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

2 Top Z0Z0 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

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 decay, t  Wb Top decays before hadronising

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+ +≥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 NObserved events Observed b-tags Background tags expected ± ± ±1.7 ≥ ±0.4 M t =176±8±10GeV/c 2 Statistical discovery - Example of selection / backgrounds:

7 Higgs searches - indirect Virtual loop diagrams Additional terms in calculation Measurements of M W and m top constrain Higgs mass logarithmic dependence W-W- t WW b H Z/W mH= GeV (Aug. 2009)  m t 2  ln(m H )

8 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(<1TeV) –Couples to mass Decays into heaviest particles BR LEP LHC/Tevatron

9 Hunting the Higgs at LEP LEP beam energy raised to maximum of ~103GeV M H ~2E b -M Z  Sensitive to M H <115GeV  Dominant Higgs decay is H  bb  Explain why!  Identify b from flight distance  B lives for s  Travels d=γct Z0Z0 H0H0 In detector: Process: Primary Vertex Secondary vertex jet b quark d Focus on b’s: B tagging principle:

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

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

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

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

14 Backgrounds gg  H  bb looks ideal at around 115GeV (  ~0.03nb) BUT gg  bb  ~10 6 nb need to background rejection at level of 10 8 This is not practical, look for distinctive decays

15 H  ATLAS Low mass Higgs – example channel Search for distinctive signature H   for 100<M H <120GeV 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 ?

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

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 ?