Monte-Carlo event generators for heavy ion collisions at the LHC

Igor Lokhtin

CERN Workshop on Monte-Carlo tools for LHC

(July 2003), heavy ion working group:

“Such important effects as jet quenching, HBT correlations and elliptic flow have rather poor implementation in available event generators (in the most of generators are not represented at all) and should be introduced in the future.”

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Experimentalists and phenomenologists are forced now to search effective ways for adequate and fast simulation of above physics phenomena

Types of models for HI collisions

1. Microscopic MC models:

Linear superposition of independent NN collisions + collective nuclear effects

Initial state nuclear effects:

“nuclear shadowing”, fA(x,Q2)≠Afp(x,Q2)

“Cronin effect”, initial state scattering Final state nuclear effects:

string interactions (FRITIOF/LUCIA, QGSM, DPMJET, PSM, NEXUS)

hadron cascade (UrQMD, VENUS)

parton cascade (VNI)

medium-induced gluon radiation, “jet quenching” (HIJING) 2. Hydrodynamical (macroscopic) models: Generation of events phase space according with hydrodynamical equations + set multiplicity and “freeze-out” parameters

Using MC event generators at LHC

There are two kind of MC HI simulations in HIC at the LHC:

1. “Hard probes” signal event (jets, quarkonia, heavy quarks) is generated with PYTHIA and superimposed on background full HI event

2. Global observables (particle spectra) and multiplicity background for “hard probes” are generated with HI event generator

The most of existing HI event generators is trying to be refined to describe SPS and RHIC data (usually not successfully). In my best knowledge the list of MC HI generators, which can be used at LHC energy, is quite short

(all of them are implemented as Fortran routines and available by web) :

jet quenching flow string inter. hadron cascade

HIJING (M. Gyulassy, X.-N. Wang) + - - -

DPMJET (J. Ranft et al.) - - + -

PSM (N. Amelin et al.) - - + -

FRITIOF/LUCIE (B. Andersson et al.) - - + +

At the moment only HIJING is used by ALICE, CMS and ATLAS Collab.

Fast MC HI event generators at LHC
(I. Lokhtin, A. Snigirev)

PYQUEN - fast code to simulate jet quenching (modify PYTHIA6.2 jet event), http://cern.ch/lokhtin/pyquen already is applying for jet analysis by CMS and ALICE

HYDRO - fast code to simulate transverse and elliptic flow in central and semi-central heavy ion AA collisions at LHC, http://cern.ch/lokhtin/hydro

HYDJET - merging HYDRO (flow effects), PYTHIA (hard jet production) and PYQUEN (jet quenching) http://cern.ch/lokhtin/hydro/hydjet.html

PYQUEN (PYthia QUENched)

Initial parton configuration

PYTHIA6.2 w/o hadronization: mstj(1)=0

Hard parton rescattering and energy loss + emitted gluons

PYQUEN (input: beam atomic number and centrality)

Parton hadronization and final particle formation

PYTHIA6.2 with hadronization: mstj(1)=1, call pyexec

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More details on internal PYQUEN physics sets can be found in:

I.Lokhtin, A.Snigirev, “Nuclear geometry of jet quenching”, EPJ C16 (2000) 527; “Fast simulation of jet quenching...”, hep-ph/0406038; and references therein.

HYDJET (HYDrodynamics + JETs)

generating # of NN subcollisions, njet, and formation of

jet-induced state by calling (PYTHIA+PYQUEN) njet times

start filling JETSET arrays with npyt lines and HYDJET arrays with nl (corresponding to # of partons) lines

calculation of multiplicity of HYDRO-induced particles, nhyd=n-npyt, and adding new particles in JETSET arrays

HYDJET: input parameters

External input:

beam and target nucleus atomic weight (A=B)

impact parameter (fixed or distributed)

total mean multiplicity in central Pb-Pb events (multiplicity for other centralities and atomic weights is calculated automatically) Parameter can be varied by user:

ytfl - maximum transverse collective rapidity, controls slope of low-pt spectra (0.01<ytfl<3.0, default value is ytfl=1.)

ylfl - maximum longitudinal collective rapidity, controls width of -spectra (0.01<ylfl<5.0, default value is ylfl=3.)

fpart - fraction of multiplicity proportional to # of participants; (1.-fpart) - fraction of multiplicity proportional to # of NN subcollisions (0.0<fpart<1.0, default value is fpart=0.)

ptmin - minimal pt of hard parton-parton scattering in PYTHIA (5 GeV < ptmin < 500 GeV, default value ptmin=10 GeV)

HYDJET: output parameters

Output particle information:

final hadronic state of the event in JETSET format (common block LUJETS, #150000)

parton history of the event in JETSET format (common block HYJETS, #150000) Output global event characteristics:

bgen - generated value of impact parameter

nbcol - mean # of NN subcollisions at given bgen

npart - mean # of nucleon participants at given bgen

npyt - multiplicity of jet-induced particles in the event

nhyd - multiplicity of HYDRO-induced particles in the event

HYDJET: physics validity of the model

Internal flow parameters was selected as an estimation for LHC heavy ion beam energies. For RHIC energy additional internal sets adjusting is possible.

Hydro-type description of HIC is applicable for central and semi-central collisions. The result obtained for very peripheral collisions (b~2*RA) can be not adequate.

The model is not expected to be correct for very forward rapidity (|y|>3), where other mechanisms of particle production among hydro-flow and jets can be important.

Transverse flow: HYDRO vs.HIJING

HYDRO HIJING (||<3)

Hydro-flow: growth of <pT> with increasing mass, ΔpT(r)=m sinh YT(r)

Elliptic flow: HYDRO vs. HIJING

v2() v2(pT)

Azimuthal anizotropy coefficient v2≡<cos 2(φ-R)> ~0 for HIJING,

increases with pT and depends slightly on  in mid-rapidity for HYDRO.

HYDJET: quenching of particle spectra

30,000 minimum bias Pb-Pb events, √s=5.5A TeV (ptmin=10 GeV, ntot=30000) PT-distribution η-distribution

PT-spectra: strong hardening due to jets and softening due to jet quenching

-spectra: broadening due to jets and small influence due to jet quenching

HYDJET: elliptic flow

v2(pT) v2()

v2(PT>2 GeV): sharp drop due to jets and additional v2 due to jet quenching

v2(): ~20%-reducing due to jets and small influence due to jet quenching

Conclusions

Very few MC HI event generators is available for simulations at LHC energies, and only one (HIJING) is really applying at the moment for physics analysis of global observables and multiplicity background by ALICE, CMS and ATLAS

A number of important nuclear effects are not represented or implemented poorly in existing MC HI event generators

Fast MC tools to simulate jet quenching and flow effects has been developed and are available by the web: PYQUEN (medium-modified jets) and HYDRO/HYDJET (global observables and multiplicity background) are already using for CMS and ALICE jet HI analysis

2004-05-29T11:14:08