Jet Quenching from Collisional Energy Loss in the QuarkGluon Plasma

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Jet Quenching from Collisional Energy Loss in
the Quark-Gluon Plasma
Gunnar Ingelman, High Energy Physics, UU,
www.isv.uu.se/ingelman
BDMPS

• Previous paradigm
• Energy loss dominated by
• medium-induced radiation
• Now
• Substantial energy loss also via
• elastic scattering on plasma partons
• Work/results by collaboration
• Uppsala G. Ingelman, J. Rathsman
• Heidelberg S. Dombey, H.J. Pirner, J. Stachel,
  Korinna Zapp
• CERN U.A. Wiedemann

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Jet quenching
Hadron flow in azimuth ?? vs trigger particle/jet

• Quark/gluon from hard QCD-scattering
• multiple scattered in plasma
• energy loss
• hadrons/jet gets lower energi

back-to-back jet disappears!
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Jet quenching from soft QCD scattering in the QGP
K. Zapp, G. Ingelman, J. Rathsman, J. Stachel,
Phys.Lett. B637, 179 (2006)

• Hard parton-parton scattering via PYTHIA
• pQCD 2?2 matrix elements
• initial final state parton showers
• distributed in AA overlap region ? Ncoll(x,y)
• 2. Quark Gluon Plasma (QGP) dynamics
• 3. Interactions with the plasma
• 4. Hadronisation
• string or independent

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2. Quark-Gluon Plasma
Geometry Npart , Ncoll from Glauber model ?
impact parameter b ? centrality EOS ideal
relativistic gluon gas Expansion boost-invariant
longitudinal expansion
saturation scale hydro lattice chosen here
Parameters Energy density
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3. Interactions with the plasma

• Cronin effect of initial state interactions ? p?
  broadening
• Soft Colour Interactions SCI model
• successive, independent elastic scatterings
• screening radius Rscr
• probability P
• momentum transfer t Gaussian distr.
• Parameters

Wang, PR C61 (2001) 064910 Zhang et al., PR C65
(2002) 034903

• soft gluon exch. between partons
• rapidity gaps, charmonium
• as in ep pp data

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Jet quenching at RHIC
Nuclear modification factor
peripheral
central
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Jet quenching at RHIC
MC away-side jet suppressed, but not
disappeared!
Trigger particles 4 lt p? lt 6 GeV associated
particles 2 lt p? lt p?(trig) sensitivity to
conditions!
Energy loss and geometry
r fm
radial distance r of hard scattering
Not strong suppression due to rapid expansion !
moderate surface bias
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Energy loss of heavy quarks
RAuAu for e e-
Energy loss in a single scattering
Kinematics ? heavy quark
lose less energy

• Suppression of e e- from
• charm ? OK
• charmbottom somewhat too weak

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QCD evolution of jets in the Quark-Gluon Plasma
S. Dombey, G. Ingelman, H.J. Pirner, J. Rathsman,
J. Stachel, K. Zapp, Nucl.Phys. A808, 178 (2008)
PYTHIA parton shower

• Parton virtuality Q2 p2
• Lifetime in lab frame ? ? E/Q2
• Lifetime ? mean free path
• medium can influence
• parton shower evolution

Time dilation ? gluon emission simultaneous with
scattering in plasma
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Modified DGLAP evolution equations
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Numerical solution of modified DGLAP equations

• perturbative scattering cross-section, regulated
  by Debye mass mD
• ideal gas EOS for QGP (as before)
• T500 MeV represent average properties of
  plasma, mD ?2GeV
• fragmentation function at Q0

Kniehl, Kramer, Pötter, Nucl.Phys. B582, 514
(2000)
g?? fragmentation function
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Higher energy scale (E,Q2) ? longer lnQ2
evolution ? longer lifetime for parton
plasma ? more multiple scattering
T500 MeV
Expanding plasma and geometry of collision
requires Monte Carlo
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A Monte Carlo for Jet Quenching
K. Zapp, G. Ingelman, J. Rathsman, J. Stachel,
U.A. Wiedemann, arXiv0804. 3568, EPJ in press

