Axel%20Drees,%20%20University%20Stony%20Brook

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Energy Loss in Dense MediaJet Quenching
One of the first discoveries at RHIC!
PRL 88 (2002) 22301
PHENIX
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Outline of My Talk

• Introduction
• Quark Gluon Plasma at RHIC
• Jets and how they probe the QGP
• Jet quenching in heavy ion collisions
• pp baseline
• High pt particle suppression in Au-Au
• d-Au control experiment
• Suppression of jet-jet correlations
• New experimental results
• Medium modification of jet-correlations
• Medium modifications of charm spectra
• Summary Outlook

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The Phase Diagram of Nuclear Matter

• QGP in Astrophysics
• early universe
• time lt 106 seconds
• possibly in the interior of
  neutron stars

• Quest of heavy ion collisions
• create QGP as transient
• state in heavy ion collisions
• verify existence of QGP
• study properties of QGP

170 MeV 1Gev/fm3
Overwhelming evidence for strongly interacting
plasma produced at RHIC
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Matter at RHIC has 15 GeV/fm3
?15 GeV/fm3
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Ideal Experiment to Probe the QGP

• Rutherford experiment a ? atom discovery of
  nucleus
• SLAC electron scattering e ?
  proton discovery of quarks

QGP
penetrating beam (jets or heavy particles)
absorption or scattering pattern
Nature needs to provide penetrating beams and the
QGP in Au-Au collisions

• QGP created in Au-Au collisions as transient
  state for 10 fm
• penetrating beams created by parton scattering
  before QGP is formed
• high transverse momentum particles ? jets
• Heavy particles ? charm and bottom

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Jets A Penetrating Probe for Dense Matter

• What is a jet?
• Incoming partons may carry large fraction x of
  beam momentum
• These partons can scatter with large momentum
  transfer
• Results in large pT of scattered partons
• appears in laboratory as jet of particles
• Jet production can be observed as
• high pT leading particles
• angular correlation

• In a gold gold collision
• Scattered partons travel through dense matter
• Expected to loose a lot of their energy
• Energy loss observed as
• suppression of high pT leading particles
• suppression of angular correlation
• Depending on path length, i.e. centrality and
  angle to reaction plane

reaction plane
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Particle Spectra from p-p Collisions

• Jet production measured indirectly by transverse
  momentum (pT) spectrum
• Identified particles (p0)
• Charged particles (h p, K, p, .. )
• At RHIC energies different mechanisms are
  responsible for different regions of particle
  production
• Thermally produced soft particles
• hard particles from jet production
• Hard component can be calculated with QCD
• Data agrees with QCD calculation
• calibrated reference

p0 from p-p collisions
QCD calculation
soft
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Scaling from p-p to Heavy Ion Collisions

• Hard-scattering processes in p-p
• quarks and gluons are point-like objects
• small probability for scattering in p-p
• p-p independent superposition of partons
• Minimum bias A-A collision
• assume small medium effects on parton density
• superposition of independent p,n collisions
• collision probability increases by A2
• cross section scales by number of binary
  collisions
• Impact parameter selected A-A collisions
• superposition of p,n collisions among
  participants
• calculable analytically by nuclear overlap
  integral
• or by MC simulation of geometry Glauber Model

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Binary Scaling in Au-Au tested with Direct Photons

• pp collisions
• qg-Compton scattering
• Direct g production described by NLO pQCD

• Au-Au collisions
• Direct g rates scale with Nbinary
  
• Similar scaling observed for charm quark
  production

Hard processes in Au-Au scale with Nbinary
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Suppression of p0 in Central AuAu Collisions
PRL 91 (2003) 72301
Nuclear modification factor
PHENIX
PHENIX preliminary
High pT suppressed by factor 5 pp to central
AuAu and peripheral to central Au-Au
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Control Experiment with d-Au

• Final state effect jet quenching
• Medium created in d-Au has small volume
• Jets easily penetrate short distance
• No suppression of jet yield expected in d-Au
• Initial state saturation effect
• Gluon density saturated in incoming gold nucleus
• Deuteron shows no or little saturation
• Expect suppression of jet yield, but with reduced
  magnitude

Final state effect no suppression Initial state
effect suppression
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Suppression at Parton Level

