From W polarization to tWb anomalous couplings

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From W polarization to tWb anomalous couplings
F. Hubaut, E. Monnier, P. Pralavorio (CPPM)

1. W polarization with semileptonic tt events
2. Sensitivity to tWb anomalous couplings
3. Conclusions and perspectives

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1. W Polarization in tt semilep (see presentation
Feb.05)
SB, 10 fb-1
Results (stat syst) Standard Model
F0 0.699 ? 0.005 ? 0.023 0.703 0.002 (Mtop-175)
FL 0.299 ? 0.003 ? 0.028 0.297 - 0.002 (Mtop-175)
FR 0.002 ? 0.003 ? 0.013 0.000
Fit results
F00.699 0.005 FL0.299 0.003 FR0.002 0.003

• ATLAS can measure W polarization component F0
  with an accuracy 3 and FR with a precision
  1.3 in 1 LHC year (10 fb-1)
• Measurements largely dominated by systematic
  uncertainties

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1. Measurement of aW, AFB
Information which can be derived from the W
polarization
1. Spin analyzing power of the W (aW) in the
polarized top decay
with qW angle between W and top spin polarization
(12 accuracy)
2. Forward Backward Assymetry (AFB), related to
the angle between the charged lepton and the
b-jet in W rest-frame (PRD67(2003) 091501)
(9 accuracy)
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2. Sensitivity to tWb anomalous couplings (1)
One of the main motivation for top physics
search for anomalous interactions

• BR (t?Wb) 100 in SM

• Many models beyond SM ? study the tWb vertex in a
  model independent approach, i.e. effective
  Lagrangian (PRD45 (1992), 124)

)
and 4 couplings (in SM LO
using W polarization
Goal determine sensitivity to
measurement (insensitive to )
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2. Sensitivity to tWb anomalous couplings (2)
Express F0, FL and FR as a function of
(hep-ph/0503040) and assume a
variation of each coupling independently
1s
1s
1s
From uncertainties on FR and F0 ? sensitivity to
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2. Sensitivity to tWb anomalous couplings (3)
statsyst --- stat (10 fb-1)
Coupling
2s limit (statsyst) 0.31 0.14 0.07
10 fb-1
Best sensitivity to (linear behavior)
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2. Sensitivity to tWb anomalous couplings (4)
Comparison with other expectations (2s limit)

Our study (low lumi,tt) 0.31 0.14 0.07
Tev (Run II, tt) 0.5 0.3 0.3
Tev (Run II, single top)1 ?? 0.35 0.25
LHC (High Lumi, single top)2 ?? 0.07 0.13
1 2 fb-1, assuming a 10 systematic
uncertainty 2100 fb-1, assuming a 5 systematic
uncertainty
Preliminary studies
Remark very stringent limits (lt 1) exist with
measurement using b?sg, but indirect and Standard
Model dependent
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3. Conclusions - Perspectives

• Conclusions
• W polarization in top decay is measured precisely
  at LHC (10 fb-1) with tt semileptonic events
  (clean signature, high stat, high S/B) ? search
  for anomalous couplings in tWb vertex (model
  independent approach)
• Sensitivity to these couplings 2-4 times better
  than Tevatron RunII and competitive with single
  top at LHC (100 fb-1,high luminosity)
• ATLAS note on W polarization in the semilep
  tt channel written (ATL-COM-PHYS-2005-015)

• Perspectives
• Include this study in the scientific note and
  combine with dileptonic tt
• Similar approach will be used for gtt vertex
  using tt spin correlation

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Spare Parton level
TopReX, parton level
F00.704 0.001 FL0.296 0.001 FR0.000
0.001
Fit results with 2 parameters (constraint
F0FLFR1)
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Spare actual limits on W pola
F0, FL, FR fractions of longitudinal, left and
right-handed polarized W
F0 FL FR
Standard Model (Mtop175 GeV) 0.703 0.297 0.000
VA component almost unchanged 0.297 ? 0.00 0.00 ? 0.297
Measurement or actual limit F00.89 ? 0.30 ? 0.17 (Tev. run II, 162 pb-1) FR lt 0.18 _at_ 95 CL (Tev. run I , 109 pb-1) FR lt 0.01 from b s ? (CLEO, BELLE, BABAR) ? indirect limit FR lt 0.18 _at_ 95 CL (Tev. run I , 109 pb-1) FR lt 0.01 from b s ? (CLEO, BELLE, BABAR) ? indirect limit
ATLAS study with single top (M. Lefebvre et al)
stat. error 1-1.5 (30 fb-1)
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Spare Event simulation
Event simulation is performed using

• TopReX 4.05 or AcerMC 2.2 or AlpGen 1.33
  production and decay of tt pairs including spin
  effects
• Pythia 6.2 or Herwig 6.5 hadronisation,
  fragmentation and decay
• Tauola Photos t lepton decay and radiative
  corrections
• Atlfast 2.60 fast ATLAS simulation
  reconstruction
• using CTEQ5L structure function, ISR-FSR, no
  pile-up.

1 year of statistics (10fb-1, 3.8 Mevents)
simulated for each generator and each
hadronization scheme
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Spare Selection criteria

• pT and h cuts
• 1 lepton pT gt 20GeV (?lt2.5) PTmiss gt 20GeV
• 1 b-jet pT gt 30GeV (?lt2.5)
• 2 non b-jets pT gt 30GeV (?lt2.5)
• 1 b-jet pT gt 30GeV (?lt2.5)
• Event topology reconstruction
• Other cuts
• lMwREC - Mwl lt 20GeV
• lMtREC - Mtl lt 35 GeV

LEPT.
HAD.
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Spare Event selection and reconstruction

• Exactly the same as in spin correlation study
• Only exception cut on tt invariant mass, used to
  enhance spin correlation, has no meaning here
  ? not performed

• e(sig)4.5, 115000 events per 10 fb-1
• main background tt? t X (S/B12)

• Selection cuts will distord the parton level
  distribution
• Leptons from longitudinal W harder than those of
  left-handed W, and softer than those of
  right-handed W
• Biases extracted parameters towards higher
  average W helicity

mandatory to perform a correction
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Spare Measurement method
In the same way as in spin correlation study, use
MC generator to parametrize reconstruction cuts
effects
parton level, no ISR/FSR
Correction function
Fit with 3rd order polynom in range -0.90.9
Reconstruction and cuts
apply weights event by event
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Spare Systematics (1)
b-jet miscalibration expected behaviour
positive miscalibration ? Mlb? ? cos ? ? ?
FR? FL ?
Top mass uncertainty expected behaviour
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Spare Systematics (2)
TopReX PYTHIA Alpgen PYTHIA AcerMC
PYTHIA AcerMC HERWIG CTEQ6L MRST 02 GRV
98 ISR FSR b-frag. eb-0.0035 b-tag.
efficiency 55 65 b-jet.
miscalib. -3 3 light-jet.
miscalib. -1 1 Top mass
173 GeV 177 GeV 2.3 pile-up events