sarah porteboeuf*, klaus werner, tanguy pierog

Sarah Porteboeuf*, Klaus Werner, Tanguy Pierog

Rencontres de Moriond, La Thuile, March 13th-20th 2010

Producing hard Producing hard processes regarding the processes regarding the

complete event:complete event:The EPOS event The EPOS event

generator generator *now at LLR, École polytechnique

I – Hard processes

II – EPOS event generator

III – Production of hard processes with EPOS : selection method

IV – Conclusions

[email protected]

OUTLINE

16/03/2010

pQCD

How to produce a jet connected to the event?

td

dQxfQxftddxdx qqqq

qqppˆ

ˆ),(),(ˆ 432122

21 221143

Jet production cross section:

Inclusive cross section: pp -> Jet +X

Difficult to access the X part : exclusive computation

No information on multiple interaction

316/03/2010 [email protected]

Multiple Interactions

4

Acosta, Darin E. and others for the CDF collaboration, Phys. Rev. D65 (2002) 072005Aaltonen, T. and others, for the CDF collaboration, 2009, arXiv hep-ex 0904.1098

Acosta, Darin E. and others for the CDF collaboration, Phys. Rev. D65 (2002) 072005

EPOS


Gribov-Regge TheoryTreat multiple interactions

Effective field theory

Pomeron = objet that represents an elementary interaction, but it’s true nature is not known

Limitations: Energy conservation, in the cross section computation, each pomeron takes the total energy

How to consider hard processes?516/03/2010 [email protected]

Parton-based Gribov-Regge Theory

EPOS theoritical framework

Mixed approche between parton model and Gribov-Regge

Energy shared between all elementary interactions

Same formalism for cross section computation and particle production

Elementary interaction = soft Semi-hard

soft: parametrisedhard: parton modelsemi-hard: soft pre-evolution before the hard part

S. Ostapchenko, T. Pierog, K. Werner, H.J. Drescher, M. Haldik Phys. Rep. 350 (2001) 616/03/2010 [email protected]

EPOS, the events generatorEnergy conserving quantum mechanical multiple scattering approach

Partons, parton ladders, strings

Off-shell remnants

Splitting of parton ladder

Based on

7

Who uses EPOS ?

• Heavy ion physics (Last Call for Prediction ,« EPOS predictions at LHC Energies », S. Porteboeuf, T. Pierog and K.Werner (ed.) Armesto N. et al. J.Phys, G35 : 054001, 2008, Topcite 50+.)

• Cosmic rays physics, excess muon problem, Auger.Pierog, Werner, Phys. Rev. Lett, 101:171101 (2008) astro-ph/0611311 | hep-ph/0905.1198

[email protected]

EPOS implemented in ALICE soft (ALIROOT)• pp physics : Study of strangeness with EPOS and PYTHIA in pp, Strasbourg, GSI

16/03/2010

EPOS frameworkParton-based Gribov Regge theory

Energy conservation

I1 I2 I3 I4

Elementary object = ladderSame formalisme for soft and hard interactions

Same formalism for cross section formalism and particle production


In EPOS collectivity, even in pp.(see Tanguy’s talk)

In the following:• Focus on description of a semi-hard ladder, • and difficulties at LHC energies.

Parton Ladder

ji

ijhard

jsoft

isoftPE

hpartPE

hpartPEPE

hhseasea QszzzEzEdzdzxFxFbsxxG

,

20

1

0

1

0

2121 ),ˆ()()()()(),,,(

Expression of one interaction

Momentum distribution of parton (i,j) in the soft

part pQCD, parton-parton cross section

h1

h2

σhard

Esoft(z+)

Esoft(z-)

I1 I2 I3 I4

xPE-

xPE+

Enter the ladder, proba of having a parton with xPE

+/-


Problematic

Need a solution to produce directly rare events with keeping multiple interactions aspects.

Multiples interactions : essential element to describe entirely an event Old description of semi-hard

ladder: create all partons step by step

For rare events production, need to produce a lot of statistics to observe few cases.

No control on what is produced.

