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Top Quark Physics Suyong Choi Korea University 1 • • • • Top Quark in the Standard Model Measurement of Production Cross Sections Properties of the Top Quark Summary and Outlook Contents 2 TOP QUARK IN THE STANDARD MODEL • Discrete quantum numbers • Spin • Weak isospin • Charge • Mass • Lifetime or Decay width • Branching fraction • Coupling – QCD, EW Properties of Top Quark • Top quark carries “color” and interacts with gluons • Production of top quark pairs is important probe of QCD interactions QCD • Fermion part 𝑢𝑖 𝜈𝑖 𝜓𝑖 = ℓ− 𝑜𝑟 𝑑 ′ 𝑖 𝑖 • Interaction with H, W, , Z Electroweak Lagrangian • Strong interaction: 𝑔 → 𝑡𝑡 • Coupling to neutral gauge boson: 𝛾/𝑍 → 𝑡𝑡 • Coupling to charged gauge boson: 𝑡 → 𝑊 + 𝑏 • Coupling to Higgs field (boson): 𝐻 → 𝑡𝑡 • Coupling to Higgs field: 𝑔𝑚𝑡 2𝑀𝑊 ≈ 0.995 Interactions of Top Quark in SM • Decays to physical states • Top decays almost 100% to W+b Top Quark Decays Decays of Top Quark • 1.3 GeV width for 172 GeV top quark • 𝜏𝑡𝑜𝑝 = ? s • Top quarks are produced and decay like free quarks with spin a t production information intact • Hadron formation time 1 Λ𝑄𝐶𝐷 ≫ 1 Γ𝑡 • Hadron formation is governed by light-quark dynamics • In contrast, B mesons decay isotropically Top Decay Width • Top quark polarization is reflected in angular distribution of decay products 1 𝑑Γi 1 + 𝛼𝑖 cos 𝜃 ∗ = ∗ Γi 𝑑 cos 𝜃 2 Particle Charged lepton 1 Neutrino -0.31 B quark -0.41 Polarized Top Quark 11 TOP QUARK AND ELECTROWEAK PRECISION DATA • Radiative corrections to W and Z propagator • Quadratic sensitivity to fermion masses 𝑔2 + 𝑔′2 𝜌= 8𝑀𝑍2 2 𝐺𝐹 𝜌 ≈1+ 3𝐺𝐹 𝑚𝑡2 8𝜋 2 2 Top Quark Corrections to Electroweak Measurements • W and Z masses 2 𝑀𝑊 = 𝜋𝛼 2𝐺𝐹 sin2 𝜃 𝑀𝑍2 = 𝜋𝛼 2𝐺𝐹 𝜌 sin2 𝜃 cos 2 𝜃 • Z mass was measured very precisely at LEP experiments • 𝑚𝑡 could be inferred with knowledge of 𝑀𝑍 , 𝐺𝐹 , sin2 𝜃 • Test of EW theory Top Quark and Electroweak Measurements • Top quark mass could be predicted from precision measurements 170 ± 10+17 −19 𝐺𝑒𝑉 𝑚𝑡 Prediction from LEP CDF CDF DØ DØ Top Quark Runfrom 2 results Top Mass Distributions 1995 observation paper 16 Success of the SM 17 • LEP EW precision res ults from 𝑒 + 𝑒 − → 𝑍 0 Top Quark Mass from Electroweak Data • In conjunction with W and Z, we can gain information on Higgs mass Δ𝜌 = − 3 𝑀𝐻 ln 8𝜋 cos 2 𝜃 𝑀𝑊 Connection with Higgs • Top quark is special • • • • • Most massive Interaction only within 3rd generation top-Higgs coupling ~ 1 2 𝑚𝑡 × 𝑚𝑍 ≈ 𝑚𝐻 Boundary between metastability and stability The Top Quark 20 5 fb-1 @ 7 TeV 20 fb-1 @ 8 TeV LHC and Experiments 21 • Properties • • • • Mass Decay width Spin Coupling • Cross section measurements • Production and decays Physics with Top Quarks 22 • Higher cross section and higher luminosity at LHC • Top quark factory • Rare processes with top quarks • New physics with top quarks • Tevatron and LHC are complementary Cross Sections at Tevatron and LHC 23 PRODUCTION 