Top at startup of LHC

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Top at startup of LHC

• Stan Bentvelsen (NIKHEF)
• October 2nd, 2004

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Top commissioning studies

• Top physics at LHC
• Top one of easiest bread and butter
• Cross section 830100 pb
• Used as calibration tool
• Rich in precision and new physics
• Top mass Mt, cross section st
• Resonances decaying into top
• Commissioning for the top group
• Summarize studies already performed
• Tasks to do before startup
• What do we need as input from others?
• What can/should we provide to collaboration?
• Goals

Semi-leptonic top channel detector tools
involved Lepton reconstruction Missing ET Jets
calibration B-tagging
Work done together with Marina Cobal
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Initial commissioning studies

• First evaluation of statistical uncertainty on
  stop and Mtop

• Gold-plated channel single lepton
• pT (lep) gt 20 GeV
• pTmiss gt 20 GeV
• 4 jets with pT gt 40 GeV
• 2 b-tagged jets
• mjjb-ltmjjbgtlt 20 GeV

P. Grenier
Period evts dMtop(stat)
1 year 3x105 0.1 GeV
1 month 7x104 0.2 GeV
1 week 2x103 0.4 GeV

• For initial run at LHC
• (L 1033 cm-2s-1)
• Pretty small uncer-tainties after very short
  time of LHC running!

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Systematic error on Mtop (TDR performance, 10
fb-1)
Initial performance uncertainty on b-jet scale
expected to
dominate
Cfr 10 on q-jet scale ? 3 GeV on Mtop
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Various scenarios currently under study

• pp collisions
• What variations in predictions of t-tbar which
  generator to use?
• Underlying event parameterization
• Background estimation from MC
• Try to be as independent from MC as possible.
• Detector pessimistic scenarios
• Partly or non-working b-tagging at startup
• Dead regions in the LArg
• Jet energy scale
• Get good feel for important systematic
  uncertainties use data to check data
• Software tools
• Many studies (not all!) only in fast simulation
• It is clear we need to redo most important
  studies with full simulation
• Estimate realistic potential for top physics
  during the first few months of running

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Status of top event generators

• Old Leading Order MC
• Pythia full standalone MC
• Herwig full standalone MC
• TopRex (include spin correlations interfaced to
  Pythia)
• New NLO QCD calculations implemented in MC
• MC_at_NLO interfaced to Herwig shower and
  fragmentation
• This is relevant theoretical improvement
• Superseeds the old Pythia and Herwig MCs.
• Validation done for this generator
• Currently DC2 processes 106 MC_at_NLO t-tbar events
• Crucial for us to analyse these
• Waiting for Tier0 exercise to obtain
  reconstructed objects

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Generators MC_at_NLO, Herwig, Pythia
Example distributions on top-anti-top
characteristic PT of the whole system PT of
t-tbar system is balanced by ISR FSR

• PT(tt system)
• Herwig MCatNLO agree at low PT,
• At large PT MCatNLO harder
• PYTHIA completely off

Many more comparisons see talks in top meeting
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Underlying event (UE) / minimum bias

• Extremely difficult to predict the magnitude of
  the UEat LHC
• Will have to learn much more from Tevatron before
  startup
• Various models exists
• Herwigs UE and minimum bias shows much less
  activity compared to Pythia.
• This has always been a problem in Herwig.
• Jimmy is developed as alternative model for UE at
  ep collisions
• Various tunings exist leading to wildly
  different results
• More work is mandatory here
• Wish list to generate fully simulated events with
  Jimmy during DC2 post-production

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Tune of Butterworth
Standard Jimmy
Standard Pythia
Standard Herwig
Running at LHC energies
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Jimmy UE Cells Jets in Atlfast

• Herwig vs Jimmy
• LO t-tbar
• At jet-level effectreduced

Cell multiplicity
Cluster multiplicity
Jet multiplicity
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Reconstruct the top

• Top peak for various reconstruction methods
• Difference in mass can be as large as 5 GeV
• Really need data to check data on UE
• Study effect better (as said)

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Background events

• Top physics background
• Mistags or fake tags
• Non-W (QCD)
• Wjets, Wbbar, Wccbar
• Wc
• WW,WZ,ZZ
• Z ? tt
• Single top
• AlpGen W4 jets samples produced
• Very CPU intense (NIKHEF grid)
• Un-weighting to W lepton (e,?,?) decay
• Production
• Effective ? 2430 pb
• 380740 unweighted events generated (2.6 10-5
  efficiency)
• 3.41 (13002) events pass first selection
• 150 pb-1 W4jet background available

Largest background is W4 jet. This background
cannot be simulated by Pythia or Herwig shower
process. Dedicated generator needed e.g. AlpGen.
Large uncertainties in rate Ultimately, get this
rate from data itself. For example, measure Z4
jets rate in data, and determine ratio (Z4
jets)/(W4 jets) from MC
W4 extra light jets Jet Ptgt10, ?lt2.5, ?Rgt0.4
No lepton cuts Initial grid 2000003 Events
150106 Jobs 98
1.5 1010 events!
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Non-W (QCD-multijet) background

