High-p T probes of heavy-ion collisions at RHIC and LHC Marco van Leeuwen, LBNL.
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Transcript of High-p T probes of heavy-ion collisions at RHIC and LHC Marco van Leeuwen, LBNL.
High-pT probes of heavy-ion collisions at RHIC and LHC
Marco van Leeuwen, LBNL
Marco van Leeuwen, High-pT probes at RHIC and LHC 2
Introduction
Motivation:Initial production well-calibratedHard processes (high Q2) are only sensitive to short distances and times
Final state particles (partons and/or hadrons) probe the medium through interactions
Marco van Leeuwen, High-pT probes at RHIC and LHC 3
Introduction
Can we check our understanding of hard processes?
For hard process, expect to scale as number of binary collision Ncoll for A+A
Yes, by showing that p+p results can be explained by pQCD
c
chbbaa
abcdba
T
hpp
z
Dcdab
td
dQxfQxfdxdxK
pdyd
d
0
/222
)(ˆ
),(),( Parton density function Matrix element Fragmentation
function
Measured in DIS e+e-pQCD
Marco van Leeuwen, High-pT probes at RHIC and LHC 4
Hard processes at RHIC
• High-ptt 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 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 A+A- Determine medium properties (density, dynamics) in heavy-ion collisions
Marco van Leeuwen, High-pT probes at RHIC and LHC 5
STARSTAR
RHIC accelerator and experiments
PHENIX
Focus: rare probes , e±, Partial coverage High-granularity calorimetry and trackingForward muon detectors
STAR
Focus:global observables
Large volume TPC (2)+EM calorimetry (coarse)
Maximum energy: sNN=200 GeV for Au+Aus=500 GeV for p+p (default 200 GeV)
Recent runs2004: large statistics Au+Au (~80M events), most results in this presentation2005: large statistics Cu+Cu, analysis in progress2006: dedicated polarised p+p run, data-taking in progress
Marco van Leeuwen, High-pT probes at RHIC and LHC 6
p+p jet spectrum @ s=200 GeV
4.0 , 200 RGeVs
First direct measurement of jet spectrum at RHIC
Statistics out to pT=50 GeV… more being collected
Measured spectrum agrees with NLO pQCD
Dominant uncertainty: jet energy scale
Prefer particle spectra, di-hadron correlationsfor Au+Au baseline
(backgrounds too large for jet reconstruction in Au+Au)
Marco van Leeuwen, High-pT probes at RHIC and LHC 7
Light hadron production in p+p
NLO calculations: W. Vogelsang
Star, PRL 91, 172302Brahms, nucl-ex/0403005
Light hadron production at RHIC in good agreement with NLO pQCD
Caveat: gluon fragmentation not so well constrained from e+e-
PRL 91, 241803
Marco van Leeuwen, High-pT probes at RHIC and LHC 8
Baryon production in p+pAlbino, Kniehl, Kramer, Nucl Phys B725, 181 hep-ph/0510173
{
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
also well described
FF parametrisation is an ongoing activity
Baryon production at RHIC also described by pQCD
Marco van Leeuwen, High-pT probes at RHIC and LHC 9
Direct photons
Direct in p+p agree with pQCD
q + g q + PHENIX, PRL 94, 232301
ppTcoll
AuAuTAA dpdNN
dpdNR
/
/
Direct in A+A scales with Ncoll
Centrality
RAA=1 (Ncoll scaling) for incoherent superposition
of p+p collisions
Production through
q + q g +
Marco van Leeuwen, High-pT probes at RHIC and LHC 10
Light hadron production in A+A
Photons and hadron production measured to well in the (expected) perturbative regime
: RAA = 1
0, h±: RAA ≈ 0.2
Light hadron production suppressed by factor 4-5 in central Au+Au
Au+Au 200 GeV, 0-5% central
Marco van Leeuwen, High-pT probes at RHIC and LHC 11
