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Z  b  b Physics and Calibration

Z  b  b Physics and Calibration. Small physics reminder Z  b  b vs  + jet Selection w/o STT, Background rejection Calibration procedures A few plots on the Hit-Root-tuples. Higgs Search. If M higgs < 135 GeV promising channel: p  p  WH  l   b b

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Z  b  b Physics and Calibration

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  1. Z bb Physics and Calibration • Small physics reminder • Z bb vs  + jet • Selection w/o STT, Background rejection • Calibration procedures • A few plots on the Hit-Root-tuples

  2. Higgs Search If Mhiggs < 135 GeV promising channel: pp  WH  l  b b Signal to background depends crucially on b b mass reconstruction resolution Can we go from 15% resolution to <10% ?

  3. Higgs Search Significance evolution with resolution at Mhiggs=120 GeV bb mass Resolution: 15% 10% Signal events/fb-1 4 4 Wbb 59 32 WZ 11 6 tt 34 24 single top 14 10 For 10 pb-1

  4. Z bb vs  + jet  + jet : high statistics, allows for a tight b-jet selection (b-tagging). Systematics are not straightforward. Z bb: very low signal/noise but systematics closer to physics processes (H or Top), due to high pT . Resonance mass independent of mult. interactions. Complementarity !!

  5. Z bb selection D0 will start without STT (and run this way about a year) Can we select these events w/o STT? CDF Run I S/N below 10-3 implies very stringent selection. On the total Run I lumi, only 45 events left, starting from about 120 000 Z bb events Trigger: central muon (pT> 7.5 GeV)  5.5 M evnts Offline: request 2 tagged (0.7 cone) jets  5479 evnts According to Pythia: expect 124 +/-14 evnts Needs further topological cuts

  6. 3ET and 12 • Signal and background are produced differently: Z is produced by a time-like q-qbar anihilation, Background is QCD induced. Color flow between initial and final partons only in the bckgd. Z is expected to have soft radiation between the jets, Background will also have strong radiation between IS and FS partons. Use 2 kinematic variables to discriminate • 3ET : sum of ET of the clusters outside the 2 leading jets • 12 : azimuthal angle difference between the 2 jets

  7. QCD and E_Weak Samples • Discriminating power of the 2 variables 3ET and 12 on 2 samples of different origin

  8. Z bb Signal • 3ET < 10 GeV 12 > 3 rad • Now S/N=1/6 at the Z mass peak • Select/antiselect w.r.t. the 2 var., tag probability • 3.2  excess • Background only

  9. Z bb Sample 3ET 

  10. Likelihood fit to the Dijet Sample • Signal + Bckgd • Background only • Results: MZ=90.0 +/- 2.4 GeV Z = 9.4 +/- 3.5 GeV NZ=91+/30(st.)+/19(sys.)

  11. Neural Net ? • With only 2 topological cuts, strong background reduction. • Neural net on further variables will improve purity, but input variables must be decorrelated from calibration variables. • With 0.4 fb-1, if S/N better than 1/2 precision on absolute scale may be better than 1% • With the STT, much purer sample, i.e. expect furter improvement

  12. How to calibrate??? • On detailed MC, compare b-jets with light-q jets. Root-tuples available for b-jets (Hit), on request for q-jets. • Expect: wider b-jets (due to the large b-mass) , muons from semileptonic decays (to be corrected). study track multiplicity, and E/p to improve the jet response using Track+Calo • Devise a relative calibration b-jets vs q-jets from MC • Check it on the Z bb data sample selectionned with a Neural Net, while awaiting for the STT

  13. Back To Earth: Root-tuples Hit Group Root-tuples : follow the colour code: Z-> bb , Z-> tau-tau , Z-> ee , Z-> mu-mu 8000 Events in each sample. Includes min. bias events To start: Compare Basic distributions

  14. Conclusions • Hit Group Root-tuples are a good Tool to start • A few variables could be added, others improved • Need of Z -> qq to make real progress • Selection of Z->bb will require Neural Net • Calibration of b-jets will take time, but start now to implement track+cell for jets

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