HighpT probes of heavyion collisions at RHIC and LHC

1
High-pT probes of heavy-ion collisions at RHIC
and LHC

• Marco van Leeuwen, LBNL

2
Introduction
Motivation Initial production well-calibrated Har
d processes (high Q2) are only sensitive to short
distances and times
Final state particles (partons and/or hadrons)
probe the medium through interactions
3
Introduction
Can we check our understanding of hard processes?
Yes, by showing that pp results can be
explained by pQCD
Fragmentation function
Parton density function
Matrix element
ee-
pQCD
Measured in DIS
For hard process, expect to scale as number of
binary collision Ncoll for AA
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Hard processes at RHIC

• High-pt light hadron production
• Most abundant process, lots of data
• Inclusive production, di-hadron correlations,
  elliptic flow
• Baryon production sensitive to quark vs gluon
  jets
• Direct g production
• Non-interaction probe, test Ncoll scaling
• Heavy quark production
• Main results so far from semi-leptonic decays
• Test pQCD theory for production and suppression
  (dead-cone effect)

• Goal
• Understand production rates and suppression in
  AA
• Determine medium properties (density, dynamics)
  in heavy-ion collisions

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RHIC accelerator and experiments
Maximum energy ?sNN200 GeV for AuAu ?s500 GeV
for pp (default 200 GeV)
PHENIX
STAR
Focus global observables
Large volume TPC (2p) EM calorimetry (coarse)
Focus rare probes g, e, m
Partial coverage High-granularity calorimetry
and tracking Forward muon detectors
Recent runs 2004 large statistics AuAu (80M
events), most results in this presentation 2005
large statistics CuCu, analysis in
progress 2006 dedicated polarised pp run,
data-taking in progress
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pp jet spectrum _at_ ?s200 GeV
First direct measurement of jet spectrum at RHIC
Statistics out to pT50 GeV more being collected
Measured spectrum agrees with NLO pQCD
Dominant uncertainty jet energy scale
Prefer particle spectra, di-hadron
correlationsfor AuAu baseline (backgrounds too
large for jet reconstruction in AuAu)
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Light hadron production in pp
PRL 91, 241803
Star, PRL 91, 172302 Brahms, nucl-ex/0403005
NLO calculations W. Vogelsang
Light hadron production at RHIC in good agreement
with NLO pQCD
Caveat gluon fragmentation not so well
constrained from ee-
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Baryon production in pp
Albino, Kniehl, Kramer, Nucl Phys B725, 181
hep-ph/0510173

L also well described
Proton spectra used to be problematic (KKP FF)
New parameterisation of FF (AKK) from full
flavour separated datasets (OPAL), (no SU(3)
flavour symmetry assumption) shows much better
agreement
Baryon production at RHIC also described by pQCD
FF parametrisation is an ongoing activity
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Direct photons
Production through
PHENIX, PRL 94, 232301
q g ? q g
Centrality
RAA1 (Ncoll scaling) for incoherent
superposition of pp collisions
Direct g in pp agree with pQCD
Direct g in AA scales with Ncoll
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Light hadron production in AA
AuAu 200 GeV, 0-5 central
g RAA 1
p0, h RAA 0.2
Photons and hadron production measured to well in
the (expected) perturbative regime
Light hadron production suppressed by factor 4-5
in central AuAu
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Radiative energy loss in QCD

• Calculational frameworks
• Multiple soft scattering (BDMPS, Wiedemann,
  Salgado,)
• Few hard scatterings,opacity expansion (Gyulassy,
  Vitev, Levai, Wang,)
• Twist expansion (Wang, Wang,)

Soft radiation suppressed by phase space
requirement kT lt w
Salgado and Wiedemann, Phys. Rev. D68, 014008
wdI/dw
Medium properties can be characterized by a
single constant
e.g. transport coefficient
average kT -kick per mean-free-path
DE does not depend on parton energy DE ? L2 due
to interference effects (for a static medium)
w/wC
w1 GeV at RHIC
Radiative energy loss is due to moderate number
(3) of finite energy gluons (w0.1-1 GeV)
Plus details Longitudinal expansion reduces
DEL2 to DEL Finite energy effects may lead to
E-dependent energy loss
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Non-perturbative dynamics at intermediate
pTIntermezzo
dAu, ?s200 GeV
p
Hadron production in dAu enhanced compared to
Ncoll scaling

