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Sarah Porteboeuf*, Klaus Werner, Tanguy Pierog

Producing hard processes regarding the complete event: The EPOS event generator. Sarah Porteboeuf*, Klaus Werner, Tanguy Pierog. Rencontres de Moriond, La Thuile, March 13th-20th 2010. *now at LLR, École polytechnique. OUTLINE. I – Hard processes. II – EPOS event generator.

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Sarah Porteboeuf*, Klaus Werner, Tanguy Pierog

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  1. Producing hard processes regarding the complete event: The EPOS event generator Sarah Porteboeuf*, Klaus Werner, Tanguy Pierog Rencontres de Moriond, La Thuile, March 13th-20th 2010 *now at LLR, École polytechnique

  2. OUTLINE I – Hard processes II – EPOS event generator III – Production of hard processes with EPOS : selection method IV – Conclusions sarah.porteboeuf@llr.in2p3.fr

  3. pQCD Inclusive cross section: pp -> Jet +X Jet production cross section: Difficult to access the X part : exclusive computation No information on multiple interaction How to produce a jet connected to the event? sarah.porteboeuf@llr.in2p3.fr

  4. Multiple Interactions EPOS Acosta, Darin E. and others for the CDF collaboration, Phys. Rev. D65 (2002) 072005 Acosta, Darin E. and others for the CDF collaboration, Phys. Rev. D65 (2002) 072005 sarah.porteboeuf@llr.in2p3.fr Aaltonen, T. and others, for the CDF collaboration, 2009, arXiv hep-ex 0904.1098

  5. Gribov-Regge Theory Treat multiple interactions Effective fieldtheory Pomeron = objet thatrepresents an elementary interaction, but it’strue nature is not known Limitations: • Energy conservation, in the cross section computation, eachpomerontakes the total energy • How to consider hard processes? sarah.porteboeuf@llr.in2p3.fr

  6. Parton-based Gribov-Regge Theory Mixed approche between parton model and Gribov-Regge Energysharedbetween all elementary interactions Sameformalism for cross section computation and particle production Elementary interaction = soft Semi-hard soft: parametrised hard: parton model semi-hard: soft pre-evolutionbefore the hard part EPOS theoriticalframework S. Ostapchenko, T. Pierog, K. Werner, H.J. Drescher, M. Haldik Phys. Rep. 350 (2001) sarah.porteboeuf@llr.in2p3.fr

  7. EPOS, the events generator Energy conserving quantum mechanical multiple scattering approach Based on Partons, parton ladders, strings Off-shell remnants Splitting of parton ladder 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+.) EPOS implemented in ALICE soft (ALIROOT) • pp physics : Study of strangenesswith EPOS and PYTHIA in pp, Strasbourg, GSI • Cosmic rays physics, excess muon problem, Auger. • Pierog, Werner, Phys. Rev. Lett, 101:171101 (2008) astro-ph/0611311 | hep-ph/0905.1198 sarah.porteboeuf@llr.in2p3.fr

  8. EPOS framework I1 I2 I3 I4 Parton-based Gribov Regge theory Sameformalism for cross section formalism and particle production Energy conservation Elementaryobject = ladder Same formalisme for soft and hard interactions In EPOS collectivity, even in pp. (seeTanguy’s talk) In the following: • Focus on description of a semi-hard ladder, • and difficultiesat LHC energies. sarah.porteboeuf@llr.in2p3.fr

  9. Parton Ladder I1 I2 I3 I4 σ hard Momentum distribution of parton (i,j) in the soft part Enter the ladder, proba of having a parton with Expression of one interaction pQCD, parton-parton cross section x x x + +/- - h1 PE PE PE Esoft(z+) Esoft(z-) h2 sarah.porteboeuf@llr.in2p3.fr

  10. Problematic Multiples interactions : essential element to describeentirely an event Old description of semi-hard ladder: create all partons step by step No control on whatisproduced. For rare events production, need to produce a lot of statistics to observe few cases. Condition not ok for LHC and study of physics of hard processes Need a solution to producedirectly rare eventswithkeeping multiple interactions aspects. sarah.porteboeuf@llr.in2p3.fr

  11. Generating semi-hard ladders… … A 2 step procedure Computation of partial cross section over total one: gives the probability of n collisions withmomentum fraction for ladder end 1) E Number of ladders and 2) Generation of partons along the ladder : Method of independant block K Detremine all the variables of the ladderindependently + x x x x x +/- + - - IB IB PE PE PE E sarah.porteboeuf@llr.in2p3.fr

  12. Approach Certification Jets cross-section production • Star Data : R=0.4 for jet finder, High Tower Trigger • Phys. Rev. Lett. 97 (2006) 252001 • Hep-ex/0608030 • NLO QCD (Vogelsang), R=0.4 • Phys. Rev. D70 (2004), 034010 • Hep-ph/0404057 • EPOS : hard parton production sarah.porteboeuf@llr.in2p3.fr

