The first results on inclusive photoproduction of π 0 at transverse momenta up to 4 GeV/ c , using incident γ energies between 50 and 150 GeV are presented. A comparison is made with inclusive π 0 production obtained, in the same experiment, with incident π − . Using the π − data to parametrize the hadronic behaviour of the photon, significant differences are observed in quantitative agreement with QCD Compton scattering and corrections thereof.
FC,IC,BC REFER TO FORWARD,INTERMEDIATE AND BACKWARD CALORIMETERS.
FC,IC,BC REFER TO FORWARD,INTERMEDIATE AND BACKWARD CALORIMETERS.
Results are presented on the inclusive photoproduction of λ and λ for incident photon energies between 25 and 70 GeV. The slope parameter of the p T 2 distribution is found to be 2.83±0.1 GeV −2 for λ and 3.28±0.25 GeV −2 for λ . The x F distributions, measured in the range −0.2 to 0.7, show that while λ are produced centrally, λ production extends to more negative values of x F ; the shapes show no energy dependence and are similar to those in pion-induced reactions. The polarization of the produced λ is less than 10%. The results are discussed in terms of vector dominance and quark fusion models.
No description provided.
No description provided.
No description provided.
The nature of b-quark jet hadronisation has been investigated using data taken at the Z peak by the DELPHI detector at LEP. Two complementary methods are used to reconstruct the energy of weakly decaying b-hadrons, E^weak_B. The average value of x^weak_B = E^weak_B/E_beam is measured to be 0.699 +/- 0.011. The resulting x^weak_B distribution is then analysed in the framework of two choices for the perturbative contribution (parton shower and Next to Leading Log QCD calculation) in order to extract measurements of the non-perturbative contribution to be used in studies of b-hadron production in other experimental environments than LEP. In the parton shower framework, data favour the Lund model ansatz and corresponding values of its parameters have been determined within PYTHIA~6.156 from DELPHI data: a= 1.84^{+0.23}_{-0.21} and b=0.642^{+0.073}_{-0.063} GeV^-2, with a correlation factor rho = 92.2%. Combining the data on the b-quark fragmentation distributions with those obtained at the Z peak by ALEPH, OPAL and SLD, the average value of x^weak_B is found to be 0.7092 +/- 0.0025 and the non-perturbative fragmentation component is extracted. Using the combined distribution, a better determination of the Lund parameters is also obtained: a= 1.48^{+0.11}_{-0.10} and b=0.509^{+0.024}_{-0.023} GeV^-2, with a correlation factor rho = 92.6%.
The combined unfolded and weighted results, per bin, for $f(x^{\rm weak}_{\rm B})$. Quoted uncertainties have been scaled by 1.31.
The average value of the $x^{\rm weak}_{\rm B}$ distribution.
Measurements of the Xi- and anti-Xi+ masses, mass differences, lifetimes and lifetime differences are presented. The anti-Xi+ sample used is much larger than those used previously for such measurements. In addition, the Xi production rates in Z -> b anti-b and Z -> q anti-q events are compared and the position xi* of the maximum of the xi distribution in Z -> q anti-q events is measured.
Corrected LN(1/X) distribution for (XI- + XIBAR+) production.
Measured and extrapoplated production rates.
Extrapolated production rate in b-bbar events.
Infrared and collinear safe event shape distributions and their mean values are determined in e+e- collisions at centre-of-mass energies between 45 and 202 GeV. A phenomenological analysis based on power correction models including hadron mass effects for both differential distributions and mean values is presented. Using power corrections, alpha_s is extracted from the mean values and shapes. In an alternative approach, renormalisation group invariance (RGI) is used as an explicit constraint, leading to a consistent description of mean values without the need for sizeable power corrections. The QCD beta-function is precisely measured using this approach. From the DELPHI data on Thrust, including data from low energy experiments, one finds beta_0 = 7.86 +/- 0.32 for the one loop coefficient of the beta-function or, assuming QCD, n_f = 4.75 +/- 0.44 for the number of active flavours. These values agree well with the QCD expectation of beta_0=7.67 and n_f=5. A direct measurement of the full logarithmic energy slope excludes light gluinos with a mass below 5 GeV.
1-THRUST distribution.
THRUST-MAJOR distribution.
THRUST-MINOR distribution.
