High pT at the SPS, RHIC and LHC

1
High pT at the SPS, RHIC and LHC

• Peter Jacobs
• CERN and Lawrence Berkeley National Laboratory
• Marco van Leeuwen
• Lawrence Berkeley National Laboratory

2
What is known Hadrons are suppressed, photons
are not
Well described by pQCDradiative energy
loss Initial medium density is high
3
What is known recoiling hadrons are suppressed
Compare to dAu suppression is final-state effect
4
Outline

• Do we understand the mechanisms underlying jet
  quenching?
• Radiative energy loss is the dominant paradigm.
  How do we test it and alternatives?
• Inclusive hadron suppression
• Multi-hadron correlations
• Systematic dependences on ?s, pT, system size,
  quark mass,
• Concentrate on measurements at mid-rapidity,
  highest available pT
• What do we learn about the medium?
• What remains to be done?

Needless to say (?) This presentation features
only a selection of data, in light of the above
questions and with emphasis on new results
5
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,)

Medium properties can be characterized by a
single constant
average kT-kick per mean-free-path
e.g. transport coefficient
DE does not depend on parton energy DE ? L2 due
to interference effects (for a static medium)
Longitudinal expansion reduces DEL2 to DEL
6
Radiative energy loss contd
Soft radiation suppressed by phase space
requirement kT lt w
Radiative energy loss is due to moderate number
(3) of finite energy gluons (w0.1-1 GeV)
Finite energy effects cannot be ignored ?
pT dependence of DE
7
Collisional energy loss revisited
Mustafa and Thoma, Acta Phys.Hung. A22, 93
(2005)
Earlier estimates collisional (elastic) energy
loss is negligible relative to radiative
effects New estimate Boltzmann transport 1-D
Bjorken expansion
Q(p?) RAA
RAA 0.25-0.4 for reasonable pathlength, similar
to radiative E-loss
Can we really ignore collisional energy loss?
8
The SPS RAA/RCP puzzle
See also X. N. Wang, Phys. Rev. C61,
064910(2000)

• WA98 p0 RAA enhanced, RCP suppressed for
  central PbPb
• Calculations with Cronin effect quenching do
  not describe data

9
RAA at SPS pp reference adjustment
D. DEnterria, nucl-ex/0504001
I.Vitev nucl-th/0404052

• pp reference at ?s17.3 GeV needs large
  extrapolation
• D. dEnterria pp reassessment brings RAA down
  (larger quenching)
• dNg/dy400-600, 3-4 compatible with Bjorken
  energy density estimate

10
New SPS data RCP for h-, p, K, K0s, p, L

• RCP(baryon)gtRCP(meson)
• same systematics as original Cronin data at
  similar ?s
• same systematics as RHIC but scaled upward

11
Hadron production at RHIC where is fragmentation
dominant?
Talk Barannikova
Poster Lamont

• pTlt5 GeV
• strongly centrality-dependent baryon/meson ratio
• deviation from from vacuum fragmentation
  ? strong non-perturbative effects
• Larger pT vacuum fragmentation dominates

12
Hadron suppression ?sNN200 GeV AuAu
Opacity, twist expansions

• Finite partonic energy ? significant theoretical
  uncertainties
• pT and centrality dependence broadly described by
  both theoretical approaches
• Energy/gluon densities dNg/dy1100, 14-15

13
Testing L-dependence? RAA for CuCu
Phenix talk, M. Shimomura
M. van Leeuwen, STAR poster
STAR preliminary Charged hadrons

• Suppression observed for central CuCu
• Models scale density from central AuAu
• All models show reasonable to good agreement

14
RAA scales with Npart

• CuCu adds significant precision at intermediate
  Npart100
• Fit to Nparta prefers a1/3 (a2/3 mildly
  excluded)
• Suggests strong surface bias circumference/area
  A1/3 Npart1/3

15
Surface emission (trigger bias)
RAA0.2-0.3 for broad range of
Large energy loss inclusive measurements
insensitive to opacity
Need more penetrating probes
16
Azimuthal correlations at yet higher pT
Phenix talk, N. Grau, poster J Jia
1/Ntrig dN/dj

• Large year-4 dataset allows to push trigger,
  associated pT higher
• Beyond intermediate pT and well into
  fragmentation region
• Combinatorial background is negligible

STAR talk, D. Magestro

• Clear, unambiguous recoil peak dijets in central
  collisions
• Away-side yield is suppressed but finite and
  measurable