• Enhancement depends on
• Particle type (different for p, p)
• Centrality

p
Cronin effect known from fixed target at
Fermilab, but mechanism unclear
Effect small compared to effects in AuAu
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Baryon production in AuAu
AuAu, 0-5 central, ?sNN200 GeV
Intermediate pT (2-4 GeV) p/p much larger in
AuAu than pp (vacuum fragmentation)
At pt6 GeV p/p similar in pp, dAu and central
AuAu
Non-perturbative effects large at intermediate pT
Note p/p ratio sensitive to gluon/quark ratio.
Probes differences in coupling to medium
This presentation focus at highest pT
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Hadron suppression ?sNN200 GeV AuAu
High statistics year-4 data
Reasonable agreement between data and
calculations for pT up to 20 GeV
Different calculations lead to similar medium
densities dNg/dy1100,
, approx. 30 times nuclear density
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Centrality dependence
Path length, density dependence leads to
centrality dependence of suppression
Data agree with calculated suppression patterns
More differential tests (e.g. from v2) are under
way
On theory side need to quantify constraints on
L-dependence
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Surface emission (geometric bias)
Eskola et al., hep-ph/0406319
RAA0.2-0.3 for broad range of
Large energy loss ? opaque core
Inclusive measurements insensitive to opacity of
bulk ? Need coincidence measurements to probe
deeper
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Azimuthal di-hadron correlations
associated
Dj
4 lt pT,trig lt 6 GeV pT,assoc gt 2 GeV
trigger
pp
AuAu
Phys Rev Lett 91, 072304
2002 result No modification of near side Strong
suppression of away side
Need to subtract background in AuAu
No measurable away-side yield cannot quantify
suppression
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Jet-like di-hadron correlations
New results, year-4
0-5
dAu
AuAu 20-40
Larger data sample extends pT-range
8 lt pT,trig lt 15 GeV
Emergence of the away side peak
Background negligible at higher pT,assoc
Larger pT allows quantitative analysis of jet
energy loss
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Di-hadron correlations centrality dependence
Fit scaledby x2
8 lt pT,trig lt 15 GeV/c
Away-side Increasing suppression with centrality
Near side yields essentially unmodified
Again surface bias
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Di-hadron fragmentation
Scaling factors
0.54
0.25
8 lt pT,trig lt 15 GeV/c
Near side fragmentation unmodified
Away-side strong suppression,but shape similar
above zT0.4
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A closer look at azimuthal peak shapes
8 lt pT(trig) lt 15 GeV/c pT(assoc)gt6 GeV
p
Df
No away-side broadening
Large energy loss without observable
modification of longitudinal and azimuthal
distributions
Observations constrain energy loss fluctuations
and geometrical bias
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Discussion of di-hadron results
Strong suppression (factor 4-5, similar to
inclusive hadron suppression) without
modification of longitudinal and azimuthal
fragmentation shapes
Observation
In contrast to several model expectations
Majumder, Wang, Wang, nucl-th/0412061
Vitev, hep-ph/0501225
Broadening due to fragments of induced radiation
Near-side enhancementdue to trigger bias
Induced acoplanarity (BDMPS)
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Confronting IAA and RAA
Dainese, Loizides and Paic, QM poster
IAA RAA 0.20-0.25
First look from IAA and RAA in quantitative
agreement
5-7 GeV2/fm in central AuAu _at_ RHIC
Need to further assess theory uncertainties
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Heavy quark suppression (non-photonic electrons)
Wicks, et al, nucl-th/0512076
Collisional energy loss revisited
Suppression of non-photonic electrons larger than
expected
Compatible with charm-dominance up to pT 10 GeV
Comparison of light and heavy quark suppression
elucidates energy loss mechanism
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Intermezzo II Jet structure at intermediate pT
absolute ridge yield
dAu 100-40
AuAu 0-5
3 lt pT,trig lt 6 GeV2 lt pT,assoc lt pT,trig
STAR Preliminary
STAR Preliminary
pt,assoc gt 2 GeV
New feature in AuAu long range Dh correlation
Persist to high pT,trigger ? likely jet-related

• Scenarios
• Parton radiates energy before fragmenting and
  couples to the longitudinal flow Armesto et al,
  nucl-ex/0405301
• Heating of the medium Chiu Hwa Phys. Rev.
  C72034903,2005
• Radial flow jet-quenching Voloshin
  nucl-th/0312065

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RHIC Summary

• pQCD applicable for pp at RHIC
• Strong suppression effects seen for light and
  heavy flavours
• Testing radiative energy loss
• Path length dependence confirmed
• Heavy flavour suppression stronger than expected
• No modifications of away-side shapes in di-hadron
  correlations
• Additional dynamics at intermediate pT ? medium
  response

Newest results at RHIC start to provide
quantitative tests of in-medium energy loss
Detailed evaluation ongoing
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Jets in nuclear collisions at the LHC
CMS
2007 pp collisions _at_ 14 TeV 2008 PbPb
collisions _at_ 5.5 TeV
ALICE
ATLAS
ALICE is the dedicated Heavy-Ion experiment
(high-density tracking and PID)
CMS and ATLAS are likely to participate in HI
runs as well
Complementary capabilities in high-Q2 probes
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Hard process rates at the LHC
Annual yields for PbPb at LHC
Jet rates and kinematic reach at LHC are huge
compared to RHIC
ETjetgt100 GeV 106/year
High statistics measurements over large
kinematic range for precision test of theory
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Inclusive hadron suppression at LHC
I. Vitev and M. Gyulassy, PRL 89, 252301(2002) A.
Dianese et al., Eur.Phys.J. C38, 461(2005)
First test of jet quenching theory at
LHC Different formalisms give different
expectations

