what do we understand about parton energy loss at rhic? an experimentalists viewpoint marco van...
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![Page 1: What do we understand about parton energy loss at RHIC? An experimentalists viewpoint Marco van Leeuwen, Utrecht University](https://reader035.vdocuments.net/reader035/viewer/2022062305/5697c02a1a28abf838cd7fab/html5/thumbnails/1.jpg)
What do we understand about parton energy loss at RHIC?
An experimentalists viewpoint
Marco van Leeuwen,Utrecht University
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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
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Energy loss in QCD matter
radiated gluon
propagating parton
kTQCD bremsstrahlung(+ LPM coherence effects)
Density of scattering centers:
Nature of scattering centers, e.g. mass: radiative vs elastic loss
Or no scattering centers, but fields synchrotron radiation?
1
2
ˆTk
q
2ˆ~ LqE Smed
Transport coefficient
Energy loss
Energy loss probes:
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Questions about energy loss
• What is the dominant mechanism: radiative or elastic?– Heavy/light, quark/gluon difference, L2 vs L dependence
• How important is the LPM effect?– L2 vs L dependence
• Can we use this to learn about the medium? – Density of scattering centers?– Temperature?– Or ‘strongly coupled’, fields are dominant?
Phenomenological questions:Large vs small angle radiationMean E?How many radiations?Virtuality evolution/interplay with fragmentation?
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0 RAA – high-pT suppression
Hard partons lose energy in the hot matter
: no interactions
Hadrons: energy loss
RAA = 1
RAA < 1
0: RAA ≈ 0.2
: RAA = 1
Factor 4-5 suppression – A large effect!
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Parton energy loss and RAA modeling
Qualitatively:
)/()( , jethadrTjetshadrT
EpDEPdEdN
dpdN
`known’ from e+e-knownpQCDxPDF
extract
Parton spectrum Fragmentation (function)Energy loss distribution
Contains medium propertiese.g. density profile
‘Same’ in p+pand Au+Au
Vacuum fragmentation (?)
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Four theory approaches
• Multiple-soft scattering (ASW-BDMPS)– Full interference (vacuum-medium + LPM)
– Approximate scattering potential
• Opacity expansion (GLV/WHDG)– Interference terms order-by-order (first order default)
– Dipole scattering potential 1/q4
• Higher Twist– Like GLV, but with fragmentation function evolution
• Hard Thermal Loop (AMY)– Most realistic medium
– LPM interference fully treated
– No interference between vacuum frag and medium
Allo
w d
efin
ition
of
‘que
nchi
ng w
eigh
ts’ P
(E
)
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Extracting , TB
ass e
t al, P
RC
79
, 02
49
01
All can be fit to RAA – RAA is not decisive
Large differences between formalisms
q̂
Not all approaches can be ‘correct’
Can we decide which formalism(s) is/are correct?
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Two extreme scenarios
p+p
Au+Au
pT
1/N
bin
d2 N/d
2 pT
Scenario IP(E) = (E0)
‘Energy loss’
Shifts spectrum to left
Scenario IIP(E) = a (0) + b (E)
‘Absorption’
Downward shift
(or how P(E) says it all)
P(E) encodes the full energy loss process
RAA not sensitive to details of mechanism
Would need E/E ~ 0.2 to get RAA ~ 0.2
Would need a = 0.2, b = 0.8to get RAA ~ 0.2
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Energy loss spectrum
BrickL = 2 fm, E/E = 0.2E = 10 GeV
Typical examples with fixed L
E/E> = 0.2 R8 ~ RAA = 0.2
Different theoretical approximation (ASW, WHDG) give different results – significant?
Significant probability to lose no energy (P(0))
Broad distribution, large E-loss (several GeV, up to E/E = 1)
Theory expectation: mix of partial transmission+continuous energy loss– Can we see this in experiment?
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How geometry complicates mattersM
. V
erw
eij,
UU
L < R, small q L > R, smaller q
For ‘typical partons’ L and q are correlated
L ~ R, large q
Resulting P(E/E) peaked at ~ 0 and 1Black-white scenario
No sensitivity to continuous E-loss?
