Summary of Commissioning Studies Top Physics Group

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Summary of Commissioning StudiesTop Physics Group

• M. Cobal, University of Udine

Top Working Group, CERN October 29th, 2003
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Top Quark Event Yields

• NLO Xsect for t-tbar production 833 pb
• 8 million t-tbar pairs produced per 10 fb-1
• We reconstruct the top mass in the leptonjets
  channel Clean sample (1 isolated lepton, high
  Etmiss).

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Statistical Error
In the single lepton channel, where we plan to
measure m(top) with the best precision
Period tt events
1 year 8x106
1 month 2x106
1 week 5x105
Period evts dMtop(stat)
1 year 3x105 0.1 GeV
1 month 7.5x104 0.2 GeV
1 week 1.9x103 0.4 GeV
L 1x1033 cm-2s-1
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Top mass precision
One top can be directly reconstructed Reconstruc
t t ? Wb ? (jj)b Selection cuts 1 iso lep,
Pt gt 20 GeV, h lt 2.5, Etmiss gt 20 At least 4
jets with Pt gt 40 GeV and h lt 2.5 At least 2
b-tagged jets Selection effic.
5 ? 126k events, with S/B 65
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• Two methods
• Reconstruction of the hadronic part
• W from jet pair with the closest invariant
• mass to m(W) cut on mjj-mW lt 20 GeV
• Association of W with a b-tagged-jet
• Cut on mjjb-ltmjjbgt lt 35 GeV
• Kinematic fit
• The leptonic part is reconstructed
• mlnb-ltmjjbgt lt 35 GeV
• -30k signal events
• -14k bkgnd events

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Systematics for the lepton jet analyses
At the beginning the jet energy scale will be not
known as well as 1
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Energy scale

• From M. Bosman
• Will start to calibrate calorimeter with weights
  from MC
• Assume
• EM scale correct to the percent level from the
  very beginning
• fragmentation correctly described in MC
• corrections for calorimeter non-compensation and
  dead material
• ? correct calibration coefficients should be
  predicted
• First check fragmentation function with the
  tracker, then dijet
• differential cross-section, h distribution,
  check pT balancing across
• different detectors, etc.
• Start lo look at in-situ calibration samples At
  the very beginning, start
• with W-gtjj.

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Taking TDR numbers 1500
ttbar-gtbW(ln)bW(jj) requiring 4 jets above 40
GeV/day at low L. In 1 week 10k W to
jj decays In 1 month 35k W to
jj decays Jets have a pT distribution 40 to
140 GeV with changing calibration. Consider pT
bins of 10 GeV, and h bins of 0.3. There are 150
"samples" to consider After a week, about 70 W
per "sample" or a statistical error on m(W)
sigma(about 8 GeV with perfect calibration)
divided by sqrt(70) This makes 1 of
statistical error On top there is the
systematic errors due to FSR and jet overlap...
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b-jet scale
Observed linearity dependence of the top mass
shift on the b-jet absolute scale error for the
inclusive sample.
Can scale correspondingly
Hadronic Kin fit 1 jet energy
uncertainty ? dM(top) 0.7 0.7
GeV 5 jet energy uncertainty ? dM(top) 0.75
3.5 3.5 GeV 10 jet energy uncertainty ?
dM(top) 0.710 7 7 GeV
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Light-jet scale
Here as well linear dependence If one performs
constrained fit on W-mass, is less important than
b-jet scale.
Can scale correspondingly Hadronic 1
jet energy uncertainty ? dM(top) lt 0.7
GeV 10 jet energy uncertainty ? dM(top) 3
GeV
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B-tagging
From S. Rozanov Main effects of initial layout
2 pixel barrel layers rejection of
light jets reduced by 30. Another important
parameter is the efficiency of the pixel chips
and modules (not predicted). Effect of
alignment precision Precise alignment of ID
could be reached only after a FEW MONTHS work.
(studies undergoing) Impact of misalignment much
higher than effect of 2 or 3 layers. Can also
compromise a jet energy calibration based on W
from tt at startup could be difficult to
select Ws over background.
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Estimates for initial ?(t-tbar) measurement

