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

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

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

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 × 10 -25 s. It’s the only quark that decay before the hadronization. life time ≈ 10 -25 s. Hadronize time ≈ 10-23 s

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%

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

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

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

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

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)

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

W± Boson Life time → ≈ 3 × 10 -25 s Composition → elementary particle Statistics → Bosonic Interaction → weak interaction Theorized → Glashow,Weinberg,Salam(1968) Discovered → 1983 Mass → W:80.385±0.015 GeV/ c2 Electric charge → ± 1e Spin → 1 Life time → ≈ 3 × 10 -25 s

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).

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

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 = 173.3 GeV/c2 F- = FR = 0.00

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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.

Thank You