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PH599 Graduate Seminar presents: Discovery of Top Quark

PH599 Graduate Seminar presents: Discovery of Top Quark. Karen Chen Stony Brook University November 1, 2010. Abstract.

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PH599 Graduate Seminar presents: Discovery of Top Quark

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  1. PH599 Graduate Seminar presents:Discovery of Top Quark Karen Chen Stony Brook University November 1, 2010

  2. Abstract • The third generation of quarks was predicted by Kobayashi and Maskawa to explain CP violation. When the bottom quark was discovered, the search to find its isospin partner began. The discovery of the top quark completes the family of six quarks in the Standard Model. Measurements of the top quark mass were conducted by the CDF and D0 experiments at the Fermilab Tevatron. The top quark mass was measured from events consistent with top pair production. The top pairs decay into a pair of bottom quarks and a pair of W bosons with a nearly 100% branching ratio. The experiments looked at events that result in either dilepton or lepton plus jets final states. It is possible for the W bosons to both decay into quarks but measurements based on events with all jets have low precision. More precise mass measurements were conducted after the top quark’s initial discovery. The experimental uncertainty associated with the top quark and W boson mass puts constraints on the mass of the Higgs boson. The high precision of the top quark mass may have important implications on the validity of the Standard Model.

  3. Outline • Discovery • 3rd generation quarks to explain CP violation • Direct Measurement of Top Quark Mass • Experiments at the Tevatron, detector basics • Decay of top pairs • Possible final states: Dilepton, l+jets • Event Selection • Signatures of signal and background processes • Likelihood fits for mt • Top Quark Mass Relevance Today • Constraints on Higgs mass

  4. Discovery of the top quark • 1964 - CP violation found in kaons • 1973 – (Cabibbo), Kobayashi and Maskawa • Need a third generation of quarks to explain CP violation • 1977 – Bottom quark discovered! • And thus begins the search for its isospin partner, the top quark

  5. Discovery of the top quark • Tevatron – particle accelerator at Fermilab • Two experiments: • Collider Detector at Fermilab • D0 (or DZero) • Proton, antiproton collisions • CDF and D0 measured the top quark mass

  6. Measurement of Top Quark Mass • Invariant Mass, m = m(E,p) • E2 = (pc)2 + (mc2)2 • Or more conveniently*: • m2 = E2 – p2 = P2, where P is four vector momentum, • P2 = E2 – p2 = E2 - px2 - py2 - pz2 • Example of a two body decay • A  B+C • mA2 = (PB+ PC)2 • To find top quark mass, we need the energy and momentum of the decay products. • Note: It is convenient to measure everything in units of GeV, so c is set to 1.

  7. Decays of top pairs • Top pair decay with branching ratio ~100% • W decay Branching ratios • We ~1/9 • Wm ~1/9 • Wt ~1/9 • Wqq ~2/3

  8. Decays of top pairs • Top pair decay with branching ratio ~100% • Final decay products • Both W’s decay into leptons • tt->bb + ll “Dilepton final state” • One decays into a lepton, the other into quarks • tt->bb + l+ qq “Lepton + jets” • Both W’s decay into quarks • tt->bb + qqqq “All Jets”, “fully hadronic”

  9. Decays of top pairs • What are “jets?” • Why don’t you see a single quark? • Quark confinement • Gravity, EM ~ 1/r2 • Strong force increases with distance! • Collision produces quarks • Energy grows with distance • More quarks are created • Can combine to form hadrons

  10. Measurement of Top Quark Mass • Dilepton (~4.5%) • Muons or electrons • Pure signal, low yield • l+jets (~30%) • Moderate yield and bg • All jets (~44.5%) • Large backgrounds • Tau channels (~21%) • At least one W decays into a t • Hard to identify t decays • Short lifetime, hadronize quickly • Tevatron Average: mt = 173.1 ± 0.6 (stat.) ± 1.1(syst.) GeV/c2 Hobbs et al.

  11. Signal (Dilepton) Both have final states of ee pair and two jets. What’s the difference? Neutrinos appear in the signal process. Problem: Neutrinos are weakly interacting, we can’t really see them! Background (Diboson) Background sources

  12. Abazov 2009 Background discrimination: ET • Total ET = 0, Missing ET must be from neutrinos. • Dilepton: ET > 35GeV • l+jets: ET > 15GeV

  13. Signal Both have W boson pair, so ET may be the same. What’s the difference? The signal has two bottom quarks. You expect more jets in the signal than in the background. Can you check if the jet is from a bottom quark or a lighter quark? Background Background sources

  14. b-tagging efficiency ~50% per jet Misidentification P(b-tag|q) = 1% P(b-tag|c) = 15% t t~10-13s C W+ t W- t~10-12s b Vertex is farther b-tagged jet b-tagging

  15. ≥ Background discrimination: # jets l + jets final state with one b-tagged jet Dilepton final state Expect 2 jets Expect 4 jets http://www-d0.fnal.gov/

  16. Background discrimination: # jets η = -ln(tan(θ/2)) θ: azimuthal angle from beam line CDF detector crack at η = 1.1 http://www-cdf.fnal.gov/physics/new/top/2004/jets/cdfpublic.html

  17. Measurement of Top Quark Mass • At this point: • Have candidate tt events (using ET, b-tagging, and other cuts) with lowered background • Few unknowns • Neutrino momentum • Jet combinatorics • What you can do: • A mix of template method and weighing methods that depend on the kinematic observables to determine a best fit for mt.

  18. Hobbs et al. Measurement of Top Quark Mass • Example: l+jets • Consider jet combinatorics under these constraints: Likelihood function as a function of jet energy scale and Mt. Energy = [1+DJES] f(s) DJES = 0 -> perfect calibration

  19. Constraints on Higgs mass • Higgs boson explains why weak force carriers, W and Z, are massive. • The mass of the top quark is HUGE! compared to other elementary particles. • Higgs mass is related to W boson and top mass • DMW ~ log(MH) • DMW ~ Mt2

  20. Constraints on Higgs mass Results from 1995 Results from 2006 Heinemeyer et al., 2006 P. Renton 1995

  21. Summary • The main decay channel used for top quark mass measurement: • With appropriate cuts (ET, # of jets, b tagging), you can increase the purity of the tt signal. • Mass measurement of top quark was done with likelihood fits of mt using a combination of template and weighting methods. • Precision of top quark and W boson mass puts constraints on Higgs mass.

  22. References • M. Kobayashi, T. Maskawa (1973). "CP-Violation in the Renormalizable Theory of Weak Interaction". Progress of Theoretical Physics 49 (2): 652–657. • P Renton, arXiv:hep-ph/0206231v2 1 Aug 2002 • P. Renton, Review of Experimental Results on Precision Tests of Electroweak Theories, Lepton-Photon 95, p35 (1995), published by World Scientific. • P. Renton, arXiv:0809.4566v1 [hep-ph] 26 Sep 2008 • S. Heinemeyer, W. Hollik, D. St ockinger, A.M. Weber, G. Weiglein, arXiv:hep-ph/0604147v2 10 Oct 2006 • V. Abazov et al. Measurement of the top quark mass in final states with two leptons. Phys. Rev., D80:092006,2009. • http://www-d0.fnal.gov/Run2Physics/WWW/results/summary.htm • John D. Hobbs, Mark S. Neubauer, Scott Willenbrock, Tests of the Standard Electroweak Model at the Energy Frontier, arXiv:1003.5733, 2010.

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