The tau lepton lifetime has been measured using three different methods with the DELPHI detector. Two measurements of one-prong decays are combined, accounting for correlations, giving a result of
τ
...τ
= 291.8 ± 3.3 (stat.) ± 2.0(sys.) fs while the decay length distribution of three-prong decays gives the result
τ
τ
= 286.7 ± 4.9 (stat.) ±3.3 (sys.) fs. Combining the results presented here with previous DELPHI measurements, we get
τ
τ
= 291.4 ± 3.0 fs and find that the ratio of the coupling constant for tau decay relative to that for muon decay is 0.990 ± 0.009, compatible with lepton universality.
The ratio of the CKM quark-mixing matrix elements |
V
ub
|/|
V
cb
| has been measured using
B hadron semileptonic decays. The analysis uses the reconstructed mass
M
X
of the secondary hadronic system ...produced in association with an identified lepton. Since
B→X
uℓ
ν
̄
transitions are characterised by hadronic masses below those of the
D mesons produced in
B→X
cℓ
ν
̄
transitions, events with a reconstructed value of
M
X
significantly below the
D mass are selected. Further signal enrichments are obtained using the topology of reconstructed decays and hadron identification. A fit to the numbers of decays in the
b→
u enriched and depleted samples with
M
X
above and below 1.6 GeV/
c
2 and to the shapes of the lepton energy distribution in the
B rest frame gives
|V
ub|/|V
cb|=
0.103
+0.011
−0.012
(stat.)±0.016
(syst.)±0.010
(model)
and, correspondingly, a charmless semileptonic
B decay branching fraction of BR(
B→X
uℓ
ν
̄
)=
(1.57±0.35
(stat.)±0.48
(syst.)±0.27
(model))×10
−3
.
An update of the searches for charginos and gravitinos is presented, based on a data sample corresponding to the 158 pb
−1 recorded by the DELPHI detector in 1998, at a centre-of-mass energy of 189 ...GeV. No evidence for a signal was found. The lower mass limits are 4–5 GeV/
c
2 higher than those obtained at a centre-of-mass energy of 183 GeV. The (
μ,
M
2) MSSM domain excluded by combining the chargino searches with neutralino searches at the Z resonance implies a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 31.0 GeV/
c
2 for tan
β≥ 1.
Promptly decaying lightest neutralinos and long-lived staus are searched for in the context of light gravitino scenarios. It is assumed that the stau is the next to lightest supersymmetric particle ...(NLSP) and that the lightest neutralino is the next to NLSP (NNLSP). Data collected with the Delphi detector at centre-of-mass energies from 161 to 183 GeV are analysed. No evidence of the production of these particles is found. Hence, lower mass limits for both kinds of particles are set at 95% C.L.. The mass of gaugino-like neutralinos is found to be greater than 71.5 GeV/\(c^2\). In the search for long-lived stau, masses less than 70.0 to 77.5 GeV/\(c^2\) are excluded for gravitino masses from 10 to 150 eV/\(c^2\). Combining this search with the searches for stable heavy leptons and minimal supersymmetric standard model staus a lower limit of 68.5 GeV/\(c^2\) may be set for the stau mass independent of the mass of the gravitino.
A study of
b semileptonic decays into
D,
Dπ
± and
D
∗π
±
final states is presented. The
D
0,
D
+ and
D
∗+
mesons are exclusively reconstructed in
Z decay data recorded from 1992 to 1995 in the DELPHI ...experiment at LEP. The overall branching fractions are measured to be:
BR(b→D
0ℓ
−
ν
̄
ℓX)=(7.04±0.34
(stat)±0.36
(syst.exp)±0.17
(
BR
D))%
,
BR(b→D
+ℓ
−
ν
̄
ℓX)=(2.72±0.19
(stat)±0.16
(syst.exp)±0.18
(
BR
D))%
,
BR(b→D
∗+ℓ
−
ν
̄
ℓX)=(2.75±0.17
(stat)±0.13
(syst.exp)±0.09
(
BR
D))%
,
where the
D
0 and
D
+ results include also contributions from
D
∗0
and
D
∗+
decays. A fit to the distribution of the
π
± impact parameter to the primary interaction vertex provides a measurement of the
b semileptonic branching fractions into the
D
0
π
±
X,
D
+
π
±
X and
D
∗+π
±X
final states. Assuming that single pion decay modes of
B mesons dominate, the partial rates for
B
̄
→Dπℓ
−
ν
ℓ
̄
and
B
̄
→D
∗πℓ
−
ν
ℓ
̄
have been obtained, corresponding to a total branching fraction:
BR(
B
̄
→Dπℓ
−
ν
ℓ
̄
)+
BR(
B
̄
→D
∗πℓ
−
ν
ℓ
̄
)=(3.40±0.52
(stat)±0.32
(syst))%
.
This result agrees well with the observed difference between the total
B semileptonic branching fraction and the sum of the
B
̄
→Dℓ
−
ν
ℓ
̄
and
D
∗ℓ
−
ν
ℓ
̄
branching fractions.
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 Detector and 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.
The DELPHI detector at LEP has collected 54 pb
−1 of data at a centre-of-mass energy around 183 GeV during 1997, 158 pb
−1 around 189 GeV during 1998, and 187 pb
−1 between 192 and 200 GeV during ...1999. These data were used to measure the average charged particle multiplicity in
e
+e
−→b
b
̄
events,
〈n〉
b
b
̄
, and the difference
δ
bl
between
〈n〉
b
b
̄
and the multiplicity,
〈n〉
l
l
̄
, in generic light quark (u,d,s) events:
δ
bl
(183
GeV)
=4.55±1.31(
stat)±0.73(
syst)
,
δ
bl
(189
GeV)
=4.43±0.85(
stat)±0.61(
syst)
,
δ
bl
(200
GeV)
=3.39±0.89(
stat)±1.01(
syst)
.
This result is consistent with QCD predictions, while it is inconsistent with calculations assuming that the multiplicity accompanying the decay of a heavy quark is independent of the mass of the quark itself.
The study of the directional dependence of two-particle correlations in the hadronic decays of the
Z
0 boson is performed using the data collected by the DELPHI experiment in the 1992–1995 running ...periods. The comparison between the transverse,
R
⊥, and longitudinal,
R
∥, correlation radii confirms the string model prediction that the transverse correlation length is smaller than the longitudinal one, with the measured values of
R
⊥=0.53±0.08
fm
and
R
∥=0.85±0.08
fm
, for selected
Z
0→q
q
̄
events.
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