Measurements of the cross section and forward-backward asymmetry for the reaction e + e − → μ + μ − using the DELPHI detector at LEP are presented. The data come from a scan around the Z 0 peak at seven centre of mass energies, giving a sample of 3858 events in the polar angle region 22° < θ < 158°. From a fit to the cross section for 43° < θ < 137°, a polar angle region for which the absolute efficiency has been determined, the square root of the product of the Z 0 → e + e − and Z 0 → μ + μ − partial widths is determined to be (Γ e Γ μ ) 1 2 = 85.0 ± 0.9( stat. ) ± 0.8( syst. ) MeV . From this measurement of the partial width, the value of the effective weak mixing angle is determined to be sin 2 ( θ w ) = 0.2267 ± 0.0037 . The ratio of the hadronic to muon pair partial widths is found to be Γ h / Γ μ = 19.89 ± 0.40(stat.) ± 0.19(syst.). The forward-backward asymmetry at the resonance peak energy E CMS = 91.22 GeV is found to be A FB = 0.028 ± 0.020(stat.) ± 0.005(syst.). From a combined fit to the cross section and forward-backward asymmetry data, the products of the electron and muon vector and axial-vector coupling constants are determined to be V e V μ = 0.0024 ± 0.0015(stat.) ± 0.0004(syst.) and A e A μ = 0.253 ± 0.003(stat.) ± 0.003 (syst.). The results are in good agreement with the expectations of the minimal standard model.
Fully corrected cross sections.
Forward-backward asymmetries corrected to full solid angle, but not for cuts on momenta and acollinearity.
Effective weak mixing angle.
The multiplicity distributions of charged particles in restricted rapidity intervals inZ0 hadronic decays measured by the DELPHI detector are presented. The data reveal a shoulder structure, best visible for intervals of intermediate size, i.e. for rapidity limits around ±1.5. The whole set of distributions including the shoulder structure is reproduced by the Lund Parton Shower model. The structure is found to be due to important contributions from 3-and 4-jet events with a hard gluon jet. A different model, based on the concept of independently produced groups of particles, “clans”, fluctuating both in number per event and particle content per clan, has also been used to analyse the present data. The results show that for each interval of rapidity the average number of clans per event is approximately the same as at lower energies.
Data for both hemispheres.
Data for both hemispheres.
Data for both hemispheres.
In four-jet events from e + e − →Z 0 →multihadrons one can separate the three principal contributions from the triple-gluon vertex, double gluon-bremsstrahlung and the secondary quark-antiquark production, using the shape of the two-dimensional angular distributions in the generalized Nachtmann-Reiter angle θ NR ∗ and the opening angle of the secondary jets. Thus one can identify directly the contribution from the triple-gluon vertex without comparison with a specific non-QCD model. Applying this new method to events taken with the DELPHI-detector we get for the ratio of the colour factor N c to the fermionic Casimir operator C F : N c C F = 2.55 ± 0.55 ( stat. ) ± 0.4 ( fragm. + models ) ± 0.2 ( error in bias ) in agreement with the value 2.25 expected in QCD from N c =3 and C F = 4 3 .
NC, CF, and TR are the color factors for SU(3) group.
A determination of the partial width Γ c c ̄ of the Z 0 boson into charm quark pairs is presented, based on a total sample of 36 900 Z 0 hadronic decays measured with the DELPHI detector at the LEP collider. The production rate of cc̄ events is derived from the inclusive analysis of charged pions coming from the decay of charmed meson D ∗+ → D 0 π + and D ∗− → D ̄ 0 π − where the π ± is constrained by kinematics to have a low p T with respect to the axis. The probability to produce these π ± from D ∗± decay in cc̄ events is taken to be 0.31 ±_0.05 as measured at √ s =10.55 GeV. The measured relative partial width Γ c c ̄ Γ h = 0.162± 0.030 ( stat. ) ±0.050 ( syst. ) is in good agreement with the standard moel value of 0.171. Together with our previous measurement of the total hadronic width Γ h this implies Γ c c ̄ = 282±53 ( stat. )±88( syst. ) MeV .
No description provided.
A sample of 2.2 million hadronic Z decays, selected from the data recorded by the Delphi detector at LEP during 1994-1995 was used for an improved measurement of inclusive distributions of pi+, K+ and p and their antiparticles in gluon and quark jets. The production spectra of the individual identified particles were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, xi^*, of the xi-distributions for kaons in gluon and quark jets are observed to be different.
Jet flavor tagging is used. (C=DUSCB), (C=DUSC), (C=UDS) mean quark-jet flavors. CONST(C=GLUON/JET) is the ratio gluon/jet for all charged particles. 'Y' events, mirror symmetric events, the angle between the most energetic jet and other two jets is 150 +- 15 deg.
Jet flavor tagging is used. (C=DUSCB), (C=DUSC), (C=UDS) mean quark-jet flavors. CONST(C=GLUON/JET) is the ratio gluon/jet for all charged particles. 'Y' events, mirror symmetric events, the angle between the most energetic jet and other two jets is 150 +- 15 deg.
Jet flavor tagging is used. (C=DUSCB), (C=DUSC), (C=UDS) mean quark-jet flavors. CONST(C=GLUON/JET) is the ratio gluon/jet for all charged particles. 'Y' events, mirror symmetric events, the angle between the most energetic jet and other two jets is 150 +- 15 deg.
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