Radiative energy loss in QCDCalculational frameworks:• Multiple soft scattering (BDMPS, Wiedemann, Salgado,…)
• Few hard scatterings,opacity expansion (Gyulassy, Vitev, Levai, Wang,…) • Twist expansion (Wang, Wang,…)
Plus details:Longitudinal expansion reduces E~L2 to E~LFinite energy effects may lead to E-dependent energy loss
CS
coherent
LPM Nq
dzd
dI
ldzd
dI ˆHeitlerBethe
2
ˆ q
2ˆ~ˆ~ LqLqdzd
dIddzE SCS
LPML
med
C
cf Lt
Medium properties can be characterized by a single constant
e.g. transport coefficient
‘average kT -kick per mean-free-path’
E does not depend on parton energyE L2 due to interference effects (for a static medium)
321 ˆLqR
Salgado and Wiedemann, Phys. Rev. D68, 014008
dI
/d
~1 GeV at RHIC C
Soft radiation suppressed by phase space requirement kT <
Radiative energy loss is due to moderate number (~3) of finite energy gluons (~0.1-1 GeV)
43
~~ˆ glueq
Marco van Leeuwen, High-pT probes at RHIC and LHC 12
Non-perturbative dynamics at intermediate pT
Intermezzo
Enhancement depends on:- Particle type (different for , p)- Centrality
d+Au, s=200 GeV
Hadron production in d+Au enhanced compared to Ncoll scaling
‘Cronin effect’ known from fixed target at Fermilab, but mechanism unclear
Effect small compared to effects in Au+Au
p
Marco van Leeuwen, High-pT probes at RHIC and LHC 13
Baryon production in Au+Au
Intermediate pT (2-4 GeV) p/ much larger in Au+Au than p+p (vacuum fragmentation)
At pt=6 GeV: p/ similar in p+p, d+Au and central Au+Au
Non-perturbative effects large at intermediate pTNote: p/ ratio sensitive to gluon/quark ratio. Probes differences in coupling to medium
This presentation: focus at highest pT
Au+Au, 0-5% central, sNN=200 GeV
Marco van Leeuwen, High-pT probes at RHIC and LHC 14
Hadron suppression: sNN=200 GeV Au+Au
Different calculations lead to similar medium densitiesdNg/dy=1100, , approx. 30 times nuclear densityfmGeVq 2155ˆ
Reasonable agreement between data and calculations for pT up to 20 GeV
High statistics year-4 data
Marco van Leeuwen, High-pT probes at RHIC and LHC 15
Centrality dependence
Dainese, Loizides and Paic, Eur.Phys.J. C38, 461 (2005)
pT>4.5 GeV
Data agree with calculated suppression patterns
Path length, density dependence leads to centrality dependence of suppression
More differential tests (e.g. from v2) are under way
On theory side: need to quantify constraints on L-dependence
Marco van Leeuwen, High-pT probes at RHIC and LHC 16
Surface emission (geometric bias)
? Inclusive measurements insensitive to opacity of bulk Need coincidence measurements to probe deeper
RAA~0.2-0.3 for broad range of q̂
Large energy loss opaque core
Eskola et al., hep-ph/0406319
fmGeVq 20ˆ
fmGeVq 21ˆ
fmGeVq 2155ˆ
Marco van Leeuwen, High-pT probes at RHIC and LHC 17
Azimuthal di-hadron correlations
Phys Rev Lett 91, 072304
4 < pT,trig < 6 GeVpT,assoc > 2 GeV
p+p
trigger
associated
Au+Au
Need to subtract background in Au+Au
2002 resultNo modification of near sideStrong suppression of away side
No measurable away-side yield; cannot quantify suppression
Marco van Leeuwen, High-pT probes at RHIC and LHC 18
Jet-like di-hadron correlations
Larger pT allows quantitative analysis of jet energy loss
New results, year-4
Background negligible
at higher pT,assoc
8 < pT,trig < 15 GeVLarger data sample extends pT-range
Emergence of the away side peak
d+Au Au+Au 20-40% 0-5%
Marco van Leeuwen, High-pT probes at RHIC and LHC 19
Di-hadron correlations: centrality dependence
Fit scaledby x2
8 < pT,trig < 15 GeV/c
