Top Quark Physics: An Overview Young Scientists’ Workshop, Ringberg castle, July 21 st 2006 Andrea Bangert

2 Why Study the Top Quark? Top is a fundamental particle of the SM: as such its parameters should be measured. Top may play a role in the mechanism of electroweak symmetry breaking. More precise constraints on m t help to constrain m H. Top decay provides a unique opportunity to study the decay of a bare quark. Top will provide a large background for searches for physics beyond the SM. Top provides a useful calibration tool for the LHC detectors.

3 A Brief History of Top 1977:  meson was discovered (b + anti-b). T 3 (b) = -1/2 → T 3 (t) = +1/2 1993: Constraints from electroweak precision data gave m t = 164 ± 27 GeV [Mele] 1995: The top quark was observed directly at the Tevatron. 2006: m t = ± 2.3 GeV 2008: The LHC will be a top factory, producing 8 million pairs/year at luminosity L~10 33 cm -2 s -1.

4 Electroweak Precision Measurements  EM, G F, m Z → sin 2 θ W  1 - (m W / m Z ) 2  m W 2 ~ 1 / G F sin 2 θ W (1-▲r)  ▲r contains all one-loop corrections to m W.  (▲r) top ~ G F m t 2 / tan 2 θ W  (▲r) Higgs ~ G F m W 2 ln (m H 2 / m Z 2 )

5 Electroweak Precision Measurements (II) LEP: m w = ± GeV TeVatron: m t =172.5 ± 2.3 GeV

6 Top Pair Production Top-antitop pair production proceeds via the strong interaction.  Tevatron ~ 6.8 pb,  LHC ~ 800 pb gluon fusion: Tevatron 10%, LHC 90% quark-antiquark annihilation: Tevatron 90%, LHC 10%

7 W Single Top Production Top is produced singly via the weak charged-current interaction. W t-channel, W-gluon fusion Wt production W* process, s-channel  Tevatron ~ 2 pb  LHC ~ 250 pb  Tevatron ~ 0 pb  LHC ~ 60 pb  Tevatron ~ 1 pb  LHC ~ 10 pb W

8 Decay Topologies Top decays via t→Wb. Final states in decay of top-antitop pair are: Dileptonic: Both W bosons decay via W→lν, Γ ~ 4.9 %. Lepton + jets: 1 W decays via W→lν, 1 W decays to produce hadron jets. Γ ~ 50.7 %. l = e, μ  Γ ~ 29.9 % Hadronic: Both W bosons decay to produce hadron jets. Γ ~ 44.4 %.

9 TeVatron Results: Top Mass m t = ± 2.3 GeV Good agreement of indirect determination with direct measurement: m t = – 10.2 GeV [LEP 1 and SLD]

10 Tevatron Results: Top Cross Section CDF theoretical cross section:  theory = 6.78 ± 1.2 pb [Bonciani 1998, Kidonakis 2003, Cacciari 2004]

11 Monte Carlo Analysis in preparation for LHC Analyses to be performed include m t and  t. Lepton + jets channel, l = e,μ; Γ ~ 29.9%. Selection criteria: At least 1 isolated lepton, p T > 20 GeV, |η| < 2.5 Missing E T > 20 GeV At least 4 jets with p T > 40 GeV, |η| < 2.5 b-tagging is not required. Reconstruction: Choose the three jets which maximize the total p T.

12 Monte Carlo Analysis: Top Mass Reconstruction (continued): Choose the two jets which have invariant mass closest to m W. Apply m jj = m W ± 10 GeV as selection criteria. Jets are reconstructed using the Cone or K T algorithms. Top mass distribution (Gaussian) and combinatorial background (Chebyshev) are depicted. Only signal events and combinatoric background are depicted. Plot by Nabil Ghodbane.

13 Monte Carlo Analysis: s t Top pair events from all channels were generated using / HERWIG events / m t. No detector simulation or event reconstruction was performed. s t and m t will both be measured: this analysis will be used as consistency check.

Summary Top is worth studying because: it is a fundamental particle of the SM, it may play a role in electroweak symmetry breaking, It helps to constrain m H, It will provide a background for future studies, It offers a useful tool for calibrating the detector. Top-antitop pair production proceeds via the strong interaction. Single top production proceeds via the weak interaction. Top decays via the weak interaction, t  Wb. m t = ± 2.3 GeV [CDF and D0 combined result] s t = 7.3 ± 0.9 pb [CDF combined result] m t and s t will be among first measurements made at LHC.

15 Backup Slides

16 Reconstructed Top Mass Variation of parameter D in the K T jet reconstruction algorithm affects m t. [Nabil Ghodbane, ATLAS MPI Group]

17 Charge of the Top Quark (I) The SM top quark has charge Q t = + 2/3. The particle of mass m t = GeV which was first observed in 1995 at the TeVatron is believed to be the SM top quark, i.e. the weak isospin partner of the b quark with Q t = + 2/3. The charge of the particle observed in 1995 at the TeVatron has never been directly measured. It is possible that the observed particle is instead an exotic quark Q with charge Q Q = - 4/3 which decays via Q →W - b.

18 Charge of the Top Quark (II) Experiments at the Tevatron will be able to exclude to 95% CL the possibility that Q Q = - 4/3. Experiments at the LHC will be able to measure the charge of the top quark to an accuracy of 10%. One possibility for directly measuring the top charge at the LHC is by considering gg → ttg events,  ~ Q t 2.

19 W Helicity in Top Decays The W boson in top quark decay must be left-handed or longitudinal; it cannot be right-handed. Weak vertex factor for t → Wb: - ig W V tb   ½ (1 -  5 ) m b ~ 0 → b quark is left-handed