Energy loss distributionsummed over all partons
Static medium
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Path length dependence I
Centrality
Au+Au
Cu+Cu
<L>, density increase with centralityVary L and density independently by changing Au+Au Cu+Cu
Turns out to be not very precise
In-plane
Out of plane
Change L in single system in-plane vs out of plane
Collision geometry
2ˆ~ LqE Smed
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RAA vs reaction plane angle
Azimuthal modulation, path length dependence largest in ASW-BDMPS
Data prefer ASW-BDMPS
C. V
ale
, PH
EN
IX, Q
M0
9
But why? – No clear answer yet
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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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Di hadron yield suppression
No suppressionSuppression 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?
Note: per-trigger yields can be same with energy-loss
Near sideassociated
trigger
Away side associated
trigger
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RAA and IAA in a single model
Armesto, Cacciari, Salgado et al.
RAA and IAA give similar densityModel has L2 built in – But how sensitive is it?
Hydro + ASW-BDMPS
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L scaling: elastic vs radiativeT. Renk, PRC76, 064905
RAA: input to fix density Radiative scenario fits data; elastic scenarios underestimate
suppression
Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions
Confirms L2 dependence radiative loss dominates
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L-dependence from surface bias
Near side trigger, biases to small E-loss
Away-side large L
Away-side suppression IAA samples different path-length distribution than inclusives RAA
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Dead cone effectRadiated wave front cannot
out-run source quark
Heavy quark: < 1
Result: minimum angle for radiation
light
M.D
jordjevic PR
L 94
Wicks, H
orowitz et al, N
PA
784, 426
Expected energy loss
Most pronounced for bottom
Dead cone effect reduces E-loss
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Heavy quark suppressionP
HE
NIX
nucl-ex/0611018, ST
AR
nucl-ex/0607012
Djordjevic, Phys. Lett. B632, 81
Armesto, Phys. Lett. B637, 362
Measured suppression of non-photonic electrons larger than
expected
Using non-photonic electrons
Expect: heavy quarks lose less energy due to dead-cone effect
Radiative (+collisional) energy loss not dominant? E.g.: in-medium hadronisation/dissociation (van Hees, et al)
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Heavy Quark comparison
No minimum – Heavy Quark suppression too large for ‘normal’ medium density
Armesto, Cacciari, Salgado et al.
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Charm/bottom separation
Combine rB and RAA to extract RAA for charm and bottom
arXiv:0903.4851 hep-ex
Bottom/charm ratio in p+p agrees with theory expectations (FONLL)
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I: Djordjevic, Gyulassy, Vogt and Wicks, Phys. Lett. B 632 (2006) 81; dNg/dy = 1000II: Adil and Vitev, Phys. Lett. B 649 (2007) 139III: Hees, Mannarelli, Greco and Rapp, Phys. Rev. Lett. 100 (2008) 192301
pT > 5 GeV/c
RAA for c e and b eB
.Biritz Q
M09
Combined data show:electrons from bothB and D suppressed
Large suppression suggestsadditional energy loss mechanism
(resonant scattering, dissociative E-loss)
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Summary so far
• Large suppression of light hadrons parton energy loss
• Rough estimates: E/E ~ 0.2, or 80% absorption
• QCD predicts broad distribution P(E)
• Geometry: many small path lengths– Effectively black/white NB: means no sensitivity to dynamics!
• IAA vs RAA: L2 preferred – radiative dominates
• RAA vs reaction plane: modulation in data larger than expected?
• Heavy quarks: suppression larger than expected
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JetsBasic idea: recover radiated energy – parton energy before E-loss
Out-of cone radiationSuppression of jet yield
RAAjets < 1
In-cone radiation: Softening of fragmentation function and/or broadening of jet structure
Alternative: use recoil photon in -jet events (low statistics)
Sa
lga
do
, Wie
de
ma
nn
, P
RL
93
, 04
23
01
Early prediction: out-of-cone radiation small effect
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Fragmentation functions
Qualitatively: )()()( zDEPzD vacmed
Dashed lines: include gluon fragments (assuming 1 gluon emitted)
Fragmentation functions sensitive to P(E)Distinguish GLV from BDMPS?