Near side yields essentially unmodified
Away-side: Increasing suppression with centrality
Again ‘surface bias’
Marco van Leeuwen, High-pT probes at RHIC and LHC 20
Di-hadron fragmentation
~0.54
~0.25
8 < pT,trig < 15 GeV/c
Scalingfactors
Near side fragmentation unmodified
Away-side: strong suppression,but shape similar above zT≈0.4
Marco van Leeuwen, High-pT probes at RHIC and LHC 21
A closer look at azimuthal peak shapes8 < pT(trig) < 15 GeV/c
pT(assoc)>6 GeV
Large energy loss without observable modification of longitudinal and azimuthal distributions
Observations constrain energy loss fluctuations and geometrical bias
No away-side broadening
Marco van Leeuwen, High-pT probes at RHIC and LHC 22
Discussion of di-hadron resultsStrong suppression (factor 4-5, similar to inclusive hadron suppression) without modification of longitudinal and azimuthal fragmentation shapes
jetTcs p
N
dz
dE 2
8
In contrast to several model expectations
Broadening due to fragments of induced radiation
Induced acoplanarity (BDMPS):
= STAR preliminary
Near-side enhancementdue to trigger bias
Majumder, Wang, Wang, nucl-th/0412061
Observation:
Vitev, hep-ph/0501225
Marco van Leeuwen, High-pT probes at RHIC and LHC 23
Confronting IAA and RAA
Dainese, Loizides and Paic, QM posterEskola et al., hep-ph/0406319
IAA ≈ RAA ≈ 0.20-0.25
First look: from IAA and RAA in quantitative agreementq̂
≈ 5-7 GeV2/fm in central Au+Au @ RHICq̂
Need to further assess theory uncertainties
Marco van Leeuwen, High-pT probes at RHIC and LHC 24
Heavy quark suppression (non-photonic electrons)
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
Wicks, et al, nucl-th/0512076
Collisional energy loss revisited
Marco van Leeuwen, High-pT probes at RHIC and LHC 25
Au+Au 0-5%
STAR Preliminary
d+Au 100-40%
Intermezzo II: Jet structure at intermediate pT
3 < pT,trig < 6 GeV2 < pT,assoc < pT,trig
pt,assoc > 2 GeV
absolute ridge yield
New feature in Au+Au: long range correlation
Persist to high pT,trigger likely jet-related
STAR Preliminary
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. C72:034903,2005
– Radial flow + jet-quenching Voloshin nucl-th/0312065
Marco van Leeuwen, High-pT probes at RHIC and LHC 26
RHIC Summary
• pQCD applicable for p+p 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
Jets in nuclear collisions at the LHC
ATLAS
CMS
ALICE
2007: p+p collisions @ 14 TeV2008: Pb+Pb collisions @ 5.5 TeV
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
Marco van Leeuwen, High-pT probes at RHIC and LHC 28
ETjet>100 GeV ~ 106/year
Hard process rates at the LHCAnnual yields for Pb+Pb at LHC
Jet rates and kinematic reach
at LHC are huge compared to RHIC
High statistics measurements over large kinematic range for precision test of theory
Marco van Leeuwen, High-pT probes at RHIC and LHC 29
Inclusive hadron suppression at LHC
Initial gluon density at LHC ~ 5-10 x RHIC:
/fmGeV10~ˆ 2RHICq
Surface bias leads to relatively small change in RAA: Use full jet structure for more differential measurements
I. Vitev and M. Gyulassy, PRL 89, 252301(2002)A. Dianese et al., Eur.Phys.J. C38, 461(2005)
/fmGeV70~ˆ 2LHCq
{
First test of jet quenching theory at LHC:Different formalisms give different expectations
Marco van Leeuwen, High-pT probes at RHIC and LHC 30
Jet reconstruction at LHC
Jet yields at high energies (>50 GeV) are large enough for full jet reconstruction
En
erg
y (G
eV)
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– -jet correlations (calibrate kinematics)– Suprises?
