Hadronic Z decay data taken with the ALEPH detector at LEP1 are used to measure the three-jet rate as well as moments of various event-shape variables. The ratios of the observables obtained from b-tagged events and from an inclusive sample are determined. The mass of the b quark is extracted from a fit to the measured ratios using a next-to-leading order prediction including mass effects. Taking the first moment of the y3 distribution, which is the observable with the smallest hadronization corrections and systematic uncertainties, the result is: mb(MZ) = [3.27+-0.22(stat) +-0.22(exp)+-0.38(had)+-0.16(theo)] GeV/c2. The measured ratio is alternatively employed to test the flavour independence of the strong coupling constant for b and light quarks.
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An experimental investigation of the structure of identified quark and gluon jets is presented. Observables related to both the global and internal structure of jets are measured; this allows for test
The measured jet broadening distributions (B) in quark and gluon jets seperately.
Measured distributions of -LN(Y2), where Y2 is the differential one-subjet rate, that is the value of the subjet scale parameter where 2 jets appear from the single jet.
The mean subjet multiplicity (-1) for gluon jets and quark jets for different values of the subject resolution parameter Y0.
The inclusive production of η mesons has been studied using 1.6 million hadronic Z decays collected with the L3 detector. The η multiplicity per event, the multiplicity for two-jet and three-jet events separately, and the multiplicity in each jet have been measured and compared with the predictions of different Monte Carlo programs. The momentum spectra of η in each jet have also been measured. We observe that the measured η momentum spectrum in quark-enriched jets agrees well with the Monte Carlo prediction while in gluon-enriched jets it is harder than that predicted by the Monte Carlo models.
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We present the first measurement of the correlation between the $Z^0$ spin and the three-jet plane orientation in polarized $Z^0$ decays into three jets in the SLD experiment at SLAC utilizing a longitudinally polarized electron beam. The CP-even and T-odd triple product $\vec{S_Z}\cdot(\vec{k_1}\times \vec{k_2})$ formed from the two fastest jet momenta, $\vec{k_1}$ and $\vec{k_2}$, and the $Z^0$ polarization vector $\vec{S_Z}$, is sensitive to physics beyond the Standard Model. We measure the expectation value of this quantity to be consistent with zero and set 95\% C.L. limits of $-0.022 < \beta < 0.039$ on the correlation between the $Z^0$-spin and the three-jet plane orientation.
Asymmetry extracted from formula: (1/SIG(Q=3JET))*D(SIG)/D(COS(OMEGA)) = 9/16*[(1-1/3*(COS(OMEGA))**2) + ASYM*Az*(1-2*Pmis(ABS(COS(OMEGA))))*COS(OMEGA)], where OMEGA is polar angle of [k1,k2] vector (jet-plane normal), Pmis is the p robability of misassignment of of jet-plane normal, Az is beam polarization. Jets were reconstructed using the 'Durham' jet algorithm with a jet-resol ution parameter Yc = 0.005.
Quark and gluon jets with equal energies are identified in three-jet hadronicZ0 events, using reconstructed secondary vertices from heavy quark decay in conjunction with energy orderi
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Distributions are presented of event shape variables, jet roduction rates and charged particle momenta obtained from 53 000 hadronicZ decays. They are compared to the predictions of the QCD+hadronization models JETSET, ARIADNE and HERWIG, and are used to optimize several model parameters. The JETSET and ARIADNE coherent parton shower (PS) models with running αs and string fragmentation yield the best description of the data. The HERWIG parton shower model with cluster fragmentation fits the data less well. The data are in better agreement with JETSET PS than with JETSETO(αS2) matrix elements (ME) even when the renormalization scale is optimized.
Sphericity distribution.
Sphericity distribution.
Aplanarity distribution.
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Three different methods are used for extraction Alphas value (see text for details). Systematical errors with C=HADR and C=THEOR are due to hadronization correction and theoretical uncertainties.
We have measured the ratio of the strong coupling constants α s for bottom quarks and light quarks at the Z 0 resonance, in order to test the flavour independence of the strong interaction. The coupling strength α s has been determined from the fraction of events with three jets, measured for a sample of all hardronic events, and for inclusive muon and electron events. The b purity is evaluated to be 22% for the first data set and 87% for the inclusive lepton sample. We find α s ( b ) α s ( udsc ) =1.00± 0.05 ( stat. )±0.06 ( syst. ) .
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We present a study of 43 000 3-jet events from Z 0 boson decays. Both the measured jet energy distributions and the event orientation are reproduced by second order QCD. An alternative model with scalar gluons fails to describe the data.
Jets are ordered according their energy: E1 > E2 > E3.
We have observed hadronic final states produced in the decays of Z bosons. In order to study the parton structure of these events, we compare the distributions in sphericity, thurst, aplanarity, and number of jets to the predictions of several QCD-based models and to data from lower energies. The data and models agree within the present statistical precision.
Corrected event shape distributions.
Corrected event shape distributions.
Corrected event shape distributions.