Jets in Nuclear Collisions: Experimental Aspects

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Jets in Nuclear Collisions Experimental Aspects

• Peter Jacobs
• CERN and Lawrence Berkeley National Laboratory

Lecture 2
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Jets in Nuclear Collisions

• Introduction jets in elementary collisions
• what is a jet?
• pdfs and fragmentation functions
• characteristics of gluon, light quark and heavy
  quark jets
• Hard processes in nuclear collisions
• nuclear geometry and scaling rules
• experimental issues collider parameters,
  luminosity
• Partonic energy loss and heavy ion collisions
• leading hadrons
• correlations
• what have we learned?
• Open questions and future prospects at RHIC and
  LHC

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Jets at RHIC
Find this.in this
pp ?jetjet (STAR_at_RHIC)
AuAu ???? (STAR_at_RHIC)
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Partonic energy loss in a colored medium
(discussed in detail by Nestor Armesto)
Bjorken, Gyulassy, Pluemer, Wang, Baier,
Dokshitzer, Mueller, Pegne, Schiff, Levai, Vitev,
Zhakarov, Wang, Salgado, Wiedemann, Armesto

• Bjorkens collisional energy loss generates only
  small effects
• But medium-induced bremsstrahlung is more
  effective

• Essential physics radiated gluon decoheres due
  to multiple interactions with medium
• DE sensitive to color-charge density of the
  medium
• Unique non-abelian feature system size
  dependence DE L2

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Partonic energy loss in cold nuclear matter?
Hermes semi-inclusive DIS off nuclei
Charged hadron yields in N and Kr relative to
deuterium

• z fraction of ?? energy carried by hadron
• strong nuclear-dependent suppression for hard
  fragments
• Theory
• partonic energy loss ? L2 dependence E Wang and
  XN Wang, PRL 89, 162301
• hadronic absorption rescaled fragmentation A
  Accardi et al NuclPhys A720 131
• Data consistent with partonic energy loss but not
  decisive

P. DiNezza JPhysG 30, S783
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Partonic energy loss in hot matter
Multiple soft interactions
(without expansion)
Gluon bremsstrahlung
Opacity expansion (few hard scatters)
(with expansion)
linear dependence of energy loss on gluon
density ?glue measure DE ? color charge density
at early hot, dense phase
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Partonic energy loss via leading hadrons
Energy loss ? softening of fragmentation ?
suppression of leading hadron yield

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pp inclusive spectra vs NLO pQCD
NLO W. Vogelsang
p0
charged hadrons
NLO calculations OK pp reference under control
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Inclusive hadron yields in 200 GeV AuAu
PHENIX
PHOBOS
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Inclusive hadrons yields in central AuAu
collisions are suppressed
Factor 5 suppression large effect

• Qualitatively inconsistent with conventional
  nuclear effects
• initial state multiple scattering (Cronin
  enhancement)
• shadowing

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Initial or final state effect?
Initial state?
Final state?
e.g. gluon saturation
How to discriminate? Turn off final state ? dAu
collisions
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Inclusive yields not suppressed in dAu
STAR
PHOBOS
PRL 91, 072302/3/4/5
BRAHMS

PHENIX
Hadron suppression in central AuAu is a final
state effect
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Cross check direct photon production
Direct g dominant channel for pTgt10 GeV is
Compton process
Photon does not carry color charge ? production
should not be suppressed by medium-induced
radiation
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Direct photons are not suppressed
Photons scale as binary collisions while p0 are
suppressed ? consistent with partonic energy
loss
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Another test h production
PHENIX preliminary
h/p0 invariant with system
h suppression p0 suppression ? partonic energy
loss followed by fragmentation in vacuum
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What do we learn from inclusive hadron
suppression?
see lectures by Nestor Armesto
Partonic energy loss calculations observed
suppression requires initial density gt30 times
cold nuclear matter density
Suppression only supplies lower bound on
density
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Surface emission (trigger bias)
Large energy loss ? opaque core
Inclusive measurements insensitive to opacity of
bulk ? need coincidence measurements to probe
deeper
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Jets via dihadron azimuthal distributions
pp ? dijet

• trigger highest pT track, pTgt4 GeV/c
• Df distribution 2 GeV/cltpTltpTtrigger
• normalize to number of triggers

