Sarah Porteboeuf*, Klaus Werner, Tanguy Pierog Rencontres de Moriond, La Thuile, March 13th-20th 2010 Producing hard processes regarding the complete event: The EPOS event 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 16/03/2010

pQCD How to produce a jet connected to the event? Jet production cross section: Inclusive cross section: pp -> Jet +X Difficult to access the X part : exclusive computation No information on multiple interaction

Multiple Interactions 4 Acosta, Darin E. and others for the CDF collaboration, Phys. Rev. D65 (2002) Aaltonen, T. and others, for the CDF collaboration, 2009, arXiv hep-ex Acosta, Darin E. and others for the CDF collaboration, Phys. Rev. D65 (2002) EPOS

Gribov-Regge Theory Treat 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?

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 =softSemi-hard soft: parametrised hard: parton model semi-hard: soft pre-evolution before the hard part S. Ostapchenko, T. Pierog, K. Werner, H.J. Drescher, M. Haldik Phys. Rep. 350 (2001)

EPOS, the events generator Energy 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 : , 2008, Topcite 50+.) Cosmic rays physics, excess muon problem, Auger. Pierog, Werner, Phys. Rev. Lett, 101: (2008) astro-ph/ | hep-ph/ EPOS implemented in ALICE soft (ALIROOT) pp physics : Study of strangeness with EPOS and PYTHIA in pp, Strasbourg, GSI 16/03/2010

EPOS framework Parton-based Gribov Regge theory Energy conservation I1I2I3I4 Elementary object = ladder Same 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 Expression of one interaction Momentum distribution of parton (i,j) in the soft part pQCD, parton-parton cross section h1 h2 σ hard Esoft(z+) Esoft(z-) I1I2I3I4 x PE - x + Enter the ladder, proba of having a parton with x PE +/-

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 x PE +/- 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) x PE + x - E E K x IB + x -

Star Data : R=0.4 for jet finder, High Tower Trigger Phys. Rev. Lett. 97 (2006) Hep-ex/ NLO QCD (Vogelsang), R=0.4 Phys. Rev. D70 (2004), Hep-ph/ EPOS : hard parton production Jets cross-section production Approach Certification

Selection Method Principle 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 MC x PE +/- 13 x IB + x - 4. Procedure to generate is modified: x IB +/- C x New MC based on block S Force a cut > Force production of high products x IB + x - 5. Procedure to generate is modified : p T OB C p T p T New MC Force a cut >

Summury EPOS : goal to be a generator of complete event 1 experimental event = 1 generator event Soft, 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 Method Old model : iterative generation of all partons along ladders Solution : selection method that gives directly access to different variables, can implement cuts Method 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 TeV 10 TeV 5.5 TeV 1.8 TeV Ratio for different energies 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 √s NN =200 GeV in the STAR detector.

19 Why studing jets? An observable at the frontiere of several aspects In heavy ion: jet-quenching, γ-jetIn pp : QCD, pQCD, PDF 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) High Energies : problème !!! More than one interaction per collision

EPOS General

Particles Production Up to know : speak about partons, but in nature, one detects hadrons Hadronization by the string model PrincipleApplication to EPOS 22 u u Hadronisation of hard processes following the same model Jet produced connected to the event Color 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 EPOS An 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 energies from 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 CollidersCosmic 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 p p N d Au dN/d p 2 T dAu

26 PYTHIA Standard event generator in particle physics Only for proton-proton collisions Order for event generation : 1. Hard process selection and inclusive cross section computation 2. Initial and final state radiation 3. Remnant and multiple interaction 4. Hadronization Based on parton model, hard processes is central element of event generation Benefits : objections : Generator of inclusive spectrum Multiple Interaction Heavy ion collisions Control the subprocess you want to study, easy selection Easy to interace with other model Very powerfull for high pt jets Connexion of the jet to the event

EPOS vs. PYTHIA EPOS PYTHIA model structure BaselineMultiple InteractionHard 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-evolution u, d, s, g, gamma, c in progress Based on inclusive cross section Almost everything, if not in the code, can couple with extra code Initial and Final state radiation Iterative procedure A posteriori reconstruction Available 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 PYTHIA model structure 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 Ion Yes Same framework extended No Need to use something else : HIJING, HYDJET, …

Hard processes in EPOS

Iterative procedure : emission determined step by step z1 z2 σ hard (S,Q2+,Q0-) h1 σ hard fisrt émission z1,Q0+ (S,Q1+,Q0-),Q0- h2 Generation of a semi-hard ladder (old model)

Structure of Semi-Hard Parton Ladder Define different sub-structure as independent block. PE + x - x + x - x E IB + x - x OB + x - x E x PE +/- : Light cone momentum of parton entering the ladder x IB +/- : Light cone momentum of parton entering the Born Process x OB +/- : 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 - x + x - x E IB + x - x OB + x - x E Structure N No resoltution on the inner part of the ladder Structure S Resolution on the inner part Direct access to variables

33 Variables Quantity discussed Equivalent to Integrated : Count the number of particles In EPOS : particles produced by ladders Count the number of ladders Jets come from hard partons Each semi-hard ladder produces two partons In the following Integrated version shows different choices of variables Direct acces to some variable of the ladder

In case of factorisation … Hard process Evolution of the parton from the hadron to the hard process Identification : f PDF 34 Hadron-ladder coupling Multiple Interaction

Approach Certification Analytical test : internal test 35 pp 200 GeV f : from hadron to hard process K : hard process F : hadron-ladder coupling E : Evolution along the ladder

Semi-analytical test: internal 36 pp 200 GeV Approach Certification

New Monte Carlo New 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 x IB + x - x PE + x - x IB + x - x +/-

In case of factorisation : comparison to PDF Changement de variable : pt ! 38 1 semi-hard ladder gives 2 partons Same bloc K f replaced with other PDF pp 200 GeV 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 p T p T p T p T 39 pp 200 GeV p T

40 Jets STAR et UA1

UA1 and STAR on the same Plot Nucl. Phys. B309 (1988) 405Phys. Rev. Lett. 97 (2006) X 2

Comparison with UA1 DATA Data : UA1, Nucl. Phys. B309 (1988) 405 Jet Spectrum : use of a Jet Finder R=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 Trigger Phys. Rev. Lett. 97 (2006) Hep-ex/ NLO QCD (Vogelsang), R=0.4 Phys. Rev. D70 (2004), Hep-ph/ 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 Vogeslgang Take 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

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 TeV 10 TeV 5.5 TeV 1.8 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 understand Hydro part/ Jet part, Soft/Hard Difference between medium and jet fragmentation Difference with Strangeness production Informations we get