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Event Generators Event Generators XI SERC School on Experimental High-Energy Physics NISER Bhubaneswar, November 07-27, 2017 Event Generators Session 1 Introduction IntroductionIntroduction Typical high energy event (p+p collision) and possible processes: Long Island, New York, USA Typical highTypical energy high event energy ( eventp+p collision)(p+p collision) and and possible possible processes: processes: M. H. Seymour and M. Marx, arXiv:1304.6677 [hep-ph] 1. Hard process 1. Hard process 2. Parton shower 2. Parton shower 3. Hadronization 3. Hadronization 4. Underlying event 4. Underlying event 5. Unstable particle decays 5. Unstable particle decays If we could create a model that rightly incorporates most physics processes and rightly predicts the physics results… Helps Increase the physics understanding of the high energy collisions2 and data results obtained 2 2 Event Generators – In General Long Island, New York, USA ü Computer programs to generate physics events, as realistic as could be using a wide range of physics processes. ü Use Monte Carlo techniques to select all relevant variables according to the desired probability distributions and to ensure randomness in final events. ü Give access to various physics observables ü Different from theoretical calculations which mostly is restricted to one particular physics observable ü The output of event generators could be used to check the behavior of detectors – how particles traverse the detector and what physics processes they undergo -- simulated in programs such as GEANT. 3 Need of Event Generators Long Island, New York, USA ü Interpret the observed phenomena in data in terms of a more fundamental underlying theory; Give physics predictions for experimental data analysis. ü To generate distributions that look sufficiently close to data to allow for detector calibration etc.; planning a new detector. ü Estimate detector acceptance/efficiency corrections to be applied on raw data to extract “true” physics signal. ü Device analysis strategies to be used on real data e.g. signal-to- background conditions for rare signals. 4 Real Experiment vs. Event Generators Real Experiment LongEvent Island, Generator New York, USA Collisions happening in machine Based on possible Physics processes, generate events Events/tracks detected by detectors – written on tape Physics analysis – through data acquisition system comparison/predictions.. (DAQ) Ø Output can also be put through Events/tracks reconstructed - same detector configuration & event electric signals translated into reconstruction chain and Physics tracks,energy deposition inferring Analysis chain (Here we know what momenta and particle species’ the “right answer” is) – Information Information is available at both initial (generation) and (final (reconstruction) level Physics analysis Information is only available at A powerful tool to gain a detailed final stage (reconstruction level) and realistic understanding of Physics 5 Early Evolution of Event Generators The need for event generators was apparent inLong early Island, 50’s Newwhen York, the USAfinal state multiplicity became large. Y. Pang, BNL-65351, CONF-97123, 1997/2 ü 1950: To locate and identify various hadronic resonances, a Monte-Carlo code was used to generate appropriate background corresponding to a uniform phase space distribution. ü 1960: As colliding energy increases, the spectra deviate from the uniform distribution and extra parameters were required to simulate the leading particle behavior and to limit transverse momenta. ü 1970: Hadronic string phenomenology alternative to phase-space parameterization was used. ü 1980: Focus got shifted to jet production from particle distribution Since then many new versions, modifications, and models focusing on different physics aspects of the high energy collisions have appeared 6 High Energy Event Generators’ Category Long Island, New York, USA ü Experimental High Energy Physics (EHEP) field is studied broadly in two areas : Particle Physics (elementary collisions) and Heavy-ion Physics (heavy-nuclei collisions) ü Since heavy-ion collisions in principle represent many elementary collisions, almost all event generators for relativistic heavy-ion collisions contain parts borrowed from event generators in particle physics. ü There are some key differences between Event Generators used in these two fields 7 Differences: Elementary and heavy-ion Generators Key differences between particle physics and heavyLong Island,-ion event New generatorsYork, USA Term Particle Physics Heavy-Ion (Elementary) Physics Medium formation: No medium Medium is formed Hadronization simple Complicated enviornment: Hadronic final state Almost negligible Very important interaction: Background processes: Can be simply defined Not easily defined Note: Recent LHC results on multiplicity dependence of pp collisions suggest effects like heavy-ion 8 Commonly Used Event Generators Particle Physics (Elementary Collisions): Long Island, New York, USA PYTHIA: http://home.thep.lu.se/~torbjorn/pythia.html; arXiv:0710.3820 [hep-ph] PHOJET: https://wiki.bnl.gov/eic/upload/Phoman5c-2.pdf HERWIG: Hadron Emission Reactions With Interfering Gluons http://projects.hepforge.org/herwig; arXiv:0803.0883 Only a few event generators are listed here, there exist many more… 9 Commonly Used Event Generators Heavy-Ion Collisions: Long Island, New York, USA HIJING: Heavy-Ion Jet Interaction Generator http://ntc0.lbl.gov/~xnwang/hijing/index.html; X. N. Wang and M. Gyulassy, Phys.Rev.D 44, 3501 (1991) AMPT: A Multi-Phase Transport Model http://myweb.ecu.edu/linz/ampt/ Z. W. Lin et al. Phys.Rev.C72, 064901 (2005) UrQMD: Ultra Relativistic Quantum Molecular Dynamics https://urqmd.org/ S. A. Bass et al., arXiv:nucl-th/9803035 Only a few event generators are listed here, there exist many more… 10 Open Standard Codes And Routines (OSCAR) q There had been appearance of many event generatorsLong Island, in New the York, market USA q People felt need for - Accessibility of source code and documentation - Systematic version controls - Standardized tests, and - Common interfaces q Open Standard Codes And Routines for event generators was developed: https://karman.physics.purdue.edu/OSCAR-old/models/list.html ü Set of minimum requirements for the accessibility of the source codes and documentation, and the reproducibility of the published results for event generators in OSCAR ü Series of Standard Tests for event generators in OSCAR Good collective information is available but not being maintained…. 11 Primary charged hadrons are defined as all charged hadrons produced in the collision, including the products of strong and electromagnetic decays, but excluding products of weak decays. Feed-down corrections from weakly 0 Λ Λ Σ+ Σ decaying strange resonances (mainly KS, , and , )havetobeaccountedforinordertoobtainthefinalhadron spectrum. Such corrections, which depend on the strange particle composition in the MC, reduce by about 8% the total charged yield at midrapidity. In all the simulations, one takes this into account by decaying all unstable∼ particles for which5 cτ < 10 mm. The sole contribution from charged leptons to the reconstructed tracks in the low-p range, comes from the Dalitz π0 decay amounting to about 1.5% of the charged yield. ALICE doesnotcorrectfor⊥ this contribution, whereas CMS does. We have removed this small contribution from all our model predictions by counting only the produced charged hadrons. 4. Data versus models 4.1. Particle pseudorapidity densities2 / η The pseudorapidity densities, dNchparticlesd ,ofchargedhadronsmeasuredinNSDcollisionsattheLHC(0. including pions, kaons, protons, and antiprotons. We see that the theoretical9, 2.36 and 7.0 TeV) by ALICE and CMS (asresults well as shown by UA5 by solid at curves 900 GeV) agree are reasonably shown with in Fig. the expe2 comparedrimental data to two [6].pythia On the 6.4 tunes, pythia 8andtophojet.Inthepythiaothercase, hand, the the NSD HIJING predictions model with are default obtained parameters, switching shown offbythe dashed single-di curvesffractive in contributions6 without any hadron-levelFig. trigger. 2, underpredicts Since the the eff inverseects of slopes the LHC of the MB-selections transverse momentum have spectra been corrected for kaons for pythiaand protons inphojet these collisions. Final state hadronic scatterings are thus important in by the experiments themselves using describing(and theSome transverseas aExamples momentum cross-check), spectra. of this Published is a consistent Results comparison. Long Island, New York, USA 5 500 50 10 NA49 prel. data NA49 data =0 =0 =0 NA44 data η η NA49 data NA49 prel. data η | ± | 400 ± 40 | η pp → h , s = 900 GeV η pp → h , s = 2.36 TeV η p AMPT results + − /d /d /d 4 HIJING CMS (NSD) 300 h +h CMS (NSD)30 10 ch ch ch /dy 6 ALICE (NSD) ch 6 p−p 6 ALICEdN/dy (NSD) dN dN 200 20 dN dN − UA5 (NSD) h 3 100 10 10 p K+ 0 0 −3 −2 −10 1 2 3 −3 −2 −10 1 2 3 1<y<1) (a.u.) 4 4 − 4 2 ± y y ( 10 pp → h , s = 7.0 TeV T 200 40 CMS (NSD) + NA49 prel. data NA49 prel. data 1 π − N/dydm π PYTHIA 6.422 (Atlas-CSC) + 2 10 PYTHIA 6.422 (Atlas-CSC) 150 30 K + d PYTHIA 6.422 (Atlas-CSC) π T 2 2 PYTHIA 6.422 (Perugia-0) 2 PYTHIA 6.422 (Perugia-0) PYTHIA 6.422 (Perugia-0) 1/m 100 PYTHIA 8.13020 (Tune-1) 0 dN/dy dN/dy 10 PYTHIA 8.130 (Tune-1) PYTHIA 8.130 (Tune-1) K− PHOJET 1.12 (+PYTHIA 6.11) PHOJET 1.12 (+PYTHIA 6.11) 50 10 PHOJET 1.12 (+PYTHIA 6.11) 10−1 0 00 0 0 −3 −2 −10 1 2 3 −3 −2 −10 1 2 3 0 0.25 0.5 0.75 1 1.25 -4 -2 0 2 4 -4 -2 0 2 4 -4 -2 02 2 4 η y η y mT−m0 (GeV/cη ) D. d'Enterria et al. Astropart. Phys. 35, 98 (2011) Z. w. Lin et al. Nucl. Phys. A 698, 375 (2002) + Figure 2: Pseudorapidity distributions of chargedFigure hadrons 1., Rapidityh± (h distributions+ h−), measured at SPS. in NSD pFigure-p events 2. at Transverse the LHC ( momentum√s = 0.9, 2.36 spectra and 7at TeV) by ALICE [36, 37] and CMS [38, 39] (and by UA5 [42] in p-¯p≡at 900 GeV) compared to three diSPS.fferent versions of pythia and to the phojet MC.
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