Phys Rev Lett 90, 082302
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Dihadrons in AuAu vs pp
AuAu peripheral
AuAu central
pedestal and flow subtracted
Phys Rev Lett 90, 082302
Near-side peripheral and central AuAu similar
to pp ? trigger bias recoil heads towards core
Strong suppression of back-to-back correlations
in central AuAu
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Initial or final state effect?
Initial state?
Final state?
e.g. gluon saturation
How to discriminate? Turn off final state ? dAu
collisions
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Final state suppression? dAu dihadrons
Phys Rev Lett 91, 072304
Near-side pp, dAu, AuAu similar Back-to-back
AuAu strongly suppressed relative to pp and dAu
Suppression of the back-to-back high pT
correlation in central AuAu is a final-state
effect
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Away-side suppression non-central collisions
Back-to-back suppression strength correlated with
reaction plane orientation ? suppression is
sensitive to propagation length in medium
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Jet quenching at RHIC

• High pT measurements
• inclusive hadrons suppressed
• direct photons unsuppressed (no color charge)
• near-side dihadron correlations unchanged
• back-to-back dihadron correlations suppressed
• azimuthal modulation of correlations vis a vis
  reaction plane

• Consistent picture core of reaction volume is
  opaque to jets
• ? surface-biased trigger
• observed jets fragment in vacuum

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Where do jet energy and momentum go? Look at
lower momentum correlated hadrons
4lt pT,trig lt 6 GeV
pT,assoc gt 2 GeV
pT,assoc gt 0.15 GeV
STAR nucl-ex/0501016

• Suppression of high momentum?enhancement of low
  momentum pairs
• recoil distribution soft and broad cos (Df)
  (momentum conservation)
• but S/B1/200 difficult background subtraction

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Low pT dihadron correlations uncertainties
Recall CDF dihadron analysis from lecture 1

• Ambiguities
• hadrons from jets vs underlying event
• momentum conservation effects
• resonances

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Dihadron correlations uncertainties (contd)

• low pTassoc
• signal/bkgd 1/200
• large v2 corrections
• normalization is
• ambiguous

Too much energy in recoil peak pickup from
medium?
My personal view this analysis is interesting
and provocative but not yet quantitative
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Evidence for shock waves?
M. Horner (STAR) see also PHENIX
lectures by Edward Shuryak, Nestor Armesto
Broad recoil peak exhibits possible substructure
Work in progress look for news at Quark Matter
from STAR and PHENIX
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Where does jet-like behavior emerge?
Time scale for hadronization for pT few GeV/c
is few fm/c ? hadronization in
medium? Factorization in nuclear collisions?
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Recall indications of factorization
h/p0 invariant with system
PHENIX prelim.
near-side peaks unchanged
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But simple jet phenomenology (factorization)
breaks down at intermediate pT 2-5 GeV/c
Mesons are suppressed, baryons are not
Limited to 2ltpTlt5 GeV
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Intermediate pT II constituent quark scaling of
elliptic flow
Scale by n3 for baryons, n2 for mesons
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Intermediate pT IIIMeson vs baryon-led dihadrons
Intermediate pT 2.5ltpTtriglt4.0 GeV/c
1.7ltpTassoclt2.5 GeV/c

• Associated yields similar for meson and baryon
  triggers

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Intermediate pT IV hadronization via quark
coalescence

• recombination from thermal hard scattering
  sources
• provides natural explanation of baryon
  enhancement, elliptic flow scaling

Correlation data require recombination of soft
and hard partons interplay between hard
scattering and medium
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Near-side correlations at intermediate pT
Dan Magestro, STAR
dAu, 40-100
Near side small Df New puzzle two distinct
components in Dh 1. dAu, AuAu short range,
jet-like 2. AuAu only long range, flat
STAR preliminary
AuAu, 0-5
3 lt pT(trig) lt 6 GeV2 lt pT(assoc) lt pT(trig)
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Dh correlations (contd)

• Recombination effects? Coupling of radiation to
  flow medium?
• Long-range correlation interplay of jet
  quenching and transverse radial flow? Voloshin,
  nucl-th/0312065

Armesto et al.
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Jets _at_ RHIC summary to date

• jet structure is strongly modified in dense
  matter
• signals are large and statistically robust,
  testable multiple ways
• consistent with partonic energy loss via induced
  gluon radiation
• ? medium is very dense gt 30 times cold nuclear
  matter
• intermediate pT complex phenomena, interplay
  between bulk medium and hard processes ? window
  into partonic equilibration?
• Open issues
• differential measurement of DE (not lower bound)
• shock waves in recoil direction?
• coupling of induced radiation to collective
  flow?
• no direct observation of induced radiation
• no accurate accounting of full jet energy
• dependence on color charge (q/g) and quark mass
  of probe
• .

