1. W – boson Helicity of top quark decay
Qasim Ali
&
Sobia khanum
National Centre for Physics

2. Outline Top Quark: W–boson Helicity: Introduction of top quark
Production of top quark
Decay of top quark
W–boson Helicity:
What is Helicity?
Fraction of the Helicity
Decay angle
How to measure the Helicity?
Helicity in the Standard Model
Distribution of the decay angle
Measurement method of cosθ
Flow chart of my work
Histograms

3. Introduction What’s Special about this one?
A set of Fermions and force carriers which is described by the Standard Model.

4. Introduction All the spin information of the top quark:
transfer to its decay products
Like all quarks the top quark is an
elementary fermion with spin - 1/2.
It almost exclusively decays to a W boson and bottom quark, and a strange quark and on the rarest of occasions, into a down quark.
Discovered in 1995 by the CDF and D0 exp at Fermilab.
The top quark charge is +2/3 e and is the most massive Particle in all observed elementary Particles.
The SM predict its life time to be
0.5 × s.
It’s the only quark that decay
before the hadronization.
life time ≈ s.
Hadronize time ≈ s

5. Introduction V CKM = twb b Top quark
Top quark decay: bottom quark and W strange quark and W down quark and W. W
d s b
b spin =1/2
W+ spin=1
t spin =1/2
V-A u c t
Vud Vus Vut
Vcd Vcs Vcb
Vtd Vts Vtb -¡
V CKM =
From CKM matrix it is clear that the probability of the decay of the top into b is 99%

6. Top quark production at Tevatron and LHC
The pair-production of the top quark was
discovered in 1995 at the Fermilab
Tevatron by proton- antiproton collider.
Centre of mass energy is √s =1.96 TeV
& cross section is 6.7 pb.
The dominant process in the Tevatron
quark–anti quark annihilation.
qq-bar  tt-bar (85%)
gg tt-bar (15%)
At LHC the collision occur between
proton –proton.
At centre of mass energy of √s =14 TeV
& cross section 833 pb.
The dominant process is
gluon –gluon fusion.
qq-bar  tt-bar (10%)
gg tt-bar (90%)
pp-collision
At Tevatron Collider
pp -collision
At LHC Collider

7. Top Quark Decay Channels
Depending on the W decay modes the top quark decay can be classified into three channels.
The Dileptonic channel
The semileptonic channel
The hadronic channel

8. Dileptonic Decay Channel
If both W bosons decay into a pairs
of the leptons and its relative
neutrino then such type of decay
mode is called the Dileptonic channel.
Since τ leptons are difficult to identify.
(tt →W+W-bb → lѴ lѵ bb)
The Dileptonic channel has low branching ratio
but a high signal to background ratio . μ- e
leptons
Where l τ

9. Hadronic Channel
If both W boson decay into jet pairs, we have a total of six jets, then this is called the “Hadronic decay”.
All hadronic channels have highest
branching fraction but is very
difficult to distinguish from background.
(tt → W+W-bb → j1 j2 j3 j4bb).
j1,j2
q1,q2
Where j1,j2,j3,j4
quarks
j3,j4
q3,q4

10. Semi Leptonic channel If one W boson decay into an electron-neutrino
or a muon neutrino pair, while the other
decay into two jets, we have four jets and a
lepton-neutrino then such type of decay
mode is called the Semileptonic channel.
The semileptonic channel is also called
“Golden Channel "for top pair analyses due
to high branching ratio &presence of high
transverse momentum leptons.
(tt → W+W- b+b- → lѵ j1 j2 bb)

11. Branching Ratio Channel b- jets Non-b jets Charged leptons Neutrinos %
Dileptonic 2 5%
Semileptonic 1
30%
Hadronic 4
44%

12. W± Boson Life time → ≈ 3 × 10 -25 s Composition → elementary particle
Statistics → Bosonic
Interaction → weak interaction
Theorized → Glashow,Weinberg,Salam(1968)
Discovered →
Mass → W:80.385±0.015 GeV/ c2
Electric charge → ± 1e
Spin →
Life time → ≈ 3 × s

13. Helicity
Def: The Helicity is the projection of the spin “S” of a particle on
the motion of the particle “P”. H
Stand for Helicity ˆ
H Ξ S.P S
Spin ˆ
S.P ˆ
Direction of motion
Of the particle P
There are three Helicity states :
In above figure simple (open) arrow denote particle direction of motion (spin).