  13. Selection Method Principle of the selection method 1. Take one standard event 2. In this event, take one standard ladder 3. Take determined by usual MC 4. Procedure to generate is modified: • New MC based on block S C x • Force a cut > x x x x x x +/- +/- + + - - • Force production of highproducts PE IB IB IB IB IB C 5. Procedure to generate is modified : • New MC • Force a cut > 6. A weight is attributed to the whole event p p p T sarah.porteboeuf@llr.in2p3.fr T T OB

  14. Summury Hard processes : important physics for pQCD and QGP studies EPOS : goal to be a generator of completeevent 1 experimental event = 1 generator event Soft, hard, multiple interactions and collective effects in the same event Selection Method Old model : iterative generation of all partons along ladders Hard process = rare event, no control on the production Solution : selection method that gives directly access to different variables, can implement cuts EPOS : a dedicated event generator for hard process – underlying event interaction study Method tested and validated sarah.porteboeuf@llr.in2p3.fr

  15. One outlook for such an event generator: 14 TeV 10 TeV Ratio for different energies 5.5 TeV 1.8 TeV “Bulk” in full line hydro “Jet” in dotted lines 200 GeV sarah.porteboeuf@llr.in2p3.fr

  16. Back Up sarah.porteboeuf@llr.in2p3.fr

  17. 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 sarah.porteboeuf@llr.in2p3.fr

  18. 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 GeVdi-jet reconstructed from a central Au+Au event at √sNN=200 GeV in the STAR detector. http://www.bnl.gov/rhic/news/102108/story2.asp sarah.porteboeuf@llr.in2p3.fr

  19. Why studing jets? An observable at the frontiere of several aspects In pp : QCD, pQCD, PDF In heavy ion: jet-quenching, γ-jet Jet finders sarah.porteboeuf@llr.in2p3.fr

  20. Multiples Interactions High Energies : problème !!! More than one interaction per collision => Total cross section and partial one, inclusive and exclusive cross scetion Attention : d’abitude gribov-regge pour soft, Ici, pour tout X.N.Wang, M.Gyulassy, Phys. Rev. D45 (1992) 844-856 One need the completeevent : with multiple interactions sarah.porteboeuf@llr.in2p3.fr

  21. EPOS General sarah.porteboeuf@llr.in2p3.fr

  22. Particles Production Up to know : speak about partons, but in nature, one detects hadrons Hadronization by the string model Principle Application to EPOS u u Hadronisation of hard processesfollowing the same model Jet producedconnected to the event Color flux conservation Differentfrom fragmentation function F(z) Partons Hadrons sarah.porteboeuf@llr.in2p3.fr

  23. String model and baryon production String fragmentation by pair production Meson production Diquarks A q-aq pair isreplaced by a diquark - anti-diquark pair In EPOS An option inPythia Popcorn sarah.porteboeuf@llr.in2p3.fr

  24. Collective Effect of a dense core Modification of the string procedure : Nuclei • Corona = low density, classical EPOS particle production • Core = high density, hydro expansion 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. sarah.porteboeuf@llr.in2p3.fr

  25. Experimental Certification Colliders Cosmic Rays EPOS testedwith a lot of experimental data sets Collision between a particlefromcosmic radiation and an atmosphericatomwhichleeds to an atmosphericshower • Differentsenergies from 20 GeV (SPS) to 1.8 TeV (Tevatron) Particle collision atveryhighenergies • Differentssystems e p Use of the sameeventgenerator : EPOS p p dN/d p 2 dAu N N T d Au + - e e • Differents observables Rapidity, multiplicity, Pt spectrum, transversemasse, v2 Everythingthatcanbemeasured • Differentsspecies p, K, p, L, X, … sarah.porteboeuf@llr.in2p3.fr

  26. PYTHIA Standard eventgenerator in particlephysics Only for proton-proton collisions Order for eventgeneration : 1. Hard processselection and inclusive cross section computation 2. Initial and final state radiation 3. Remnant and multiple interaction 4. Hadronization Based on parton model, hard processesis central element of eventgeneration Benefits : Verypowerfull for high pt jets Control the subprocessyouwant to study, easyselection objections : Easy to interacewithother model Generator of inclusive spectrum Multiple Interaction Connexion of the jet to the event Heavy ion collisions sarah.porteboeuf@llr.in2p3.fr

  27. EPOS vs. PYTHIA model PYTHIA structure EPOS Baseline Multiple Interaction Hard process Parton-based Gribov-Regge Theory Reconstructed after the hard process, Interaction ordered in hardness. In the new model : color reconnection Multiple Interaction Hard process Hard and semi-hard ladder with soft pre-evolution Based on inclusive cross section Almost everything, if not in the code, can couple with extra code u, d, s, g, gamma, c in progress Initial and Final state radiation A posteriori reconstruction Iterative procedure Available for MPI in the new model (6.4) String model with aera law, diquark for baryon production String model with fragmentation function, popcorn for baryon production Hadronization Yes, a toy model with parametrisation, event by event hydro in developpement Collectivity No sarah.porteboeuf@llr.in2p3.fr

  28. EPOS vs. PYTHIA model PYTHIA structure EPOS Selection of the hard process, Can vary all parameters : different tunes optimized for observables Pt cut in implementation and testing Selection 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 Yes No Heavy Ion Need to use somethingelse : HIJING, HYDJET, … Sameframeworkextended sarah.porteboeuf@llr.in2p3.fr

