We report the results of a pion-electron scattering experiment to measure the charge radius of the pion. The experiment was performed in a 50 GeV/ c negative, unseparated beam at the IHEP accelerator, Serpukhov, and has been briefly reported in an earlier publication [1]. A magnetic spectrometer instrumented with wire spark chambers was used to record the incident pion trajectory and the angles and momenta of the scattered particles. Events are reconstructed by detailed trackfinding programs, and a set of kinematic and geometric cuts define the elastic sample. Electrons are identified both by kinematic criteria and pulse height information from total absorption lead glass Čerenkov counters. The final elastic sample consisted of 40 000 πe events in the region of four-momentum transfer squared 0.013 (GeV/ c ) 2 ⩽ q 2 ⩽ 0.036 (GeV/ c ) 2 . A full error matrix fit to the form factors of the pion gave the r.m.s. charge radius of the pion: 〈r π 2 〉 1 2 = (0.78 −0.10 +0.09 ) fm .
Axis error includes +- 0.7/0.7 contribution (DUE TO ACCIDENTAL ANTI-COINCIDENCES).
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A sample of Z0→τ+τ− events observed in the DELPHI detector at LEP in 1991 and 1992 is analysed to measure the τ polarisation in the exclusive decay channels\(ev\bar v\),\(\mu v\bar v\), πν, ρν and a1ν. The τ polarisation is also measured with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision than the use of the exclusive decay modes. The results have been combined with those published on the 1990 data. A measurement of the τ polarisation as a function of production angle yields the values for the mean τ polarisation 〈P〉τ=−0.148±0.022 and for the Z0 polarisationPZ=−0.136±0.027. These results are used to determine the ratio of vector to axial-vector effective couplings for taus\(\bar v_\tau/\bar a_\tau= 0.074 \pm 0.011\), and for electrons\(\bar v_e /\bar a_e= 0.068 \pm 0.014\), compatible with e-τ universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons\(\bar v_l /\bar a_l= 0.072 \pm 0.008\) is obtained, implying a value of the effective weak mixing angle sin2θefflept=0.2320±0.0021.
Results are for both TAU+ and TAU- decay.
The systematic error contains a systematic error of 0.003 common to all channels.
Errors are statistical only.
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.
Transverse momentum PTIN w.r.t. the Thrust axis. For the first table Thrust axis definition is from seen charged particles corrected to final state particles. For the second table Thrust axis definition is from seen charged plus neutral particles corrected to final state charged plus neutral particles.
Transverse momentum PTOUT w.r.t. the Thrust axis. For the first table Thrust axis definition is from seen charged particles corrected to final state particles. For the second table Thrust axis definition is from seen charged plus neutral particles corrected to final state charged plus neutral particles.
Transverse momentum PTIN w.r.t. the Sphericity axis. For the first table Sphericity axis definition is from seen charged particles corrected to final state particles. For the second table Sphericity axis definition is from seen charged plus neutral particles corrected to final state charged plus neutral particles.
In a sample of 3.02 million hadronic Z 0 decays collected by the DELPHI detector, 270 J ψ → ℓ + ℓ − candidates have been selected. A search for fully reconstructed B c ± mesons has yielded one B c ± → J ψ π ± candidate, no B c ± → J ψ ℓ ± ν ℓ candidates, and one B c ± → J ψ , π + π − π ± candidate, consistent with expected background in each channel. The following 90% confidence level upper limits are determined: Br(Z 0 → B c ± X) × Br(B c ± → J ψ π ± ) < (1.05 to 0.84) × 10 −4 and Br(Z 0 → B c ± X) × Br(B c ± → J ψ ℓ ± ν ℓ ) < (5.8 to 5.0) × 10 −5 , where the ranges quoted correspond to the range of predicted B c ± lifetimes from 0.4 to 1.4 ps, and Br(Z 0 → B c ± X) × Br(B c ± → J ψ π + π − π ± ) < 1.75 × 10 −4 , constant over the range of predicted B c ± lifetimes.
B/C life-time equals (0.4 to 1.4) ps.
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Low x domain.
