Measurement of the {\rm\bf B_{\bf d}^{\bf 0}- {\bf \bar{B}}_{\bf d}^{\bf 0}} oscillation frequency

Time-dependent mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of meson oscillations is Received: 20 February 1998 / Revised version: 23 March 1998 / Published online: 12 August 1998     

Precise determination of the Z resonance parameters at LEP: “Zedometry”

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in collisions with the OPAL detector makes use of the full LEP 1 data sample comprising of integrated luminosity and selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: GeV, GeV, nb, , and . Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, . Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, . Interpreting the data within the context of the Standard Model allows the mass of the top quark, GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: and GeV. Received: 1 December 2000 / Published online: 6 April 2001     

Inclusive semileptonic branching ratios of b hadrons produced in Z decays

A measurement of the inclusive semileptonic branching ratios of b hadrons produced in Z decay is presented, using four million hadronic events collected by the ALEPH detector from 1991 to 1995. Electrons and muons are selected opposite to b-tagged hemispheres. Two different methods are explored to distinguish the contributions from direct and cascade decays to the total lepton yield. One is based on the lepton transverse momentum spectrum, the other makes use of the correlation between the charge of the lepton and charge estimators built from tracks in the opposite hemisphere of the event. The latter method reduces the dependence on the modelling of semileptonic b decays. The results obtained by averaging the two techniques are Received: 1 August 2001 / Published online: 14 December 2001     

Improved search for $B_{s}^{0} - \overline{B}_{s}^{0}$ oscillations

An improved search for B _s ⁰ oscillations is performed in the ALEPH data sample collected during the first phase of LEP, and reprocessed in 1998. Three analyses based on complementary event selections are presented. First, decays of B _s ⁰ mesons into hadronic flavour eigenstates are fully reconstructed. This selection yields a small sample of candidates with excellent decay length and momentum resolution and hi gh average B _s ⁰ purity. Semileptonic decays with a reconstructed D _s ^- meson provide a second sample with larger statistics, high average B _s ⁰ purity, but a poorer momentum and decay length resolution due to the partial decay reconstruction. Finally, semileptonic b-hadron decays are inclusively selected and yield the data sample with the highest sensitivity to B _s ⁰ oscillations, as the much higher statistics compensate for the low average B _s ⁰ purity and poorer time resolution. A lower limit is set atps^-1 at 95% C.L., significantly lower than the expected limit of 15.2 ps^-1. Received: 21 February 2003 / Published online: 11 June 2003     

Measurement of inclusive K*0(892), ϕ(1020) and ${⤪ K}_{2}^{⇒t 0}(1430)$ production in hadronic Z decays DELPHI Collaboration

The inclusive production of the neutral vector mesons K*⁰(892) and ?(1020), and of the tensor meson ${? K}_{2}^{?t 0}(1430)$, in hadronic decays of the Z has been measured by the DELPHI detector at LEP. The average production rates per hadronic Z decay have been determined to be 0.77 ± 0.08 K*⁰(892), 0.104 ± 0.008 ?(1020) and ${? K}_{2}^{?t 0}(1430)$. The ratio of the tensor-to-vector meson production yields, $«ngle {? K}_{2}^{?t 0}(1430)»ngle$, is smaller than the 〈f₂(1270)〉/〈ρ⁰(770)〉 and $«ngle f_{2}^{?ime}(1525)»ngle$ ratios measured by DELPHI. The production rates and differential cross sections are compared with the predictions of JETSET 7.4 tuned to the DELPHI data and of HERWIG 5.8. The K*⁰(892) and ?(1020) data are compatible with model predictions, but a large disagreement is observed for the ${? K}_{2}^{?t 0}(1430)$.     

Investigation of inclusive CP asymmetries in \rm B^0 decays

A search for CP violating effects in the mixing of neutral B mesons is performed using a sample of 4.1 million hadronic Z decays collected with the ALEPH detector from 1991 to 1995. By studying time-dependent asymmetries in flavour-tagged samples of semileptonic and fully inclusive b-hadron decays, two measurements of the semileptonic asymmetry are extracted. No evidence for CP violation is observed, and the combined value is obtained. Received: 26 July 2000 / Published online: 8 June 2001     

Production of orbitally excited charm mesons in semileptonic B decays

A sample of 3.6 million hadronic Z decays recorded between 1991 and 1995 with the ALEPH detector at LEP is used to investigate semileptonic decays of B mesons into final states involving orbitally excited charm mesons. Topological vertex criteria are used to search for decays involving narrow D** states as well as wide D** resonances and non-resonant D(*)π final states. The sum of the branching ratios for these processes is measured to be ${? Br}({?erline {? B}}?ghtarrow {? D}? ?l^{-}{?erline v}) + {? Br}({?erline {? B}}?ghtarrow {? D}^{?t}? ?l^{-}{?erline v})?op =(2.26 pm 0.29({? stat}) pm 0.33({? syst}))%,$which accounts for a significant fraction of the deficit between inclusive measurements and the sum of exclusive semileptonic B decay modes.     

