Improved measurements of the b quark mass at LEP

María José Costa (IFIC-València)

ICHEP 2002 Amsterdam

• Motivations to measure mb at MZ.• Observables sensitive to mb at MZ.• Experimental strategy.• Systematic uncertainties.• Results (DELPHI and OPAL).• Conclusions.

27 July 2002 M.J. Costa 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 unificationpredicted by GUT models at MGUT ~1016 GeV.

27 July 2002 M.J. Costa 3

Observables sensitive to mb at MZ

llZtotc

gnllZnj

bbZtotc

gnbbZnj

cbln

y

yyR

00

00

/)(

/)()(

)2(

)2(

DELPHI: n = 3, 4 jets Durham, Cambridge.

OPAL: n = 3 jetsJade, E0,P, P0, E,Geneva,Durham.

• Due to the mass effect, massive quarks radiate less gluons than light quarks.

mb(MZ) s universality

• For inclusive observables, b mass effects are negligible at LEP (mb

2/mZ2 0.3%).

• For jet rates: yc (mb2/mZ

2)/yc sizeable effects.• The observable used:

e+

e-

q

q

NLO for n = 3 jetsLO for n = 4 jets

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Experimental Strategy

Data

Hadronicselection

Jet reconstuction

qqZ 0

llZ

bbZ0

0

Tagging

detblnR

Detector simulation

hadblnR

partblnR

DELPHIDELPHITAGGING

PythiaHerwig

blR3

Hadronization

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Systematic uncertainties• Detector resolution.• Tagging (b, Rb, nch

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.

yc

blR3

DE

LP

HI

OPAL DURHAM R3bl(yc=0.01)

ResultStatisticsDetector simulation|cos(thrust)|0.750.05L/L

8Ncharged (decay) = 4.9550.062

b = (1.5640.014) psRb = 0.21750.013b quark fragmentationc quark fragmentationb = (0.52 0.05) GeV2

q = (0.40 0.05) GeV

Q0 = (1.90 0.05) GeVMb

JETSET = (5.0 0.05) GeV

0.95320.00560.01430.00110.00410.00100.00050.00070.00590.00010.00130.00120.00360.0000

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DELPHI results

)theo( 28.0)syst()stat(99.2)( 34.037.0

25.027.0

Zb Mm

)theo( 06.0)syst()stat( 19.082.2)( 31.033.0

Zb Mm

Durham:

Cambridge:)theo( 002.0)syst( 007.0)stat( 004.0001.1/ ls

bs

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Modelling of the generatorsFirst look into 4 jets

Data lies between the LOpredictions for Mb = 3 - 4.8 GeV.

DELPHI

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OPAL results

The 7 results of mb(MZ)were combined:

)theo( 19.0)syst()stat( 03.067.2)( 29.037.0

Zb Mm

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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.

• b mass effects have been measured with 4 jet events!NLO calculations are welcome!

• 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) ofabout 250-300 MeV.