0000000001184718
AUTHOR
J Grahl
Investigation of the splitting of quark and gluon jets
The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays L with the {\sc Delphi} detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation $C_A/C_F$. The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resol…
An upper limit for the τ neutrino mass from τ → 5π(π0)ντ decays
An upper limit for the tau-neutrino mass has been determined from the decay tau -> 5pi+- \nu_tau using data collected with the OPAL detector from 1991 to 1995 in e+e- collisions at \sqrt{s} ~ M_Z. A limit of 43.2 MeV at 95% CL is obtained using a two-dimensional method in the 5pi invariant mass and energy distribution from 22 selected events. Combining this result with OPAL's previously published measurement using tau+tau- -> 3h+- \bar{nu}_tau 3h-+ \nu_tau decays, a new combined limit of m_{nu_tau} < 27.6 MeV (95% CL) is obtained.
MEASUREMENT OF ALPHA(S) FROM SCALING VIOLATIONS IN FRAGMENTATION FUNCTIONS IN E(+)E(-) ANNIHILATION
A study of scaling violations in fragmentation functions performed by the ALEPH collaboration at LEP is presented. Data samples enriched in uds, c, b and gluon jets, respectively, together with measurements of the longitudinal and transverse inclusive cross sections are used to extract the fragmentation function for the gluon and for each flavour. The measurements are compared to data from experiments at energies between 22 GeV and 91 GeV and scaling violations consistent with QCD predictions are observed. From this, a measurement of the strong coupling constant alpha(s) (M(z)) = 0.126 +/- 0.009 is obtained.
A measurement of |Vcb| from
Abstract From approximately 3 million hadronic decays of Z bosons recorded with the aleph detector at lep , a sample of 410 ± 32 B 0 → D ∗+ l − ν l candidates is selected, where l is either an electron or a muon. The differential decay rate dΓ ( B 0 → D ∗+ l − ν l ) dω from this sample is fitted, yielding a value for the product of the CKM matrix element |Vcb| and the normalisation of the decay form factor at the point of zero recoil of the D∗+ meson F (ω = 1)|Vcb| = (31.4 ± 2.3stat ± 2.5syst) × 10−3. A value for |Vcb| is extracted using theoretical calculations of the form factor normalisation, with the result |Vcb| = (34.5 ± 2.5stat ± 2.7syst ± 1.5theory) × 10−3. From the integrated s the…
A study of D*+π- production in semileptonic B decay
Abstract In a sample of 1.5 million hadronic decays of the Z collected by the ALEPH detector, a search is carried out for the decays B → D 1 0 (2420)l − ν X and B → D 2 ∗0 (2460)l − ν X . The product branching ratio for D10 production is measured to be Br (b → B ) × Br( B → D 1 0 l − ν X) × Br(D 1 0 → D ∗+ π − ) = (2.04 ± 0.58 stat ± 0.34 syst ) × 10 −3 , and a 95% confidence level limit of Br(b → B ) × Br( B → D 2 ∗0 l − ν X) × Br(D 2 ∗0 → D ∗+ π − ) ≤ 0.81 × 10 −3 is obtained for D2∗0 production. A topological search sensitive to the processes above, but also to wide resonances that decay to D ∗+ π − and to non-resonant D ∗+ π − production is also carried out, yielding Br(b→ B ) ×Br( B →D…
A Study of Bose-Einstein Correlations In e+e- Annihilation At 91 Gev
This paper describes a study of Bose-Einstein correlations made using the ALEPH detector at LEP. The correlations are found to enhance the two particle differential cross section for pairs of identical pions by a factor which can be roughly parametrized by R(Q) = 1 + lambda exp(-Q2-sigma(2)), where Q is the difference in the 3-momenta of the two pions in their centre of mass frame, lambda = 0.51 +/- 0.04 +/- 0.11 and sigma = 3.3 +/- 0.2 +/- 0.8 GeV-1, which corresponds to a source size of 0.65 +/- 0.04 +/- 0.16 fm. The large systematic errors on these results reflect their strong dependence on the choice of the reference sample used in the analysis. This problem is believed to occur primari…
UPDATE OF ELECTROWEAK PARAMETERS FROM Z DECAYS