Measurement of the fraction of hadronic Z decays into charm quark pairs

The full statistics of hadronic Z decays collected with the ALEPH detector are analysed to measure, by three methods, the ratio, , of the partial decay width of the Z into quarks to the total hadronic width. One method uses the inclusive and spectra of electrons. The other two use a double-tagging technique to measure independently of the charm-tagging efficiency. In one, the tagging procedure uses the reconstruction of high momentum charmed mesons (, and to detect the charm quarks. In the other, a charm quark is tagged from the 's reconstructed in the decay channel and the other from the slow pion from the decay. The combination of these measurements leads to . Received: 6 March 1998 / Published online: 2 July 1998     

A precise measurement of the ${⊼ B}_{d}^{⊼ 0}$ meson lifetime using a new technique

From data recorded by DELPHI between 1991 and 1994, which corresponds to 3.2 million hadronic Z⁰ decays, a measurement of the $?erline {{? B}_d^0}$ meson lifetime, based on the inclusive reconstruction of 3520 ± 150 semileptonic decays of the type $?erline {{? B}_d^0}?ghtarrow {? D}^{?t +}{? ? X} ?l ?erline {v_{?l}},$ has been performed. The result is: $?u ?erline {({? B_d^0})}=1.532 pm 0.041(stat.) pm 0.040(syst.)ps.$. The contribution to the systematic uncertainty which depends on external errors is ±0.015.     

Study of f_{0} decays into four neutral pions

A simultaneous analysis of two different final states from annihilation at rest is used to study the decays of isoscalar states into . The data demand two scalar states, the with , and the with mass and width compatible with previous findings. Apart from the dominant scalar intensity, the (1300) plays an important role in the 5 dynamics. We find a mass of and a width of with a dominant decay into . The decay of the is less than 15% of its decay. Received: 13 September 2000 / Revised version: 4 January 2001 / Published online: 6 April 2001     

Tau hadronic branching ratios

From 64492 selected τ-pair events, produced at the Z⁰ resonance, the measurement of the tau decays into hadrons from a global analysis using 1991, 1992 and 1993 ALEPH data is presented. Special emphasis is given to the reconstruction of photons and π⁰s, and the removal of fake photons. A detailed study of the systematics entering the π⁰ reconstruction is also given. A complete and consistent set of tau hadronic branching ratios is presented for 18 exclusive modes. Most measurements are more precise than the present world average. The new level of precision reached allows a stringent test τ-μ, universality in hadronic decays, ${cal G}_{?u}/{cal G}_{?}=1.0013pm 0.0095$, and the first measurement of the vector and axial-vector contributions to the non-strange hadronic τ decay width: R_τ,V = 1.788 ± 0 0.025 and R_τ,A = 1.694 ± 0.027. The ratio (R_τ,V ? R_τ,A)/(R_τ,V + R_τ,A), equal to (2.7 ± 1.3) %, is a measure of the importance of QCD non-perturbative contributions to the hadronic τ decay width.     

Improved direct measurement of leptonic coupling asymmetries with polarized Z bosons

We present final measurements of the Z boson-lepton coupling asymmetry parameters A(e), A(mu), and A(tau) with the complete sample of polarized Z bosons collected by the SLD detector at the SLAC Linear Collider. From the left-right production and decay polar angle asymmetries in leptonic Z decays we measure A(e) = 0.1544+/-0.0060, A(mu) = 0.142+/-0.015, and A(tau) = 0.136+/-0.015. Combined with our left-right asymmetry measured from hadronic decays, we find A(e) = 0.1516+/-0.0021. Assuming lepton universality, we obtain a combined effective weak mixing angle of sin (2)theta(eff)(W) = 0.230 98+/-0.000 26.     

