charm: m ~ 1.5 GeVBottom: m ~ 4.5 GeVTop: m ~ 170 GeV
M. Cacciari, CTEQ-MCNet summer school 2008
Complications exist: QCD, EW corrections; quark mass defined in different ways
‘Perturbative’ hadronisation
Heavy quarks are (mostly) produced in hard scattering, no (or small)contribution from hadronisation and thermal production in QGP
Heavy quarks: m >> TQGP
5
Dead cone effect
Radiated wave front cannot out-run source quark
Heavy quark: < 1
Result: minimum angle for radiation Mass regulates collinear divergence
6
Heavy quark suppressionM
.Djordjevic P
RL 94
Expect: heavy quarks lose less energy due to dead-cone effect
light
Wicks, H
orowitz et al, N
PA
784, 426
Calculated energy loss
Most pronounced for bottom
Interference effect:Radiated wave front cannot out-run source quark
Heavy quark: < 1
Result: minimum angle for radiation Mass regulates collinear divergence
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Heavy-to-Light ratio expectations at LHC
Heavy-to-light ratios: )()()( )(/)( t
hAAt
BDAAthBD pRpRpR
gq EE mass effect
For pT > 10 GeV charm is ‘light’ RD/h : colour-charge dependence of E lossRB/h : mass dependence of E loss
Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027
Colour-charge and mass dep. of E loss
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Charm decay reconstruction
KD 0 softsoft KDD 0*
D meson mass ~ 1.8 GeV/c2
Decay length ~ 100 mmBranching ratios ~ 4%
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Heavy vs light comparison
Charm meson and light hadron RAA similar at
LHC
No dead cone effect?or
Not sensitive?
Jury still out...
Experiment: measure BeautyChallenging, but not impossible,
see Andrea Beraudo’s talk
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D meson RAA
RAA < 1: charm also loses energyAgrees with (most) model calculations
However: some models have no prediction for light quarks. Calibration?
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So how about beauty?
Would like to measure beauty – Heavier, so larger dead cone effect
:Very few all-hadron decays; tiny branching ratios:
Experimentally very challenging
Current techniques:- Semi-leptonic decays; displaced electrons- Electron-hadron correlations ‘partial reconstruction’ of the decay- Secondary J/- Beauty in jets (displaced electrons, muons)
0DB BR: 0.48%
KJB / BR: 0.1%
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Heavy quark fragmentation
Light quarks Heavy quarks
Heavy quark fragmentation: leading heavy meson carries large momentum fraction
(Dead cone in vacuum)
Can we use this to get a handle on parton pT? To be studied...
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Part II: Path length dependence
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Geometry
Density profile
Profile at ~ form known
Density along parton path
Longitudinal expansion dilutes medium Important effect
Space-time evolution is taken into account in modelling
15
RAA vs and elastic elossT
. Renk, P
RC
76, 06
4905, J. Auvinen et al, P
RC
82, 051901
Elastic E-loss givessmall v2
Data require L2 or stronger path length
dependence
However, also quite sensitive to medium density evolution
In Plane
Out of Plane
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Path length dependence: RAA vs PHENIX, arXiv:1208.2254
In Plane
Out of Plane
Suppression depends on angle, path length
Not so easy to model: calculations give different results
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RAA vs : heavy flavour
In- vs out-of-plane difference also seen for charm Additional constraint for models?
18
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
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pT
assoc > 3 G
eVp
Tassoc >
6 GeV
d+Au Au+Au 20-40% Au+Au 0-5%
Suppression of away-side yield in Au+Au collisions: energy loss
High-pT hadron production in Au+Au dominated by (di-)jet fragmentation
Di-hadrons at high-pT: recoil suppression
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Di hadron yield suppression
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?
