W polarization in semileptonic tt events

1
W polarization in semileptonic tt events
F. Hubaut, E. Monnier, P. Pralavorio (CPPM)

1. Motivations
2. SM expectations and actual limits
3. Observables and measurement method
4. Systematics Final results
5. Beyond SM
6. Conclusions and perspectives

2
Motivations

• Main conclusions of our study on spin correlation
  (ATL-COM-PHYS-2005-002)
• Precision of 15 on SM parameter can be reached,
  setting the limit of sensitivity to physics
  beyond SM
• Test of top production and decay if deviation
  from SM, both processes have to be tested
  independently
• A direct test of the Wtb top decay vertex is
  provided by the measurement of the W polarization
  
• Direct test of the V-A structure
• Search for new physics (e.g. VA component)
• Use the SAME semileptonic tt events

Never done in ATLAS (only single top)
3
Helicity of W from top decay in the SM
b spin 1/2
Top Standard Model weak decay ? V-A coupling as
for all fermions
V-A
t spin 1/2
W spin1
b
W
W
Suppressed by V-A coupling
t
t
t
(mb0)
W
b
b
Left-Handed W
Longitudinal W
Right-handed W
SM only left-handed and longitudinal W may be
produced
(For W- Left and Right are inverted. Use W
terminology in the following)
4
SM expectations and actual limits
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)
5
Observable
W polarization is measured through angular
distribution of charged lepton
? angle between lepton in W rest frame and
W direction in top rest frame
Left-handed W
Longitudinal W
Right-handed W
l (1/2)
l (1/2)
? (-1/2)
W
W
W
W direction of flight
? (-1/2)
? (-1/2)
l (1/2)
l opposite to W (? p)
l transverse to W (? p/2)
l same as W (? 0)
6
Parton level
TopReX, parton level
F00.704 0.001 FL0.296 0.001 FR0.000
0.001
Fit results with 2 parameters (constraint
F0FLFR1)
7
Related observables

• In the Mb0 approx.
• Mlb2 (lepton-b invariant mass) simpler
• assume Mt175 GeV in the formula
• can even look at the lepton PT

Tevatron studies

• not necessary to reconstruct the whole event
  topology
• high systematics due to jet energy scale and top
  mass

• cos ?
• use the reconstructed top mass in the formula

• have to reconstruct the whole event topology
• lower systematics due to jet energy scale and
  top mass

We tested both observables. The second one gives
a final total systematics 2 times lower.
8
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
9
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/B15)

• 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
10
Measurement method (1)
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
11
Measurement method (2)
TopReX, selection recons. correction
F00.699 0.005 FL0.299 0.003 FR0.002
0.003
Compatible with parton level

• Error due to the method
• Vary number of bins
• Vary fit limits
• Vary correction polynom order

Changes are within the fit errors
12
Systematics (1)
MC level
Reconstruction level

1. Parton generation
2. Structure function
3. ISR, FSR
4. b-fragmentation
5. Hadronization scheme

1. Input top mass
2. b-tagging efficiency
3. b-jet mis-calibration
4. Light jet mis-calibration

Others

1. Pile-up
2. Background level

13
Systematics (2)
b-jet miscalibration expected behaviour
positive miscalibration ? Mlb? ? cos ? ? ?
FR? FL ?
Top mass uncertainty expected behaviour
14
Systematics (3)
TopReX PYTHIA Alpgen PYTHIA AcerMC
PYTHIA AcerMC HERWIG MRST 98 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
15
Results
W polarization for SB at 10 fb-1 (stat syst)
Results Theory Mtop175 GeV ?Mtop
F0 0.699 ? 0.005 ? 0.024 0.703 ? 0.002 ?Mtop
FL 0.299 ? 0.003 ? 0.028 0.297 0.002 ?Mtop
FR 0.002 ? 0.003 ? 0.014 0.000
16
Beyond SM VA component (1)
The correction we used is extracted from a V-A
hypothesis (70 FO - 30 FL)
V-A
Correction
VA
Correction changes in case of VA component ?
bias expected
17
Beyond SM VA component (2)
Introduce a VA component in the Wtb vertex (with
AlpGen)
Pure V-A
Pure VA
Pure V-A
Pure VA
More general correction (V-A/VA)
SM correction function
Input FR
Input FR

• Small bias (expected) due to correction function
  
• Sensibility FR lt 0.05 _at_ 95 C.L. in 1 LHC year
• Tevatron run I FR lt 0.18 _at_ 95 C.L.
• Tevatron run II expectations (2 fb-1) FR lt
  0.09 _at_ 95 C.L.

dilep. semilep.
18
Conclusions - Perspectives

• Interesting study, very complementary to spin
  correlation
• Same events, same reconstruction easy!
• ATLAS can measure W polarization components F0
  and FL with a precision 2.5 and FR 1.4 in 1
  LHC year (10 fb-1)
• Measurements dominated by systematics

• Perspectives
• ATLAS note in preparation
• Include this in the scientific note we began to
  write ?

Spin studies in tt pairs
19
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 and lMtREC - Mtl lt 35 GeV

LEPT.
HAD.

• e(sig)4.5, 115000 events per 10 fb-1
• main background tt? t X (S/B15)