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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, . The average value of is measured to be 0.699±0.011. The resulting 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: with a correlation factor ρ =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 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: with a correlation factor ρ =92.6%.

Single photons detected by the DELPHI experiment at LEP2 in the years 1997–2000 are reanalysed to investigate the existence of a single extra dimension in a modified ADD scenario with slightly warped large extra dimensions. The data collected at centre-of-mass energies between 180 and 209 GeV for an integrated luminosity of ∼650 pb −1 agree with the predictions of the Standard Model and allow a limit to be set on graviton emission in one large extra dimension. The limit obtained on the fundamental mass scale M D is 1.69 TeV/ c 2 at 95% CL, with an expected limit of 1.71 TeV/ c 2 .

Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2< E γ ≤1 GeV and transverse momentum with respect to the parent muon p T <40 MeV/ c , and 1< E γ ≤10 GeV and p T <80 MeV/ c . A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z 0 decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06±0.12±0.07 in the photon energy range 0.2< E γ ≤1 GeV and 1.04±0.09±0.12 in the photon energy range 1< E γ ≤10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP.

Moments of the hadronic invariant mass and of the lepton energy spectra in semileptonic B decays have been determined with the data recorded by the DELPHI detector at LEP. From measurements of the inclusive b-hadron semileptonic decays, and imposing constraints from other measurements on b- and c-quark masses, the first three moments of the lepton energy distribution and of the hadronic mass distribution, have been used to determine parameters which enter into the extraction of |Vcb| from the measurement of the inclusive b-hadron semileptonic decay width. The values obtained in the kinetic scheme are:\\(\\begin{array}{*{20}l}{{m_{b} {\\left( {1\\;{\\text{GeV}}} \\right)}} \\hfill} & { = \\hfill} & {{4.591 \\pm 0.062 \\pm 0.039 \\pm 0.005\\;{\\text{GeV/c}}^{2} ,} \\hfill} \\\{{m_{c} {\\left( {1\\;{\\text{GeV}}} \\right)}} \\hfill} & { = \\hfill} & {{1.170 \\pm 0.093 \\pm 0.055 \\pm 0.005\\;{\\text{GeV/c}}^{{\\text{2}}} {\\text{,}}} \\hfill} \\\{{\\mu ^{2}_{\\pi } {\\left( {1\\;{\\text{GeV}}} \\right)}} \\hfill} & { = \\hfill} & {{0.399 \\pm 0.048 \\pm 0.034 \\pm 0.087\\;{\\text{GeV}}^{{\\text{2}}} {\\text{,}}} \\hfill} \\\{{ \\ifmmode\\expandafter\\tilde\\else\\expandafter\\~\\fi{\\rho }^{3}_{D} } \\hfill} & { = \\hfill} & {{0.053 \\pm 0.017 \\pm 0.011 \\pm 0.026\\;{\\text{GeV}}^{{\\text{3}}} ,} \\hfill} \\\\\end{array} \\)and include corrections at order 1/mb3. Using these results, and present measurements of the inclusive semileptonic decay partial width of b-hadrons at LEP, an accurate determination of |Vcb| is obtained: \\({\\left| {V_{{cb}} } \\right|} = 0.0421 \\times {\\left( {1 \\pm 0.014_{{{\\text{meas}}{\\text{.}}} \\pm 0.014_{{{\\text{fit}}} \\pm 0.015_{{{\\text{th}}{\\text{.}}} } \\right)}\\)

An analysis of the direct soft photon production rate as a function of the parent jet characteristics is presented, based on hadronic events collected by the DELPHI experiment at LEP1. The dependences of the photon rates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadron multiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photon excess, a similarity of the observed soft photon behavior to that of the inner hadronic bremsstrahlung predictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photon rate on the jet neutral and total hadron multiplicities a prominent difference is found for the observed soft photon signal as compared to the expected bremsstrahlung from final state hadrons. The observed linear increase of the soft photon production rate with the jet total hadron multiplicity and its strong dependence on the jet neutral multiplicity suggest that the rate is proportional to the number of quark pairs produced in the fragmentation process, with the neutral pairs being more effectively radiating than the charged ones.

Single top quark production via four-fermion contact interactions associated to flavour-changing neutral currents was searched for in data taken by the DELPHI detector at LEP2. The data were accumulated at centre-of-mass energies ranging from 189 to 209 GeV, with an integrated luminosity of 598.1 pb −1 . No evidence for a signal was found. Limits on the energy scale Λ , were set for scalar-, vector- and tensor-like coupling scenarios.

Soft photons inside hadronic jets converted in front of the DELPHI main tracker (TPC) in events of qq disintegrations of the Z(0) were studied in the kinematic range 0.2 < E-y < I GeV and transverse momentum with respect to the closest jet direction P-T < 80 MeV/c. A clear excess of photons in the experimental data as compared to the Monte Carlo predictions is observed. This excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/- 0.27) x 10(-3) gamma/jet in the specified kinematic region, while the expected level of the inner hadronic bremsstrahlung (which is not included in the Monte Carlo) is (0.340 +/- 0.001 +/- 0.038) X 10(-3) gamma/jet. The ratio of the excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8), which is similar in strength to the anomalous soft photon signal observed in fixed target experiments with hadronic beams.

The effect of the heavy b-quark mass on the two, three and four-jet rates is studied using LEP data collected by the DELPHI experiment at the Z peak in 1994 and 1995. The rates of b-quark jets and light quark jets (ℓ=uds) in events with n=2, 3, and 4 jets, together with the ratio of two and four-jet rates of b-quarks with respect to light-quarks, R n bℓ , have been measured with a double-tag technique using the CAMBRIDGE jet-clustering algorithm. A comparison between experimental results and theory (matrix element or Monte Carlo event generators such as PYTHIA, HERWIG and ARIADNE) is done after the hadronisation phase. Using the four-jet observable R 4 bℓ , a measurement of the b-quark mass using massive leading-order calculations gives: This result is compatible with previous three-jet determinations at the M Z energy scale and with low energy mass measurements evolved to the M Z scale using QCD renormalisation group equations.

Neutral triple-gauge-boson couplings ZZZ, ZZγ and Zγγ have been studied with the DELPHI detector using data at energies between 183 and 208 GeV. Limits are derived on these couplings from an analysis of the reactions e + e - →Zγ, using data from the final states γff̄, with f=q or ν, from e + e - →ZZ, using data from the four-fermion final states qq̄qq̄, qq̄μ + μ - , qq̄e + e - , qq̄νν̄, μ + μ - νν̄ and e + e - νν̄, and from e + e - →Zγ * , in which the final state γ is off mass-shell, using data from the four-fermion final states qq̄e + e - and qq̄μ + μ - . No evidence for the presence of such couplings is observed, in agreement with the predictions of the Standard Model.