An updated analysis using about 1.5 million events recorded at $\sqrt{s} = M_Z$ with the DELPHI detector in 1994 is presented. Eighteen infrared and collinear safe event shape observables are measured as a function of the polar angle of the thrust axis. The data are compared to theoretical calculations in ${\cal O} (\alpha_s^2)$ including the event orientation. A combined fit of $\alpha_s$ and of the renormalization scale $x_{\mu}$ in $\cal O(\alpha_s^2$) yields an excellent description of the high statistics data. The weighted average from 18 observables including quark mass effects and correlations is $\alpha_s(M_Z^2) = 0.1174 \pm 0.0026$. The final result, derived from the jet cone energy fraction, the observable with the smallest theoretical and experimental uncertainty, is $\alpha_s(M_Z^2) = 0.1180 \pm 0.0006 (exp.) \pm 0.0013 (hadr.) \pm 0.0008 (scale) \pm 0.0007 (mass)$. Further studies include an $\alpha_s$ determination using theoretical predictions in the next-to-leading log approximation (NLLA), matched NLLA and $\cal O(\alpha_s^2$) predictions as well as theoretically motivated optimized scale setting methods. The influence of higher order contributions was also investigated by using the method of Pad\'{e} approximants. Average $\alpha_s$ values derived from the different approaches are in good agreement.
The weighted value of ALPHA-S from all the measured observables using experimentally optimized renormalization scale values and corrected for the b-mass toleading order.
The value of ALPHA-S derived from the JCEF and corrected for heavy quark mass effects. The quoted errors are respectively due to experimental error, hadronization, renormalization scale and heavy quark mass correction uncertainties.
Energy Energy Correlation EEC.
An analysis of the data collected in 1997 and 1998 with the DELPHI detector at e+e- collision energies close to 183 and 189 GeV was performed in order to extract the hadronic and leptonic fermion-pair cross-sections, as well as the leptonic forward-backward asymmetries and angular distributions. The data are used to put limit on contact interactions between fermions, the exchange of R-parity violating SUSY sneutrinos, Z' bosons and the existence of gravity in extra dimensions.
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Muon pair production in the process e+e- -> e+e-mu+mu- is studied using the data taken at LEP1 (sqrt(s) \simeq m_Z) with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb^{-1}. The QED predictions have been tested over the whole Q^2 range accessible at LEP1 (from several GeV^2/c^4 to several hundred GeV^2/c^4) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function F_2^\gamma. Azimuthal correlations are used to obtain information on additional structure functions, F_A^\gamma and F_B^\gamma, which originate from interference terms of the scattering amplitudes. The measured ratios F_A^\gamma/F_2^\gamma and F_B^\gamma/F_2^\gamma are significantly different from zero and consistent with QED predictions.
The measured QED photon structure function at Q**2 = 12.5 GeV for the combine SAT and STIC data.
The measured QED photon structure function at Q**2 = 120 GeV for the combine SAT and STIC data.
Ratio of the structure functions FA and FB to F2.
During the 1992 running period of the LEP e + e − collider, the DELPHI experiment accumulated approximately 24 pb − of data at the Z 0 peak. The decays into hadrons and charged leptons have been analysed to give values for the cross sections and leptonic forward-backward asymmetries which are significantly improved with respect to those previously published by the DELPHI collaboration. Incorporating these new data, more precise values for the Z 0 resonance parameters are obtained from model-independent fits. The results are interpreted within the framework of the Standard Model, yielding for the top quark mass m t = 157 −48 +36 (expt.) −20 +19 (Higgs) GeV, and for the effective mixing angle sin 2 θ eff lept = 0.2328 ± 0.0013 (expt.) −0.0003 +0.0001 (Higgs), where (Higgs) represents the variation due to Higgs boson mass in the range 60 to 1000 GeV, with central value 300 GeV.
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First result corresponds to the total cross section (i.e. S+T channel), while second one corresponds to S-channel only. An acollinearity less that 10 deg.
Forward-backward asymmetry within the polar angular range 44 < THETA < 136 degrees and acollinearity < 10 degrees.. First result corresponds to the total cross section (i.e. S+T channel), while second one corresponds to S-channel only.
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