Event Generators in ALICE
Andreas Morsch
Special Application Area Meeting Dedicated to
LCG Generator Services
CERN, June 1, 2005
Simulation of Heavy Ion Collisions
 Shortcomings of existing generators

None of the existing generators do give detailed account
of the expected multiplicities, pt and rapidity dependence
at LHC energies

Most of the hard probes (heavy flavor, jets ...) are not
properly reproduced by existing generators.

Existing generators do not provide for event topologies
like momentum correlations, flow etc.
 The small cross-section of hard processes would demand
prohibitively long runs to simulate a number of events that is
commensurable with the expected number of detected events in
the experiment
The ALICE Approach




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The simulation framework provides an interface to external generators, like
HIJING and DPMJET.
A parameterised ''signal free'' underlying event with multiplicity as a parameter
is provided.
Rare signals can be generated using

External generators like PYTHIA

Libraries of parameterized pt and rapidity distributions
The framework provides a tool to assemble events from different signal
generators

On the primary particle level (cocktail)

On the digit level (merging)
After-Burners are used to introduce

Soft particle correlations

Jet quenching
Event Generator Interfaces:
External Generators
Interface to external generators
using the TGenerator class from
ROOT
HIJING
 Hijing (v1.36) used as
 Underlying event

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Particle Correlation studies

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Inclusive
Reconstructed jets
Nuclear effects

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Realistic fluctuations (N,E) from jets
Pessimistic multiplicity (dN/dy ~ 6000)
Shadowing
Quenching (parton energy loss)
Had to correct several bugs


Standard HIJING with quenching has “flow-signal” in dN/df for
b=0.
Many protections to avoid floating point exceptions.
pp
 Minimum Bias
Pythia (v6.214) with ATLAS Tuning
 Hard Probes (pp and embedded into Pb-Pb)
 Pythia tuned to NLO (MNR)

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NLO topology
Modification of nuclear structure functions via EKS in PDFlib
After Burners for Jet Quenching

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AliPythia::Quench
pyquench (Igor Lokhtin)
 In the future

Interest in NLO generators
NMR Tuning (Beauty)
MNR
PYTHIA
pair creation (PC) “LO”
flavour excitation (FE) “NLO”
gluon splitting (GS) “NLO”
TOTAL
ALICE baseline: N.Carrer and A.Dainese, ALICE-INT-2003-019, hep-ph/0311225.
Gluon shadowing
fgPb(x)/fgp(x)
Jet Quenching
Nuclear Geometry
(Glauber)
Afterburner A
Pythia hard scattering
Initial and Final State Radiation
Afterburner B
Pythia Hadronization
Afterburner C
Jet (E) → Jet (E-DE) + n gluons (“Mini.Jets”)
 Two extreme approaches
.


Quenching of the final jet system
and radiation of 1.
5 gluons. (AliPythia::Quench + Salgado/Wiedemann
- Quenching weights with q = 1.5 GeV2/fm)
Quenching of all final state partons and radiation of
many (~40) gluons (I. Lokhtin: Pyquen)*
)*I.P. Lokhtin et al., Eur. Phys. J C16 (2000) 527-536
I.P.Lokhtin et al., e-print hep-ph/0406038
http://lokhtin.home.cern.ch/lokhtin/pyquen/
Event Generator Interfaces:
Parameteriations
Interface to parametrisations
and decayer
Example: MUON Library
Parameterisations:
kPhi, kOmega, kEta,
kJpsi, kJpsiFamily, kPsiP, kJpsiFromB,
kUpsilon, kUpsilonFamily, kUpsilonPP,
kCharm, kBeauty,
kPion, kKaon
Event Generator Interfaces
 Cocktail class to assemble events, for
example:


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Underlying event + hard process
Different muon sources
pA + slow nucleons
pA Collisions
AliGenCocktail
The event generators
AliGenHijing
AliCollisionGeometry
AliGenSlowNucleon
AliSlowNucleonModel
Impact parameter, number
of wounded nucleon, participants
provided by HIJING or other model
Calculates for given Collision Geometry
the number of gray protons and neutrons
Example ALICE PDC’05
 Event generator is cocktail out of

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HIJING with heavy flavor production removed
Pythia for heavy flavor production
Hyperons from parameterisation
AliGenCocktail *gener = new AliGenCocktail();
AliGenPythia * genpyc = new AliGenPythia(230);
genpyc->SetProcess(kPyCharmPbPbMNR);
genpyc->SetStrucFunc(kCTEQ4L);
genpyc->SetPtHard(2.1,-1.0);
genpyc->SetEnergyCMS(5500.);
genpyc->SetNuclei(208,208);
genpyc->SetYRange(-999,999);
genpyc->SetForceDecay(kAll);
genpyc->SetFeedDownHigherFamily(kFALSE);
genpyc->SetCountMode(AliGenPythia::kCountParents);
AliGenPythia * genpyb = new AliGenPythia(9);
genpyb->SetProcess(kPyBeautyPbPbMNR);
genpyb->SetStrucFunc(kCTEQ4L);
genpyb->SetPtHard(2.75,-1.0);
genpyb->SetEnergyCMS(5500.);
genpyb->SetNuclei(208,208);
genpyb->SetYRange(-999,999);
genpyb->SetForceDecay(kAll);
genpyb->SetFeedDownHigherFamily(kFALSE);
genpyb->SetCountMode(AliGenPythia::kCountParents);
AliGenSTRANGElib *lib = new AliGenSTRANGElib();
Int_t particle;
particle = AliGenSTRANGElib::kXiMinus;
AliGenParam *genXi = new AliGenParam(16,particle,lib->GetPt(particle),lib->GetY(particle),lib->GetIp(particle));
genXi->SetPtRange(0., 12.);
genXi->SetYRange(-1.1, 1.1);
genXi->SetForceDecay(kNoDecay);
particle = AliGenSTRANGElib::kOmegaMinus;
AliGenParam *genOmega = new AliGenParam(10,particle,lib->GetPt(particle),lib->GetY(particle),lib->GetIp(particle));
genOmega->SetPtRange(0, 12.);
genOmega->SetYRange(-1.1, 1.1);
genOmega->SetForceDecay(kNoDecay);
AliGenHijing *genHi = HijingStandard();
genHi->SwitchOffHeavyQuarks(kTRUE);
genHi->SetImpactParameterRange(0.,5.);
// Add everything to the cocktail and shake ...
gener->AddGenerator(genHi,
"Hijing cent1", 1);
gener->AddGenerator(genpyc,
"Extra charm", 1);
gener->AddGenerator(genpyb,
"Extra beauty", 1);
gener->AddGenerator(genXi,
"Xi"
, 1);
gener->AddGenerator(genOmega, "Omega",
1);
Conclusions
 Shortcomings of present generators for Heavy
Ion Collisions

Most interesting observables are nor simulated
 Solution

Hard probes

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Assemble events as signal + underlying event
Jet quenching as afterburner
Soft probes: Afterburners, parameterisations, ...
 The ALICE simulation framework provides
collaborating classes tailored to these needs.