Near sideassociated
trigger
Away side associated
trigger
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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
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Di-hadron modelingT
. Renk, P
RC
, arXiv:1106.1740
L2 (ASW) fits dataL3 (AdS) slightly below
Modified shower generates increase at low zT
L (YaJEM): too little suppresionL2 (YaJEM-D) slightly above
Model ‘calibrated’ on single hadron RAA
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Di-hadrons and single hadrons at LHC
Need simultaneous comparison to several measurements
to constrain geometry and E-loss
Here: RAA and IAA
Three models:ASW: radiative energy lossYaJEM: medium-induced virtualityYaJEM-D: YaJEM with L-dependent virtuality cut-off (induces L2)
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Di-hadron with high-pT trigger
pttrig > 20 GeV at LHC: strong signals even at low pT
assoc 1-3 GeVCMS-PAS-HIN-12-010
19.2 - 24.0 GeVpTtrig (GeV): 14.0 - 28.8 GeV 28.8-35.2 GeV 35.2-48.0 GeV
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CMS di-hadrons: near side19.2 - 24.0 GeV
pTtrig (GeV):
14.0 - 28.8 GeV 28.8-35.2 GeV 35.2-48.0 GeV
Transition enhancement → suppression @ pT ~ 3 GeV
also compatible with IAA=1 at pT > 3 GeV?
peripheral 50-60%
central 0-10%
CM
S-P
AS
-HIN
-12-010
26
CMS di-hadrons: away side
Transition enhancement → suppression @ pT ~ 2 GeV
CM
S-P
AS
-HIN
-12-010
19.2 - 24.0 GeVpT
trig (GeV):14.0 - 28.8 GeV 28.8-35.2 GeV 35.2-48.0 GeV
peripheral 50-60%
central 0-10%
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Part III: Low, intermediate pT
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Identified hadron RAA
M. Ivanov, A. Ortiz@QM2012
Low-intermediate pT (1-6 GeV):Large baryon/meson ratio
Probably due to:1) radial flow2) parton recombination
M. Ivanov, A. Ortiz@QM2012
Baryon, meson RAA similar at pT > 8 GeV
As expected from parton energy loss
29
Identified hadron RAA (strangeness)
Kaon, pion RAA
similar
: RAA~1 at pT~3 GeV/cSmaller suppression,/K enhanced at low pT
pT ~8 GeV/c:All hadrons similar
partonic energy loss + pp-like fragmentation?
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Hadronisation by recombination
fragmenting parton:
ph = z p, z<1
recombining partons:
p1+p2=ph
Fries, Muller et alHwa, Yang et al
MesonpT=2pT,parton
Recombination of thermal (‘bulk’) partons
produces baryons at larger pT
Hot matter
Baryon pT=3pT,parton
Expect large baryon/meson ratio associated with high-pT trigger
(Hwa, Yang)
Baryon pT=3pT,parton
MesonpT=2pT,parton
Hard parton
Hot matter
‘Shower-thermal’ recombination
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Di-hadrons: p/ in jets
associated
trigger
Jet peak
Background/Bulk region(v2, v3 peak here)
5 < pTtrig < 10 GeV
Use TOF+dE/dx to identify particles
32
p/ bulk vs jetsM
. Veld
hoen, HP
p/ ratio in Bulk regionagrees with inclusive
p/ ratio in jet* agrees with Pythia
*after background subtraction
No effect of shower-thermal recombination and/or modified color flow observed
33
Near-side ridge in AA – Flow
d+Au, 200 GeV
3 < pt,trigger < 4 GeVpt,assoc. > 2 GeV
Au+Au 0-10%STAR preliminary
trigger
d+Au: ‘jet’-peak, symmetric in ,
Au+Au: extra correlation strengthat large ‘Ridge’
First seen at RHIC – Unexplained for a while
Most likely: flow, v3
34
Di-hadron correlations and flow at low pT
ALIC
E, P
RL 107, 032301
Alver and
Roland
,P
RC
81, 054
905
Low pT <~ 3 GeV: di-hadron correlations dominated by flow
Important contributions from v3, v4
Also NB: v1 can mimick jet (near or away-side)
35
Low pT di-hadron shapes at LHC
pT
0-10% 60-70% pp
2 <
pT
,tri
g <
31
< p
T,a
sso
c <
24
< p
T,t
rig <
82
< p
T,a
sso
c <
3
36
• The lowest pT bin shows a structure with a flat top in
• This feature is reproduced by AMPT
•
• Qualitative and quantitative agreement of peak shapes with AMPT compatible with hypothesis of interplay of jets with the flowing bulk
Departure from Gaussian
AMPTData
0-10%2 < pT,t < 3 GeV/c1 < pT,a < 2 GeV/c
37
Peak Deformation
PRL 93,242301 (2004)
dNch/d
rms (calc.)
rms (calc.)
