Event generators

Simulation of pp events Many programs exist for the simulation of pp collisions at high energy For the study of global event properties, simulations may be carried out by PYTHIA or HERWIG event generators

What is an event generator? Event generators are programs suitable to generate events as detailed as they could be observed by an ideal detector. The output is in the form of an event, i.e. a set of particles with their momenta, exhibiting average properties and statistical fluctuations as in real data, provided by Monte Carlo techniques used to sort out the informations according to probability distributions A typical output from an event generator may be the following: Particle # Particle type px py pz 1 ID1 .. .. .. 2 ID2 .. .. .. …

An example of output from PYTHIA

Why use event generators? To provide a feeling of the type of events one may expect to find in real data To estimate the production rates of specific processes in real events To help in the planning of new detectors and estimate the detector performances To train on simulated events the strategies for the analysis of real data To estimate detector corrections (acceptance, efficiency,…) To provide background events for rare processes …

Generated events may be a convenient input for detector simulation programs (GEANT,…) to understand how the event would be seen in a real detector

PYTHIA event generator PYTHIA can generate different types of high-energy events. It is based on perturbative QCD, but includes also soft interactions, parton showers, multiple interactions, fragmentation and decays. Due to the composite nature of hadrons, several interaction between parton pairs are expected to occur in a hadron-hadron collision. Since the understanding of multiple interactions is still very rough, several approaches (Models 1-4) may be used. One of the main parameter in this model is ptmin, a sort of cut-off introduced to regularize cross sections which diverge at pt->0. To compare PYTHIA with data, ptmin must be chosen to reproduce the observed charged particle multiplicity. The result depend on c.m. energy, multiple interaction model and parton distribution functions.

PYTHIA predictions for charged particle multiplicity at η=0 as a function of c.m. energy and different tuning of the model.

PYTHIA predictions for pseudorapidity and transverse momentum distributions charged particle and different tuning of the model. No large differences observed in pt-distribution for different tuning of the model

However, large differences are observed in the multiplicity

HERWIG HERWIG is a general purpose event generator, which takes into account hard lepton-lepton, lepton-hadron and hadron-hadron hard scattering and soft hadron-hadron scattering Some difference is observed with respect to PYTHIA

HERWIG = Solid line PYTHIA = Dashed line

In conclusion, no single event generator is able to describe all the details of the available data. Work is still in progress to refine the models and tune their parameters, to try to reduce the uncertainties in the predictions at LHC energies. A lot of refinements came after first data from RHIC

What about event generators for heavy ion collisions? A lot of different codes: HIJING, VENUS, DPMJET, SFM, … For a short summary and comparison between different model predictions see the ALICE Physics Performance Report Vol. I J.Phys. G 30(2004)1517-1763 Strong differences observed between different models

HIJING (Heavy Ion Jet INteraction Generator) and parametrized HIJING HIJING combines a QCD model for jet production with the Lund model for jet fragmentation Hard and semihard parton scatterings with pt up to a few GeV/c dominate high energy heavy ion collisions HIJING model is especially suited to treat mini-jets in pp, pA and AA collisions at high energies HIJING is able to reproduce many inclusive spectra, two-particle correlations

The formulation of HIJING is derived from the FRITIOF and DPM models at lower energies (c.m. energy around 20 GeV) Hadronic interactions were treated according to QCD processes in PYTHIA. Binary scattering with the Glauber geometry for multiple collisions were used to extrapolate from pp to pA and AA collisions Two important aspects of HIJING: Jet quenching Nuclear shadowing

Jet quenching Jet quenching is the energy loss by partons in nuclear matter. It is responsible for the increased particle multiplicities at central rapidities Nuclear shadowing Shadowing describes the modifications of the free nucleon parton density in the nucleus. Shadowing results in a decrease of the multiplicity.

HIJING 1. 36 predictions for Pb-Pb central (b<3fm) collisions at c HIJING 1.36 predictions for Pb-Pb central (b<3fm) collisions at c.m. energy of 5.5 A TeV With jet-quenching Without jet-quenching Pseudorapidity distribution

HIJING 1. 36 predictions for Pb-Pb central (b<3fm) collisions at c HIJING 1.36 predictions for Pb-Pb central (b<3fm) collisions at c.m. energy of 5.5 A TeV Solid line= With jet-quenching Dashed line= Without jet-quenching Pt distribution

HIJING 1. 36 predictions for Pb-Pb central (b<3fm) collisions at c HIJING 1.36 predictions for Pb-Pb central (b<3fm) collisions at c.m. energy of 5.5 A TeV Solid line= With jet-quenching Dashed line= Without jet-quenching Net baryon pseudo rapidity distribution

Different scenarios for jet quenching in HIJING, inspired by recent RHIC results at cm energy of 200 AGeV New parameters decrease the expected charged multiplicity at η=0 by 25 %

Parametrized HIJING Sometimes a parametrized version of HIJING event generator is used for the simulation of heavy ion collisions, producing only the most abundant particles (neutral and charged pions and kaons) according to parametrized rapidity and pt distributions with proper mt scaling. This choice is usually good to inject the bulk of the emitted particles into the detector, to provide a background description of the expected events in the detector, on top of which one can superimpose specific particles,…