• Not possible to realistically generate this
  background
• Crucially depends on Atlas capabilities to
  minimize mis-identification
  and increase e/? separation
• This background has to be obtained from data
  itself
• E.g. method developed by CDF during run-1
• Rely now on e/? separation of 10-5

Use missing ET vs lepton isolation to define 4
regions A. Low lepton quality and small missing
ET Mostly non-W events (i.e. QCD background) B.
High lepton quality and small missing
ET Observation reduction QCD background by
lepton quality cuts C. Low lepton quality and
high missing ET W enriched sample with a
fraction of QCD background D. High lepton quality
and high missing ET W enriched sample, fraction
of QCD estimated by (BC)/(AD)
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Detector scenarios HV problems

• Effects of dead HV regions om Mtop
• Argon gap (width 4 mm) is split in two half
  gaps by the electrode
• HV by D? x Df 0.2 x 0.2 (or 0.1 x 0.2) sectors,
  separately in each half gap
• 33 / 1024 sectors where we may be unable to set
  the HV on one half gap ? multiply energy by 2 to
  recover

particle
A.I. Etienvre, J.P. Meyer, J.Schwindling
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Analysis
EM clusters

• 100 000 tt events ( 1.5 days at LHC at low L)
• Simulated using PYTHIA ATLSIM
• / G3 (initial detector, h lt 3.2)
• Reconstructed using ATHENA 7.0.0
• Preselection of events
• At least one recontructed e or µ
• with PT gt 20 GeV and ? lt 2.5
• ETmiss gt 20 GeV
• At least 4 jets with PT gt 20 GeV and h lt 2.5

Jets
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Results

• If the 33 weak HV sectors die (very
  pessimistic), the effects on the top mass
  measurement, after a crude recalibration, are
• Loss of signal lt 8
• Increase in background not studied
• Displacement of the peak of the mass
  distribution -0.2 GeV

Mtop(without ) mtop(with dead regions)

• This effect on the Top mass is (much) better
  known than other systematic uncertainties

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Detector scenarios b-tagging

• Precise alignment of ID can be reached only after
  few months of data taking.
• The impact of misalignment can be much larger
  than having 2 instead of 3 layers
• Top events to evaluate b-tagging efficiencies
  from data
• Select a pure t-tbar sample with tight
  kinematical cuts
• Count the number of events with at least 1 tagged
  jet
• Compare 0 vs 1 vs 2 b-tagged jets in top events
• Can expect the b-tagging efficiency different in
  data from MC
• In most pessimistic scenario b-tagging is absent
  at start
• Can we observe the top without b-tagging?

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Non b-tag tops
t ? bjj
M (bjj)
V. Kostiouchine

• Selection
• Isolated lepton with PTgt20 GeV
• Exactly 4 jets (?R0.4) with PTgt40 GeV
• Reconstruction
• Select 3 jets with maximal resulting PT

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Non btag top sample

• Most important background for top W4 jets
• Leptonic decay of W, with 4 extra light jets

• Signal plus background at initial phase of LHC

With extreme simple selection and reconstruction
the top-peak should be visible at LHC
L 150 pb-1 (2/3 days low lumi)
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Extraction of top signal

• Fit to signal and background
• Gaussian signal
• 4th order polynomal Chebechev background
• In this fit the width of top is fixed at 12 GeV

150 pb-1
Extract cross section and Mtop?
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Can we see the W? (4 jets sample)

• Select the 2 jets with highest resulting PT
• W peak visible in signal
• No peak in background
• Better ideas well possible!
• E.g. utilizing 2 body decay in top rest frame.

150 pb-1

• Select 2 jets with invariant mass closest to Mw
  (80.4 GeV)
• Large peak in background
• Enormous bias
• Not useable!

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Fit to W mass

• Fit signal and background also possible for
  W-mass
• Not easy to converge fit
• Fix W width to 6 GeV

150 pb-1 mean s(stat)
? in peak 3.0 5
Mtop 167.0 0.8
Mw 77.8 0.7
These numbers for statistical uncertainties are
consistent to the earlier study
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Jet Energy scale / MC dependence

• Variation of the jet energy scale to infer
  systematics
• Bjet scale 0.92 0.96 1.00 1.04 1.08
• Light scale 0.94 0.98 1.00 1.02 1.04
  
  
• (1) (2)
  (3) (4) (5)
• Analysis with jet energyscaled
• All with MC_at_NLO, Herwigand Pythia
• Redo analysis with doubled W4jet background
  (stat indep)

Determine Mtop and s(top) Raw, i.e. no
correction for jet scale Corrected, i.e. apply
percentage difference of W-peak to the
reconstructed top Dependence on top mass reduced
by scaling with W Rms Raw 6.2 GeV Rms
Scaled 1.2 GeV Large dependence s(top)
on jet energy scale Via event selection.
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Some results (still no b-tag)

• Using 150 pb-1 of data
• Statistic uncertainty already smaller than these
  systematic variations
• Note these numbers are very preliminary
• Luminosity uncertainty (15-20) to be added!