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Are FF sensitive to P(E) ?Toy model curves
P(E) toy model Fragmentation function ratio
Fragmentation functions are sensitive to P(E) – Somewhat
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-hadron results
STAR Preliminary
Large suppression for away-side: factor 3-5
Results agree with model predictions
Would like to see z-dependence, uncertainties still large
A. H
am
ed
, ST
AR
, QM
09
8 < ET, < 16 GeV
PH
EN
IX, P
RC
80
, 02
49
08
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Jet finding in heavy ion events
η
p t p
er g
rid c
ell [
GeV
]
STAR preliminary~ 21 GeV
FastJet:Cacciari, Salam and Soyez; arXiv: 0802.1188http://rhig.physics.yale.edu/~putschke/Ahijf/A_Heavy_Ion_Jet-Finder.html
Jets clearly visible in heavy ion events at RHIC
Use different algorithms to estimate systematic uncertainties:• Cone-type algorithms
simple cone, iterative cone, infrared safe SISCone
• Sequential recombination algorithmskT, Cambridge, inverse kT
Combinatorial backgroundNeeds to be subtracted
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p+p Au+Au central
Jet spectra
Note kinematic reach out to 50 GeV
• Jet energy depends on R, affects spectra• kT, anti-kT give similar results
Take ratios to compare p+p, Au+Au
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Jet RAA at RHIC
Jet RAA >> 0.2 (hadron RAA)
Jet finding recovers most of the energy loss measure of initial parton energy
M. P
loskon, ST
AR
, QM
09
Some dependence on jet-algorithm? Under study…
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Radius dependence
RAA depends on jet radius:Small R jet is single hadron
Jet broadening due to E-loss ?
M. P
losko
n, S
TA
R, Q
M0
9
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Fragmentation functions
STAR Preliminary
pt,rec(AuAu)>25 GeV20<pt,rec(AuAu)<25 GeV
Use recoil jet to avoid biases
Recoil suppression in reconstructed jets small
E. B
runa, ST
AR
, QM
09
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Di-jet suppression
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Ele
na B
run
a fo
r the S
TA
R C
olla
bora
tion
- Q
M0
9STAR PreliminarySTAR Preliminary
E. B
runa, ST
AR
, QM
09
Jet IAA
Away-side jet yield suppressed partons absorbed
... due to large path length(trigger bias)
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Emerging picture from jet results
• Jet RAA ~ 1 for sufficiently large R – unbiased parton selection
• Away side jet fragmentation unmodified – away-side jet emerges without E-loss
• Jet IAA ~ 0.2 – Many jets are absorded (large E-loss)
Study vs R, E to quantify P(E) and broadening
Ongoing developments of event generators for modified fragmentation important for measurement, interpretation
JEWEL, q-PYTHIA, YaJEM, PYQUEN, MARTINI
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RHIC future
• Machine upgrades– Stochastic cooling to increase luminosity– Several polarisation upgrades
• Experiment upgrades– Vertex detectors for charm, bottom– Forward calorimeters– STAR: TOF for PID– DAQ upgrades to deal with rates
Increased luminosity: -hadron, jets, charmonia ()
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Delivered Integrated Luminosity
0
20
40
60
80
100
120
140
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Weeks into run
Nu
cleo
n p
air
lum
inos
ity
LN
N [
pb-1
]
d-Au2008
Au-Au2007
Cu-Cu2005
Au-Au2004
d-Au2003
Au-Au2001/02
0
10
20
30
40
50
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Weeks into runN
ucl
eon
pai
r lu
min
osit
y L
NN
[p
b-1]
2003P=34%
2005P=46%
2006P=60%
2008P=45%
Nucleon-pair luminosity allows comparison of luminosities of different species
Inte
grat
ed n
ucl
eon
-pai
r lu
min
osit
y L
NN [
pb
-1]
Inte
grat
ed n
ucl
eon
-pai
r lu
min
osit
y L
NN [
pb
-1]
Collider luminosity increases as experience grows– Often beyond original design!