  29. Hard processes in EPOS sarah.porteboeuf@llr.in2p3.fr

  30. Generation of a semi-hard ladder σ σ hard hard (old model) ,Q0+ h1 fisrt émission z1 z1 z2 (S,Q1+,Q0-) (S,Q2+,Q0-) ,Q0- h2 Iterative procedure : emission determined step by step sarah.porteboeuf@llr.in2p3.fr

  31. Structure of Semi-Hard Parton Ladder Definedifferentsub-structure as independent block. : Light cone momentum of parton entering the ladder x +/- PE + + x x + x PE PE E x +/- IB : Light cone momentum of parton entering the Born Process IB + x OB - x x +/- OB : Light cone momentum of parton out the Born Process OB - x IB - - x x PE PE E E : Evolution of the parton, branching, gluon emissison sarah.porteboeuf@llr.in2p3.fr

  32. Description of structures (as a function of independant block) Structure N No resoltution on the inner part of the ladder + + x x + x PE PE E IB Structure S Resolution on the inner part Direct access to variables + x OB - x OB - x IB - - x x PE PE E sarah.porteboeuf@llr.in2p3.fr

  33. Variables Quantitydiscussed Integrated : Count the number of particles Equivalent to In EPOS : particlesproduced by ladders Count the number of ladders Each semi-hard ladderproducestwo partons Jets come from hard partons In the following Integrated version shows differentchoices of variables Direct acces to some variable of the ladder sarah.porteboeuf@llr.in2p3.fr

  34. In case of factorisation … Hadron-laddercoupling Multiple Interaction Hard process Evolution of the parton from the hadron to the hard process Identification : f PDF sarah.porteboeuf@llr.in2p3.fr

  35. Approach Certification Analytical test : internal test f : from hadron to hard process K : hard process F : hadron-laddercoupling E : Evolution along the ladder pp 200 GeV sarah.porteboeuf@llr.in2p3.fr

  36. Approach Certification Semi-analytical test: internal pp 200 GeV sarah.porteboeuf@llr.in2p3.fr

  37. New Monte Carlo New MC Principle • S functionisused as a probability distribution to directyproduced a couple ( , ) for a given ( , ) • the produced ( , ) isused in the eventgeneration • To randomlydistribute as S : Monte Carlo technics Results for pp collisions at 200 GeV x x x x x x x +/- - - + + - + IB IB IB IB IB PE PE sarah.porteboeuf@llr.in2p3.fr

  38. Approach Certification In case of factorisation : comparison to PDF pp 200 GeV Same bloc K f replacedwithother PDF ! Changement de variable : pt 1 semi-hard laddergives 2 partons sarah.porteboeuf@llr.in2p3.fr

  39. Which Selection? Pt contribution fromdifferentsproducts at the ladder end • lowproducts : don’tcontribute to high • highproducts : contributeeverywhere Selectionproductswhichcontributes in the chosen zone pp 200 GeV Then : selection on Can’t do direct selection on : Thenneed to reject couple thatwill not contribute to the zone p p p p p sarah.porteboeuf@llr.in2p3.fr T T T T T

  40. Jets STAR et UA1 sarah.porteboeuf@llr.in2p3.fr

  41. UA1 and STAR on the same Plot X 2 Nucl. Phys. B309 (1988) 405 Phys. Rev. Lett. 97 (2006) 252001 sarah.porteboeuf@llr.in2p3.fr

  42. 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 sarah.porteboeuf@llr.in2p3.fr

  43. Comparison with STAR DATA • Star Data : R=0.4 for jet finder, High Tower Trigger • Phys. Rev. Lett. 97 (2006) 252001 • Hep-ex/0608030 • NLO QCD (Vogelsang), R=0.4 • Phys. Rev. D70 (2004), 034010 • Hep-ph/0404057 • EPOS : pt out born : Rmax sarah.porteboeuf@llr.in2p3.fr

  44. 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 sarah.porteboeuf@llr.in2p3.fr

  45. Comparison with DATA First good test for EPOS jet production : Good agreement between EPOS and NLO Vogelsgang (R=0.7) Lessthan a factor 2 between EPOS and Star Data over 9 orders of magnitude Difficulttask : EPOS : out Born parton = total jet (Rmax) Need to do the Study by using jet finder on EPOS events DATA : Reconstructed Jets, Jet Finder, variable R NLO Vogeslgang : Computation with variable R sarah.porteboeuf@llr.in2p3.fr

  46. Ratio lambda/k0s sarah.porteboeuf@llr.in2p3.fr

  47. Ratio lambda/k0s 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 Is this ratio the same if we look at the bulk, if we look at jets ? sarah.porteboeuf@llr.in2p3.fr

  48. 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 sarah.porteboeuf@llr.in2p3.fr

  49. All : no selection Jet sarah.porteboeuf@llr.in2p3.fr

  50. All : no selection Jet All with hydro off sarah.porteboeuf@llr.in2p3.fr

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