17
Jet yields at higher pT
STAR talk, D. Magestro
8 lt pT(trig) lt 15 GeV/c

• No significant suppression of near side
• Away-side suppressed
• Suppression pattern independent of pT,assoc

18
Characterization of recoiling jet
STAR talk by D. Magestro
First differential measurement of energy loss
dN/dzT

• Recoil yield is suppressedIAA 0.25 RAA
  (central collisions)
• Fragmentation distribution essentially unchanged
  (zTgt0.4)

0.25
8 lt pT(trig) lt 15 GeV/c
STAR preliminary
19
Comparison to energy loss calculations
Recoil fragmentation fn
Near side IAA
STAR talk, D. Magestro
Near-side (Majumder et al.) energy-dependent
energy loss generates strong centrality variation
of correlation ? in contrast to
measurement Recoil (Wang) suppression factor
0.4, no variation in frag. fn (zTgt0.4) ?
qualitative but not quantitative agreement with
measurement
20
Angular distribution of recoil is invariant

• I. Vitev (hep-ph/0501225) for large energy
  loss, recoil dominated by fragments of induced
  radiation up to pT10 GeV/c
• expect strong azimuthal broadening relative to
  vacuum
• in contrast to measurement
• Acoplanarity is connected to suppression
• BDMPS
• analogous relationship for collisional energy
  loss? ? Can we discriminate mechanisms by this
  measurement?
• ? need quantitative calculations

21
Surface emission for dijets?
Poster by Dainese, Loizides and Paic
Emission points for dijets also show surface
bias, tangential emission IAA dominated by
tangential pairs for large
22
Putting the pieces together
Poster by Dainese, Loizides and Paic
IAA RAA 0.20-0.25
23
Heavy quark suppression (Theory) via
non-photonic electrons
M. Djordjevic et al., nucl-th/0507019

• Theory expectations
• Charm RAA 0.2-0.3
• Beauty RAA 0.4-0.6

(at high pT)
Note using large medium densities dNg/dy3500,
14
24
Heavy quark suppression (Data) via non-photonic
electrons
STAR talk by J. Bielcik
Data indicate Large suppression of beauty
or charm dominance up to electron pT 10 GeV
Medium density inferred from heavy quark energy
loss larger than from light quark RAA and IAA
25
Next steps _at_ RHIC
Di-hadron correlations in CuCu
g-jet correlations
Talk T. Dietel
Inclusive g-hadron correlations
Poster J. Jia
ET,trig gt 10 GeV pT,assoc gt 4 GeV
Reducing L with a more penetrating probe
Reducing the couplingto the medium
Methods need further developmentLarger data
samples welcome (RHICII)
First results available, need differential
studies, model comparisons
26
Jets in nuclear collisions at the LHC
CMS
ALICE
ATLAS
2007 pp collisions _at_ 14 TeV 2008 PbPb
collisions _at_ 5.5 TeV
27
Jet rates at the LHC

• Jet and dijets very broad kinematic reach
• Huge jet statistics for ET100 GeV ? access to
  rare but sensitive fragmentation patterns
• e.g. very high pT dihadron pairs probe coupling
  in medium
• Evolution of quenching over
• very broad ?s
• logarithmically large jet energy range

100
28
Full jet reconstruction in heavy ion events?

• leading hadrons correlations strong
  fragmentation and surface biases
• fully reconstructed jets unbiased view of
  medium modifications, interaction of radiation
  with medium

Use small cone radius 0.2 to suppress
backgrounds

• CDF 80 of jet energy contained in Rlt0.2
• Background from 5.5 TeV PbPb
• dET/dh 3700 GeV, ET(Rlt0.2) 75 GeV
• saturation model scaling (Eskola et al,
  hep-ph/0506049)

• Significant irresolutions due to
• out-of-cone radiation
• background fluctuations
• broadening due to energy loss (!)

