From QGP to the Detector: a Journey

1
From QGP to the Detector a Journey

• Craig Ogilvie, MIT
• 7 Oct 1998

2
Review of Last Week

• Range of momentum transfer in pp collisions
• Hard scattering probes short distances
• structure functionspQCDfragmentation
• high pt spectra
• Soft scattering, color field between correlated
  quarks
• formation, decay of strings
• In AA reaction at RHIC, both occur
• collision zone gt dense, colored material
• proto-QGP

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AA at RHIC
wide range of soft/hard collisions occur
partons from hard scattering color-field
between quarks
what is this material ? not color singlets!!
proto-QGP?
4
Todays Outline

• Interactions in this dense colored matter
• overview of proto-QGP to hadrons
• more details in lectures 5 signatures of QGP
• Families of dynamical models
• Next week
• properties of QGP
• phase diagram
• lattice calculations

5
Difference between NN and AA ?

• Imagine a fictitious AA reaction where
• after each N-N interaction stringspartons evolve
  independently.
• each N-N has no idea of any other N-N collisions
• strings decay into qq pairs, eventually to
  hadrons
• partons hadronize
• how will AA results compare to NN results?

6

• ResultsAA ( of collisions) Results NN
• System is not a state with identifiable
  properties
• analogy row of houses is not a community

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Scattering in Dense Matter

• Key is subsequent interactions
• Constituents strongly interact
• calculation is highly non-trivial
• e.g. how do strings interact?
• strings as color-field, gluons are color-field
  carriers gt strings should interact

partons from hard scattering color-field
between quarks SCATTER
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Parton Cascade

• Calculate scattering of partons using pQCD
• Classically propagate partons in time
• cm-reference frame, dependence on frames..
• collisions occur in a time-step
• if distance closest approach
• where s is calculated for each pQCD reaction

VNI K. Kinder-Geiger Comp Phys. Comm 104 (97)
70 qgt1.5 GeV/c
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Internetted Cascade of Partons

• Major problems knowledge of Au structure
  functions, quantum interference of amplitudes,
  soft scattering.

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Scattering is Key but Difficult VNI

• Large model variations in produced particles
• Bottom line - RHIC will be expt. driven for a
  while

nucleon structure funct Au structure functions
include 2-gt1 processes mock-up quantum
interference
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Multiple Strings
1) Allow projectile nucleon to interact
successively with several target nucleons 2)
Creation of multiple strings. Wounded proj.
nucleon. Is 2nd interaction same as 1st? 3)
String-string interaction, fusion
p
t
t
Very difficult to model, justify,
parameterize may be possible to fix from pA
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Properties of Interacting Matter
energy-density?

• RHIC AuAu 200 GeV
• Model and time dependent- HIJING
• when quark distribution is isotropic,
  tiso0.7fm/c
• collisions lead to quasi-thermal system??
• e higher at earlier times but not isotropic
• energy density e3.2 GeV/fm3
• Energy density gt lattice QGP phase transition

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Gluon-Rich Material

• Many initial hard scatterings are gluon-gluon
  (low-x)
• g-gt gg also occurs at large rate
• In a chemically equilibrated plasma can predict
  density (fthermal(e)) of gluons, density of
  quarks
• gluon-rich plasma
• fugacities are low in HIJING ?

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Hydrodynamics (aside)

• In limit of many collisions in dense material
• local equilibrium established
• can apply hydrodynamics to evolution of material
• Discussed later in dynamical signature lectures

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Hadronization
Fragmentation function
pQCD

• Fragmentation functions - developed over years
• parton-gt hadron PYTHIA, Phys. Rev.D36,2019 (87)
• coalescence of multiple partons, VNI, Comp.Phys
  Comm 104,70, (97)
• Strings break until energy is at a hadron mass
  with same quantum numbers

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Densely Packed Hadronic Phase
m

• Below phase boundary the hadronic density is high
• any small increase in density makes a plasma !!!
• tightly packed hadrons strongly scatter.
• Scattering in exit hadronic stage removes
  information
• Use AGS, SPS as test-beds for hadronic scattering

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Hadronic Cascade

• Collisions between hadrons, propagated
  classically
• Allow
• secondary collisions enhance strangeness
• models not yet under control, multiple
  collisisions?

AuAu 11.6AGeV/c
HSD W. Cassing
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Effect on Observables at RHIC
AuAugt K VNI with hadronic scattering without
hadronic scattering

• Some signatures radically changed in hadron exit
  stage
• More in Lecture 5

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Evolution of AA at RHIC
t
exit-stage hadronic collisions hadronization parto
n-cascade and breaking strings initial
hard-scatteringsstring formation
Different models attempt parts of these
dynamics VNI K.Kinder-Geiger Phys Rep
258, 237 (95) Comp.Phys Comm 104,70
(97) HIJING X.-N.Wang Phys. Rep.280, 287
(97) VENUS K.Werner Phys. Rep.232, 87
(93) RQMD H.Sorge Ann. Phys 192, 266 (89)
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Range of Initial Scattering in Models
soft
RQMD
VENUS
HIJING
VNI
0
hard scattering
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Types of Secondary Collisions
string-string
RQMD
HIJING
VENUS
VNI
0
parton-parton
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Scattering in Exit Hadronic Channel
RQMD
VENUS
VNI
HIJING
none
Full range of resonances
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Summary

• AA should be different than pp
• secondary scattering of colored objects
• many soft scatterings non-pQCD
• multiple collisions with amplitudes that
  interfere
• need many-body quantum dynamics
• have semi-classical transport calculations
• Models differ in many areas
• RHIC will be expt. driven
• models will change include discovered key
  features of first RHIC data

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Next Week

• Recent studies on phase-diagram of QGP
• Review of Lattice QCD