ELHC ≈ 40 GeV need ET,Jet~200 GeV for E>>E
100 GeV jet in central Pb+Pb
Marco van Leeuwen, High-pT probes at RHIC and LHC 31
ALICE+EMCal
Pb+Pb, 5.5 TeV
Rjet=0.3
Jet reconstruction in heavy ion events
CDF, Phys Rev D65, 092002 (2002)
Full jet reconstruction removes fragmentation and geometric biases
PYTHIAHERWIG
pTcharged>5 GeV
pTcharged>30 GeV
80%
80%TeV1.8p,p
Jet cone:22 R
• CDF: ~80% of jet energy containedin R<0.2
• Background from 5.5 TeV Pb+Pb: ~ 75 GeV
Use small cone radius ~ 0.3 to suppress backgrounds:
Further optimisation of jet-finding parameters awaits data
pT-cut for charged hadrons: pT > 2 GeV{
With cuts, only modest influence of background fluctuations
Marco van Leeuwen, High-pT probes at RHIC and LHC 32
ALICE EMCalLead-scintillator sampling calorimeter||<0.7, =110o
Shashlik geometry, APD photosensor~13k towers (x~0.014x0.014)
ALICE-EMCal upgrade project in full swing:-First module by 2008-Full detector by 2009(Depending on funding)
US contribution to ALICE
Marco van Leeuwen, High-pT probes at RHIC and LHC 33
MLLA: parton splitting+coherence angle-ordered parton cascadeGood agreement with fragmentation function data
=ln(EJet/phadron)
pThadron~2 GeV for
Ejet=100 GeV
Fragmentation strongly modified at pThadron~1-5 GeV even for the
highest energy jets
Measuring jet quenching
Borghini and Wiedemann
Introduce medium effects in parton splitting
Use large kinematic reach of LHC to test theory
z
Marco van Leeuwen, High-pT probes at RHIC and LHC 34
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,-jet to calibrate recoil energyand change geometric bias
Marco van Leeuwen, High-pT probes at RHIC and LHC 35
Conclusion
pQCD and jet quenching at RHIC reaches quantitative era:– Jet measurements in p+p– 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
Marco van Leeuwen, High-pT probes at RHIC and LHC 36
RHIC outlookDi-hadron correlations
in Cu+Cu-jet correlations
Reducing L with a more penetrating probe
Inclusive -hadron correlations
ET,trig > 10 GeVpT,assoc > 4 GeV
T. Dietel, QM talk
Reducing the couplingto the medium
First results available, need differential studies, model comparisons
Methods need further development and large data samples
Marco van Leeuwen, High-pT probes at RHIC and LHC 37
Extra slides
Marco van Leeuwen, High-pT probes at RHIC and LHC 38
Hot and dense QCD matter
Phase diagram of nuclear matter
Baryon density
tem
per
atu
re
Hadronic matter
(Quasi-)free quarks and gluons
Nuclear matter
Neutron stars
Elementary collisions(accelerator physics) High-density
phases?
Thermodynamic approach Microscopic picture
Binding force between quarks
in protons and neutrons
Confinement: isolated quarks cannot exist in vacuum
The strong interaction (QCD)
Nuclear matter Quark Gluon Plasma
High density: large overlap between hadrons quarks are ‘quasi-free’
Goal: understand dense bulk matter of the Standard Model
Ea
rly u
niv
ers
e RNC research
Fundamental phase transition of the Standard Model
Marco van Leeuwen, High-pT probes at RHIC and LHC 39
Surface and other bias effectsPQM: Dainese, Loizides and Paic
X-N Wang, PLB 595, 165 (2004)
= STAR preliminary
Note also: possible low-z enhancement from fragmentation of induced gluons. Outside measured range, awaits confirmation
‘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, Gyulassy
nucl-th/0512076
Are we selecting pairs, events with small energy-loss?
Alternative:Shape of di-hadron fragmentation changes little if underlying partonic spectrum shape unmodified
This calculation underpredicts suppression
Partonic spectrumEjet
Nuclear geometry
L
Energy lossE(Ejet)
FragmentationD(Ejet,E)
General form:
Need full calculations, a la PQM Different observables probe different parts of convolution