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Exploring Particle Physics: Top Quark Discovery and Higgs Searches

Delve into the discovery of the top quark, its unique decay properties, and the search for the elusive Higgs boson in high-energy particle collider experiments. Learn about indirect evidence for the top quark’s existence and the challenges in identifying Higgs decays. Explore the techniques used to hunt for the Higgs at the LEP, Tevatron, and LHC colliders, including b-tagging principles and event selection strategies.

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Exploring Particle Physics: Top Quark Discovery and Higgs Searches

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  1. Particle Physics II 2nd Handout • Top Quark • Discovery • Decay • Higgs Searches • Indirect • mW and mt • Direct • LEP & LHC searches Chris Parkes

  2. Top • Top very heavy • 171.4±2.1GeV • Similar to mass of Gold atom • t-->Wb • Decays before hadronisation • Very different event structure from other quark decays Z0

  3. Indirect Evidence for Top The rate at which various processes happen depend on the top quark mass • Will discuss B oscillations later From precision measurements can obtain top mass inside SM Sensitivity through virtual loops • Will discuss same concept for Higgs

  4. Top quark decay B-decays b in bound state with q Top decays before hadronising Top decay, tWb

  5. Discovery of the top quark • Top discovered at the Tevatron • p-anti p • 1.8TeV collision energy • Initial event selection • Large backgrounds from all hadronic events • Use l+n+≥3jets • Large top mass  large transverse momentum

  6. Backgrounds in top quark • Backgrounds from W+jets • q+qW+≥3jets • Use b-tagging to reject W+jet events and retain top events • B tagging discussed later Statistical discovery - Example of selection / backgrounds: Mt=176±8±10GeV/c2

  7. Higgs searches - indirect W- H t Z/W Z/W W W b • Virtual loop diagrams • Additional terms in calculation • Measurements of MW and mtop constrain Higgs mass logarithmic dependence  ln(mH)  mt2 mH=87+35-27 GeV (Aug. 2009)

  8. Searching for Higgs boson BR • Higgs boson is the missing piece of the electroweak model • Required for W and Z masses • Mass is not predicted • Unitarity/width arguments  O(<1TeV) • Couples to mass • Decays into heaviest particles LHC/Tevatron LEP

  9. Hunting the Higgs at LEP • LEP beam energy raised to maximum of ~103GeV • MH~2Eb-MZ • Sensitive to MH<115GeV • Dominant Higgs decay is Hbb • Explain why! • Identify b from flight distance • B lives for 10-12 s • Travels d=γct Process: In detector: H0 Z0 B tagging principle: jet Focus on b’s: b quark d Secondary vertex Primary Vertex

  10. LEP decay channels Znn Ztt Zee, mm Zqq Hbb Signal: b • Signal must be statistically significant compared with background • Separate with b tag, mass, angular distributions…. b Background: q q Background: Z g Z q Z q Z

  11. Higgs searches at LEP 2 Jets + 2 muons: Jets not good b tag • LEP sees a few possible events but not enough! MH>114.4GeV at 95%CL If no events observed then 115.3GeV Four Jets: Possible b-tags

  12. Indirect measurements give chi square curve Direct searches LEP & Tevatron give yellow exclusion region Higgs mass limit

  13. Higgs searches at the Tevatron & LHC BR Events for 10 fb-1 105 104 103 Leading order 10 R. StDenis, A. Robson et al. T. Doyle et al. • Most common:production: ggH~30pbdecay: Br(Hbb)~0.99s(ggHbb)~30pb • Sometimes the most common reaction is not the one we can see! • ggHbb

  14. Backgrounds • ggHbb looks ideal at around 115GeV (s~0.03nb)BUTggbb s~106nbneed to background rejection at level of 108 • This is not practical, look for distinctive decays

  15. Low mass Higgs – example channel ATLAS H • Search for distinctive signature Hgg for 100<MH<120GeV • Produced ggHbut rare decay ~ Hgg/Hbb~10-3but distinctive! • Requires high resolution electromagnetic calorimetry • Narrow peak on top of huge background • Higgs couples to mass • How can we get two photons ?

  16. ZZ - Golden channel CMS, H  ee H • If nature is kind and MH>2MZ • Golden channel • HZZ4leptons • Leptons are distinctive and well measured • Look for peak in invariant mass • Reconstruct 2lZ • Reconstruct 2ZH Z Z e+e-/m+m- e+e-/m+m-

  17. LHC Higgs Searches • Probing possible mass range requires many channels • Combination of searches over entire Higgs mass range~100GeV-1TeV • May take many years to find at LHC, but will eventually cover full range Q) What is relative BR of H-> tau tau compared with b b ? Q) Why is there a dip in the ZZ(*) curve ?

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