Showing papers by "F. Hahn published in 1996"

Performance of the DELPHI detector

Abstract: DELPHI (DEtector with Lepton, Photon and Hadron Identification) is a detector for e^+e^- physics, designed to provide high granularity over a 4\pi solid angle, allowing an effective particle identification. It has been operating at the LEP (Large Electron-Positron) collider at CERN since 1989. This article reviews its performance.

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.

Current achievements of the DELPHI ring imaging Cherenkov detector

Abstract: The DELPHI experiment has already collected 2.5 million Z 0 decays with the ring imaging Cherenkov detector (RICH) operational. This detector, covering most of the solid angle, is designed to perform π K separation from 0.8 to 20 GeV/ c and K/p separation from 0.8 to 35 GeV/ c . After a brief detector description we discuss the actual operating conditions, the data monitoring and the signal treatment. The collected data from Z 0 decays and the detector response to signals from the calibration system are used to evaluate the performance of the RICH system.

Mean lifetime of the $B_s^0$ meson

Abstract: This paper presents an update of the measurement of the mean lifetime of theBs0 meson. CombiningDs−l,Ds−h,φ−l and inclusiveDs final states from the 3.2 million hadronic Z decays collected by DELPHI between 1991 and 1994, theBs0 mean lifetime was measured to be:τ(Bs0)=1.67±0.14 ps.

Determination of the average lifetime of b -baryons

Abstract: The average lifetime ofb-baryons has been studied using 3×106 hadronicZ0 decays collected by the DELPHI detector at LEP. Three methods have been used, based on the measurement of different observables: the proper decay time distribution of 206 vertices reconstructed with aΛ, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by aΛ in the same jet; and the proper decay time distribution of 125Λc-lepton decay vertices with theΛc exclusively reconstructed through itspKπ,pK0 andΛ3π decay modes. The combined result is: $$\begin{gathered} \tau (b - baryon) = (1.254_{ - 0.109}^{ + 0.121} (stat) \hfill \\ \pm 0.04(syst)_{ - 0.05}^{ + 0.03} (syst)) ps \hfill \\ \end{gathered} $$ where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of theb-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the averageb-baryon lifetime is: $$\tau (b - baryon) = (1.255_{ - 0.102}^{ + 0.115} (stat) \pm 0.05) ps$$ .

Particle identification algorithms for the DELPHI RICH detector

Abstract: Particle identification in the DELPHI experiment at LEP relies heavily on the use of RICH counters. Information about the mass of a charged particle traversing the RICH is obtained both by the observed number of Cherenkov photons and by their emission angles. Algorithms in use for the extraction of mass information from these quantities are presented, and examples of their application in different areas of physics analysis are given.

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: $$\Delta m_d = 0.531_{ - 0.046}^{ + 0.050} (stat.) \pm 0.078 (syst.) ps^{ - 1} .$$ .

Determination of /Vcb/ from the semileptonic decay B0 → D*-ℓ+ν

Abstract: Semileptonic decays B→ D*−l+νX were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D*−π+ states was performed and the ratio of the branching fractions: $$\frac{{Br(B \to D^* \ell ^ + X)}}{\begin{gathered} Br(B \to D^* \ell ^ + vX) + Br(B^0 \to D^{* - } \ell ^ + v) \hfill \\ = 0.19 \pm 0.10(stat) \pm 0.06(syst) \hfill \\ \end{gathered} }$$ was determined. Taking into account this contribution, the differential decay width of B0→D*−l+l+ν was measured as a function of the momentum transfer from the B to the D*− in two separate analyses, using exclusive and inclusive methods of D*− reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of |Vcb| times the normalization of the decay form factorF(qmax2): $$F(q_{max}^2 )|V_{cb} | = (35.4 + 1.9(stat) \pm 2.4(syst)) \cdot 10^{ - 3} $$ . The value of |Vcb| was computed using theoretical calculations ofF(qmax2), giving: $$|V_{cb} | = (39.9 \pm 2.0(stat) \pm 2.6(syst) \pm 1.7(theory)) \cdot 10^{ - 3} $$ . The total branching fractionBr(B0→D*−l+gn) was determined to be: $$Br(B^0 \to D^{* - } \ell ^ + v) = (5.52 \pm 0.17(stat) \pm 0.68(syst))\% $$ .

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