• Initial lum 1x1033 cm-2 s-1 ? t-tbar production
  rate 0.85 Hz
• 500k t-tbar events produced per week
• With same analysis and detector performance as in
  Physics TDR, predict
• Selection of 8000 single lepton plus jets events,
  S/B 65
• In 35 GeV window around m(top), would have
• 1900 signal events
• 900 bkgnd events (dominated by wrong
  combinations from t-tbar events)
• ? stat error on ?(t-tbar) ? 2 after 1 week

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• What happens with degraded initial detector
  performance?
• eg. Consider case where b-tagging is not
  available in early running
• Drop b-tagging requirement signal effic.
  increases from 5 to 20, but bkgnd increases
  faster
• For one week, would select 32000 signal events,
  but with S/B 6
• Biggest problem comes from large increase in
  combinatorial bkgnd when trying to reconstruct t
  ? Wb ? (jj)b with b-tagging

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• W ? jj t ? Wb ? (jj)b
• Fit of m(jjb) spectrum provides Xsect measurement
  with stat. error ? 7
• Even with no b-tagging, can measure ?(t-tbar) to
  lt 10 with two days of integrated luminosity at
  1x1033

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Results presented

• In Athens

• An initial uncertainty of 5 on the b-jet energy
  scale, gives a top mass
• uncertainty of 3.5 for the mass reconstuction.
• If we go to 10 , the uncertainty on the top mass
  is of 7 GeV
• An initial uncertainty of 10 on the light jet
  energy scale, gives a top
• mass uncertainty of 3 GeV for the mass
  reconstuction.
• Kinematic fit less sensitive to light jet energy
  scale. But can have very
• large combinatorial background in case of
  b-tagging not working
• After 1 week of data taking we should be able to
  measure
• the cross-section with a 2 statistical error
• Even without b-tagging, with two days of data
  taking, can
• measure s at lt 10 (stat. error)

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• In Prague
• First evaluation of Mtop, assuming no b-tagging
  at the
• startup (V. Kostiouchine)
• Investigation of differences found in the
  combinatorial
• backgnd between TDR and DC1 (V. Kostiouchine)

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Mtop reconstruction in ATLAS at startup

• Work done by V. Kostioukhine
• Assumptions
• No jet energy calibration, no b-tagging.
• Uniform calorimeter response
• Good lepton identification.

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TDR signalbackgrounds estimation

• In case of no b-tag
• tt signal 500k evt ( 4 times reduction due to
  b-tag)
• Wjets 85k evt (50 times reduction due to
  b-tag)

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Signal selection without b-tag

• Lepton4jets exactly (DR0.4)
• e signal 76 with respect to ?4jet
• Wjets 83 with respect to ?4jets

Select among them 2 jets with maximal
jjj
jj
Select the 3-jet combination with maximal
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• Having 3 jets from t-quark decay,there are 3
  possible jet
• assignments for W(jj)b.

• A kinematical constraint fit can be used for a
  further selection MW1MW2 and Mt1Mt2. An
  approximate calibration is obtained with the W
  peak
• Select the combination with lowest ?2 out of the
  3 available. Event is accepted is this minimal ?2
  is less than a fixed value.

