Top Quark Charge Determination S. Tokr, Comenius Univ., Bratislava - PowerPoint PPT Presentation

Top Quark Charge Determination S. Tokár, Comenius Univ., Bratislava Motivation: CDF and D0 analyses + precision EW data do not exclude: top quark seen in Fermilab is an exotic quark with Q top =-4/3. ( D. Chang et al., Phys. Rev. D59, 091503 ) How to determine the top charge? • via radiative tt_bar events (sensitive to Q top ) • by measuring the charges of top decay products ⇒ weighting b-jet tracks charges ⇒ semileptonic b-decay 11/29/2004 S. Tokar, Fermilab,Nov 2004 1

tt Top charge via photon radiation in events Signal Processes • Radiative production → tt γ (matrix el., PYTHIA) tt pp • Radiative top decay (matrix el., PYTHIA) → Wb γ tt t , Background processes (non sensitive to Q top ) ( ) → γ ν γ • Radiative W decay in (matrix el., tt W , j j l l PYTHIA) • Other processes leading to high p T photon ( PYTHIA) tt • Non radiative bkgd (W,Z+jets+ γ ) (matrix el., PYTHIA) tt 11/29/2004 S. Tokar, Fermilab,Nov 2004 2

Why matrix elements? Pythia vs matrix elements p T ( γ ) > 10 GeV, | η |<3.5 σ PY =0.66 pb tt γ σ ME = 2.33 pb σ PY =0.66 pb Wb γ σ ME = 2.33 pb Using ISR, FSR for m t systematics is questionable ! Soft photon (gluon) approximation used in Pythia is insufficient 11/29/2004 S. Tokar, Fermilab,Nov 2004 3

Radiative Top Production Radiative top production followed by top decay • Production phase: matrix elements implemented into PYTHIA q q • In initial state : gg and/or tt γ • production cross section σ ∼ 2 Q top • Important: virtuality of radiating top is required t Wb bl ν (jj) → → • Decay of top : Top and W decay treated in narrow width approximation 11/29/2004 S. Tokar, Fermilab,Nov 2004 4

Radiative top production diagrams + diagrams with d, s, c and b production by quark tt γ production by gluon tt γ annihilation fusion LHC:10%, Tevatron:85% LHC:90%, Tevatron:15% 11/29/2004 S. Tokar, Fermilab,Nov 2004 5

Radiative top decay Radiative top decay: (production), t → Wb γ … → pp tt • Decaying top quark is on-mass-shell • Photons from W and b do not feel the top charge • Destructive interference of the diagrams is expected: σ (-4/3) < σ (2/3) 11/29/2004 S. Tokar, Fermilab,Nov 2004 6

Selection criteria Selected sample: (radiative lepton + jets ev.) → ν γ tt l jjbb general top quark cuts (C1) W reconstruction criteria njets ≤ 4, p T > 20 GeV, η < 2.5 |m(jj)-M W | < 20 GeV nbjets = 2, p T > 20 GeV, η < 2.0 m(jj)=min{|m(j i ,j k )-M W |} nlept = 1, p T > 20 GeV, η < 2.5 p m T (l; ) < M W + 20 GeV T nphot = 1, p T > 30 GeV, η < 2.5 missing p T > 20 GeV m(b 1,2 jj γ ) > 190 GeV & Radiative top production cuts m T (b 2,1 l γ ; ) > 190 GeV p T ( ) � � 2 ( ) 2 γ = γ + γ + − γ + 2 2 m ( bl ; p ) p ( bl ) m ( bl ) p p ( bl ) p T T T T T 11/29/2004 S. Tokar, Fermilab,Nov 2004 7

Radiative top production SM top radiative production tt γ statistics Sample: 10 fb -1 (1 LHC L.L. year) Exotic q radiative production tt γ statistics 30% uncertainty from scale ( ) m 2 2m , t t CDF possibilities to measure photon-top coupling: None ! 11/29/2004 S. Tokar, Fermilab,Nov 2004 8

Top charge via top decay products Suitable event samples: → ν ν • Dilepton sample ( 400 kEvnt/10fb -1 ) tt ( l )( l ) bb • Lepton +jets sample (2500 kEvnt/10fb -1 ) → ν tt ( l )( jj bb ) • All jets mode not suitable ⇒ huge QCD bkgd Cornerstone of the top charge determination: � association of l + and/or l - with correct b -jets � determine the charge of the associated b -jet � in SM the mean value of b-jet charge spectrum associated with l + ( l - ) should be negative (positive) q i ≡ i th part. Charge � � ∑ κ N ⋅ � q j p ≡ i th part. momentum b-jet charge p i i = i Q � i � � ≡ b-jet direction determination − ∑ κ b jet N j ⋅ j p κ ≡ an exponent i i 11/29/2004 S. Tokar, Fermilab,Nov 2004 9

Selection criteria Di-lepton Lepton + jets 2 isol. high p T leptons (e, µ ) 1 isol. high p T leptons (e, µ ) and < < ( ) 1 ( ) 2 p 35GeV p 25GeV T T > > E 40GeV E 20GeV T T At least 2 jets, p T > 25 GeV At least 2 b-jets, p T > 40 GeV 1 or 2 b-tagged, | η |< 2.5 In total 4 jets p T > 40 GeV, | η |< 2.5 S/B ≈ 10 ( Yellow Rep. 2000 ) S/B ≈ 65 (P. Grenier, phys-note) ATLAS experiment cuts 11/29/2004 S. Tokar, Fermilab,Nov 2004 10

