HIJING: An Ancient Tale Confronting the RHIC Era

1
HIJING An Ancient Tale Confronting the RHIC Era

• Why HIJING?
• A short introduction
• The basics of HIJING
• Importance of hard processes at RHIC
• Soft versus hard processes
• Jet quenching
• Where will HIJING fail?
• Partonic and hadronic cascade
• A simpler Parton Model
• pQCD parton model with multiple scattering and
  parton energy loss
• Conclusions

2
Why HIJING?

• String model reigns before 1989
• FRITIOF, DPM,VENUS,RQMD,
• Good for heavy-ion phenomenology at intermediate
  energies (Ecmlt20 GeV)
• Minijets in high-energy pp
• Many phenomena point to minijets
• Success of PYTHIA in high-energy pp
• Minijets in high-energy AA
• Blaizot Mueller (87),
• Kajantie, Landshoff, Lindfors (87), Eskola et al
  (89)
• How to implement minijets in a Monte Carlo model
  for AA collisions?
• HIJING -Gyulassy, XNW (91)

3
HIJING Heavy Ion Jet INteraction Generator

• Combine FRITIOF string model with pQCD processes
  (PYHTIA)
• Soft? strings Hard ? pQCD
• Nuclear effects
• Eikonal formalism softhard
• Nuclear shadowing of parton distrib.
• Jet quenching
• HIJING ? HI JING (I Ching)
• HIJING ? I Ching for heavy-ion collisions? Maybe
  too ambitious!
• PCM Very ambitious model
• Geiger Muller (92)
• NEXUSA Renewed Effort
• Werner, et al.

4
Factorization in pQCD
5
Mini-jet production
Soper et al
6
Hard versus soft

• pQCD fails at small momentum scale
• as large, factorization breaks down,
  higher-twists important
• How to incorporate mini-jets into pp total cross
  section?

7
Eikonal Approximaton
8
Unitarization
9
Onset of minijet production
10
Multiple Minijet production
KNO scaling broken
11
Inclusive spectra
XNWGyulassy91
12
pA and AA Collisions

• Multiple parton scattering coherence
• Nuclear modification of parton distribution
  function
• Final state interaction Formation of QGP

Glauber-Gribov Multiple Scattering
Multiple scattering ? coherent ? different
A-dependence
13
Multiple Scattering
14
Example n2
15
Coherent scattering
Coherent scattering
16
Nuclear shadowing
Coherent length
Coherent scattering
anti-shadowing
shadowing
Anti-shadowing
x
17
Parameterization of Shadowing
Gluon shadowing
18
Particle Production in AA

• Soft processes coherent interaction
• Cross section surface of nuclei
• Particle production number of participants
• Hard processes Incoherent interaction
• Cross section volume of nuclei
• Particle production number of binary collisions
• Parton distribution ? nuclear shadowing

19
Energy dependence
shadowing
pp collisions
AA collisions
nsoft
Li XNW01
20
Interplay bwt soft and hard processes

• Transition from soft (coherent) to hard
  (incoherent) processes

Absorptive part
21
Effective A-dependence
Coherent soft ? ltNpartgt
Incoherent hard ? ltNbinarygt
22
Nuclear Modification factor
(E. WangXNW)
23
HIJING implementation

• Soft processes
• Fritiof string phenomenology
• Number of strings proportional to number of
  participants
• pQCD parton scattering
• Incoherent processes, proportional to number of
  binary collisions
• Use PYTHIA for simulation of pQCD
• Parton distributions in nuclei
• Nuclear shadowing
• Parametrization of DIS data
• Assume same shadowing for quarks and gluons
• Parton energy loss
• Constant dE/dx, schematic gluon bremsstrahlung

24
Where will HIJING fail?
Inadequate description of multiple parton
scattering
Difficult to incorporate parton energy Loss
dE/dx LPM interference effect
No parton and hadron cascade? thermalization for
low pt particles
25
pQCD parton model
Modified frag. func.
Modified Dh/a(z,Q2) from pQCD Guo Wang00)
26
dE/dx and hadron spectra
dE/dx here is only an average value
27
Static versus Dynamic
In a static medium with density
28
pT dependence of R(pT) sensitive to energy
dependence of dE/dx
Gluon absorption will enhance the energy
dependence (E. Wang XNW01)
29
High pt anisotropy
30
Conclusions

• RHIC data confirms the onset of minijet
  production in AA collisions at RHIC
• Model constrained by pp data reasonable nuclear
  dependence can account the RHIC data well
• Is dN/dh sensitive to thermalization?
• Saturation vs. parton shadowing?
• High pt spectra and anisotropy sensitive to dE/dx
• dE/dx provide information about initial parton
  ensity