• How to judge these values?
• Systematics overestimated
• since all generators are used, with all energy
  scales double counting
• W4jets rate can be measured from data
• Systematics underestimated
• No real FSR variation
• No other backgrounds(e.g. WW, QCD)
• Trigger
• Non-uniformities
• Need further detailed studies!
• Please dont thrust any of this without
  Full simulation

    mean Stat std percent
Mtop raw 168,1 0.8 6,2 3,7
  corrected 171,9 0.8 1,2 0,7
         
s(top) raw 817,2 5 94,8 11,6
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Lower luminosity?

• Go down to 30 pb-1
• Both W and T peaks already observable
• See something!

30 pb-1 mean s(stat)
? in peak 0.8 17
Mtop 170.0 3.2
Mw 78.3 1.0
30 pb-1
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Fit the leptonhadronic top

• Full kinematic fit to t-tbar system no
  b-tagging
• Fit the neutrino Px and Py - and get Pz via W
  constraint
• Use W-mass constraints
• Require equal top masses for lepton and hadron
  side
• Repeat fit over all possible combinations of jets
• Better suited for mass than for cross section
• Looks promising but further study needed

Background shows structure
Cut on quality of fit (X2)
E. Bos
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What with b-tagging on?

• Now assume full b-tagging
• Efficiency 60, mistag 1
• Background is rapidly decreasing
• See for example the W-mass peaks for 1 and 2
  b-tags
• Same selection 4 jet events
• W reconstructed as dijet mass Mjj-80.4 minimal
  for light-jets j

150 pb-1
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Reconstruct top mass

• Sharp top mass peaks with little background
• Only use events for which Mjj-80.4 lt 20 GeV
• Standard kinematic top reconstruction for 1 and 2
  b-tags
• Background from W4jets removed by b-tag
  requirement
• These results are very sensitive to b mis-tag rate

150 pb-1
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Mtt at startup?

• Can we determine ds(tt)/dmtt without b-tagging?
• Interesting to SuSy models that modify this cross
  section
• Mtt is invariant mass over all final state to tt
  products
• In principle no assignment of b-jet to top is
  needed
• Suffer a lotfrom background
• No reliable measurement of mtt without
  b-tagging

W4jet background
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Resonances decaying to t-tbar

• Observe heavy resonances X?t-tbar during
  commissioning?
• Plot invariant mass of 4 jets lepton neutrino
• No intelligence in determining Pz neutrino here
• Insert resonance at 2000 GeV
• Cross section BR(tt) 350 pb

• Heavy resonances with large cross sections
  visible

True mtt distributions of resonance and ttbar
events
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Need checks with full simulation

• We are eagerly waiting for reconstructed DC2
  events
• Repeat with full simulation for Rome next year

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Top group in Atlas

• What we have to provide
• Top candidates enriched samples
• A pure one, obtained with quite tight selection
  criteria
• A loose one a more background enriched
  sample, to be used as control sample for
  background calculations etc
• Estimate of a light jet energy scale correction
• Assume ?10 for light and b-quark jets, look at
  effect on Mtop and stop
• Assume that at the very beginning only the EM
  scale is known (means do not put any weight on
  the hadronic scale)
• Output provide the MW peak to rescale the light
  jets
• Estimate of the b-tagging efficiency

• Inputs to the top group
• Estimate of the single electron trigger
  efficiency
• Can be done by using the Z triggered as single
  electron
• How much time is needed to arrive to a reasonable
  evaluation of this efficiency?
• Estimate of the initial lepton identification
  efficiency
• Estimate of the integrated luminosity
• At the beginning the precision on L should be
  around 10-20.
• The ultimate precision should be lt 5
• Eventually
• B-tagging efficiency
• Jet scales

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Summary / Conclusion

• Understand the interplay between using the
  top signal as tool to improve the understanding
  of the detector (b-tagging, jet E scale, ID,
  etc..) and top precision measurements
• At LHC Top more easily found than Ws in 4 jet
  channel
• Using extreme simple selection, no b-tagging
• Need more work on background estimation, both
  from Ws and QCD, e/? ratio, trigger, lepton ID,
  etc
• Remove dependence of results on MC as much as
  possible
• Using few days of data taking (150 pb-1)
• Current estimate on cross section accuracy of
  (ball park) 10 plus luminosity uncertainty
• Interestingly mass of top via fits to mass peak
  looks promising(use W-mass as constraint to
  jet-scale)
• Need better ideas to isolate very pure top sample
  without b-tagging
• Its clear we need full simulation
• Eagerly awaiting reconstructed DC2 events to
  repeat/extend these studies