Condition not ok for LHC and study of physics of hard processes


Generating semi-hard ladders… … A 2 step procedure

Number of ladders and xPE+/-

Computation of partial cross section over total one: gives the probability of n collisions with momentum fraction for ladder end

1)

Detremine all the variables of the ladder independently

Generation of partons along the ladder : Method of independant block

2)

xPE+

xPE-

E

E

K

xIB+

xIB-


• Star Data : R=0.4 for jet finder, High Tower TriggerPhys. Rev. Lett. 97 (2006) 252001Hep-ex/0608030

• NLO QCD (Vogelsang), R=0.4Phys. Rev. D70 (2004), 034010Hep-ph/0404057

• EPOS : hard parton production

Jets cross-section production


Approach Certification

Selection MethodPrinciple of the selection method

1. Take one standard event

2. In this event, take one standard ladder

6. A weight is attributed to the whole event

3. Take determined by usual MCxPE+/-

13

xIB+ xIB

-

4. Procedure to generate is modified: xIB+/-

Cx

• New MC based on block S• Force a cut >• Force production of high products xIB

+ xIB-

5. Procedure to generate is modified :pT OB

CpTpT

• New MC• Force a cut >


16/03/2010 [email protected] 14

Summury

EPOS : goal to be a generator of complete event1 experimental event = 1 generator eventSoft, hard, multiple interactions and collective effects in the same event

Hard processes : important physics for pQCD and QGP studies

Hard process = rare event, no control on the production

Selection MethodOld model : iterative generation of all partons along ladders

Solution : selection method that gives directly access to different variables, can implement cutsMethod tested and validated

EPOS : a dedicated event generator for hard process – underlying event interaction study

hydro

200 GeV

“Bulk” in full line

“Jet” in dotted lines

14 TeV10 TeV5.5 TeV1.8 TeV

Ratio for different energies02 sk


One outlook for such an event generator:

Back Up


What are jets ?Asked to a theoritician

A collimated flux of particle coming from the fragmentation of a hard parton

Fragmentation Function

F(z)

Partons Hadrons


What are jets ?Asked to Experimentalist (from heavy ion communitty)

A collimated flux of particle coming from the fragmentation of a hard parton

Swallowed up in particles coming from the underlying event

Identified by a jet finder algorithm for a defined Radius

21 GeV di-jet reconstructed from a central Au+Au event at √sNN=200 GeV in the STAR detector.

http://www.bnl.gov/rhic/news/102108/story2.asp16/03/2010 [email protected]

19

Why studing jets?An observable at the frontiere of several aspects

In heavy ion: jet-quenching, γ-jetIn pp : QCD, pQCD, PDF

[email protected]

Jet finders

16/03/2010

Multiples Interactions

=> Total cross section and partial one, inclusive and exclusive cross scetion

One need the complete event : with multiple interactions

Attention : d’abitude gribov-regge pour soft,Ici, pour tout

X.N.Wang, M.Gyulassy, Phys. Rev. D45 (1992) 844-856

High Energies : problème !!!

More than one interaction per collision



EPOS General

Particles ProductionUp to know : speak about partons, but in nature, one detects hadrons

Hadronization by the string modelPrincipleApplication to EPOS

22

Hadronisation of hard processes following the same model

Jet produced connected to the eventColor flux conservation

F(z)

Partons Hadrons

Different from fragmentation function


23

String model and baryon production

Diquarks

Popcorn

String fragmentation by pair production Meson production

A q-aq pair is replaced by a diquark - anti-diquark pair

In EPOSAn option inPythia


Modification of the string procedure :

• Core = high density, hydro expansion

• Corona = low density, classical EPOS particle production

Nuclei

One defines a freeze-out surface : transition from strongly interacting matter to free hadrons.

Parametrisation of the freeze out hypersurface.

Already available in pp interactions.

Collective Effect of a dense core


Experimental Certification

EPOS tested with a lot of experimental data sets

• Differents energiesfrom 20 GeV (SPS) to 1.8 TeV (Tevatron)

• Differents systems

• Differents observables

• Differents species

Rapidity, multiplicity, Pt spectrum, transversemasse, v2

, K, p, , …

Everything that can be measured

Colliders Cosmic Rays

Collision between a particle from cosmic radiation and an atmospheric atom which leeds to an atmospheric shower

Particle collision at very high energies

Use of the same event generator : EPOS

25

e +e -e pp pN Nd Au


dN/d

p

2T

dAu

26

PYTHIAStandard event generator in particle physics

Only for proton-proton collisions

Order for event generation :

1. Hard process selection and inclusive cross section computation2. Initial and final state radiation3. Remnant and multiple interaction 4. Hadronization

