Transcript AcerMC and ISR/FSR systematics at ATLAS

AcerMC and ISR/FSR systematics at ATLAS
Top Physics Workshop (Grenoble 2007)
Liza Mijovic, Borut Kersevan
Jozef Stefan Inst.
Univ. of Ljubljana
ATLAS approach:
• Generator level studies
• Parameters treated
• Interesting examples
Impact of different models
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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
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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..
We cannot know
offhand which answer
- Herwig/Pythia showers and UE is correct! We need the
data!
 The difference becomes really small..
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QCD-activity related systematics
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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
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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.selection criteria:
Semileptonic decay
 Studies
done< 2.5
on generator/truth
level
•pT > 40first
GeV, |eta|
for 2 b-jets and at least
twoonly.
light jets.
> 20variation
GeV for thetypical/simple
lepton from W, isolation
requirement
anywere
jet)>0.4
 In•pT
each
analysis
cuts and~cone(lepton,
procedures
•No cuts on missing ET .
used
for top mass reconstruction and the distributions were
•No jet energy rescaling.
compared.
•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)
Default : PARP(81)=1.9 GeV=D
PARP(81)=D/2
PARP(81)=2*D
t-tbar rel. pT
# of truth-jets
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Rregularization scheme: PARP(81)
Default : PARP(81)=1.9 GeV=D
PARP(81)=D/2
PARP(81)=2*D
b jet / b quark E ratio
light jet / light q. E ratio
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Rregularization scheme: PARP(81)
Default : PARP(81)=1.9 GeV=D
PARP(81)=D/2
PARP(81)=2*D
top mass: truth W + bjet
top mass: W(jets)+bjet
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PARP(61): Λ(QCD)
Default : PARP(61)=0.192 GeV
PARP(61)=0.1 GeV
PARP(61)=0.4 GeV
t-tbar rel. pt
truth-jet number
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PARP(61): Λ(QCD)
Default : PARP(61)=0.192 GeV
PARP(61)=0.1 GeV
PARP(61)=0.4 GeV
high-pt truth jet n.
top mass: W(jets)+bjet
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PARP(64): evolution scale factor
Default : PARP(64)=1=D
PARP(64)=D/2
PARP(64)=2*D
t-tbar rel. pt
top mass: W(jets)+bjet
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Pythia FSR parameters
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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)
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Top mass: final W + b jet
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AcerMC + Pythia:
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Parj(81) = 0.25 GeV
Parj(81) = 0.14 GeV
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MCatNLO+Herwig
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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)
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b quark pt, number of high-pt jets, pt of the jets, pt of b-jets
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PARP(71)
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top mass (final W + bjet)
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PARP(71) has no/little effect
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Pythia ISR and FSR parameters
PARAMETER
ISR
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MSTP(70): reg. scheme
0 : PARP(62)
1 : PARP(81)
2 : PARP(82)
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PARP(61) : Λ(QCD)
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PARP(64) : evol. Factor
FSR
 PARJ(81) : Λ(QCD)

TOP MASS
low
high
f
f
f
g
f
f
These parameters were then combined into two samples which
lower/increase the reconstructed mass to obtain endpoints..
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Systematics sample proposal
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AcerMC + Pythia, new showering
Minimum top mass:
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2*PARJ(81)
(Λ(QCD) , FSR)
0.5* PARP(61) (Λ(QCD) , ISR)
2*PARP(62)
(pt -> 0, kt cut-off, ISR)
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Maximum top mass:
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0.5*PARJ(81)
2* PARP(61)
0.5*PARP(62)
(Λ(QCD) , FSR)
(Λ(QCD) , ISR)
(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 of
varying b-fragm. Parameters
gives ~stable results. However:
Need to compare with other
models like Herwig or EvtGen
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Further studies/plans
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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...
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