24 Pair production diagams • Strongly produced • Contribution of 𝑞𝑞 and 𝑔𝑔 changes as 𝑠 𝑡𝑡 Pair Production 25 Dilepton: ee, e, tau+X Multijet mu+jets e+jets Lepton+jets • • • • 𝐵𝑟 𝑡 → 𝑏ℓ𝜈 = 10.6% per lepton flavor Multijet – Highest statistics, but large backgrounds and combinatorics Lepton+jets – Highest statistics and usually yields best measurement Dilepton – Smaller statistics but clean, less combinatoric, solving for 2 neutrino momenta not trivial 𝑡𝑡 channels 26 • Lepton+Jets • 1 charged lepton • 4 hadronic jets (2 are b-quark jets) • Missing ET • Problem • How to correctly assign jets to top or antitop • How to reconstruct neutrino momentum Reconstructing tt-bar Events 27 • 1 unknown: neutrino 𝑝𝜈,𝑧 • 𝑝𝜈,𝑥 = 𝑀𝐸𝑇𝑥 , 𝑝𝜈,𝑦 = 𝑀𝐸𝑇𝑦 • 3 constraints: 𝑚 𝑗1𝑗2 = 𝑀𝑊 𝑚 ℓ𝜈 = 𝑀𝑊 𝑚 ℓ𝜈𝑏1 = 𝑚(𝑏2 𝑗1𝑗2) • Problem of combinatorics • 2 fold ambiguity – if 2 b-jets tagged • 6 fold ambiguity – if 1 b-jet tagged Top Quark Reconstruction in L+Jets • Have to consider • experimental uncertainties on measurements • finite widths of W and top • Numerically minimize 𝜒 2 event-by-event Top Quark Reconstruction in L+Jets TOP PRODUCTION CROSS SECTION 30 • Experimental error comparable to theory error • QCD explains well the inclusive pair production Pair Production Cross Section 31 s and t channel • Electroweak production • Cross section of the same order as pair production • Sensitive probe of |𝑉𝑡𝑏 | without the assumption of 3 generation of quarks W associated Single Top Production 32 Single Top Production t-channel 33 Signal Region 𝜎 𝑝𝑝 → 𝑊𝑡 = 23.4+5.5 −5.4 𝑝𝑏 Control Region 6.0𝜎 significance Observation of Wt Single Top Production 34 • From single top quark production cross section, we can measure |𝑉𝑡𝑏 | directly without assuming 3 generation of quarks • Current best direct measurement: 𝑉𝑡𝑏 = 1.020 ± 0.046(𝑒𝑥𝑝𝑡. ) ± 0.017(𝑡ℎ𝑒𝑜𝑟. ) Measurement of |𝑉𝑡𝑏 | 35 PROPERTIES 36 • Tevatron: 𝑚𝑡 = 173.20 ± 0.87 GeV – 0.5% accuracy Mass of Top Quark 37 • CPT violated if 𝑚𝑡 ≠ 𝑚𝑡 • 𝑡 and 𝑡 distinguished by electric charged of lepton Δ𝑚𝑡 = −272 ± 196 𝑠𝑡𝑎𝑡 ± 122(𝑠𝑦𝑠𝑡) Mass Difference of 𝑡 and 𝑡 38 • In SM, top quark width at NLO is • 1.29 GeV/c2 • Lifetime of 0.5 × 10−24 𝑠 • Decay width reflected in reconstructed mass distribution • CDF measures Γ𝑡 = 2.21+1.84 −1.11 GeV • Γ𝑡 < 6.38 GeV @ 95% CL Decay Width of Top Quark 39 • 𝑄𝑐𝑜𝑚𝑏 = 𝑄ℓ × 𝑄𝑏−𝑗𝑒𝑡 • B-jet charge calculated from tracks associated with b-jet Electric Charge of Top Quark 40 • Use lepton angular distribution in top rest frame • W from top decays are either left-handed or longitudinal W Polarization from Top 41 • On average, spin of top and antitop are unpolarized, but event-by-event, their spins are correlated • Most prominent in 𝑞𝑞 initial state: aligned top spin • For gg mostly anti-aligned spins • Results depend on spin quantization axis chosen 𝑡𝑡 Produced at Rest Relativistic top 𝑡 𝑞 𝑡 𝑞 𝑡 𝑞 𝑞 𝑡 Spin Correlation in 𝑡𝑡 Production 42 • 𝑞𝑞 → 𝑡𝑡 and 𝑔𝑔 → 𝑡𝑡 the spins of top quarks are correlated • Due to 𝜏𝑡 ≪ 1/Λ𝑄𝐶𝐷 , spin state of top at production reflected in decay products • Lepton is the most sensitive probe of top spin polarization • Tevatron and LHC has different contributions of 𝑞 𝑞 and 𝑔𝑔 • ATLAS observed spin correlations at 5.1 s.d. 