• No suppression for direct photons
• Hadron suppression persists up to gt20 GeV jets
• Common suppression for p0 and h it is at
  partonic level
• Typical model calculation e gt 15 GeV/fm3
  dNg/dy gt 1100

Hot opaque partonic medium e gt 15 GeV/fm3
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Centrality Dependence of Suppression

• Hard region pT gt 7 GeV/c
• Suppression depends on centrality but not on pT
• Characteristic features of jet fragmentation
  independent of centrality
• pQCD spectral shape
• h/p0 constant
• xT scaling

Centrality dependence characteristic for jet
absorption in extremely opaque medium!
Insensitive to details of energy loss mechanism
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Azimuthal Correlations from Jets
pp ?jetjet
STAR
Jet correlations in Au-Au via statistical
background subtraction
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Disappearance of the Away-Side Jet
trigger 6 ltptlt 8 GeV partner 2 lt pt lt 6 GeV
Near-side pp, dAu, AuAu similar Back-to-back
AuAu strongly suppressed relative to pp and dAu
Suppression of the away side jet in central AuAu
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Suppression of Back-to-Back Pairs
Jet correlation strength
Near side
Compared to jet absorption model (J.Jia et al.)
Away side
Away side jets are suppressed consistent with jet
absorption in opaque medium
Mono jets point outward
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Remaining Jets from Matter Surface
8 lt pT(trig) lt 15 GeV/c
pT(assoc)gt6 GeV
D. Magestro, QM2005
STAR Preliminary
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Where Does the Energy Go?
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Modification of Jet Shape at Lower pT
Near side
Away side
PHENIX preliminary
Can jet shape be related to properties of matter?
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Sound velocity? Dielectric Constant?Jet
Tomography will be power tool to probe matter!
Theoretical Speculation

• Energy loss of jet results in conical shock wave
  in strongly interacting plasma
• Hydrodynamic mach cone?
• Longitudinal modes ?
• Cherenkov radiation ?
• Momentum conservation multiple scattering with
  meduium
• Medium evolution of radiated gluons

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How opaque is the medium? Check Charm Production!

pp

• Signal
• Background

background subtracted electron spectrum

• Default PYTHIA parameterization
• PDF CTEQ5L mC 1.25 GeV mB 4.1 GeV
• ltkTgt 1.5 GeV K 3.5
• Parameterization tuned to describe ?s lt 63 GeV
  pN world data
• Spectral shape is harder than PYTHIA
  expectation

pp PHENIX preliminary
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Open Charm in AuAu at ?sNN200 GeV

• Total yield scales with number of binary
  collisions

No indication of strong medium modification of
charm production
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Heavy Quark Energy Loss Nuclear Modification
Factor

• Strong modification of the spectral shape
• Suppression by factor 2-5, similar to pion
  suppression
• Large bottom contribution above 4 GeV?

Production of charm scales like hard
process Spectral shape modified while propagating
in medium
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Elliptic Flow A Collective Effect
dn/d? 1 2 v2(pT) cos (2 ?) ...
Initial spatial anisotropy is converted into
momentum anisotropy
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Charm Quarks flow with light quarks

• Charm flows, strength 60 of light quarks (p0)
• Drop of the flow strength at high pT due to
  b-quark contribution?
• The data favor the model that charm quark itself
  flows at low pT.

High parton density and strong coupling in the
matter
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Strongly interacting QGP produced at RHICState
of unprecedented energy density 15
GeV/fm3Opaque to colored hard probes, jets and
heavy flavor Hard probes will be critical to
study properties of QGP
Summary Outlook

On tape analysis ongoing
2004
4x larger Au-Au data sample in 2006
2001
2002
Factor 10 luminosity increase with electron
cooling after 2010
Discovery of jet quenching
Most data seen today
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Backup Slides
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Outlook into the Away Future
g-jet the golden channel for jet tomography
Quark gluon Compton scattering g-energy
fixes jet energy g Jet direction fix
kinematics measure DE as function of E, L,
flavor
70 of photons are prompt photons
Promising measurement at RHIC every low cross
section pTlt 8-10 GeV on tape luminosity and
detector upgrades extend range
to pT25 GeV and ylt3