Using the data sample of hadronic $Z^0$ decays collected by the DELPHI experiment in the 1992-1995 LEP1 period, the leptonic decay of the charged $B$ mesons (\btauex) has been studied. The analysis ...was done in both leptonic \taulm and hadronic \tauxmm decay channels. No excess was observed in data and the upper limit BR(\btauex) $ 1.1 \times 10^{-3}$ at the 90\åonfidence level was obtained. This result is consistent with Standard Model expectations and puts a constraint on the ratio $\tan \beta / M_{H^{\pm}} 0.45$~(GeV$/c^2)^{-1} $ in the framework of models with two Higgs doublets (any type II Higgs doublet model). From the missing energy distribution, the branching ratio of \btau was measured in the hadronic channel \tauxm. The result, BR(\btau)$ = (2.19 \pm 0.24 \ (stat) \pm 0.40\; (syst)) \$, is consistent with the Standard Model prediction and with previous experimental measurements.
Data taken by the {\tt 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 (${\mathrm {\tilde \ell}}\rightarrow\ell{\mathrm {\tilde \chi_1^0}}$), producing acoplanar lepton pairs plus missing energy, and the cascade decay (${\mathrm {\tilde \ell}} \rightarrow \ell{\mathrm {\tilde \chi_2^0}} \rightarrow \ell\gamma{\mathrm {\tilde \chi_1^0}}$), 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 \mbox{$m_{\tilde {e}} \le$ 87~\GeVcc\:}, \mbox{$m_{\tilde {\mu}} \le$ 80~\GeVcc\:} and $m_{\tilde {\tau}} $ 75~\GeVcc, respectively, for values of $\mu$=-200~\GeVcc\ and tan$\beta$=1.5.
A new precise measurement of |
V
cb
| and of the branching ratio BR
(
B
̄
0
→D
∗+ℓ
−
ν
̄
ℓ
) has been performed using a sample of about 5000 semileptonic decays
B
̄
0
→D
∗+ℓ
−
ν
̄
ℓ
, selected by the ...DELPHI detector at LEP I by tagging the soft pion from
D
∗+→D
0π
+
. The results are:
V
cb=(39.0±1.5
(stat.)
+2.5
−2.6
(syst. exp.)±1.3
(syst. th.))×10
−3
,
BR(
B
̄
0
→D
∗+ℓ
−
ν
̄
ℓ)=(4.70±0.13
(stat.)
+0.36
−0.31
(syst. exp.))%
. The analytic dependencies of the differential cross-section and of the Isgur–Wise form factor as functions of the variable
w=v
B
0
·v
D
∗
have also been obtained by unfolding the experimental resolution.
Searches for pair production of gauginos and squarks in $e^{+} e^{-}$ collisions at a centre-of-mass energy of 189 GeV have been performed on data corresponding to an integrated luminosity of 158 ...pb$^{-1}$ collected by the DELPHI detector at LEP. The data were analyzed under the assumption of non-conservation of \mbox{$R$-parity} through a single dominant $\bar{U} \bar{D} \bar{D}$~coupling between squarks and quarks. Typical final states contain between 4 and 10 jets with or without additional leptons. No excess of data above Standard Model expectations was observed. The results were used to constrain domains of the MSSM parameter space and derive limits on the masses of supersymmetric particles. The following mass limits at 95\% CL were obtained from these searches: \begin{itemize} \item neutralino mass: $m_{\tilde{\chi^0_{1}}} \ge 32$ GeV \item chargino mass: $m_{\tilde{\chi^+_{1}}} \ge 94 $ GeV \item stop and sbottom mass (indirect decay) with $\Delta M 5$ GeV: $m_{\tilde{t_{1}}} \ge 74$ GeV, for $\Phi_{mix}=0$ rad $m_{\tilde{t_{1}}} \ge 59$ GeV, for $\Phi_{mix}=0.98 $ rad $m_{\tilde{b_{1}}} \ge 72$ GeV, for $\Phi_{mix}=0$ rad. \end{itemize} The angle $\phi_{mix}$ is the mixing angle between left and right handed quarks.
The branching ratio for the leptonic decay of charged B mesons
(
B
−→τ
−
ν
̄
τ)
has been measured using selected leptonic
τ
−→ℓ
−ν
τ
ν
̄
ℓ
and hadronic
τ
−→
ν
τ
X decays in
Z→
b
b
̄
decays recorded ...by DELPHI at LEP1 in 1992–1995. The result,
BR(
B
−→τ
−
ν
̄
τ)<1.1×10
−3
at the 90% confidence level, is consistent with standard model expectations and puts a constraint on the ratio tan
β/
M
H
±
<0.46 (GeV/
c
2)
−1 in the framework of models with two Higgs doublets (type II Higgs doublet model). From the missing energy distribution in
Z→
b
b
̄
decays without identified leptons, the
b→τ
ν
̄
τ
X
branching ratio has been measured in the hadronic channel
τ→
ν
τ
X′. The result,
BR(
b→τ
ν
̄
τ
X)=(2.19±0.24
(
stat)±0.39
(
syst))%
, is consistent with the Standard Model prediction and with previous experimental measurements.
Searches for supersymmetric partners of top and bottom quarks are presented using data taken by the DELPHI experiment at LEP in 1997 and 1998. No deviations from standard model expectations are ...observed in these data sets, which are taken at centre-of-mass energies of 183 GeV and 189 GeV and correspond to integrated luminosities of 54 pb
−1 and 158 pb
−1. These results are used in combination with those obtained by DELPHI at lower centre-of-mass energies to exclude regions in the squark–neutralino mass plane at 95% confidence level.
A test of the QED process
e
+
e
−→
γγ(
γ) 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
−→
γγ 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
Λ
+>330 GeV and
Λ
−>320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/
c
2 (for
λ
γ
=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 (
λ=1) and
M
S
>691 GeV/
c
2 (
λ=−1).
A search for stable and long-lived heavy charged particles was performed using the data taken by the DELPHI experiment at an energy of 189 GeV. The Cherenkov light detected in the Ring Imaging ...Cherenkov Detectorand the ionisation loss measured in the Time Projection Chamber were used to identify heavy particles passing through the detector. No evidence for the production of such particles has been found, therefore exclusion limits at 95% confidence level were derived on the masses of left and right handed smuons and staus. The results were combined with previous DELPHI searches in this channel.Including previous DELPHI results, masses of left (right) handed stable smuons and staus can be excluded between 2 GeV/$c^2$ and 88 (87.5) GeV/$c^2$ at 95% CL.
Searches for charginos, neutralinos and sleptons at LEP2 centre-of-mass energies from 130 GeV to 189 GeV have been used to set lower limits on the mass of the Lightest Supersymmetric Particle and ...other supersymmetric particles within the MSSM framework. R-parity conservation has been assumed. The lightest neutralino was found to be heavier than 32.3 GeV/
c
2 independent of the
m
0 value. The lightest chargino, the second-to-lightest neutralino, the next-to-heaviest neutralino, the heaviest neutralino, the sneutrino and the right-handed selectron were found to be heavier than 62.4 GeV/
c
2, 62.4 GeV/
c
2, 99.9 GeV/
c
2, 116.0 GeV/
c
2, 61.0 GeV/
c
2, and 87.0 GeV/
c
2, respectively. These limits do not depend on
m
0 or
M
2 and are valid for 1≤ tan
β≤40, in the
μ region where the lightest neutralino is the LSP. If the sneutrino is heavier than the chargino the lightest neutralino has to be heavier than 32.4 GeV/
c
2. The effects of mixings in the third family of sfermions on these limits are discussed. The confidence level of all limits given is 95%.
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