1. Improved measurements of the b quark mass at LEP María José Costa (IFIC-València) ICHEP 2002 Amsterdam 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.

2. 27 July 2002M.J. Costa2 Motivations to measure m b at M Z Direct and independent measurement of the b mass at high energy. In the case of m b (  ), test of the evolution with the scale. m b (M Z ) is a basic input parameter to test m b -m  unification predicted by GUT models at M GUT ~10 16 GeV.

3. 27 July 2002M.J. Costa3 Observables sensitive to m b at M Z DELPHI: n = 3, 4 jets Durham, Cambridge. OPAL: n = 3 jets Jade, E0,P, P0, E,Geneva,Durham. Due to the mass effect, massive quarks radiate less gluons than light quarks. m b (M Z )   s universality For inclusive observables, b mass effects are negligible at LEP (m b 2 /m Z 2  0.3%). For jet rates:  y c  (m b 2 /m Z 2 )/y c sizeable effects. The observable used: e+e+ e-e- q q NLO for n = 3 jets LO for n = 4 jets

4. 27 July 2002M.J. Costa4 Experimental Strategy Data Hadronic selection Jet reconstuction Tagging Detector simulation DELPHI TAGGING Pythia Herwig Hadronization

5. 27 July 2002M.J. Costa5 Systematic uncertainties Detector resolution. Tagging (  b, R b, n ch B-decays,  b,,  c ). gluon splitting rates. Detector and physics modelling: Hadronization: Tuning of the fragmentation parameters. Model (string or cluster). b fragmentation function. b mass parameter in the generator. ycyc DELPHI

6. 27 July 2002M.J. Costa6 DELPHI results Durham: Cambridge:

7. 27 July 2002M.J. Costa7 Modelling of the generators First look into 4 jets Data lies between the LO predictions for M b = 3 - 4.8 GeV. DELPHI

8. 27 July 2002M.J. Costa8 OPAL results The 7 results of m b (M Z ) were combined:

9. 27 July 2002M.J. Costa9 Conclusions All measurements of m b (M Z ) are compatible. The shift m b (M  /2) -m b (M Z ) has been observed with almost 4  in agreement with the QCD prediction. b mass effects have been measured with 4 jet events! NLO calculations are welcome! The values obtained for M b (  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 m b (M Z ) of about 250-300 MeV.