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
OUTLINE
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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
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Multiple Interactions
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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
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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
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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
EPOS implemented in ALICE soft (ALIROOT)• pp physics : Study of strangeness with EPOS and PYTHIA in pp, Strasbourg, GSI
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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
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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
+/-
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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
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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-
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• 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
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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+/-
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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 >
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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
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One outlook for such an event generator:
Back Up
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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
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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]
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Why studing jets?An observable at the frontiere of several aspects
In heavy ion: jet-quenching, γ-jetIn pp : QCD, pQCD, PDF
Jet finders
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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
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EPOS General
Particles ProductionUp to know : speak about partons, but in nature, one detects hadrons
Hadronization by the string modelPrincipleApplication to EPOS
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Hadronisation of hard processes following the same model
Jet produced connected to the eventColor flux conservation
F(z)
Partons Hadrons
Different from fragmentation function
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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
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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
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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
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e +e -e pp pN Nd Au
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dN/d
p
2T
dAu
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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
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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
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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, …
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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
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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
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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]
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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
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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
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Approach CertificationAnalytical test : internal test
ijIBIBijIB
jMIB
iM
IBIB
Asemi tsxxKxfxfdtdxdx
dn),,()()( ,,
)(
ijIBIBij
jMiM
IBIB
Bsemi tsxxKzEzEdtFdxdx
dn),,()()( ,,
)(
),,,(),()(
IBIBPEPEPEPEAGKPEPEIBIB
Csemi xxxxSxxNdxdxdxdx
dn
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pp 200 GeV
f : from hadron to hard processK : hard processF : hadron-ladder couplingE : Evolution along the ladder
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Semi-analytical test: internal
),,,(),()(
IBIBPEPEPEPEAGKNONPEPEIBIB
Dsemi xxxxSxxNdxdxdxdx
dn
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pp 200 GeV
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Approach Certification
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+/-
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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
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Approach Certification
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
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pp 200 GeV
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pT
UA1 and STAR on the same Plot
Nucl. Phys. B309 (1988) 405 Phys. Rev. Lett. 97 (2006) 252001
X 2
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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
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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
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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
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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
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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
16/03/2010 [email protected]
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
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For differents energies : from RHIC to LHC
10 TeV 5.5 TeV
1.8 TeV
200 GeV
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hydro
200 GeV
“Bulk” in full line
“Jet” in dotted lines
14 TeV10 TeV5.5 TeV1.8 TeV
Ratio for different energies
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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
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