Determination of The Top Quark Charge M. Ciljak, M. Jurcovicova, S. Tokar Comenius Univ., Bratislava, Slovakia Motivation: • CDF and D0 analyses + precision EW data do not exclude: Top quark seen in Fermilab is an exotic quark with Q t = − 4 / 3. ( D. Chang et al , Phys. Rev. D59, 091503) How to determine the top charge? • by measuring the charges of top decay products • via radiative t ¯ t events (sensitive to Q top ) Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 1
Top charge via its decay products Event samples suitable for the analysis : t → ( lν )( lν ) b ¯ • Dilepton decays t ¯ (400 kEv./10 fb − 1 ) b t → ( lν )( jj ) b ¯ • Semileptonic decays t ¯ (2500 kEv./10 fb − 1 ) b All jet modes not suitable due to huge QCD bkgd ! Determination of the b -jet charge : i q i | � p i | κ � j · � q bjet = i | � � j · � p i | κ • q i ( � p i ) is the charge(momentum) of b -jet track • � j i is the b -jet direction and κ is an exponent Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 2
Selection criteria Di-lepton Semileptonic 2 isolated high p T leptons ( e , µ ) 1 isolated high p T lepton ( e , µ ) p (1) > 35 GeV , p (1) p (1) > 25 GeV > 20 GeV T T T | η | < 2 . 5 | η | < 2 . 5 Big E miss Big E miss > 40 GeV > 20 GeV T T At least two jets, p T > 25 GeV At least two 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 (Y-Book) S/B ≈ 65 (P. Grenier) Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 3
Association of lepton and b -jet di-lepton case: m ( l + , b (1) jet ) < m cr ∧ m ( l + , b (2) jet ) > m cr m ( l − , b (1) jet ) < m cr ∧ m ( l − , b (2) jet ) > m cr semileptonic case: m ( l, b (1) jet ) < m cr ∧ m ( l, b (2) jet ) > m cr 120 GeV < m ( jjb jet ) < 220 GeV l − b -jet invariant mass, solid : l and b − jet from the same top, dashed : from different tops, m cr ≈ 160 GeV . Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 4
Reconstructed b (¯ b ) -jet charge distributions Charge distributions of b -jets connected with l + ( b jet ) and l − (¯ b jet ): ¯ b -jet charge distribution b -jet charge distribution Mean b -jet charges : q (¯ q ( b jet ) = − 0 . 109 ± 0 . 007 and b jet ) = 0 . 112 ± 0 . 007 ( Independent fragment. in PYTHIA - 500 k events, cone R = 0 . 4: q ( ¯ q ( b jet ) = − 0 . 113 ± 0 . 007 and b jet ) = 0 . 117 ± 0 . 008 ) Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 5
Top charge via photon radiation in t ¯ t events Signal processes (sensitive to Q top ) : • Radiative t ¯ t production pp → t ¯ tγ (matrix el., PYTHIA) • Radiative top decay t ¯ t , t → Wbγ (matrix el., PYTHIA). Background processes (non sensitive to Q top ): • Radiative W decay t ¯ t , W → jjγ ( lνγ ) (matrix el., PYTHIA) • Other t ¯ t processes leading to high p T photon ( PYTHIA ) • Non − t ¯ t radiative bkgd ( W + jets + γ ) (matrix el., PYTHIA) Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 6
Radiative top production t γ b t W – t Figure 1: Radiative top production (an example) - production (matrix el.) and decay(PYTHIA), in initial state: q ¯ q and gg . Cross section: σ ∼ Q 2 top Important: virtuality of radiating top is needed ! Decay of final t -quark: t → Wb → blν ( jj ) Top and W decays treated in narrow width approximation. Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 7
u → t ¯ Radiative top production: gg, u ¯ tγ diagrams Diagrams by MadGraph 4 4 4 1 1 1 t t t t A t Diagrams by MadGraph t A 5 5 5 t t A 5 4 4 t 1 1 1 u t t t u A t 3 3 3 u 2 2 2 t graph 1 graph 2 graph 3 t u 4 3 t 5 u 5 3 3 1 1 1 A t t t A u t t A t u t t u t A 3 5 3 3 5 5 2 2 2 t graph 1 graph 2 graph 3 t A t t t t 4 4 4 2 2 2 graph 4 graph 5 graph 6 4 1 t 4 4 1 1 u A t t 5 t A t 5 5 t A t u t t 3 2 3 3 2 2 graph 4 graph 7 graph 8 Figure 2: pp → t ¯ tγ , 2 gluons or u ¯ u quark pair in initial state. Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 8
Radiative top decay γ W b b W W t t b t + + γ γ W Figure 3: Radiative top decay: pp → t ¯ t (production), t → Wbγ . . . • Decaying top quark is on mass shell . • Photons from the W and b lines do not feel the top charge ! • Destructive interference of the diagrams is expected: σ ( − 4 / 3) < σ (2 / 3). Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 9
