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Improved measurements of the b quark mass at LEP

Improved measurements of the b quark mass at LEP. Motivations to measure m b at M Z . Observables sensitive to m b at M Z . Experimental strategy. Systematic uncertainties. Results (DELPHI and OPAL). Conclusions. María José Costa (IFIC-València) ICHEP 2002 Amsterdam.

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Improved measurements of the b quark mass at LEP

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  1. Improved measurements of the b quark mass at LEP • Motivations to measure mb at MZ. • Observables sensitive to mb at MZ. • Experimental strategy. • Systematic uncertainties. • Results (DELPHI and OPAL). • Conclusions. María José Costa (IFIC-València) ICHEP 2002 Amsterdam

  2. Motivations to measure mb at MZ • Direct and independent measurement of the b mass at high energy. • In the case of mb(), test of the evolution with the scale. • mb(MZ) is a basic input parameter to test mb-m unification • predicted by GUT models at MGUT ~1016 GeV. M.J. Costa

  3. q e+ e- Observables sensitive to mb at MZ • Due to the mass effect, massive quarks • radiate less gluons than light quarks. q • For inclusive observables, b mass effects are negligible at LEP • (mb2/mZ2 0.3%). • For jet rates:  yc  (mb2/mZ2)/yc sizeable effects. • The observable used: NLO for n = 3 jets LO for n = 4 jets DELPHI: n = 3, 4 jets Durham, Cambridge. OPAL: n = 3 jets Jade, E0,P, P0, E,Geneva,Durham. mb(MZ) s universality M.J. Costa

  4. DELPHI DELPHI TAGGING Experimental Strategy Hadronic selection Jet reconstuction Tagging Data Detector simulation Hadronization Pythia Herwig M.J. Costa

  5. Detector and physics modelling: Hadronization: DELPHI yc Systematic uncertainties • Detector resolution. • Tagging (b, Rb, nchB-decays, b,, c). • gluon splitting rates. • Tuning of the fragmentation parameters. • Model (string or cluster). • b fragmentation function. • b mass parameter in the generator. M.J. Costa

  6. DELPHI results Durham: Cambridge: M.J. Costa

  7. First look into 4 jets Modelling of the generators DELPHI Data lies between the LO predictions for Mb = 3 - 4.8 GeV. M.J. Costa

  8. OPAL results The 7 results of mb(MZ) were combined: M.J. Costa

  9. Conclusions • All measurements of mb(MZ) • are compatible. • The shift mb(M/2) -mb(MZ) • has been observed with almost • 4 in agreement with the QCD • prediction. • The values obtained for Mb • (4.2 GeV) are lower than the • ones obtained at low energy. • A better understanding of the • mass entering in the MC could • lead to a precision on mb(MZ) of • about 250-300 MeV. • b mass effects have been • measured with 4 jet events! NLO calculations are welcome! M.J. Costa

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