Based on parton model, hard processes is central element of event generation

Benefits :

objections :Generator of inclusive spectrumMultiple InteractionHeavy ion collisions

Control the subprocess you want to study, easy selectionEasy to interace with other model

Very powerfull for high pt jets

Connexion of the jet to the event



EPOS vs. PYTHIAEPOS PYTHIA

modelstructure

Baseline Multiple Interaction Hard process

Hard process

Parton-based Gribov-Regge Theory

Multiple Interaction

Reconstructed after the hard process,Interaction ordered in hardness.In the new model : color reconnection

Hard and semi-hard ladder with soft pre-evolutionu, d, s, g, gamma, c in progress

Based on inclusive cross sectionAlmost everything, if not in the code, can couple with extra code

Initial and Final state radiation Iterative procedure

A posteriori reconstructionAvailable for MPI in the new model (6.4)

Hadronization String model with aera law, diquark for baryon production

String model with fragmentation function, popcorn for baryon production

Collectivity

Yes, a toy model with parametrisation, event by event hydro in developpement

No


EPOS PYTHIAmodelstructure

EPOS vs. PYTHIA

Selection Pt cut in implementation and testing

Selection of the hard process,Can vary all parameters : different tunes optimized for observables

Connection between hard processes and underlying event

Total by construction : several ladders soft or hard, energy conservation and color connection

In MPI color reconnection, less easy to interpret

Heavy IonYes

Same framework extendedNo

Need to use something else : HIJING, HYDJET, …


Hard processes in EPOS

Iterative procedure : emission determined step by step

z1z2

σhard

(S,Q2+,Q0-)

h1

σhard

fisrt émission

z1

,Q0+

(S,Q1+,Q0-)

,Q0-

h2

Generation of a semi-hard ladder(old model)

PEx

PEx


Structure of Semi-Hard Parton Ladder

Define different sub-structure as independent block.

PE+x

PE-x

PE+x

PE-x

E

IB+x

IB-x

OB+x

OB-x

E

xPE+/- : Light cone momentum of

parton entering the ladder

xIB+/- : Light cone momentum of

parton entering the Born Process

xOB+/- : Light cone momentum of

parton out the Born Process

E : Evolution of the parton, branching, gluon emissison


Description of structures(as a function of independant block)

PE+x

PE-x

PE+x

PE-x

E

IB+x

IB-x

OB+x

OB-x

E

Structure NNo resoltution on the inner part of the ladder

Structure SResolution on the inner partDirect access to variables

),( PEPEPEPEsemi xxNdxdxn

),,,(),( IBIBPEPEPEPEIBIBPEPEsemi xxxxSxxNdxdxdxdxn3216/03/2010 [email protected]

33

Variables

Tinel dydp

d

1

Tdydp

dN

Quantity discussed

Equivalent toIntegrated :

Count the number of particles

In EPOS : particles produced by laddersCount the number of ladders

Jets come from hard partons Each semi-hard ladder produces two partons

In the followingsemin

Integrated version shows different choices of variablesDirect acces to some variable of the ladder


In case of factorisation …

),()()(),( arg PEPEPEtPEprojPEPE xxGxFxFxxN

)()()( PEpartPEprojPE xFxFxF

)()()( arg

PEpartPEtPE xFxFxF

ij

IBIBijIBjM

IBiM

IBIBsemi tsxxKxfxfdtdxdxn ),()()( ,,

i

iMiM EFf ,,

),,(1

),,( ,, uts

dtdu

dutsK ji

inelji

Hard process

Evolution of the parton from the hadron to the hard process

Identification : f PDF34

Hadron-ladder couplingMultiple Interaction


Approach CertificationAnalytical test : internal test

ijIBIBijIB

jMIB

iM

IBIB

Asemi tsxxKxfxfdtdxdx

dn),,()()( ,,

)(

ijIBIBij

jMiM

IBIB

Bsemi tsxxKzEzEdtFdxdx

dn),,()()( ,,

)(

),,,(),()(

IBIBPEPEPEPEAGKPEPEIBIB

Csemi xxxxSxxNdxdxdxdx

dn

35

pp 200 GeV

f : from hadron to hard processK : hard processF : hadron-ladder couplingE : Evolution along the ladder


Semi-analytical test: internal

),,,(),()(

IBIBPEPEPEPEAGKNONPEPEIBIB

Dsemi xxxxSxxNdxdxdxdx

dn

36

pp 200 GeV



New Monte CarloNew MC Principle

• S function is used as a probability distribution to directy produced a couple ( , ) for a given ( , )

