AcerMC and ISR/FSR systematics at ATLAS

Liza Mijovic, Borut Kersevan Jozef Stefan Inst.Univ. of Ljubljana

ATLAS approach:• Generator level studies• Parameters treated• Interesting examples

Top Physics Workshop (Grenoble 2007)

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Impact of different models Recently a study of top mass reconstruction using tt~ was done using:

MC@NLO (Herwig+Jimmy) AcerMC (Pythia – new showering and UE model) Full detector simulation The observed discrepancy caused quite a few raised eyebrows..

We cannot know offhand which answer is correct!

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Impact of different models cont’d The first thought was that NLO corrections impact the event

shapes more than anyone suspected... ...But the difference turns out to be purely parton-shower

related! We just plugged the AcerMC events into Herwig and..

The difference becomes really small..

We cannot know offhand which answer - Herwig/Pythia showers and UE -is correct! We need the data!

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QCD-activity related systematics

From the above example one can see that the predictions are by no means unique; using the standard division one needs to have a look at: Initial state radiation Final state radiation Underlying event modeling PDFs, etc...

I think I don’t need to stress that the precision in top measurements at the LHC will be systematics-dominated..

At ATLAS the UE is handled by tuning the available models to the Tevatron data so it was excluded from these studies.

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Strategy

First thing to estimate is the prediction range the models on the marked provide – and the experimentalists can tune on the data. In this respect HERWIG is rather unflexible: It has excellent

theoretical basics but very few parameters in the shower activity that are allowed to have at least some uncertainty..

Pythia is in this respect much more flexible so at least as a start a detailed study was made on how (and how much) we can ‘push around’ the Pythia showering activity.

AcerMC + Pythia with varying parameters was thus used to check on the prediction uncertainties w.r.t. the QCD (parton-)showering activity. Studies first done on generator/truth level only. In each variation typical/simple analysis cuts and procedures

were used for top mass reconstruction and the distributions were compared.

Semileptonic decay selection criteria:•pT > 40 GeV, |eta| < 2.5 for 2 b-jets and at least two light jets.•pT > 20 GeV for the lepton from W, isolation requirement ~cone(lepton, any jet)>0.4•No cuts on missing ET .•No jet energy rescaling.•W reconstruction: truth / 2 light jets with MJJ closest to MW and MJJ < 120 GeV.•The JJB1,2 combination with highest pT chosen as the top candidate.

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Pythia ISR parameters

ATLAS uses the Pythia new pT-ordered showering and UE model!

A lot of switches: MSTP(62): level of coherence MSTP(70): regularization scheme, pT ->0

PARP(62), PARP(81), PARP(82) MSTP(72):max. pT for FSR of ISR partons PARP(61) : Λ(QCD) PARP(64) : evolution scale factor ISR master switch, ME corrections ...

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Rregularization scheme: PARP(81)

t-tbar rel. pT # of truth-jets

Default : PARP(81)=1.9 GeV=DPARP(81)=D/2PARP(81)=2*D

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Rregularization scheme: PARP(81)

b jet / b quark E ratio light jet / light q. E ratio

Default : PARP(81)=1.9 GeV=DPARP(81)=D/2PARP(81)=2*D

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Rregularization scheme: PARP(81)

top mass: truth W + bjet top mass: W(jets)+bjet

Default : PARP(81)=1.9 GeV=DPARP(81)=D/2PARP(81)=2*D

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PARP(61): Λ(QCD)

t-tbar rel. pt truth-jet number

Default : PARP(61)=0.192 GeVPARP(61)=0.1 GeVPARP(61)=0.4 GeV

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PARP(61): Λ(QCD)

high-pt truth jet n. top mass: W(jets)+bjet

Default : PARP(61)=0.192 GeVPARP(61)=0.1 GeVPARP(61)=0.4 GeV

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PARP(64): evolution scale factor

t-tbar rel. pt top mass: W(jets)+bjet

Default : PARP(64)=1=DPARP(64)=D/2PARP(64)=2*D

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Pythia FSR parameters Tunable & relevant parameters for the new showering: PARJ(81): Λ(QCD) (for external processes) (D=0.25 GeV) PARP(71): scale of the hard scattering (D=4) PARJ(82): mass cut-off below which partons don’t radiate (D=1 GeV)

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PARJ(81): Λ(QCD) Top mass: final W + b jet

AcerMC + Pythia: Parj(81) = 0.25 GeV Parj(81) = 0.14 GeV MCatNLO+Herwig

Reconstruction: True W from + truth b-jet.

MT(AcerMC) < MT(AcerMC-FSR) < MT(MC@NLO)

The FSR change in Pythia goes in the right direction!

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PARP(71) (scale of the hard scattering) b quark pt, number of high-pt jets, pt of the jets, pt of b-

jets

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PARP(71) top mass (final W + bjet) PARP(71) has no/little effect

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Pythia ISR and FSR parameters

PARAMETER TOP MASS ISR low high MSTP(70): reg. scheme 0 : PARP(62) 1 : PARP(81) 2 : PARP(82) PARP(61) : Λ(QCD) PARP(64) : evol. Factor FSR PARJ(81) : Λ(QCD)

These parameters were then combined into two samples which lower/increase the reconstructed mass to obtain endpoints..

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Systematics sample proposal AcerMC + Pythia, new showering Minimum top mass: 2*PARJ(81) (Λ(QCD) , FSR) 0.5* PARP(61) (Λ(QCD) , ISR) 2*PARP(62) (pt -> 0, kt cut-off, ISR)

Maximum top mass: 0.5*PARJ(81) (Λ(QCD) , FSR) 2* PARP(61) (Λ(QCD) , ISR) 0.5*PARP(62) (pt -> 0, kt cut-off, ISR)

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B-fragmentation studies

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B-fragmentation studies II

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B-fragmentation studies III

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B-fragmentation studies IV

The overall effect ofvarying b-fragm. Parametersgives ~stable results. However:Need to compare with other models like Herwig or EvtGen

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Further studies/plans

I don’t have time to go through all of the things we had done but to summarize other results: The impact of QCD uncertainties on the ttbar cross-

section measurements (efficiency) seems to be small (percent order) so not crucial at the initial measurement stages. Further investigation ongoing...

Studies on the impact of PDF uncertainties is planned. Impact of other models like EvtGen on b-tagging and top

reconstruction needs to be studied. We need to develop methods on extracting the QCD

model parameters from the data and/or validate different showering models.

A lot of work...