Optimal definition of jet awaits data
29
Jet quenching at the LHC some observables
Softening of fragmentation hadron suppression at
high pT, hadron excess at low pT Medium-modified
MLLA hadron excess at pTfew GeV/c for high
energy jets

• Coupling of radiation to flow of medium?
• Medium modification of hard dihadrons?
• LHC vs RHIC wider dynamic range for mass and
  color charge effects (b vs c vs g jets Armesto
  et al., PRD 71 054027 (2005) )
• Multi-hadron correlations à la RHIC and beyond

30
gjet and Zjet at LHC
g, Z no color charge ? no in-medium
interaction Zjet background negligible Precise
calibration of jet energy ? precise measurement
of modified fragmentation X.-N. Wang et al., PRL
77, 231 (1996)
Important measurements, but statistical and
kinematic reach are limited
? full exploitation of energy loss as a probe of
the medium at LHC requires understanding of
inclusive jets and dijets
31
Summary

• New RHIC data great reach and precision, very
  clear jet interpretation
• ? suppressed, vacuum-like recoil in central AuAu
  providesstringent constraints on underlying
  physics
• Is jet quenching due to radiative energy loss?
• broad agreement with calculations on vs,
  centrality dependence
• but some tests are weak at present
• no clear observation of induced radiation
• no sharp test of L2 dependence
• heavy quark energy loss larger than expected
• collisional energy loss may play a significant
  role
• LHC will provide enormous reach and qualitatively
  new observables

New SPS data qualitatively fit in the picture
Medium initially very dense response of medium
to energy loss not yet under quantitative control
Jet quenching as a precise probe of the medium is
in sight
32
Extra slides
33
Radiative energy loss in a colored medium

• Virtual gluons multiple scatter in medium
• QCD LPM effect interplay between formation time
  and ltkT2gt from multiple scattering ?
  medium-dependent radiation spectrum
• e.g. multiple soft scattering (BDMPS)

? medium-induced radiated energy
34
What is recoiling from an 8 GeV trigger?
pp ? p0 at mid-rapidity

• pTtriggt8 GeV/c
• 50 qg (qq not yet dominant)
• trigger is mostly quark, recoil is mostly gluon

Observed recoil nevertheless biased towards
quarks due to color charge?
35
Identified particle RCP
36
What do we learn from inclusive hadron
suppression?
Need more penetrating probes
37
RAA vs Npart
M. Shimomura
PHENIX plenary pTgt7 GeV/c (in hard
region) Centrality dependence of suppression in
AuAu scales as Npart2/3 CuCu consistency with
this scaling barely within 90 CL (c2/Ndof10.6/6)
38
SPS RCP(K) vs energy loss calculations

• Wang calculation pQCD Cronin
  (anti-)shadowing energy loss
• gluon density scaled by dNch/dh
• Good agreement between data and both Wang and PQM
  calculations
• ? SPS discrepancy resolved quenching compatible
  with eBj

39
SPS RCP (p) NA49 vs WA98

• NA49 p vs WA98 p0 NA49 reference more
  peripheral yet RCP larger (less suppression) ?
  data in conflict?

40
Medium response to recoiling jet?
4lt pTtrig lt 6 GeV
pTassoc gt 2 GeV
pTassoc gt 0.15 GeV
STAR, Phys Rev Lett 91, 072304
High momentum correlation suppressed ? low
momentum enhanced Recoil distribution soft and
broad cos (Df) (momentum conservation) Qualitati
ve picture consistent ? can dynamics be studied
quantitatively at low pT?
41
Low pT correlated yields technical remarks
M. Horner, STAR poster

• signal at low pT is small difference of large
  numbers with delicate uncertainties in
• background normalization (ZYAM assumption)
• v2 correction

Interplay between long range Dh correlations and
ZYAM?
42
Evidence for conical flow is marginal
pTtrig2.5-4.0 GeV/c pTassoc1.0-2.5 GeV/c
J. Ulery, STAR talk

• similar analyses in different acceptances ?
  qualitatively different shapesuncertainties
• ? imho quantitative study of medium response
  still an open issue

43
Jet-correlations at the SPS
Azimuthal correlations are measured at
SPS Trigger pT 1.5 GeV
44
LHC Inclusive hadron suppression

• But RAA insensitive to density (surface emission)
• ? qualitatively similar at RHIC and LHCShape of
  RAA tests interplay of
• E dependence of DE (finite energy constraint)
• (anti-)shadowing
• partonic spectrum shape 1/pTn (nLHCltnRHIC)
• pTlt10 GeV/c large effects due to complete
  quenching?

45
Tangential pairs?
Simplest surface emission picture only jets
seeing medium thickness ltDL are observable

• Realistic nuclear geometry tune DL to reproduce
  RAA
• ? negligible phase space for recoil escape, in
  contrast to data

46
(No Transcript)