Study of charm production in Z decays

The production rates of , , , , , and in decays are measured using the LEP I data sample recorded by the ALEPH detector. The fractional energy spectrum of the is well described as the sum of three contributions: charm hadronisation, b hadron decays and gluon splitting into a pair of heavy quarks. The probability for a c quark to hadronise into a is found to be . The average fraction of the beam energy carried by mesons in events is measured to be The energy and the hemisphere mass imbalance distributions are simultaneously used to measure the fraction of hadronic Z decays in which a gluon splits to a pair: The ratio of the Vector/(Vector+Pseudoscalar) production rates in charmed mesons is found to be . The fractional decay width of the Z into pairs is determined from the sum of the production rates for various weakly decaying charmed states to be Received: 21 September 1999 / Published online: 8 June 2000     

Strange quark mass from the invariant mass distribution of Cabibbo-suppressed tau decays

Quark mass corrections to the hadronic width play a significant role only for the strange quark. The complete determination of decays into strange hadronic final states performed by ALEPH allows the extraction of the strange spectral function. New results on strange decay modes from other experiments are also incorporated into the present analysis. Using as input moments of the spectral function the analysis leading to the determination of is conducted using reasonable theoretical constraints on the nonperturbative components. Careful attention is paid to the treatment of the perturbative expansions of the moments which exhibit convergence problems. The result obtained, , is stable over the scale from down to about 2 GeV. Its evolution yields and . Received: 22 May 2001 / Published online: 5 November 2001     

Determination of the b quark mass at the Z mass scale

In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using calculations for massive quarks, this difference was used to determine the b quark mass in the renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be . Evolving this value to the b quark mass scale itself yields , consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations. Received: 14 May 2001 / Published online: 17 August 2001     

Charmless decays B \to{\mathrm{PP}}, {\mathrm{PV}}, and the effects of new strong and electroweak penguins in topcolor-assisted technicolor model

Based on the low energy effective Hamiltonian with generalized factorization, we calculate the new physics contributions to the branching ratios and CP-violating asymmetries of the two-body charmless hadronic decays from the new strong and electroweak penguin diagrams in the topcolor-assisted technicolor (TC2) model. The top-pion penguins dominate the new physics corrections, and both new gluonic and electroweak penguins contribute effectively to most decay modes. For tree-dominated decay modes , etc. the new physics corrections are less than 10%. For decays , etc. the new physics enhancements can be rather large (from to ) and are insensitive to the variations of and within reasonable ranges. For the decays and is strongly dependent: varying from to in the range of –. The new physics corrections to the CP-violating asymmetries vary greatly for different B decay channels. For five measured CP asymmetries of the decays, is only about 20% and will be masked by large theoretical uncertainties. The new physics enhancements to interesting decays are significant in size (), insensitive to the variations of the input parameters and hence lead to a plausible interpretation for the unexpectedly large decay rates. The TC2 model predictions for branching ratios and CP-violating asymmteries of all fifty-seven decay modes are consistent with the available data within one or two standard deviations. Received: 18 August 2000 / Revised version: 22 October 2000 / Published online: 5 February 2001     

Radiative corrections to K_{\ell 3} decays

We present a complete calculation of the decays and to in chiral perturbation theory with virtual photons and leptons. We introduce the concept of generalized form factors and kinematical densities in the presence of electromagnetism, and propose a possible treatment of the real photon emission in decays. We illustrate our results by applying them to the extraction of the Kobayashi–Maskawa matrix element from the experimental decay parameters. Received: 12 October 2001 / Published online: 11 January 2002     

Observation of doubly-charmed B decays at LEP

A search for doubly-charmed B decays with both charmed mesons reconstructed is performed, using about 3.8 million hadronic Z decays recorded with the ALEPH detector at LEP. A clear signal is observed in the channels and (where D can be either a D, a D or a D), providing the first direct evidence for doubly-charmed B decays involving no production. Evidence for associated and production in the decays is also presented and some candidates for completely reconstructed decays , and are observed. Furthermore, candidates for the two-body Cabibbo suppressed decays and are also observed. Measurements of the corresponding branching fractions are extracted. Received: 6 March 1998 / Published online: 23 June 1998     

A measurement of R_{\rm b} using a double tagging method

The fraction of events in hadronic decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of was obtained, where the first error is statistical and the second systematic. The uncertainty on , the fraction of events in hadronic decays, is not included in the errors. The dependence on is where $\Delta R_{\rm c}$ is the deviation of from the value 0.172 predicted by the Standard Model. The result for agrees with the value of predicted by the Standard Model. Received: 28 August 1998 / Published online: 7 April 1999