Tau-pair production in the process \(\ensuremath{\mathrm{e}^ + \mathrm{e}^- \rightarrow \mathrm{e}^ + \mathrm{e}^-\tau^ + \tau^-}\) was studied using data collected by the DELPHI experiment at LEP2 ...during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb-1. The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.
Searches for neutral Higgs bosons produced at LEP in association with Z bosons, in pairs and in the Yukawa process are presented in this paper. Higgs boson decays into b quarks, \(\tau\) leptons, or ...other Higgs bosons are considered, giving rise to four-b, four-b + jets, six-b and four-\(\tau\) final states, as well as mixed modes with b quarks and \(\tau\) leptons. The whole mass domain kinematically accessible at LEP in these topologies is searched. The analysed data set covers both the LEP1 and LEP2 energy ranges and exploits most of the luminosity recorded by the DELPHI experiment. No convincing evidence for a signal is found, and results are presented in the form of mass-dependent upper bounds on coupling factors (in units of model-independent reference cross-sections) for all processes, allowing interpretation of the data in a large class of models.
A search for pair-produced charged Higgs bosons was performed in the data collected by the DELPHI detector at LEP II at centre-of-mass energies from 189 GeV to 209 GeV. Five different final states, ...\(\tau^ + \nu_{\tau}\tau^- \bar{\nu}_{\tau}\), \(\mathrm{c}\bar{\mathrm{s}}\bar{\mathrm{c}}\mathrm{s}\), \(\mathrm{c}\bar{\mathrm{s}}{\tau^- \bar{\nu}_{\tau}}\), \(\mathrm{W}^*\!\mathrm{A}\mathrm{W}^*\!\mathrm{A}\) and \(\mathrm{W}^*\!\mathrm{A}\tau^- \bar{\nu}_{\tau}\) were considered, accounting for the major expected decays in type I and type II Two Higgs Doublet Models. No significant excess of data compared to the expected Standard Model processes was observed. The existence of a charged Higgs boson with mass lower than 76.7 GeV/c2 (type I) or 74.4 GeV/c2 (type II) is excluded at the 95% confidence level, for a wide range of the model parameters. Model independent cross-section limits have also been calculated.
Infrared and collinear safe event shape distributions and their mean values are determined in \({\mathrm{e^+e^-}}\) collisions at centre-of-mass energies between 45 and 202GeV. A phenomenological ...analysis based on power correction models including hadron mass effects for both differential distributions and mean values is presented. Using power corrections, \(\alpha_s\) is extracted from the mean values and shapes. In an alternative approach, renormalisation group invariance (RGI) is used as an explicit constraint, leading to a consistent description of mean values without the need for sizeable power corrections. The QCD \(\beta\)-function is precisely measured using this approach. From the DELPHI data on Thrust, including data from low energy experiments, one finds \\beta_0 = 7.86 \pm 0.32\ for the one loop coefficient of the \(\beta\)-function or, assuming QCD, \n_{\mathrm{f}} = 4.75 \pm 0.44 \ for the number of active flavours. These values agree well with the QCD expectation of \(\beta_0=7.67\) and \(n_{\mathrm{f}}=5\). A direct measurement of the full logarithmic energy slope excludes light gluinos with a mass below 5GeV.
A determination of the single W spin density matrix (SDM) elements in the reaction e
+
e
-
→W
+
W
-
→lνqq̄(l=e/μ) is reported at centre-of-mass energies between 189 and 209 GeV. The data sample used ...corresponds to an integrated luminosity of 520 pb
-1
taken by DELPHI between 1998 and 2000. The single W SDM elements, ρ
ττ’
W±
(τ,τ’=± 1 or 0), are determined as a function of the W
-
production angle with respect to the e
-
beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Λ
ττ’
, which assume the V-A coupling of the W-boson to fermions. The measured SDM elements are used to obtain the fraction of longitudinally polarised Ws, with the result:
at a mean energy of 198 GeV. The SDM elements are also used to determine the triple gauge couplings Δg
1
Z
,Δκ
γ
,λ
γ
and g
4
Z
,
and
. For the CP-violating couplings the results of single parameter fits are:
The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.
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.
These final results from DELPHI searches for the Standard Model (SM) Higgs boson, together with benchmark scans of the Minimal Supersymmetric Standard Model (MSSM) neutral Higgs bosons, used data ...taken at centre-of-mass energies between 200 and 209 GeV with a total integrated luminosity of 224 pb-1. The data from 192 to 202 GeV are reanalysed with improved b-tagging for MSSM final states decaying to four b-quarks. The 95% confidence level lower mass bound on the Standard Model Higgs boson is 114.1 GeV/c2. Limits are also given on the lightest scalar and pseudo-scalar Higgs bosons of the MSSM.
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.
Inclusive J/ψ production in photon–photon collisions has been observed at LEP II beam energies. A clear signal from the reaction γγ→J/ψ+X is seen. The number of observed N(J/ψ→μ+μ−) events is 36±7 ...for an integrated luminosity of 617 pb−1, yielding a cross-section of σ(J/ψ+X)=45±9(stat)±17(syst) pb. Based on a study of the event shapes of different types of γγ processes in the PYTHIA program, we conclude that (74±22)% of the observed J/ψ events are due to ‘resolved’ photons, the dominant contribution of which is most probably due to the gluon content of the photon.
A study of the inclusive charged hadron production in two-photon collisions is described. The data were collected with the DELPHI detector at LEP II. Results on the inclusive single-particle pT ...distribution and the differential charged hadrons dσ/dpT cross-section are presented and compared to the predictions of perturbative NLO QCD calculations and to published results.
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