The inclusive cross section times branching ratio for J/ψ→e + e − has been measured in the forward region in the UA6 experiment for p p and pp collisions at √ s = 24.3 GeV. The ratio of the pp to p p cross sections is found to be 0.76 ± 0.14 ± 0.06. This demonstrates that gluon-gluon fusion dominates over quark-antiquark annihilation in the formation of the c c state.
Extrapolation to full forward hemisphere (yrap > 0.0) uses the Bourquin-Gaillard parametrization (NP B114(76)334).
Extrapolation to full forward hemisphere (yrap > 0.0) uses the Bourquin-Gaillard parametrization (NP B114(76)334).
No description provided.
The TPC/Two-Gamma Collaboration has measured the inclusive cross section for production of charmed D ∗± mesons in photon-photon collisions. The reaction utilized was e + e - →e + e - D ∗± X, with D ∗± →D O π +- , D O →K -+ π ± , and either zero or one outgoing e ± detected. The result, σ(e + e - → e + e - D ∗± X) = 74±26±19 pb , is in agreement with the quark parton mo del prediction for e + e - → e + e - c c , combined with a Lund model for the hadronization of the charmed quarks.
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This paper presents an analysis of the multiplicity distributions of charged particles produced inZ0 hadronic decays in the DELPHI detector. It is based on a sample of 25364 events. The average multiplicity is <nch>=20.71±0.04(stat)±0.77(syst) and the dispersionD=6.28±0.03(stat)±0.43(syst). The data are compared with the results at lower energies and with the predictions of phenomenological models. The Lund parton shower model describes the data reasonably well. The multiplicity distributions show approximate KNO-scaling. They also show positive forward-backward correlations that are strongest in the central region of rapidity and for particles of opposite charge.
Charged particle multiplicity distribution for the raw data in full phase space.
Charged particle multiplicity distribution for full phase space. Errors include systematics. A 2 pct correction for excess electrons from photon conversions is not included. The first two points, at N=2 and 4, were not measured but taken from the Lund PS model.
Charged particle multiplicity distribution for single hemisphere. Errors include systematics. A 2 pct correction for excess electrons from photon conversions is not included.
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.
We have measured the polarization of D*, the energy dependence of the polarization, and the spin-density matrix of D* in e+e− annihilation at a center-of-mass energy of 29 GeV using the Time Projection Chamber detector at the SLAC storage ring PEP. In 147 pb−1 of data we see no strong evidence for polarization, alignment, or final-state interactions in this fragmentation process.
Polarization is the factor alpha(z) in the expression d width (D*-->D pi)/domega = C(1+alpha(z)cos(theta)**2).
Spin density matrices for D* --> D0 pi+.
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.
Absolute π±d differential cross sections and charge asymmetries have been measured at an incident pion energy of 65 MeV, using an active target of deuterated scintillator plastic to detect recoil deuterons in coincidence with scattered pions. Statistical and systematic uncertainties in the cross sections are each typically ±3%. The charge asymmetry is consistent with theoretical predictions.
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The first spin-transfer observables for the πd→pp reaction have been measured at a number of energies spanning the Δ resonance in this system. These parameters correspond to KSL and KSS of the pp→dπ reaction for incident proton energies ranging from 600 to 800 MeV. Such data can provide an important constraint on the determination of the partial-wave amplitudes describing this fundamental reaction. The discrepancies between our data, theoretical predictions, and values calculated from published partial-wave amplitudes demonstrate the need for further work in this area.
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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.
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