• JEWEL Jet Evolution With Energy Loss
• simulation of (perturbative) jet evolution in
  medium
• produces correct result in vacuum (scatterings
  switched off)
• exact energy-momentum conservation
• produces complete partonic and hadronic final
  state
• can test different microscopic descriptions in
  same environment

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Medium modifications

• lifetime of parton in shower
• ideal relativistic gas of
• elastic scattering
• cross-section
• pQCD, regulated
• no-scattering probability
• scattering possible between splittings and
  after last splitting
• possibility to split and/or scatter the recoil
  (not explored here)

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Cont. medium modifications

• Induced radiation in medium
• - no microscopic description here
• factor (1fmed) increase of
• splitting function (probability)
• Hadronisation
• Modified string model applicable for jets also
  in nuclear collisions
• ? assume maximal colour correlation of partons
  close in phase space
• String from most energetic (p?) parton to
  closest neighbour in p-space
• until string end is reached.
• Jet can be hadronised alone or together with
  scattering centres

BDMPS
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Collisional energy loss
Average energy loss of on-shell quark of energy
E through multiple elastic collisions over 1 fm
in medium at temp. T
Probability for energy loss ??E of quark with
energy E100 GeV after medium of lenght L and
temperature T
Relatively small energy loss ? other
calculations Depends on regularisation of pQCD
cross-section
Considerable fraction of partons suffer only
small energy loss
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Recoiling scattering centres
p? spectrum of recoiling scattering centres
Angle between jet and recoiling scattering centre
E100 GeV
thermal
Developes a power-tail due to shape of
scattering cross-section
Preferred angle nearly independent of temperature
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Intra-jet distribution dN/d?
Distribution of ?ln(pmax/p) in a single 100 GeV
quark jet Medium T500 MeV, L5 fm, fmed3
small p ?

• hadronisation softens the distribution
• collisional energy loss only increases jet
  multiplicity
• when recoils are included
• radiative energy loss ? clear increase of
  multiplicity

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Transverse momentum broadening
100 GeV quark jet, medium T500 MeV, L5 fm,
fmed3
Ehadrongt2 GeV k? wrt jet axis

• only minor broadening due to collisional energy
  loss
• radiative energy loss may lead to mild k?
  broadening
• energy-momentum conservation important

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A local MC implementation of the non-abelian LPM
effect
K. Zapp, J. Stachel, U.A. Wiedemann,
arXiv0812.3888
Landau-Pomeranschuk-Migdal effect from quantum
interference between spatially separated,
inelastic radiation processes in matter

• Incoherent emission
• gluon formation time tf2?/k?2 ?? distance
  between scattterings ? probabilistic ? MC
• Coherent emission for tf gt d
• scattering centres in medium not resolved
  individually by radiated gluon
• ? scattering centres contribute coherently ?
  quantum interference important
• Add momentum transfers from different scattering
  centres within formation time tf
• ? MC algorithm
• select a scattering centre as source of gluon
  emission
• determine probabilistically d to next scattering
• if dlttf momentum transfer added coherently to
  gluon, goto 2
• if dgttf gluon is formed and independent of
  projectile, goto 1
• Iterate as long as parton is within medium

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Medium-induced energy loss coherence
energy-mom. conservation

• Full MC algorithm with LPM
• LPM but no E-p conservation
• Incoherent without E-p conservation
• Incoherent with E-p conservation

Eq100 GeV
Energy distribution of gluon radiated from 100
GeV quark
q?lt1 GeV
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Summary / Outlook

• Collisional radiative energy losses are both
  important
• Parton shower collisions at same distance
  scale in the plasma
• ? modified DGLAP evolution equations
• JEWEL Monte Carlo available
• ? partonic hadronic final state
• Plasma properties and expansion
• can be taken into account properly
• Non-abelian LPM coherence effect implemented

Korinna Zapp PhD Dec. -08
Detailed realistic studies can be made to
discover/explore the quark-gluon plasma