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Reconstructed Mtop
Big ?2 events
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• Signal selection e ( 4jets exactly?2 cut) 40
  (200k evt)

Wjets selection e with the same cuts 9 (8k
evt)
3-jet mass Wjets
?2 signal
?2 Wjets
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Preliminary results with full simulation
TDR top sample (same cuts as fast sim.)
W mass
Top mass
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W mass
DC1 sample (same cuts as fast sim.)
Top mass
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Conclusions on Mtop

• A tt signal can be selected without b-tagging and
  precise jet energy calibration
• Signal / backgnd ratio is 20 in this case (70
  in the region Mjjblt200 GeV) . Here only Wjets
  events are considered as background.
• Such a clean sample could be also used for jet
  energy calibration.
• Results confirmed by full simulation

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Combinatorial background in DC1 data

• Work done by V. Kostioukhine
• Increase of the combinatorial background in DC1
  samples
• with respect to the TDR ones
• Vadim checked better and.....

W(TDR)
W (DC1)
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TDR ?jets sample

• Selection 1 lep with Ptgt20 GeV, Pt miss gt20
  GeV, at least 4 jets with
• Ptgt40GeV, 2 b-jets (parton level). 2 non-b jets
  with minMjet-jet MW
• taken as W decay products. b jet is selected
  so that Pt jet-jet-b -gt max

jj mass
jjb mass
top
t-quark peak after application of constraint fit
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DC1 ?jets sample

• Same selection

DC1 sample with application of TDR-like
generation level cuts
DC1 sample
jjb mass
jj mass
jj mass
jjb mass
top
top
t-quark peak after application of constraint fit
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DC1 ejets sample

• Selection the same

DC1 sample with application of TDR-like
generation level cuts
DC1 sample
jjb mass
jj mass
jjb mass
jj mass
top
top
t-quark peak after application of constraint fit
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DC1 summary e,?jets sample

• Same selection

DC1 sample with application of TDR-like
generation level cuts
DC1 sample
jjb mass
jjb mass
jj mass
jj mass
top
top
t-quark peak after application of constraint fit
? agreement with TDR !!
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Next Steps

• More detailed MC study W jets background.
• Study of background level dependence on
  b-tagging e.
• Measure the cross-section and top mass
  assuming
• different efficiency for the b-tagging
  (and no b-tagging
• at all) and looking at various channels.
• What is the minimal b-tagging needed?

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First look at data in 2007

• Study of high pT isolated electrons and muons
• Select a standard top sample, and a
  golden top
• sample with tighter cuts.
• Try to reconstruct the two top masses (in
  single lepton
• events, one top decays hadronically, the
  other one
• leptonically)
• Take top events try a first measurement of
  the cross
• section, and of the mass in various
  channels (as a cross
• check, since systematic errors are
  different)

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? (tt) initial measurement dominated by L and
detector uncertainties ? 10-20?
In addition, very pessimistic scenario considered
b-tag not yet available ? S increases by 4 ?
S/B decreases from 65 to 6 ? large
combinatorial background
Still a top peak is visible Statistical error
from fit from 2.5 (perfect b-tag) to 7 (no
b-tag) for one week What about B systematics ?
difference of distributions for events in the top
peak and for events in the side-bands
Feedback on detector performance -- m (top)
wrong ? jet scale ? -- golden-plated sample to
commission b-tag
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• W ? jj t ? Wb ? (jj)b
• Fit of m(jjb) spectrum provides Xsect measurement
  with stat. error ? 7
• Even with no b-tagging, can measure ?(t-tbar) to
  lt 10 with two days of integrated luminosity at
  1x1033

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Conclusions

• An initial uncertainty of 5 on the b-jet energy
  scale, gives a top mass
• uncertainty of 3.5 for the mass reconstuction.
• If we go to 10 , the uncertainty on the top mass
  is of 7 GeV
• An initial uncertainty of 10 on the light jet
  energy scale, gives a top
• mass uncertainty of 3 GeV for the mass
  reconstuction.
• Kinematic fit less sensitive to light jet energy
  scale. But can have very
• large combinatorial background in case of
  b-tagging not working
• After 1 week of data taking we should be able to
  measure
• the cross-section with a 2 statistical error
• Even without b-tagging, with two days of data
  taking, can
• measure s at lt 10 (stat. error)
• Additional studies (e.g. di-lepton) undergoing