Showing papers by "S. O. Holmgren published in 1996"

Tuning and test of fragmentation models based on identified particles and precision event shape data

Abstract: Event shape and charged particle inclusive distributions are measured using 750000 decays of the Z to hadrons from the DELPHI detector at LEP. These precise data allow a decisive confrontation with models of the hadronization process. Improved tunings of the JETSET, ARIADNE and HERWIG parton shower models and the JETSET matrix element model are obtained by fitting the models to these DELPHI data as well as to identified particle distributions from all LEP experiments. The description of the data distributions by the models is critically reviewed with special importance attributed to identified particles.

Energy dependence of the differences between the quark and gluon jet fragmentation

Abstract: Three jet events arising from decays of the Z boson, collected by the DELPHI detector, were used to measure differences in quark and gluon fragmentation. Gluon jets were anti-tagged by identifying b quark jets. Unbiased quark jets came from events with two jets plus one photon. Quark and gluon jet properties in different energy ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same energy in symmetric three jet event topologies were also compared. Using three independent methods, the average value of the ratio of the mean charged multiplicities of gluon and quark jets is = 1.241 ± 0.015 (stat.) ± 0.025 (syst.). Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent energy. The string effect has been observed in fully symmetric three jet events. The measured ratio R_{gamma} of the charged particle flow in the qoverline{q} inter-jet region of the qar{q}g and qar{q}gamma samples agrees with the perturbative QCD expectation. The dependence of the mean charged multiplicity on the hadronic center-of-mass energy was analysed in photon plus n-jet events. The value for alpha_s(M_Z) determined from these data using a QCD prediction with corrections at leading and next-to-leading order is αs(MZ)= 0.116 ± 0.003 (stat.) ± 0.009 (syst.).

Study of rare $b$ decays with the DELPHI detector at LEP

Abstract: Rare decays of beauty particles were studied in several charmless modes using the data collected with the DELPHI detector at LEP from 1991 to 1994. These decays are mediated by both tree levelb→u and one-loop penguin b → s, d transitions. Evidence for charmlessB decays was obtained in two body hadronic modes. The branching ratios ofB d,s 0 toπ + π − orK + π − andB u − toρ 0 π − orK*0 π − were found to be (2.8 −1.0 +1.5 ±0.2)×10−5 and (1.7 −0.8 +1.2 ±0.2)×10−4 respectively. The fraction of these decays with a charged kaon in the final state that is not from the spectators quark, was measured to be 0.58±0.18. Upper limits were set at 90% confidence level on the branching ratios for three and four body charmless hadronic decays in the range of (1–3)×10−4, for inclusive radiativeb → sγ decays at 5.4×10−4, for the exclusive radiative decaysB d 0 → K*(892)0 γ andB s 0 → φ(1020)γ at 2.1×10−4 and 7.0×10−4 respectively, and for dineutrino decays b → sννˉ , in the exclusive channels B0d → K∗(892)0ννˉ and B0s → ϕ(1020)ννˉ at 1.0×10−3 and 5.4×10−3 respectively. The limits on dineutrino decays constrain theories with a newU(1) gauge boson coupling predominantly to the third family of fermions.

Measurement of the B d 0 oscillation frequency using kaons, leptons and jet charge

Abstract: A measurement of the mass difference,Δmd, between the two physical Bd0 states has been obtained from the analysis of the impact parameter distribution of a lepton emitted at large transverse momentum (pt) relative to the jet axis and from the analysis of the flight distance distribution of secondary vertices tagged by either a highpt lepton or an identified kaon. In the opposite hemisphere of the event, the charge of the initial quark has been evaluated using a highpt lepton, a charged kaon or the mean jet charge. With 1.7 million hadronic Z0 decays recorded by DELPHI between 1991 and 1993,Δmd is found to be: $$\Delta m_d = 0.531_{ - 0.046}^{ + 0.050} (stat.) \pm 0.078 (syst.) ps^{ - 1} .$$ .

First study of the interference between initial and final state radiation at the Z resonance

Abstract: The interference between initial and final state radiation in the processe+e−→μ+μ− at √s≈MZ has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons The interference is expected to be sensitive to the space-time separation of the initial and final state radiation The measured asymmetry distribution has been compared to theoretical predictions using thekoralz generator, with and without\({\mathcal{O}}(\alpha )\) interference The magnitude of the interference between initial and final state radiation was found to be of the order predicted and to follow the expected distribution Using the theoretical predictions, a value of $$\Gamma _{\rm Z} = 250 \pm 021 (stat) \pm 006 (syst) GeV$$ has been extracted The interpretation of this result is discussed There is an additional uncertainty in the estimate ofΓZ from as yet uncalculated higher order interference terms By assuming a value ofΓZ consistent with the world average, the data were used to estimate the size of these uncalculated corrections