• the produced ( , ) is used in the event generation

• To randomly distribute as S : Monte Carlo technics

Results for pp collisions at 200 GeV

xIB+ xIB

- xPE+ xPE

-

xIB+ xIB

-

xIB+/-


In case of factorisation : comparison to PDF

),(

)()( ,,

tsxxK

xfxfdtdxdxn

IBIBij

ijIB

jMIB

iMIBIBsemi

Changement de variable : pt

! semiptn nn 238

1 semi-hard ladder gives 2 partons

Same bloc Kf replaced with other PDF

pp 200 GeV

2Tsdydpdtdu



Which Selection?Pt contribution from differents products at the ladder end

• low products : don’t contribute to high• high products : contribute everywhere

Selection products which contributes in the chosen zone

Then : selection on

Can’t do direct selection on :Then need to reject couple that will not contribute to the zone

)( IBIBxx

pT

pT

pT

pT

39

pp 200 GeV


pT

40

Jets

STAR et UA1


UA1 and STAR on the same Plot

Nucl. Phys. B309 (1988) 405 Phys. Rev. Lett. 97 (2006) 252001

X 2


Comparison with UA1 DATA

• Data : UA1, Nucl. Phys. B309 (1988) 405

Jet Spectrum : use of a Jet FinderR=0.7

• EPOS : Out Born Parton, R =Rmax (everything is in the jet)

• NLO Vogeslgang : thanks to Joana Kiryluk for the plot, NLO computation with R=0.7


Comparison with STAR DATA

• Star Data : R=0.4 for jet finder, High Tower TriggerPhys. Rev. Lett. 97 (2006) 252001Hep-ex/0608030

• NLO QCD (Vogelsang), R=0.4Phys. Rev. D70 (2004), 034010Hep-ph/0404057

• EPOS : pt out born : Rmax


Comparison with STAR DATA

Data exctracted with a jet finder algorithm :• Influence of the jet fnder : efficiency, cone, kt

• Influence of the jet resolution : systematic uncertainties

• Smearing effect ?

Could we really compare with analytic computation such as EPOS?

Comparison with NLO QCD VogeslgangTake into account some effect of the small radius R=0.4

Comparison with UA1 DATA?Same observable, same energy : same plot?But : different analysis, different R, different binning


Comparison with DATA

Difficult task :

EPOS : out Born parton = total jet (Rmax)

DATA : Reconstructed Jets, Jet Finder, variable R

NLO Vogeslgang : Computation with variable R

Need to do the Study by using jet finder on EPOS events

First good test for EPOS jet production :

Good agreement between EPOS and NLO Vogelsgang (R=0.7)

Less than a factor 2 between EPOS and Star Data over 9 orders of magnitude



Ratio lambda/k0s

Ratio lambda/k0s

Is this ratio the same if we look at the bulk, if we look at jets ?

Strageness : a signature of the Quark Gluon Plasma in heavy ion collisions

In pp collisions at sqrt(s)=200 GeV : flat ratio and remainded below unity

With increasing energy : the behaviour changes!

Hélène is working on that issue


Ratio lambda/k0s

In EPOS, look at this ratio in pp collisions for :

“The Bulk” : here everything, no trigger on particles origin, effects from collectivity

“Jets” : all particles coming from semi-hard ladders jet disconnected from the medium

Preliminary study to see what could be done at LHC


All : no selection

Jet


All : no selection

Jet

All with hydro off


For differents energies : from RHIC to LHC

10 TeV 5.5 TeV

1.8 TeV

200 GeV


hydro

200 GeV

“Bulk” in full line

“Jet” in dotted lines

14 TeV10 TeV5.5 TeV1.8 TeV

Ratio for different energies


Ratio Lambda/k0s different in the “bulk” and in “jets”

Difference of fragmentation in jet and the bulk

In jets : the usual string model

In the bulk : collectivity

This difference grows with energy : collectivity more and more important

In pp : should clearly see this effect

A way to understandHydro part/ Jet part, Soft/HardDifference between medium and jet fragmentationDifference with Strangeness production

Informations we get


sarah porteboeuf*, klaus werner, tanguy pierog

Documents

parton modelsemihard

hard parton productionj

parton ladders

epos collectivity

epos predictions

parton ladderexpression

hard ladders

hard parts