Initial gluon density at LHC 5-10 x RHIC
Surface bias leads to relatively small change in
RAA Use full jet structure for more
differential measurements
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Jet reconstruction at LHC
100 GeV jet in central PbPb

• Jet yields at high energies (gt50 GeV) are large
  enough for full jet reconstruction

Energy (GeV)
Full jet reco removes fragmentation bias ? Study
jet quenching (modified fragmentation) in more
detail
Jets accessible over large energy range
(50-200 GeV from full jet reco) ? Validate jet
quenching mechanism

• And more
• Heavy quark jets
• g-jet correlations (calibrate kinematics)
• Suprises?

DELHC 40 GeV ? need ET,Jet200 GeV for EgtgtDE
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Jet reconstruction in heavy ion events
Full jet reconstruction removes fragmentation and
geometric biases
Jet cone
CDF, Phys Rev D65, 092002 (2002)
Use small cone radius 0.3 to suppress
backgrounds

• CDF 80 of jet energy containedin Rlt0.2
• Background from 5.5 TeV PbPb 75 GeV

ALICEEMCal
pT-cut for charged hadrons pT gt 2 GeV

With cuts, only modest influence of background
fluctuations
Rjet0.3
Further optimisation of jet-finding parameters
awaits data
PbPb, 5.5 TeV
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ALICE EMCal
Lead-scintillator sampling calorimeter hlt0.7,
Df110o Shashlik geometry, APD photosensor 13k
towers (DhxDf0.014x0.014)
US contribution to ALICE

• ALICE-EMCal upgrade project in full swing
• First module by 2008
• Full detector by 2009
• (Depending on funding)

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Measuring jet quenching
MLLA parton splittingcoherence ?angle-ordered
parton cascade Good agreement with fragmentation
function data
Introduce medium effects in parton splitting
Borghini and Wiedemann
pThadron2 GeV for Ejet100 GeV
?ln(EJet/phadron)
z
Fragmentation strongly modified at pThadron1-5
GeV even for the highest energy jets
Use large kinematic reach of LHC to test theory
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More jet quenching at LHC
charm/light
Armesto, Dainese, Salgado, Wiedemann, PRD 71
(2005) 054027.
Charm and beauty energy loss to distentangle
colour charge and mass (dead-cone) effects
Z,g-jet to calibrate recoil energyand change
geometric bias
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Conclusion

• pQCD and jet quenching at RHIC reaches
  quantitative era
• Jet measurements in pp
• Differential measurements of di-hadron
  fragmentation and suppression
• Heavy quark energy loss
• Baryon suppression to probe colour charge effects

But kinematic reach (dynamic range) is limited

• Qualitative improvements expected at LHC
• Large kinematic range
• Full jet reconstruction

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RHIC outlook
Di-hadron correlations in CuCu
g-jet correlations
Inclusive g-hadron correlations
T. Dietel, QM talk
ET,trig gt 10 GeV pT,assoc gt 4 GeV
Dj
Dj
Reducing L with a more penetrating probe
Reducing the couplingto the medium
Methods need further development and large data
samples
First results available, need differential
studies, model comparisons
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Extra slides
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Hot and dense QCD matter
Thermodynamic approach
Microscopic picture
Phase diagram of nuclear matter
The strong interaction (QCD)
(Quasi-)free quarks and gluons
temperature
Binding force between quarks in protons and
neutrons
Confinement isolated quarks cannot exist in
vacuum
Early universe
Hadronic matter
Nuclear matter
Quark Gluon Plasma
Elementary collisions (accelerator physics)
High-density phases?
Neutron stars
Baryon density
High density large overlap between hadrons ?
quarks are quasi-free
Nuclear matter
Fundamental phase transition of the Standard
Model
Goal understand dense bulk matter of the
Standard Model
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Surface and other bias effects
PQM Dainese, Loizides and Paic
Surface bias- Trigger, associated selection
favours short path lengths

• Surface bias is not the only possibility
• Energy-loss fluctuations (at fixed path length)
  potentially large
• Fragmentation bias

Wicks, Horowitz, Djordjevic, Gyulassynucl-th/0512
076
Are we selecting pairs, events with small
energy-loss?
X-N Wang, PLB 595, 165 (2004)
Alternative Shape of di-hadron fragmentation
changes little if underlying partonic spectrum
shape unmodified
This calculation underpredicts suppression
Note also possible low-z enhancement from
fragmentation of induced gluons. Outside measured
range, awaits confirmation
Need full calculations, a la PQM
Different observables probe different parts of
convolution