Reconstruction by ATLFAST: sets of conditions t → l ν jjb¯ Selected sample: t¯ b (semileptonic events) • General top cuts (C1) njets ≥ 4 p T > 20 GeV | η | < 2 . 5 nbjets = 2 p T > 15 GeV | η | < 2 . 0 nlep = 1 p T > 20 GeV | η | < 2 . 5 nphot = 1 p T > 30 GeV | η | < 2 . 5 p T > 20 GeV miss / • Top reconstructed via the W → jj decay ( C2 ) | m ( jj ) − m W | < 20 GeV m ( jj ) = min { m ( j i , j j ) − m W } | m T ( l ν ) − m W | < 20 GeV Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 10
Kinematic cuts Radiation in top production - top virtuality is required t ¯ tγ cuts : m ( b 1 , 2 jjγ ) > 190 GeV and m T ( b 2 , 1 lγ ; / p T ) > 190 GeV � 2 � 2 �� � m 2 p T ( blγ ) + � p 2 T ( blγ ) + m 2 ( blγ ) + / T ( blγ ; / p t ) = − p T � p T / Radiation in top decay (on-mass-shell top ) Wb γ cuts: m T ( bl γ ; / p T ) < 190 and m ( bjj γ ) > 190 or m T ( bl γ ; / p T ) > 190 and 160 < m ( bjj γ ) < 190 Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 11
Selection efficiencies - t ¯ tγ cuts t¯ t γ cuts (radiative top production selection) applied Selection efficiencies for different processes pp → t¯ t ¯ Process t γ t → Wbγ W → lνγ t Wγjets → jjγ 2 · 10 5 6 . 9 · 10 6 2 . 1 · 10 6 3 . 5 · 10 5 2 · 10 5 # events 1 . 50 · 10 − 3 1 . 2 · 10 − 3 C1 (%) 0 . 723 0 . 423 0 . 148 0 . 76 · 10 − 3 6 . 2 · 10 − 4 C2 (%) 0 . 423 0 . 292 0 . 079 0 . 38 · 10 − 3 4 . 3 · 10 − 4 C3 (%) 0 . 338 0 . 021 0 . 013 Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 12
Selection efficiencies t ¯ tγ cuts) Radiative top production vs. Radiative to decay Figure 4: Criterion (C3) applied Figure 5: Criterion (C3) applied to the ttγ spectrum to the t → Wbγ spectrum Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 13
Signal( ttγ ) vs. Bckg( t → Wbγ ) Figure 6: S vs. p T γ ( Q t = 2 Figure 7: S vs. p T γ ( Q t = − 4 3 ) 3 ) Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 14
Cross section Q t = 2 / 3 vs. Q t = − 4 / 3 Figure 8: Comparison of S vs p T γ for Q t = 2 / 3 and − 4 / 3 Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 15
Radiative top production vs Bkgd t¯ Sample: 10 fb − 1 t γ -cuts applied, Signal vs Bkgd , integrated x-sections ( σ ) and # events Q = 2 / 3 Q = − 4 / 3 process σ seen [ fb ] events σ seen [ fb ] events (1 year) (1 year) pp → t¯ t γ 7.81 78.1 24.81 248.1 pp → t¯ t ; t → Wb γ 0.62 6.2 0.244 2.4 Q top indep. bkgd 6.65 66.5 6.65 66.5 Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 16
Background for t ¯ tγ -cuts Q top independent Bkgd for Radiative top production, x-sections ( σ ) and cut and reconstr. efficiencies ( ǫ ): σ [ pb ] ( | y | < 3 . 5 ǫ [ % ] events Bkgd process p T > 10 GeV ) Atlfast (1 year) pp → t ¯ t ; W → p 1 p 2 γ 8.25 0.013 10.7 ± 3.4 pp → t ¯ 3 . 8 ∗ 10 − 4 t ; ( π o , η, .. ) 550 20.7 ± 4.5 4 . 3 ∗ 10 − 4 pp → Wγ jets 812 35.0 ± 6.0 full background 66.4 Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 17
Radiative top decay vs Bkgd Sample: 10 fb − 1 Wb γ -cuts applied, Signal vs Bkgd , integrated x-sections ( σ ) and # events: Q = 2 / 3 Q = − 4 / 3 process σ seen [ fb ] events σ seen [ fb ] events (1 year) (1 year) pp → t¯ t ; t → Wb γ 1.036 10.4 0.406 4.1 pp → t¯ t γ 0.324 3.2 1.028 10.1 Q top indep. bkgd 1.240 12.4 1.240 12.4 Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 18
Summary Top charge determination studied for ATLAS ( LHC , 10 fb − 1 ) via • charges of top decay products with samples of di-lepton ( pp → ( lν )( lν ) b ¯ b ) and one-lepton ( pp → ( lν )( jj ) b ¯ b ) t ¯ t -events • radiative t ¯ t events. The ATLAS experiment will be able ( preliminary) • to find correlations between b -quarks and W -bosons in t (¯ t )decays using top decay products charges, • to distinguish between the top charge 2 / 3 and − 4 / 3 by measuring the radiative t ¯ t process cross section. Study of helicity of top decay products. b -jet charge via leptonic b -decay Comenius Univ. May 22, 2003 S.Tokar, ATLAS Workshop, Athens, May 2003 19
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