Based on 520000 fermion pairs accumulated during the first three years of data collection by the ALEPH detector at LEP, updated values of the resonance parameters of the Z are determined to be M(Z) = (91.187 +/- 0.009) GeV, GAMMA(Z) = (2.501 +/- 0.012) GeV, sigma(had)0 = (41.60 +/- 0.27) nb, and R(l) = 20.78 +/- 0.13. The corresponding number of light neutrino species is N(v) = 2.97 +/- 0.05. The forward-backward asymmetry in lepton-pair decays is used to determine the ratio of vector to axial-vector couplings of leptons: g(V)2(M(z)2)/g(A)2 (M(Z)2) = 0.0052 +/- 0.00 1 6. Combining this with ALEPH measurements of the b and c quark asymmetries and tau polarization gives sin2theta(W)eff = 0.23…
Photon events with missing energy at root s=183 to 189 GeV
The production of single photons has been studied in the reaction e+e- -> gamma + invisible particles at centre-of-mass energies of 183 GeV and 189 GeV. A previously published analysis of events with multi-photon final states accompanied by missing energy has been updated with 189 GeV data. The data were collected with the DELPHI detector and correspond to integrated luminosities of about 51 pb^{-1} and 158 pb^{-1} at the two energies. The number of light neutrino families is measured to be 2.84 +/- 0.15(stat) +/- 0.14(syst). The absence of an excess of events beyond that expected from Standard Model processes is used to set limits on new physics as described by supersymmetric and compos…
Search for a very light CP-odd neutral Higgs boson of the MSSM
Abstract The reactions e + e − → hZ ∗ and e + e − → hA have been used to search for the neutral Higgs bosons h and A of the MSSM in the case where the CP -odd A is lighter than 2 m μ , taking into account the large h → AA decay branching ratio. No signal was found in the data sample collected until the end of 1991 by the ALEPH experiment at LEP. For tan β ⩾, m A m μ is excluded at 95% CL for any m h .
W pair production cross-section and W branching fractions in $e^{+}e^{-}$ interactions at 189 GeV
The cross-section for the process e+e- -> W+W- has been measured with the data sample collected by DELPHI at an average centre-of-mass energy of 189 GeV and corresponding to an integrated luminosity of 155 pb^{-1}. Based on the 2392 events selected as WW candidates, the cross-section for the doubly resonant process e+e- -> W+W- has been measured to be 15.83 +- 0.38 (stat) +- 0.20 (syst) pb. The branching fractions of the W decay were also measured and found to be in good agreement with the Standard Model expectation. From these a value of the CKM mixing matrix element |V_{cs}| = 1.001 +- 0.040 (stat) +- 0.020 (syst) was derived.
Search for particles with unexpected mass and charge in Z decays
During 1989 and 1990 over 180 000 hadronic and leptonic events, corresponding to 8 pb-1 of luminosity, were collected by the ALEPH detector in a scan of the Z peak at the e+e- collider LEP. This letter reports the results of a search in these data for particles with unexpected mass and charge by measurement of the ionization energy loss of charged tracks in the ALEPH TPC central tracking detector. The mass limits for the pair production of fractionally charged particles and of heavy, long lived charged particles are extended to 43 GeV/c2 at 90% confidence level. If single production of a heavy particle is considered, the mass limit is extended to more than 70 GeV/c2. RI Perrier, Frederic/A-…
Measurement of the Bs0 lifetime and production rate with Ds−ℓ+ combinations in Z decays
Abstract The lifetime of the Bs0 meson is measured in approximately 3 million hadronic Z decays accumulated using the ALEPH detector at LEP from 1991 to 1994. Seven different Ds− decay modes were reconstructed and combined with an opposite sign lepton as evidence of semileptonic Bs0 decays. Two hundred and eight Ds−l+ candidates satisfy selection criteria designed to ensure precise proper time reconstruction and yield a measured Bs0 lifetime of τ(Bs0) = 1.59−0.15+0.17 (stat) ±0.03 (syst) ps. Using a larger, less constrained sample of events, the product branching ratio is measured to be Br ( b → B s 0 ) · Br(B s 0 → D s − l + νX) = 0.82 ± 0.09 (stat) −0.14+0.13 (syst) %.