DPMJET (Dual Parton Model) event generator DPMJET is an implementation of the dual parton model for the description of nuclear interactions based on the Glauber-Gribov approach. It treats both hard and soft scattering processes in a unified way. Particle production in the fragmentation region is described by evaporation processes of light nucleons, nuclei, photons,… Important features of DPMJET: New diagrams contributing to baryon stopping Better calculations of Glauber cross sections

DPMJET-II. 5 predictions for Pb-Pb central (b<3 fm) collisions at 5 DPMJET-II.5 predictions for Pb-Pb central (b<3 fm) collisions at 5.5 A TeV Full line=baryon stopping Dashed line= without baryon stopping The baryon stopping mechanism increases the multiplicity by about 15 % Pseudorapidity distribution

DPMJET-II. 5 predictions for Pb-Pb central (b<3 fm) collisions at 5 DPMJET-II.5 predictions for Pb-Pb central (b<3 fm) collisions at 5.5 A TeV Full line=baryon stopping Dashed line= without baryon stopping Pt distribution

DPMJET-II. 5 predictions for Pb-Pb central (b<3 fm) collisions at 5 DPMJET-II.5 predictions for Pb-Pb central (b<3 fm) collisions at 5.5 A TeV Full line=baryon stopping Dashed line= without baryon stopping Net baryon pseudo rapidity distribution

SFM (String Fusion Model) The main features of SFM are: Soft interactions described by the Gribov-Regge multipomeron exchange Hard part of the interaction simulated by PYTHIA Strings formed by gluon splitting fragmented with JETSET Fusion of soft strings included Rescattering of produced particles included

SFM predictions for Pb-Pb central (b<3 fm) collisions at 5 SFM predictions for Pb-Pb central (b<3 fm) collisions at 5.5 A TeV (No rescattering) Full line= with string fusion Dashed line= without string fusion Pseudorapidity distribution Including string fusion reduces the multiplicity by about 10 %

SFM predictions for Pb-Pb central (b<3 fm) collisions at 5 SFM predictions for Pb-Pb central (b<3 fm) collisions at 5.5 A TeV (No rescattering) Full line= with string fusion Dashed line= without string fusion Pt distribution

SFM predictions for Pb-Pb central (b<3 fm) collisions at 5 SFM predictions for Pb-Pb central (b<3 fm) collisions at 5.5 A TeV (No rescattering) Full line= with string fusion Dashed line= without string fusion Net baryon pseudo rapidity distribution

Apart from the multiplicity and pt distributions, these models exhibit differences also for other observables Pion transverse mass spectra

Kaon transverse mass spectra

Nucleon and antinucleon transverse mass spectra

Inverse slope parameters obtained by fitting transverse mass spectra by Large differences observed from model to model

Recent results at RHIC (Phenix Collaboration) for different centrality selections They are not reproduced by any of the existing models ! Above 2 GeV pions and protons are comparable

Predictions from HIJING

Predictions from DPMJET

Predictions from SFM

Other event generators Among other event generators widely used in the heavy ion physics community, MEVSIM was developed for the STAR experiment at RHIC to produce in a fast way AA collisions. MEVSIM generates particle spectra according to a momentum model chosen by the user. Main input parameters: Types and number of generated particles Momentum distribution model (pt and y-distributions) Reaction plane and azimuthal asymmetry Multiplicity fluctuations … Specific signals can be introduced on top of that (Event-by-event, resonances, flow,…)

Quite often, one needs to superimpose some specific feature of heavy ion collisions, which theoretical models are not able to predict in detail. One of such cases is the study of HBT correlations, where one needs some specific two-particle correlation at small relative momenta. Correlation functions built with the output of any event generator are normally flat in the region of small relative momenta. HBT processors afterburner introduce a two-particle correlation at small relative momenta on the set of generated events. To do this, particle momenta are modified after generation, in order to fit a specific correlation function.

Correlation effects introduced in such a way normally do not alter single particle distributions and multiplicities, so that event reconstruction procedures are identical with and without correlation. However, tracking efficiency, particle identification and momentum resolution may be affected, since these are sensitive to the details of the topology involving particles at small relative momenta.

Example of an HBT processor applied to MEVSIM events Two-particle correlation Single particle distribution

References: HIJING DPMJET SFM

References/2: MEVSIM PYTHIA HERWIG

Activity project: Analyze pp or heavy ion collisions simulated events from an event generator

A real output from PYTHIA for pp collisions at LHC 0 20 N.traccia pdg pt E pz py px theta phi eta y 0 5 3.84242 75.6509 75.4038 0.471787 -3.81335 0.0509139 -0.123094 3.67055 3.20778 1 -5 3.30503 67.5437 67.2954 -0.751551 3.21845 0.0490729 2.91219 3.70739 3.14853 -523 3.05386 58.942 58.6215 0.226883 -3.04542 0.0520475 -0.0743624 3.64852 2.95242 …………………………… A real output from PYTHIA for pp collisions at LHC