Heavy ion runs Polarized proton runs
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-hadron luminosity projection
Gradual increase in p+p and Au+Au luminosity reduces measurement uncertainties
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Inner tracking upgrades at RHIC
STAR PHENIX
Goals:Charm flow, spectra, RAA
b-tagging for bottom RAA
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Heavy-light ratios
Armesto plot
Armesto et al, PRD71, 054027
light
M.Djordjevic PRL 94 (2004)
Wicks, Horowitz et al, NPA 784, 426
Heavy-light RAA ratios directly sensitive to dead-cone effect
Effect sizeable, should be measurable with vertex detectors
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ALICE
2010: p+p collisions @ 7-10 TeV2010: Pb+Pb collisions @ ? TeV
3 Large general purpose detectorsALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV
Large Hadron Collider at CERN
ATLAS
CMS
ATLAS, CMS: large acceptance, EM+hadronic calorimetry
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From RHIC to LHC
- Larger pT-reach:typical parton energy > typical E
- Energy dependenc of E-loss with high-energy jets
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)
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Energy loss distribution
BrickL = 2 fm, E/E = 0.2E = 10 GeV
Typical examples with fixed L
E/E> = 0.2 R8 ~ RAA = 0.2
Significant probability to lose no energy (P(0))
Broad distribution, large E-loss (several GeV, up to E/E = 1)
Broad distribution; typical energy loss ~5 GeV
RHIC: E ~ Ejet, LHC: Ejet > E sensitivity to P(E)
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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
RHICRHIC
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Heavy-to-Light ratios at LHC
Heavy-to-light ratios: )()()( )(/)( t
hAAt
BDAAthBD pRpRpR
gq EE mass effect
For pT > 10 GeV charm is ‘light’ RD/h probes colour-charge dep. of E lossRB/h probes mass dep. of E loss
Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027
Colour-charge and mass dep. of E loss
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TDpp
TDAA
collT
DAA dpdN
dpdN
NpR
/
/1)(
Tepp
TeAA
collT
eAA dpdN
dpdN
NpR
/
/1)(
ALICE heavy flavour performance
mb = 4.8 GeV
D0 K B e + X
1 year at nominal luminosity(107 central Pb-Pb events, 109 pp events)
Alice can measure charm from 0 < pT < 20 GeV and bottom (semi-leptonic decays) 3 < pT < 20 GeV
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ALICE performance: heavy-to-light
1 year at nominal luminosity(107 central Pb-Pb events, 109 pp events)
)()()(/ ThAAT
DAAThD pRpRpR
)()()( D from eB from e/ tAAtAAtDB pRpRpR
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=ln(EJet/phadron)
pThadron ~2 GeV for
Ejet=100 GeV
Borghini and Wiedemann, hep-ph/0506218
Medium modification of fragmentation MLLA calculation: good approximation for soft fragmentationextended with ad-hoc implementation medium modifications
Recent progress: showering with medium-modified Sudakov factors, see Carlos’s talk and arXiv:0710.3073
Trends intuitive: suppression at high z, enhancement at low z
z 0.37 0.14 0.05 0.02 0.007
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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
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Jet shapes
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Jet shapes in ATLAS
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Conclusion• Parton energy loss at RHIC is large
E ~ E Limited sensitivity to E-loss dynamics• Di-hadron suppression indicates L2 dependence radiative
dominates• Heavy flavour more suppressed than expected• Jet broadening significant• Fragmentation function modification difficult to measure –
expected ?• Future at RHIC
– Direct measurements of charm, heavy/light ratios– Increase -jet statistics– Improve understanding of jet results
• Future at LHC– Verify/test our understanding in a new regime– May reach into E > E regime– Abundant jets E ~ 100 GeV
Sensitivity to E-loss dynamics
We still have 4+ formalisms – Need to devise tests of validity
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Thanks for your attention!
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Heavy quark fragmentation
Light quarks Heavy quarks
Heavy quark fragmentation: leading heavy meson carries large momentum fraction
More handle to extract P(E)?
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Direct photons: no interactions
PHENIX
Direct spectra
Scaled by Ncoll
PHENIX, PRL 94, 232301
ppTbin
AuAuTAA dpdNN
dpdNR
/
/
Direct in A+A scales with Ncoll
Centrality
A+A initial state is incoherent superposition of p+p for hard probes
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Geometry III
Brick isolines from left to right:R7 = 0.45, 0.25, 0.15, 0.05
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Determining the medium densityPQM (Loizides, Dainese, Paic),Multiple soft-scattering 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• Typical energy loss per L: 0
• Coupling constant S
q̂
PHENIX, arXiv:0801.1665,J. Nagle WWND08
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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
Data constrain model parameters to 10-20%
Method extracts medium density given the model/calculation Theory uncertainties need to be further evaluated
e.g. comparing different formalisms, varying geometry
But models use different medium parameters– How to compare the results?
