MonteCarlo tuning using ATLAS data 1

Monte Carlo tuning usingATLAS data

Davide Costanzo(on behalf of the ATLAS collaboration)

23/08/2011

MonteCarlo tuning using ATLAS data 2

Monte Carlo simulation process

The Monte Carlo simulation of the ATLAS detector is used in most ATLAS papers/results

Event generators: Pythia6, Pythia8, Herwig, Herwig++, Alpgen, AcerMCMc@NLO, Powheg, Sherpa...

AtlasG4 simulation

Reconstruction, Analysis, Systematics

Data

Reconstruction, Analysis

Data/MC comparison exampleATLAS-CONF-2011-098Search for Supersymmetry withMET, bjets and no leptons

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Tuning a combination ofevent generator and simulation parameters

MonteCarlo tuning using ATLAS data 3

Event Generator tunings ATL-PHYS-PUB-2011-008ATL-PHYS-PUB-2011-009

PYTHIA 6 is used as the main general-purpose event generator in ATLAS. The tune is performed in four stages:1) Flavour parameters tuned to hadron multiplicities/ratios, from e+e- collisions2) Final State Radiation (FSR) and hadronisation parameters, tuned to event shapes and jet rates from e+e- collisions3) Initial state shower parameters and primordial kT, tuned to Tevatron and ATLAS data4) Multiple Partonic Interaction (MPI) parameters, tuned to Tevatron and ATLAS data

For HERWIG/JIMMY, only the MPI parameters were tuned:• The inverse proton radius squared [PRRAD]• The MPI cut-off at √s = 1800 GeV [PTJIM0]• The MPI cut-off evolution [EXP]Energy evolution of the MPI cut-off was added to HERWIG/JIMMY MPI model, to allow simultaneous tuning to 7 TeV and 900 GeV ATLAS UE data.

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MonteCarlo tuning using ATLAS data 4

Pythia 6 tuneATL-PHYS-PUB-2011-009

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AUET2B tuning to ATLAS data. To be used for 2011 analyses

AUET2B (CTEQ6L1) main Pythia 6 tune forATLAS MC11

MonteCarlo tuning using ATLAS data 5

Pythia 8 tuneATL-PHYS-PUB-2011-009

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Pythia8 has a better diffractive modelling than Pythia 6

ATLAS A1 and AU1 tunes to √s=7TeV minimum bias and underlying event dataFor ATLAS MC11

MonteCarlo tuning using ATLAS data 6

Herwig/Jimmy tuneATL-PHYS-PUB-2011-008

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MonteCarlo tuning using ATLAS data 7

The AtlasG4 simulationEPJC 70 (2010) 823

Over 109 events simulated with AtlasG4 so far

A few examples of AtlasG4 tuning using data in the next few slides23/08/2011

MonteCarlo tuning using ATLAS data 8

Inner Detector material validationUse photon conversions in the Inner Detector to map the material distribution:

Conversion rate (in colour) for R vs zThe beam pipe, the pixel barrel and part of the SCT detectors are visible

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MonteCarlo tuning using ATLAS data 9

Inner Detector material validation (2)Conversion rates for fixed eta:

Small discrepancies still visible, geometry description in continuous evolution

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10

Inner Detector hadron-graphyReconstruct secondary vertices inclusively and select those arising from secondary hadronic interactions

MonteCarlo tuning using ATLAS data

Cooling Pipe, Cables,

Carbon Fibre shell

Details of modules in 1st Pixel layer.

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Calorimeter response to single hadronsATLAS-CONF-2011-028

Measure the calorimeter response to isolated tracks in pp collisions:E=Energy deposited in the calorimeterp=track momentum

E/p for isolated tracks (2.2<p<2.8 GeV)

Translates into a jet energy scale uncertainty of 1% to 3%

Pythia AMBT1 tune Geant4 QGSP_BERT Physics list

(Central region)

Calorimeter response to single hadrons (2)ATLAS-CONF-2011-019

Use Ks -> π+ π-, Λ-> π-p and Λ -> π+p to measure response to pion, protonsand anti-protons

Response to anti-protons underestimated by MC(Only few % of the total energy in a jet)

Pythia AMBT1 tune Geant4 QGSP_BERT Physics list

(Central region)

Jet shape measurement PRD 83 052003

Differential jet shape ρ(r): Average fraction of jet PT in an annulus or radius r

Jets are composed by hadrons. The distribution of the hadrons within the jet depends on the parton-jet fragmentation process

ATL-PHYS-PUB-2011-010

MonteCarlo tuning using ATLAS data 14

Conclusions

23/08/2011

Simulation is a very important component of the ATLAS physics programme

Different components need to be tuned and validated. Tuning is a cyclic process :- Event generators- Geant4 MonteCarlo- ATLAS detector response

A good agreement is achieved between the ATLAS data and simulation

With more data available small discrepancies become visible resulting in a continuous improvement of the ATLAS Monte Carlo strategy

Response to electrons

Use J/Psi and Z decay to study the response to electrons

Response to muons

Use Z->μμ events to study theresponse to muons.

The Z mass reconstructed width, and hence the muon momentum resolution is underestimated in the simulation