Particle Physics II 2 nd Handout Top Quark

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

Top • Top very heavy – 171. 4± 2. 1 Ge. V – Similar to mass of Gold atom • t-->Wb – Decays before hadronisation – Very different event structure from other quark decays Z 0

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

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

Discovery of the top quark • Top discovered at the Tevatron – p-anti p – 1. 8 Te. V collision energy • Initial event selection – Large backgrounds from all hadronic events – Use l+n+≥ 3 jets – Large top mass large transverse momentum

Backgrounds in top quark • Backgrounds from W+jets – q+q W+≥ 3 jets • Use b-tagging to reject W+jet events and retain top events • B tagging discussed later Statistical discovery Example of selection / backgrounds: N Observed Background events b-tags expected 1 6578 40 50± 12 2 1026 34 21. 2± 6. 5 3 164 17 5. 2± 1. 7 ≥ 4 39 10 1. 5± 0. 4 Mt=176± 8± 10 Ge. V/c 2 6

Higgs searches - indirect • Virtual loop diagrams • Additional terms in calculation • Measurements of MW and mtop constrain Higgs mass W - H t W logarithmic dependence m. H=87+35 -27 Ge. V (Aug. 2009) b m t 2 W Z/W ln(m. H)

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

Hunting the Higgs at LEP • LEP beam energy raised to maximum of ~103 Ge. V Process: • MH~2 Eb-MZ Sensitive to MH<115 Ge. V In detector: H 0 Dominant Higgs decay is H bb Explain why! Identify b from flight distance B lives for Travels d=γct 10 -12 s Z 0 B tagging principle: Focus on b’s: b quark Primary Vertex d Secondary vertex jet

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

Higgs searches at LEP 2 Jets + 2 muons: Jets not good b tag • LEP sees a few possible events but not enough! MH>114. 4 Ge. V at 95%CL If no events observed then 115. 3 Ge. V Four Jets: Possible b-tags 11

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

Higgs searches at the Tevatron & LHC R. St. Denis, A. Robson et al. T. Doyle et al. • Most common: production: gg H~30 pb decay: Br(H bb)~0. 99 s(gg H bb)~30 pb • Sometimes the most common reaction is not the one we can see! – gg H bb BR Events for -1 5 fb 1010 104 103 Leading order 10

Backgrounds • gg H bb looks ideal at around 115 Ge. V (s~0. 03 nb) BUT gg bb s~106 nb need to background rejection at level of 108 • This is not practical, look for distinctive decays

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

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

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