𝑓 𝑆𝑀 = 1.30 ± 0.14 𝑠𝑡𝑎𝑡 +0.27 −0.22 (𝑠𝑦𝑠𝑡) Spin Correlation 43 Top Coupling with Vector Bosons with 𝑡𝑡𝑊and 𝑡𝑡𝑍 44 • Major background to 𝑡𝑡 + 𝐻(→ 𝑏𝑏) • Number of b-tagged jets distribution 𝑡𝑡 + 𝑏𝑏 Production 45 SEARCHES WITH TOP QUARKS 46 Search for Resonances Decaying into 𝑡𝑡 47 Anomalous Single Top Search for 𝑡 → 𝑍𝑞 • 𝑔𝑞 → 𝑡 𝐵 𝑡 → 𝑍𝑞 < 0.0021 @ 95% 𝐶𝐿 Search for FCNC 48 • Top-Higgs coupling almost 1 • Consistent with backgrounds • Cross section limits at Search for 𝑡𝑡 + 𝐻 49 • Approaching 20 years of rich physics program at hadron colliders with top quark events • Top quark production and properties consistent with SM • Many measurements systematics limited. What can you do with millions of top quark events? Summary and Outlook 50 BACKUP 51 • When Υ was discovered in 1977, it was considered as a bound state of 𝑏𝑏 quarks. Hence extra quark was thought to exist. • It took a long time until top quark was discovered in 1995 by CDF and D-Zero experiments using Fermilab Tevatron accelerators • Being the most massive quark, it may hold the key. • With the luminosity and energy reach of the LHC at CERN, top quarks can be studied with unprecedented precision. • 1.96 TeV → 8 TeV Introduction 52 • 6th quark discovered • Partner to bottom quark predicted after discovery of Υ • “gauge anomaly” of SM must be absent • Electric charge inferred from Υ → 𝑒 + 𝑒 − using quarkonium mo del • Weak isospin inferred from forward backward asymmetry • 1995 by D-Zero and CDF experiments @ Fermilab The Top Quark Channel Luminosity CDF Lepton+jets 9.4 fb-1 0.094+0.032 −0.029 D0 Lepton+jets 9.7 fb-1 0.047+0.025 −0.027 D0 Dilepton 9.7 fb-1 0.058 ± 0.053 𝐴ℓ SM prediction @ NLO: 𝐴𝑙 = 0.036 ± 0.002 Lepton AFB in 𝑡𝑡 54 • 𝜎𝑡𝑡 is a function of 𝑚𝑡 and 𝛼𝑠 𝑀𝑍 Strong Coupling Constant 55 • Dilepton channel – 2 leptons + 2 b-jets + Missing ET • 𝑀𝐸𝑇 = (𝑝𝜈,𝑥 + 𝑝 𝜈,𝑥 , 𝑝𝜈,𝑦 + 𝑝 𝜈,𝑦 ) • Unknowns • Neutrino momentum components – 2 x 3 = 6 Top Quark Reconstruction in Dilepton • Lepton asymmetry reflects • Asymmetry in production • Polarization of 𝑡𝑡: 𝑡𝑅 𝑡𝑅 vs 𝑡𝐿 𝑡𝐿 • SM predicts small asymmetry in production and no polarization Lepton ForwardBackward Asymmetry 57 𝑚 ℓ1 𝜈1 𝑏1 = 𝑚(ℓ2 𝜈2 𝑏2 ) • Constraints 2-fold ambiguity in Lepton-jet assignment 𝑚 ℓ1 𝜈1 = 𝑀𝑊 𝑚 ℓ2 𝜈2 = 𝑀𝑊 𝑝𝜈1,𝑥 + 𝑝𝜈2,𝑥 = 𝑀𝐸𝑇𝑥 𝑝𝜈1,𝑦 + 𝑝𝜈2,𝑦 = 𝑀𝐸𝑇𝑦 System is Underconstrained! Top Quark Reconstruction in Dilepton • If top quark mass is assumed, then the number of constraints = number of unknowns • But, still, up to 4 solutions are possible – intersection of 2 ellipses in 2-D space * 2 fold ambiguity • Numerically maximize likelihood event-by-event • Assume momentum and mass spectra of 𝑡𝑡 system Top Quark Reconstruction in Dilepton