Association of lepton and b-jet Di-lepton case l and b jet from ( ) ( ) the same top + + < > ( , ) 1 2 ( , ) 2 1 m l , b m & m l , b m jet cr jet cr ( ) ( ) − − < > ( , ) 1 2 ( , ) 2 1 m l , b m & m l , b m jet c r je t c r and b jet from Lepton+jets case different tops ( ) ( ) < > ( , ) 1 2 ( , ) 2 1 m l b , m & m l b , m jet cr jet cr 120 GeV < m(jjb) < 220 GeV m cr m cr ≈ 160 GeV lepton-b jet invariant mass ATLAS: Particle generation ⇒ PYTHIA detector resolutions and efficiencies ⇒ ATLFAST 11/29/2004 S. Tokar, Fermilab,Nov 2004 11

Reconstructed b -jet charge Sample: 0.87fb -1 ⇒ ≈ 160 kEvn’s (lepton+jets), ≈ 22 kEvn’s (dilepton ) b-jet charge assoc. with l + b-jet charge assoc. with l − + − = − ± = ± ( l ) ( l ) Q 0 109 . 0 007 . Q 0 112 . 0 007 . − − b jet b jet Independent fragmentation in PYTHIA ( R cone = 0.4 ) + − = − ± = ± ( l ) ( l ) Q 0 113 . 0 007 . Q 0 117 . 0 008 . − − b jet b jet For 5 σ separation we need 63 inputs for each distribution 11/29/2004 S. Tokar, Fermilab,Nov 2004 12

Top charge in CDF � radiative tt_bar events (sensitive to Q top ) � measuring the charges of top decay products � � ∑ κ N ⋅ q j p ⇒ weighting b-jet tracks charges i i = Q i � � − b jet ∑ κ N ⋅ j p i i Problems: association of W boson and b-jet (invariant mass criterion needs high statistics) ⇒ semileptonic b-decay: b → c,u +l ν charge of lepton defines charge of b-jet Problems: � Low branching (2/9 of b-decay are taken) � B 0 oscillation (change of lepton charge) 11/29/2004 S. Tokar, Fermilab,Nov 2004 13

CDF: invariant mass criterion only Sample lepton +jets: 6248 selected MC events (CDF cuts) ( ) ( ) Invariant mass criterion applied: < > ( , ) 1 2 ( , ) 2 1 m l b , m & m l b , m jet cr jet cr < Q b > N evnt < Q b > N evnt σ σ -0.0615 0.0379 227 l+ 0.0795 0.0395 203 l- -0.0738 0.0470 136 e+ 0.0800 0.0527 111 e- -0.0432 0.0633 91 µ + 0.0789 0.0596 92 µ - 11/29/2004 S. Tokar, Fermilab,Nov 2004 14

W and b-jet association via kinematic fit Kinematic fit (event by event) for lepton+jets sample: Lepton and jets energy, jets direction ( ϕ , η ), neutrino p x , p y , p z vary freely (in errors inter.) χ 2 uses constraints M jjb =m t , M l ν b =m t , M jj =M W ⇒ m t Results: m t , χ 2 found for each of 24 combinations Association: � topology with the lowest χ 2 taken as the correct one � lepton sign defines sign of W and associated b-jet: (bl ν ) giving m t � Tracks pointing to the b-jet are weighted → the found charge is accumulated to the distribution associated with (+) or (-) sign accordingly to the lepton sign. � “charge” of b-jet - part of 2 nd top branch (bjj) - is accumulated in the distribution with opposite (to previous case) sign. Dilepton sample : a similar association procedure can be applied 11/29/2004 S. Tokar, Fermilab,Nov 2004 15

11/29/2004 S. Tokar, Fermilab,Nov 2004 16

Thank you ! Some additional slides follow 11/29/2004 S. Tokar, Fermilab,Nov 2004 17

Top Quark Decay SM: by far dominant t → bW e-e(1/81) mu-mu (1/81) G m Γ t → bW 3 ( ) ≈ × F t = GeV 0 807 . 1 42 . tau-tau (1/81) 2 π V 8 2 e -mu (2/81) tb τ top ≈ 5 × 10 -25 sec << τ hadr (10 -23 sec) e -tau(2/81) mu-tau (2/81) (m t =175 GeV, M W =80.4 GeV, α S =0.03475) e+jets (12/81) mu+jets(12/81) Top decays before hadronization !!! tau+jets(12/81) • No tt-bar bound states ( gluon exchange ) jets (36/81) • t,W helicity from SM V-A (no depolarization via hadronization) – Dilepton channels (ee, e µ , µµ ) tt-bar samples } topological variables and b-tagging defined via W decays – Lepton + jets ch. (e+jets, µ +jets) topological analysis and b-tagging – All hadronic channel 11/29/2004 S. Tokar, Fermilab,Nov 2004 18

t  t Production Cross Section Big m t ⇒ α S (m t )~0.1 ⇒ pExpansion coverges rapidly : – t  t cross section is a test of QCD predictions (Inclusive and differential cross sections) – Acurate Xsection ⇒ indirect determination of m t (in SM is expected: ∆ m/m ≈ ∆ σ (tt)/ σ (tt) ) – Rapidity distr. assymetries of t and  t are sensitive to light- quark PDF – A discrepancy may indicate a new physics • Production via a high mass intermediate state • Non Wb decay model ATLAS: Statistical uncertainties < 1% → Systematics (Exp.& Theo.) will be dominant 11/29/2004 S. Tokar, Fermilab,Nov 2004 19