0000000001253887
AUTHOR
José ÁNgel Hernando Morata
First evidence of hard scattering processes in single tagged gamma-gamma-collisions
For the first time, multihadronic production from single tagged gammagamma collisions has been studied, where one of the scattered leptons was tagged at very low virtual photon absolute mass squared ( = 0.06GeV/(c2))2 ). Data collected during 1991 and 1992 in the DELPHI experiment at LEP are shown to agree well with predictions which included the non-perturbative vector meson dominance model in which the interacting photons are assumed to have converted into a vector meson (rho, omega or pi), a quark-parton model which describes direct photon interactions and a QCD-based model which considers the photon to have quark and gluon structure functions. Five different parametrizations of these st…
A measurement of b+ and b-0 lifetimes using (d)over-bar-l(+) events
A measurement of B meson lifetimes is presented using data collected from 1991 to 1993 by the DELPHI detector at the LEP collider. Samples of events with a D meson and a lepton in the same jet are selected where D¯0ℓ+ and D*ℓ+ events originate mainly from the semileptonic decays of B+ and B0 mesons, respectively. From the reconstructed B decay length and the estimated B momentum, taking into account the dilution due to B decays into D¯∗∗ℓ+v , the following B meson lifetimes and lifetime ratio are measured: τ(B+)=1.61+0.16−0.16(stat.)±0.12(syst.)psτ(B0)=1.61+0.14−0.13(stat.)±0.08(syst.)psτ(B+)/τ(B0)=1.00+0.17−0.15(stat.)±0.10(syst.) and an average lifetime of B+ and B0 mesons is obtained: τ(…
Search for pair-produced heavy scalars in Z° decays
A search for charged Higgs bosons decaying into quarks is described, based on statistics of around 1.7 million hadronic Z0 decays detected in DELPHI. Despite the very high background from standard hadronic decays of the Z0, masses in the range up to 43.5 GeV/c2 are excluded at the 95% con dence level. After combination with a search for leptonic decays, this mass limit is extended to cover all branching ratios. A similar analysis sets new limits on the possible production of any pair-produced heavy scalar decaying into a pair of jets, such as neutral Higgs bosons in a two doublet scheme and diquarks.
Study of prompt photon production in hadronic Z° decays.
From a sample of 1.5 million Z0 ! qq decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated nal state photons is measured and is compared with the O( ; s) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to the Z0 boson. The measured values c2=3 = 0:91+0:25 0:36 and c1=3 = 1:62+0:24 0:17 are compatible with the Standard Model prediction, c2=3 = 1:145 and c1=3 = 1:477.
Measurement of the B0 - anti-B0 mixing using the average electric charge of hadron jets in Z0 decays
From the data recorded with the DELPHI detector at LEP in the years 1991-1992, 46 497 events were selected having a high-momentum muon in hadron jets. A fit to the average electric charge sum of the jets recoiling against a b-quark jet tagged by a high-PT muon results in an average mixing parameter of chiBAR = 0.144 +/- 0.014(stat.)+0.017/-0.011(syst.).
A Study of radiactive muon-pair events at Z° energies and limits on an additional Z' gauge boson
An analysis is reported on the channele + e −→μ+μ−(nγ), n=1,2..., using data taken with the DELPHI detector at LEP from 1990 to 1992. Differential cross sections of the radiative photons as a function of photon energy and of the angle between the photon and the muon are presented. No significant deviations from expectations are observed. The data are also used to extract the muon-pair cross section and asymmetry below the Z0 peak by using those events with relatively hard initial state radiative photon(s). The measured cross section and asymmetry show no significant deviation from the Standard Model expectations. These results together with the DELPHI cross section and asymmetry measurement…
Production of strange b-baryons decaying into xi-/+-l-/+ pairs at LEP
An excess of events containing, in a jet, a same-sign Xi(-/+) - l(-/+) pair as compared to those with an opposite-sign Xi(-/+) - l(-/+) pair has been observed in an analysis of 1.7 million hadronic Z(o) decays collected by the DELPHI detector at LEP between 1991 and 1993 inclusive. The probability for this signal to come from non B-baryon decays is less than 5 x 10(-4). The measured production fraction corresponds to: P(b --> B - baryon) x BR(B - baryon --> Xi(-) l(-) X) = (5.9 +/- 2.1 +/- 1.0) x 10(-4), per lepton species, averaged for electrons and muons and assuming the two channels have an equal contribution. Semileptonic decays of Lambda(b) baryons can account for less than 10% of thes…
Upper limits on the branching ratios tau-]mu-gamma and tau-]e-gamma
The DELPHI collaboration has searched for lepton flavour violating decays tau -> mugamma and tau -> egamma using a data sample of about 70 pb-1 of integrated luminosity corresponding to 81 000 produced tau+tau- events. No candidates were found for either of the two modes. This yields branching ratio upper limits of B(tau -> egamma) mugamma) < 6.2 × 10-5 at 90% confidence level.
A precise measurement of the muon neutrino nucleon inclusive charged current cross section off an isoscalar target in the energy range 2.5<E<40 GeV by NOMAD
We present a measurement of the muon neutrino-nucleon inclusive charged current cross section, off an isoscalar target, in the neutrino energy range 2.5 <= E-v <= 40 GeV. The significance of this measurement is its precision, +/- 4% in 2.5 <= E-v <= 10 GeV, and +/- 2.6% in 10 <= E-v <= 40 GeV regions, where significant uncertainties in previous experiments still exist, and its importance to the current and proposed long baseline neutrino oscillation experiments.
Measurement of gamma(b(b)over-bar)/gamma(had) using impact parameter measurements and lepton identification
The partial decay width of theZ to bb¯quark pairs has been measured by the DELPHI detector at LEP.b-hadrons, containingb-quarks, were tagged by leptons with high transverse momentum relative to the hadron or by tracks with large impact parameters to the primary vertex.The ratio of the numbers of events with a single such tag to those with two tags was used to estimate the efficiency of the method and to reduce the systematic uncertainty. Combining all methods, the value: Γbb¯Γhad=0.2210±0.0033±0.0003(model)±0.0014(Γcc¯) was found, where the third error corresponds to a±8% uncertainty on the cc production width. A maximum likelihood in to the single and di-lepton distributions gave the branc…