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Some pocket formula results
Large differences between models
GLV/WHDG: dNg/dy = 1400
2
1)(
Rdy
dN g
3
0 fm4.12)fm1( 32
202.116T
T(0) = 366 MeV
PQM: (parton average) /fmGeV2.13ˆ 2q
32202.172
ˆ Tq s
T = 1016 MeV
AMY: T fixed by hydro (~400 MeV), s = 0.297
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Naive picture for di-hadron measurements
PT,jet,1
PT,jet,2
Fragment distribution(fragmentation fuction)
Out-of-cone radiation:PT,jet2 < PT,jet1
jetT
hadrT
P
pz
,
,
dzdN
Ref: no ElossIn-cone radiation:PT,jet2 = pT,jet1
Softer fragmentation
Naive assumption for di-hadrons: pT,trig measures PT,jet
So, zT=pT,assoc/pT,trig measures z
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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 fromaway-side suppression
and single hadron suppression agree
Theory: ZOWW, PRL98, 212301
Data: STAR PRL 95, 152301
8 < pT,trig < 15 GeV
zT=pT,assoc/pT,trig
(Experiment and theory updates in the works)
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Heavy Quark Fragmentation II
Significant non-perturbative effects seen even
in heavy quark fragmentation
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• Use e-K invariant mass to separate charm and bottom
• Signal: unlike-sign near-side correlations
• Subtract like-sign pairs to remove background
• Use Pythia to extract D, B yields
arXiv:0903.4851 hep-ex
D/B from e-K correlations
B → e + D D → e + KD → e + K
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Luminosity projections
Also: -hadron correlations
Projection of uncertainties in Upsilon(1S) RAA for two sets of integrated luminosity.
Heavy Flavor signalsstudy color screening with quarkonia
J/Ψ
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Testing Ncoll scaling II: Charm
PRL 94 (2005)
NLO prediction:m ≈ 1.3 GeV, reasonably hard scale at pT=0
Total charm cross section scales with Nbin in A+A
Scaling observed in PHENIX and STAR – scaling error in one experiment?
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B
DX.Y. Lin, hep-ph/0602067
e h rBe hB (1 rB )e h
D
rB eB /(eD eB )
Charm/bottom separation
Idea: use e-h angular correlations to tag semi-leptonic D vs B decay
D → e + hadrons
B peak broader due to larger mass
Extract B contribution by fitting:
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Charm-to-Bottom Ratio
PHENIX p+p measuments agree with pQCD (FONLL) calculation
arXiv:0903.4851 hep-ex
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STAR TOF – now fully installedTOF 1/β cut rejects hadrons providing nearly
complete and accurate electron identification for di-lepton program.
Large statistics for di-hadron, fragmentation studiesIdentification of decay products from charm, bottom
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STAR inner tracker upgrade (HFT)
2 30.5
~ 30 microns pointing resolution at 0.7 GeV/c
~ 30 microns secondary vertex resolution (large p)
3 Layers: SSD: existing double sided strip detector IST: intermediate strip layer PIXEL: 2 inner layers of high resolution
Pixel (MAPS) (18*18 m) and thin 0.4% Xo per layer
Main goal: heavy flavour spectra and v2 at low and high pT
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PHENIX Silicon Vertex Detectors
• VTX: silicon VerTeX barrel tracker– Fine granularity, low occupancy
• 50m×425m pixels for L1 and L2• R1=2.5cm and R2=5cm
– Stripixel detector for L3 and L4• 80m×1000m pixel pitch • R3=10cm and R4=14cm
– Large acceptance• ||<1.2, almost 2 in plane
– Standalone tracking
• FVTX: Forward silicon VerTeX tracker – 2 endcaps with 4 disks each– pixel pad structure (75m x 2.8 to
11.2 mm)FVTX endcaps1.2<||<2.7 mini strips
VTX barrel ||<1.2
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Physics Projections with HFT+TOF
Charm collectivity Medium properties, light flavor thermalization
Charm energy loss Energy loss mechanisms, Medium properties
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PHENIX VTX Performance
Large suppression of heavy flavourCurrent result mix of b and cVTX can separate b and c
Expected with VTX (0.4/nb ~3 weeks in RUN11)
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ALICE charm performancepp, s = 14 TeV
charm (D0 K) beauty (B e+X)
1 year at nominal luminosity(109 pp events)
A. DaineseAlice can measure charm from 0 < pT < 20 GeV
and bottom (semi-leptonic decays) 3 < pT < 30 GeVin p+p events