Particle Physics II

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 ?