W polarization study in ttbar semi-lep decay

BoLi

Shandong University

W polarization in top decay Test the top decay (in fully reconstructed tt) with W polarization ...

Standard Model (Mtop=175 GeV ) Longitudinal W+ (F0) Left-handed W+ (FL)

Right-handed W+ (FR)

LO 0.703 0.297 0.000

...by measuring angular distribution of charged lepton in W rest frame

222

0 2

cos1

2

cos1

2

sin

2

3

cos

1RL FFF

d

dN

N

1/2W+b t

spin

Angle between: •lepton in W rest frame and W in top rest frame

Data

MC@NLO tt samples:mc08.105200.T1_McAtNlo_Jimmy.merge.AOD.e357_s462_s520_r809_r838

AcerMC tt samples:mc08.105205.AcerMCttbar.recon.AOD.e429_s462_s520_r808mc08.105205.AcerMCttbar.merge.AOD.e429_s462_s520_r808_r838

MC08 background samples(s520_r808_r838): w+jets (Alpgen 106280-3,107682-5,107692-5,107702-5) z+jets (Alpgen 107652-5, 107662-5,107672-5) dibosons(Herwig 105985-7) single top(AcerMC 105500,105502)

New data sample (re-reconstructed MC08 with rel. 15.3.1.6) and rel.15.6.0 for analysis

event selection68%(64%) =1 lepton isolated |eta|<2.5pT>20GeV for μ

pT>25GeV for e

90%(90%)etmiss>20GeV

57%(57%) ≥ 4 jets pt>30GeV |eta|<2.5

33%(33%)≥ 2 bjets btagged with jetprob<0.014@eff~60%

Final kinematic cut efficiency 11.5(11.1)%

Event reconstructionwhad = 2 jets

(mass closest to 80.4GeV)

Possible wlep = l +ν (constraint of Mw=80.4GeV)

thad = whad + bjet

(mass closest to 175GeV)

tlep = bjet + wlep

(bjet dR closest to lepton)

(mass closest to 175GeV)

reconstruction efficiency 3.39%(3.14%)

Plus quality cut ( |Mwhad-80.4 |<20 and |Mt – 175|<35 )

DataTotal integral luminosity 700 Pb-1

After selectionProcess Run# Req x-sec (pb) Req # events

tt (non-hadronic) AcerMC 105205 218.37 152856

Signal 2686

Tau background 228

Single top Wt 105500 14.27 9986 24

Single top t-chan 105502 40.59 28414 31

Unfiltered W(e/m/t+nu)+(2/3/4/5)parton 3501.69 2451185 9

Z(ee,mm,tt)+(0/1/2/3/4/5) parton 4324.51 3027157 0

Wbb+(0/1/2/3) parton 17.86 12502.56 7

WW 105985 26.40 18478.4 0

ZZ 105986 1.93 1347.86 0

WZ 105987 8.82 6172.82 0

Total 5708100 s/b =9.0

Selection cuts and reconstruction distort the parton lever distribution Use an independent sample to correct this distortion.

Measurement method

parton level

Reconstruction and cuts

Correction function :

Measurement method

The tau events not correctly reconstructed as the neutrino from tau decay contaminate the neutrino from w decay

Taking e/mu/tau as signal instead of e/mu

at parton level distribution

W polarization e/mu/tau as signal

e/mu as signal

F0 0.681 0.690

FL 0.315 0.308

FR 0.004 0.003

Truth values get from parton level distribution

Result Function fit for w polarization

Measured value with statistical error:

FL= 0.281± 0.023

F0= 0.754± 0.042

FR=-0.034± 0.023

Result

FL F0 FR

Theoretical error 0.02 0.02 0.01

Bjet energy scale (5%) 0.018 0.010 0.008

Btag efficiency (5%) 0.017 0.013 0.005

Systematic error

ResultCorrection function use MC@NLO tt sample

F0 FL FR

Truth 0.697 0.301 0.002

Measured

0.614 0.332 0.054

Summary Use new samples and software at r15 The selection efficiency increase from 11.1% to 11.5%

reconstruction efficiency increase from 3.14% to 3.39%

s/b from 8.6 to 9.0

Taking e/mu/tau as signal result matched well with theoretical value