Search for neutral heavy leptons produced in ZZ decays

Weak isosinglet Neutral Heavy Leptons (νm) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 106 hadronic Z0 decays at LEP1. Four separate searches have been performed, for short-lived νm production giving monojet or acollinear jet topologies, and for long-lived νm giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z0 → νmν̄) of about 1.3 × 10-6 at 95% confidence level for νm masses between 3.5 and 50 GeV/c2. Outside this range the limit weakens rapidly with the νm mass. The results are also interpreted in terms of limits for the single production of excited neutrinos. © Springer-Verlag 1997

Search for sleptons in e+ e- collisions at s**(1/2) = 183-GeV to 189-GeV

Data taken by the DELPHI experiment at centre-of-mass energies of 183 GeV and 189 GeV with a total integrated luminosity of 212 pb^{-1} have been used to search for the supersymmetric partners of the electrons, muons, and taus in the context of the Minimal Supersymmetric Standard Model (MSSM). The decay topologies searched for were the direct decay ({\tilde \ell} -> \ell {\tilde \chi_1^0}), producing acoplanar lepton pairs plus missing energy, and the cascade decay ({\tilde \ell} -> \ell {\tilde \chi_2^0} -> \ell \gamma {\tilde \chi_1^0}), producing acoplanar lepton and photon pairs plus missing energy. The observed number of events is in agreement with Standard Model predictions. The 95% CL excluded mass limits for selectrons, smuons and staus are m_{\tilde {e}} \leq 87 GeV/c^2, m_{\tilde {\mu}} \leq 80 GeV/c^2 and m_{\tilde {\tau}} \leq 75 GeV/c^2, respectively, for values of \mu=-200 GeV/c^2 and tan(beta)=1.5.Comment: 25 pages, 9 figure

Study of final state photons in hadronic Z0 decay and limits on new phenomena

The differential cross section for final state radiation from primary quarks is obtained from a study of isolated energetic photons produced in the reaction e+ e- --> Z0 --> hadrons + gamma, as measured in the DELPHI detector at the CERN LEP collider. When combined with the measurement of the total hadronic width of the Z0, the observed rate determines the electroweak coupling constants of up and down type quarks, i.e., upsilon(1/3)2 + a(1/3)2 = 1.13 +/- 0.29 and upsilon(2/3)2 + a(2/3)2 = 1.65 +/- 0.43. No evidence is seen for additional photon production from anomalous decays of the Z0 or from decays of new particles. This measurement leads to upper limits on the production cross section times branching fraction of (a) the Higgs boson in the reaction e+ e- --> Z0 --> H +gamma, H --> hadrons, (b) an excited quark, q* --> q + gamma, and (c) the contribution of an anomalous decay of the Z0 into a photon and hadrons. These limits, all at the 95% confidence level, vary from 3 to 10 pb as the mass of the intermediate state (H, q* or Z*) varies from 10 GeV/c2 to 80 GeV/c2

Determination of the e+ e- --> gamma gamma (gamma) cross-section at centre-of-mass energies ranging from 189-GeV to 202-GeV

A test of the QED process e(+)e(-) --> gamma gamma>(*) over bar *(gamma) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb(-1). The differential and total cross-sections for the process e(+)e(-) --> gamma gamma were measured, and found to be in agreement with the QED prediction. 95% confidence level (C.L.) lower limits on the QED cut-off parameters of Lambda (+) > 330 GeV and Lambda_ > 320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/c(2) (for lambda gamma = 1) was obtained, s-channel virtual graviton exchange was searched for, resulting in 95% C.L. lower limits on the string mass scale, M-S: M-S > 713 GeV/c(2) (lambda = 1) and M-S > 691 GeV/c(2) (lambda = -1). (C) 2000 Published by Elsevier Science B.V

Searches for neutral Higgs bosons in e+ e- collisions around s**(1/2) = 189-GeV

Searches for neutral Higgs bosons in the Standard Model and the MSSM have been performed using data collected by the DELPHI experiment at a centre-of-mass energy of 188.7 GeV, corresponding to an integrated luminosity of 158 pb^{-1}. These analyses are used, in combination with our results from lower energies, to set new 95% confidence level lower mass bounds on the Standard Model Higgs boson (94.6 GeV/c^2) and on the lightest neutral scalar (82.6 GeV/c^2) and neutral pseudoscalar (84.1 GeV/c^2) Higgs bosons in a representative scan of the MSSM parameters. The results are also interpreted in the framework of a general two-Higgs doublet model.Comment: 36 pages, 21 figures, see also Addendum in hep-ex/010402

Measurement of the strange quark forward backward asymmetry around the Z0 peak

A precise measurement of the strange quark forward-backward asymmetry used 3.2M multihadronic events around the Z° peak collected by the DELPHI experiment from 1992 to 1995. The ring imaging Cherenkov detectors in the barrel and end-cap regions identify high energy charged kaons which tag the s quark. The s quark asymmetry was measured at different centre-of-mass energies; m particular at the Z° pole A°ss̄ = 0.1008 ± 0.0113 (stat.) ± 0.0040 (syst.), (1) taking the expected d and u quark asymmetries from the Standard Model. The quark flavour fractions are assumed from the Standard Model and the fragmentation process is modelled by JETSET. From the s quark pole asymmetry the electroweak mixing angle was determined: sin2 θlepteff = 0.2321 ± 0.0029. (2) The parity violating coupling of the s quark to the Z was determined to be: As = 0.909 ± 0.102 (stat.) ± 0.036(syst.). (3).0SCOPUS: ar.jinfo:eu-repo/semantics/publishe