14. Polarization state reflected in the angular distribution
W direction
W direction
W direction
Right-handed W
Left-handed W
Longitudinal W

15. Decay angle
Def: The measurement method uses the cosine of the decay angle which is defined as the angle between direction of the lepton in the W -rest-frame and W in the top-rest-frame.
Equation for Helicity fraction: θ
The angular distribution is given by the following expression.
Where F0 =
Mt = GeV/c2
F- =
FR =

16. Measurement Method ι During the analysis the relationship
b/w the mother and daughter
particles is ensured. t
First top quark was selected and built its
4-vector from its components px, py, pz
and E.
top vertex
A boost vector is calculated from its
4-vector and stored in the 3-vector
with the name t_rest. W b
W vertex
Then the decay vertex of the top quark was
selected and it was ensured that this vertex
is the production vertex for the “W” and “b”. ѵ ι
Then the “W” and “b” were selected and
built its 4-vector from its components px, py, pz
and E.
In leptons I take only
muon.
The boost transformation is applied and “W” is boosted by boost vector of top quark and is stored in the 4-vector with the name w_t.

17. Measurement Method
After this the decay vertex of “W” was chosen and It was ensured that (ι, ѵ)
were come out from the Decay vertex of the “W” boson.
Then muon were chosen and built its 4-vector.
The muon is boosted by using boost vector of “W” and this boost is saved as
4-vector with the name m_w. ι b ѳ t ѵ
Finally angle is obtained between the two 3-vectors which are obtained from 4-vector
w_t and m_w. And this angle is plotted as cosθ and the histogram is filled with
this angular distribution.
In the last step this angular distribution is fitted with a function. This equation is a
second order polynomial in cosθ. Final fit gives the values of F0, FL and FR.

18. Flow Chart
The analysis has been performed using proton proton collisions data which is reconstructed by CMSSW software version 4_2_9 _HLT3
patRefSel_muJets_cfg.py
Histograms
analyzeTopHypothesis.root
patRefSel_muJets.root
HypothesisAnalyzer.cc
ttSemilepEvtBuilder_cfg.py
analyzeTopHypotheses_cfg.py
ttSemiLepEvtBuilder.root

19. Selection Criteria for MuHad & SingleMu data stream
Muon Cut
Pt > 25
Loose Muon Cut
Pt > 10
eta < 2.1
Chi square < 10
Tight Muon Cut
Pt > 25
Muon Cut
Pt>10
eta<2.5
Loose Muon Cut
Pt > 20
eta <2.1
chi square <10
Tight Muon Cut
(trackIso + caloIso)/Pt<0.05

20. Selection Criteria for MuHad & SingleMu data stream
Electron Cut
et > 15
eta < 2.5
Jet cut
Pt >30
Eta<2.4
Jet Cut PF
pt >30
Eta <2.4
Electron cut
et >15
eta<2.5
Jet Cut
Pt >30
Jet Cut PF
Pt>30

21. Distributions of (a) lep Top Pt (b) Pt of Top in CMS note

22. Distributions of (a) Lep Top Eta (b) Eta of Top in CMS note

23. Distributions of (a) lep W Pt (b) Pt of the W in CMS note

24. Distributions of (a) lep W Eta (b) Eta in CMS note

25. B Pt b a
Distribution of the (a) lep B pt by data (b) show the B pt of the CMS note.

26. Muon Pt a b
Distribution of (a) lep muon pt and (b) lep pt of muon in CMS note

27. Neutrino Pt a b
Distribution of the (a)neutrino Pt and (b) neutrino Pt in CMS note

28. Costheta Distribution
Fo =1.156
F- = 0.70
F+ = 0.56 a b
Distribution of the Cosθ done by us and (b) Distribution in CMS note

29. CONCLUTION In this work we learn that what is Helicity?
How experimentally these values are extracted?
How to deal with the real data?
The physics observabels which help us to extract W helicity.
But our values are not correct as in the SM values but we are trying to improve our result.

30. Thank You