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

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. The dashed band is the systematic uncertainty of the CMS experiment which is similar to those of the two other measurements. 12 With parameters in the AMPT model constrained by experimental data from heavy The Perugia-0 tune underpredicts allion the collisions measured at SPS midrapid energies,ity we densities then studied – by heavy about ion col 20%lisions at √ ats RHIC= 0.9 energies. and 2.36 We TeV and by about 35% at 7 TeV – whereasfirst the show Atlas-CSC in Fig. 3 tune by the overpred solid curveicts the by pseudorapidity 10% the central distribution multiplicities of total at charged 0.9 TeV particles in central (b 3fm)Au+Aucollisionsat130A GeV. The theoretical result but reproduces well the data at higheris c.m. consistent energies. withpythia the data≤ 8is15%(10%)belowtheexperimentalcentraldensities from the PHOBOS collaboration [7]showninthefigureby at √s = 0.9 and 2.36 TeV but agrees wellthe full with circle. the We experimental note that the shape total charged and normalization particle multiplicity at 7 TeV. increasesphojet appreciablyis compat- ible within uncertainties with the measuredwithout hadron final hadronicdNch/d scatteringsη distributions but is at hardly 0.9 and affected 2.36 by TeV, partonic whereas scatterings. at 7 TeV The it is some 15% lower. The Perugia tunes werelatter obtained is partly duemostly to the with absence Tevatron of inelastic non-di scatteringffractive intheZPCmodel.Tomodelthe processes using hadrons with effect of parton energy loss due to inelastic scatterings, we introduce the jet quenching as p > 0.4 GeV/c, which affects their predictionin default HIJING. accuracy In Fig.for the 3, we lowest show by multipliciti dashed curveses and the lowerresults fromtransverse AMPT momenta model ⊥ considered here (the ALICE and CMSwith experimental a jet quenching hadron of dE/dx distributions=1GeV/fm.AlsoshowninFig.3aretheresultsobtained are measured from p 100 MeV/candex- ⊥ ≈ trapolated down to zero p ). The betterby agreement neglecting nuclear of the shadowing Atlas-CSC on parton tune and productionpythia in8withtheobservationsislinked the AMPT model. In both cases, to the faster rise of particle⊥ productionthe with total√ chargeds,asgivenbytheirsmallerexponents, particle multiplicity is larger than thatεfrom= 0 the.11 default and 0.08 AMPT respectively, model. The PHOBOS data is thus consistent with a significant nuclear shadowing effect but a rather weak jet quenching. Our predictions for the multiplicities of pions, kaons, protons and antiprotons are also shown by solid curves in Fig. 3. It is seen that thep/p ¯ ratio is 5MSTJ(22)=2,PARJ(71)=10 in pythiasignificantly6.4, and ParticleDecays:limitTau0 increased in comparison with that = on, in central ParticleDecays:tau0Max Pb+Pb collisions at SPS. = 10 in pythia 8. 6MSUB(92)=MSUB(93)=0 in pythia 6.4, SoftQCD:singleDiffraction=off in pythia 8.

10 Running of Event Generators Long Island, New York, USA The Event Generators are used to run in two ways

A). Stand-alone: Run the model with desired parameters, get the output file and do the Physics analysis.

B). With Detector Response: Run the model with desired parameters, get the output file, pass this output to the other simulation program GEANT to get the detector response. The output will include the information of how the particle traverse the detector and underwent Physics processes (behavior in magnetic field, showers in calorimeter etc.)

In this tutorial: ü Run the stand-alone Event Generators covering particle physics and heavy-ion physics i.e. PYTHIA, PHOJET, HIJING, and AMPT ü Analyze the output file to perform different exercises

13 WRITE(*,*) HINT1(31), HINT1(32), HINT1(33), HINT1(34) STOP END

Acknowledgements

During the development of this program, we benefited a lot fromdiscussionswithJ.Car- WRITE(*,*) HINT1(31), HINT1(32), HINT1(33), HINT1(34) roll, J. W. Harris, P. Jacobs, M. A. Bloomer, and A. Poskanzer.Wewouldliketothank T. Sjöstrand for making available JETSET and PYTHIA Monte Carlo programs on which STOP WRITE(*,*) HINT1(31), HINT1(32), HINT1(33), HINT1(34) HIJING is based on. We would also like to thank K. J. Eskola for helpful comments and STOPEND discussions. END Acknowledgements Appendix: Flavor Code Acknowledgements For users’ reference, a selection of flavor codes from JETSET 7.2 are listed below. For full During the development of this program, we benefited a lot fromdiscussionswithJ.Car- list please check JETSET documentation. The codes for anti-particles are just the negative Duringroll, J. the W. development Harris, P. Jacobs, of this M.program, A. Bloomer, we benefited and A. a lot Poskanzer fromdiscussionswithJ.Car-.Wewouldliketothankvalues of the corresponding particles. roll,T. Sjöstrand J. W. Harris, for making P. Jacobs, available M. A. JETSET Bloomer, and and PYTHIA A. Poskanzer Monte.Wewouldliketothank Carlo programs on which Quarks and leptons T.HIJING Sjöstrand is based for making on. We available would JETSET also like and to thank PYTHIA K. J. Monte Eskola Carlo for programshelpful comments on which and 1d 11e− HIJING is based on. We would also like to thank K. J. Eskola for helpful comments and discussions. 2u 12νe discussions. 3s 13µ−

