Bertrand Martin

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p17 tt? ee cross section p20 W helicity
prospects
Bertrand Martin for the LPSC group Dø France
Strasbourg 11/26/07
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Outline

• Status of the p17 ee selection
• success
• failures
• Around MET
• jet resolution
• Prospects for p20 W helicity
• improvements in the dilepton channel

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The dielectron signature

• The main physics backgrounds
• Drell-Yan Z/? jets ? ee / tt jets
• Diboson processes (WW , WZ , ZZ)
• Instrumental backgrounds (fake leptons and fake
  MET)

2e top_tight pT gt 15 GeV
pT(1) gt 30 GeV pT(2) gt 20 GeV
2 high pT jets

• Further cuts
• 15 lt Meelt 80 GeV ? MET gt 40 GeV
• or
• Mee gt 100 GeV ? MET gt 35 GeV
• Sphericity gt 0.15

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Status of stt
Preliminary results approved for Moriond E.W.
2007

• 16 observed events , 9.5 expected ttbar ( for s
  7 pb ) , 3.0 background
• No optimization w.r.t. p14 analysis
• Still statistics dominated. However, large
  systematics were also quoted
• EMID 5.5 per object
• (?det , Fdet) ? accounted for (scale factors)
• ?R(e,closest jet) , inst. lumi ? not corrected
  systematics

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EMID systematics
Assuming an efficiency depending on 3
variables e (?,F,?R)
(?,F) and ?R are independent variables
Efficiencies from the (?,F) spc files are
averaged over the ?R variation
Cant make a 3D .spc file
spc files may not be directly applied for ttbar
as the hidden variable is integrated over the
Zee spectrum
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EMID contd
Finally, after some more equations, one gets
Average reco ID efficiency of a Zee electron
Can be used for ttbar now

• Applying additional corrections on ttbar (?R ,
  lumi)
• does not change much the shape of the
  distributions,
• but rather the normalization 2.3 between
  full / (?,F)

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Post-Moriond life
Along with the final JES / SSR , came a new
version of ReComputeMET , with a right way to
propagate jet smearing to MET. See the effect
for ee 2 jets
Post-Moriond propagation
Before Moriond no propagation
Numerous attempts to explain the MET oversmearing
in vain Some people suspect that jet
smearing might be too large.
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Jet Resolution Or How to absorb Mikkos work in
1 week ?
I dont know
Mikkos code developed for JCCA with special JES
corrections Switch to JCCB and usual JES, to
compare jet resolutions in data and MC
SSR Compute asymmetries in a dijet sample
extract jet resolution
y lt 0.4
Of course, not so simple
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Jet resolution , bias corrections
Equivalent gaussian core resolution
Reco cut 6 GeV
pT imbalance expected at particle level ( missing
? , µ )
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Raw Jet Resolution in CC
Data from sgaussian
Data from RMS
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Raw Jet Resolution in CC
MC no SSR from sgaussian
MC no SSR from RMS
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Jet Resolution - status

• This study is at its very beginning
• Cant conclude anything yet
• low pT region driven by ksoft corrections
• needs further checks
• CC needs punch-through tails correction at
  high pT
• ICR needs the double gaussian structure
• EC needs kbias corrections
• Dijet sample ? access to high pT jets resolution
• ( fits constrained above 20 30 GeV )
• While dielectron jets rather low pT

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Now, the future
( Once we are done with p17 MET)
p20 prospects get involved in the W Helicity
measurement
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Extracting f with log(L)
ljets
Dilepton
ljets and dilepton results are marginally
consistent (2.4s stat) ? Assumed it originates
from a statistical fluctuation proceed to
combination What about adding another 1 fb-1 in
each channel
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p17 W helicity
ttbar multivariate selection likelihood
discriminant Reconstruction of the 2 W
leptonic decays ( to find cos? ) 2 neutrinos
unknown momenta kinematically
underconstrained Algebraic resolution
needs mass assumptions ( MW , Mtop )
lepton / b jet pairing ambiguity
More powerful approaches Decorrelated
likelihood , N.N. , Boosted decision trees
Also room for improvement (see next slides)
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Kinematics
6 final state particles (2b , 2l, 2v) 3
independent variables ( px , py , pz ) 12
directly measured quantities ( 2b and 2l ) 2 MW
2 Mtop assumptions Last constraints in p17
18 unknowns
? 2 unknowns left
Quadratic equations give several solutions for
neutrino momenta
p17 way of picking up the right solution
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Alternative approach
Try an approach a-la neutrino weighting ( top
mass analysis )
Abandon the METx,y constraints

• Make an assumption on the neutrinos rapidities
  gaussian
• ? Enough constraints now
• Compute the neutrinos momenta
• Then one can compare the expected METx,y with
  the observed MET (weight w)

Should give better results than the current
average ?
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Summary
Preliminary p17 cross section measurement (1.04
fb-1)
OLD

• Main analysis improvement EMID systematics
• BUT
• Cross section analysis stuck due to the MET
  oversmearing
• ? Waiting for the jet resolution results
• Optimization is the next step
• Publication
• W helicity
• the end

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Backup slides
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Fake electron background

• Fake electron jet faking electron, or a
  non-isolated electron (b decay).
  Includes QCD, Wjets ? ljets, ttbar? ljets
• Signal/fake separation electron likelihood (1
  for signal, 0 for background)
• 2 steps
• Determination of likelihood shapes for
  signal-like background-like samples
• Likelihood fit to extract the background
  contamination in the analysis sample.

Estimated from data
Signal sample Z?ee (incl)
Bkg sample (incl)

• Signal sample Z?ee , METlt15
• Background sample same topology as the
  analysis sample, but require
• 1 anti-isolated electron , MET lt 15

Likelihood fit determine ne and ns
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Fake MET background

• Main background Z/? jets ? ee jets
• no real MET (no ?) but fake MET can appear
  due to resolution effects
• MET may not be accurately modeled in the MC
• Fake rate measurement
• sample kinematically close to Zjets , with no
  real MET ? jets sample

Estimated from data
Major impact of jets on MET

• Event yield calculation from analysis sample
  in the low MET regime (reversed MET cut)

Yield in the low MET regime
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tt cross section
The cross section is extracted with a likelihood
minimization
where
The statistical error on s arises from the
likelihood minimization procedure
Compared to p14 results (425 pb-1)
Preliminary p17 result (1.04 fb-1)
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Z pT 0 jet
Parameterization with generated Z pT
Z pT 1 jet
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Z pT 2 jets
jets
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Jet 1 pT
Jet 2 pT