Calculation + STAR prel:
2 < pT,t < 3 1 < pT,a < 2 GeV/c4 < pT,t < 8 2 < pT,a < 3 GeV/c
Centrality | 100 = pp
,
(f
it)
Significant increase of towards central events
– > (eccentricity ~ 0.2)Armesto, Salgado, Wiedemann
Longitudinal flow deforms jet shape
38
AMPT Comparison
• AMPT (A MultiPhase Transport Code) describes collective effects (e.g. v2, v3, v4) in HI collisions at LHC
– Here version with string melting (2.25) is shown
• RMS of the near-side peak reasonably described by AMPT– Detailed mechanism not known; Interplay of jet and flow ?
Centrality | 100 = pp
(fit
) (r
ad.)
(fit
) (r
ad.)
Centrality | 100 = pp
Lines: AMPT 2.25 and Pythia P-0 (for pp)
2 < pT,t < 3 1 < pT,a < 2 GeV/c3 < pT,t < 4 2 < pT,a < 3 GeV/c4 < pT,t < 8 2 < pT,a < 3 GeV/c
39
Integrated vs differential
• Inclusive hadron suppression RAA
– Overall magnitude + pT dependence: limited dynamical information
– Only useful when the energy loss mechanism is understood
• Di-hadrons; IAA
– Two ‘degrees of freedom’
– Adds constraints when compared to RAA; mostly geometry?
• Low pT, shape info– More differential, but also more difficult to model
40
Summary
• Heavy flavour– Expect dead cone effect: reduced energy loss– No clear evidence of dead cone effect for charm in data
• Path length dependence– RAA vs
– Di-hadron suppression
Favours: L2 or stronger
• Low, intermediate pT < 6-8 GeV/c– Large B/M ratio: flow or recombination?– B/M in jets ~0.2– Jet-like correlations:
• Large effect of bkg flow
• Shapes change (broadening in )
41
Extra slides
42
Comparing single- and di-hadron @ RHIC
Armesto, Cacciari, Salgado et al.
RAA and IAA fit with similar density
Confirms ~L2 dependence
Calculations with elastic loss give too little suppression
43
D mesonsQM2012, Zaida CdelV
44
Jet Quenching
1) 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?
High-energy
parton(from hard scattering)
Hadrons
45
Heavy Quark Fragmentation II
Significant non-perturbative effects seen even
in heavy quark fragmentation
46
Di-hadron correlations
associated
trigger
Background
Di-hadron correlations:• Simple and clean way to access di-jet
fragmentation• Background clearly identifiable• No direct access to undelying kinematics
(jet energy)
Compare AA to pp
After background subtraction
Energy loss+fragmentation
Quantify/summarise: IAA
Near side: yield increases
Away side: yield decreases
ALIC
E, arX
iv:1110.0121
47
Comparing single- and di-hadron @ RHIC
Armesto, Cacciari, Salgado et al.
RAA and IAA fit with similar density
Confirms ~L2 dependence
Calculations with elastic loss give too little suppression
48
Spectra at intermediate pT
Low-intermediate pT (1-6 GeV):Large baryon/meson ratio
Probably due to:1) radial flow2) parton recombination
Schukraft et al, arXiv:1202.3233
49
Parton energy loss – main questions
• Understand production rates• Understand parton energy loss process
– Energy loss as a function of density– Path length dependence
• Elastic, radiative, other?
– Mass dependence– Interplay between vacuum and
medium radiation– Broadening of shower:
• Out-of-cone radiation
– Leading hadron vs softening of FF
• Use as a probe to determine medium density (and other properties)
High-energy
parton(from hard scattering)
Hadrons
50
Di-hadron yields at LHCA
LICE
, PR
L 108,092301
Near side Away side
8 < pTtrig < 15 GeV
Near side: ~20% yield enhancement
Away side: suppressionby factor ~2
Fragmentation after energy loss Recoil parton energy loss