4c 14νµ Appendix: Flavor Code − Appendix: Flavor Code 5b 15τ 6t 16ν For users’ reference, a selection of ﬂavor codes from JETSET 7.2 are listed below. For full τ MesonsFor users’ reference, a selection of ﬂavor codes from JETSET 7.2 are listed below. For full list please check JETSET documentation. The codes for anti-particles are justGauge the bosons negative 211 π+ 213 ρ+ listvalues please ofthe check corresponding JETSET documentation. particles. The codes for anti-particles are just the negative 21 g 311 K0 313 K∗PID0 (Flavor) Codes values of the corresponding particles. 22 γ 321 K+ 323 K∗+ Quarks and leptons411 D+ 413 D∗+ Quarks and leptons ∗ 421 D1d0 423 D 0 11e− DiquarksLong Island, New York, USA + ∗+ − 4311d Ds 433 Ds 11e 1103 dd1 2u 12νe 511 B0 513 B∗0 2101 ud 2103 ud 2u 12νe − 0 1 521 B3s+ 523 B∗+ 13µ 3s 13µ− 2203 uu1 531 B0 533 B∗0 4cs s 14νµ 3101 sd0 3103 sd1 0 0 111 4cπ 113 ρ 14νµ − 5b 15τ 3201 su0 3203 su1 221 5bη 223 ω 15τ − 3303 ss1 331 η6t′ 333 φ 16ντ 6t 16ντ 441 η 443 J/ψ c Mesons 33 551 ηb 553 Υ Gauge bosons 211 π+ 213 ρ+ Gauge bosons 661 ηt 663 Θ 210 g 311 K0 313 K∗0 13021 KL g 310 K0 + ∗+ 2222S γ γ 321 K 323 K 411 D+ 413 D∗+ Baryons 0 0 ∗0 4132 Ξc 421 D 423 D DiquarksDiquarks 1114 ∆− 4312 Ξ′0 4314 Ξ∗0 + ∗+ 0 c c 431 Ds 433 Ds 2112 n 2114 ∆ 1103+ dd 11034232 ddΞ 1 1 0 ∗0 2212 p 2214 ∆+ c 511 B 513 B ′+ ∗+ 2101 ud 43222103Ξ ud 4324 Ξ ∗ 2101 ud0 0 2224 ∆++ 2103 udc1 1 c 521 B+ 523 B + 0 ∗0 − ∗− 4332 Ω 4334 Ω 3112 Σ 3114 Σ 22032203 uuc1 uu1 c 0 ∗0 − ∗− 531 Bs 533 Bs 0 5112 Σ 5114 Σ 3122 Λ b b 0 0 31013101 sd sd0 0 31033103 sd01 sd1 111 π 113 ρ 3212 Σ0 3214 Σ∗0 5122 Λb 32013201 su su0 32033203 su01 su ∗0 221 η 223 ω 3222 Σ+ 0 3224 Σ∗+ 5212 Σ 1 5214 Σ b b ′ − ∗− 3303 ss+ ∗+ 331 η 333 φ 3312 Ξ 3314 Ξ 52223303Σb1 ss1 5224 Σb 3322 Ξ0 3324 Ξ∗0 441 ηc 443 J/ψ − 3334 Ω 33 33 551 ηb 553 Υ 0 ∗0 4112 Σc 4114 Σc 661 η 663 Θ + t 4122 Λc 0 130 KL 4212 Σ+ 4214 Σ∗+ c c 0 ++ ∗++ 310 K 4222 Σc 4224 Σc S

34 Baryons 111414 ∆− 2112 n 2114 ∆0 2212 p 2214 ∆+ 2224 ∆++ 3112 Σ− 3114 Σ∗− 3122 Λ0 3212 Σ0 3214 Σ∗0 3222 Σ+ 3224 Σ∗+ 3312 Ξ− 3314 Ξ∗− 3322 Ξ0 3324 Ξ∗0 3334 Ω− 0 ∗0 4112 Σc 4114 Σc + 4122 Λc + ∗+ 4212 Σc 4214 Σc ++ ∗++ 4222 Σc 4224 Σc

35 Long Island, New York, USA

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