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Jets in Nuclear Collisions

• Introduction jets in elementary collisions
• what is a jet?
• pdfs and fragmentation functions
• characteristics of gluon, light quark and heavy
  quark jets
• Hard processes in nuclear collisions
• nuclear geometry and scaling rules
• experimental issues collider parameters,
  luminosity
• Partonic energy loss and heavy ion collisions
• leading hadrons
• correlations
• what have we learned?
• Open questions and future prospects at RHIC and
  LHC

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84 days to QM05
AuAu results to date are from here
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RHIC II
R. Bellwied, RHIC II workshop
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Jets in nuclear collisions at the LHC
CMS
ALICE
ATLAS
2007 pp collisions _at_ 14 TeV 2008 PbPb
collisions _at_ 5.5 TeV
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Hard process rates at the LHC
ALICE EMCal convenient example I found on my
laptop Rates in CMS and ATLAS acceptances are
yet larger
Jet rates and kinematic reach at LHC are large!
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Jets in nuclear collisions at the LHC (in one
slide)

• LHC is a new physics regime ? surprises
• higher density ? stronger medium effects?
• Jet cross sections are huge robust statistics
  enable precise, microscopic studies
• Detailed probes of energy loss mechanisms
• Kinematic reach in jet ET is huge from RHIC
  (large quenching effects) to asymptotia (small
  quenching effects?)
• Robust tests of quark mass dependence, color
  charge coupling
• g/Zjet ? fragmentation function
• Hadronization of high energy jets (gt 100 GeV)
• many fragments still have modest pTlt10 GeV/c
• intermediate pT breakdown of factorization?
• coupling of radiation to medium?
• ? new phenomena?

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Some obvious comments on preparing for the LHC
It is crucial to continue developing new ideas
and to anticipate where the most exciting physics
lies in LHC heavy ion collisions However, it is
equally crucial to build flexible instruments
that can respond to the surprises when they come
? especially important but difficult to
maintain flexibility at trigger level for rare
processes
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Jets in ALICE
Large backgrounds ? optimal resolution using
small jet cones R0.3?

• Complex underlying event fluctuations in heavy
  ion events
• full jet reconstruction is difficult
• jet trigger is tricky (large background
  fluctuations)
• real jet capabilities will only be known with
  first data

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Observables jet broadening and softening
Medium Modification of Jet Shapes and Jet
Multiplicities C.A. Salgado, U. A. Wiedemann
hep-ph/0310079
Longitudinal momentum fraction z along the
thrust axis of a jet
pT relative to thrust axis

Cleanest measurements gjet, Zjet (but low-ish
cross sections even at LHC)
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Smaller energy loss for heavy quarks ?
Dokshitzer, Khoze, Troyan, JPG 17 (1991)
1602. Dokshitzer and Kharzeev, PLB 519 (2001) 199.

• In vacuum, gluon radiation suppressed at q lt
  mQ/EQ
  
• ? dead cone effect
• Dead cone implies lower energy loss
  (Dokshitzer-Kharzeev, 2001)
• energy distribution wdI/dw of radiated gluons
  suppressed by angle-dependent factor
• suppress high-w tail

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Ratio of heavy/light meson yields
Armesto, Dainese, Salgado, Wiedemann, PRD 71
(2005) 054027.
More detailed considerations multiple scattering
fills dead cone fragmentation q vs g color
charge
pT10-20 GeV/c light mesons from glue, charm
effectively massless ? well-controlled
discimination of color-charge and mass effects
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Summary of Lecture 2

• Jet structure is strongly modified in dense
  matter
• Signals are large and statistically robust,
  testable multiple ways
• very high parton density early in collision
  evolution
• Intermediate pT complex phenomena, interplay
  between bulk medium and hard processes ? window
  into partonic equilibration?

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Phys Rev Lett 91, 072302/3/4/5