MEASUREMENT OF THE TAU-POLARIZATION AT THE Z-RESONANCE
Using 18.8 pb-1 of data collected in 1990 and 1991, ALEPH has measured the tau polarisation in the decay modes tau-->enunuBAR, tau-->mununuBAR, tau-->rhonu tau-->a1nu, using both the individual tau decay kinematics and the event acollinearity. The measurement of the tau polarisation as a function of the production polar angle yields the two parameters A(tau) and A(e), where A(l) = 2g(V)l g(A)l/[(g(V)l)2 + (g(A)l)2]. The results A(tau) = 0.143 +/- 0.023 and A(e) = 0. 120 +/- 0.026 are consistent with the hypothesis of electron-tau universality. Assuming universality yields a measurement of the effective weak mixing angle sin 2theta(W)eff = 0.2332 +/- 0.0022.
The Scale Dependence of the Hadron Multiplicity in Quark and Gluon Jets and a Precise Determination of $C_{A}/C_{F}$
\frac{C_A}{C_F} = 2.246 \pm 0.062~(stat.) \pm 0.080~(syst.) \pm 0.095~(theo.) Data collected at the Z resonance using the DELPHI detector at LEP are used to determine the charged hadron multiplicity in gluon and quark jets as a function of a transverse momentum-like scale. The colour factor ratio, \cacf, is directly observed in the increase of multiplicities with that scale. The smaller than expected multiplicity ratio in gluon to quark jets is understood by differences in the hadronization of the leading quark or gluon. From the dependence of the charged hadron multiplicity on the opening angle in symmetric three-jet events the colour factor ratio is measured to be: C_A/C_F = 2.246 \pm 0.0…
Measurement of charge asymmetry in hadronic Z decays
A significant charge asymmetry is observed in the hadronic Z decays with the ALEPH detector at LEP. The asymmetry expressed in terms of the difference in momentum weighted charges in the two event hemispheres is measured to be − = −0.0084±0.0015 (stat.) ±0.0004 (exp. sys.). In the framework of the standard model this can be interpreted as a measurement of the effective electroweak mixing angle, sin2Ow (Mz2=0.2300±0.0034 (stat.) ±0.0010 (exp. sys.) ±0.0038 (theor. sys.) or of the ratio of the vector to axual- vector coupling costants of the electron, gvegAe=+0.073±0.024.
Update of the search for supersymmetric particles in scenarios with Gravitino LSP and Sleptons NLSP
An update of the search for sleptons, neutralinos and charginos in the context of scenarios where the lightest supersymmetric particle is the gravitino and the next-to-lightest supersymmetric particle is a slepton, is presented, together with the update of the search for heavy stable charged particles in light gravitino scenarios and Minimal Supersymmetric Standard Models. Data collected in 1999 with the DELPHI detector at centre-of-mass energies around 192, 196, 200 and 202 GeV were analysed. No evidence for the production of these supersymmetric particles was found. Hence, new mass limits were derived at 95% confidence level.
MEASUREMENT OF TAU BRANCHING RATIOS
著者人数：112名
Measurement of α from the structure of particle clusters produced in hadronic Z decays
Abstract Using 106 000 hadronic events obtained with the ALEPH detector at LEP at energies close to the Z resonance peak, the strong coupling constant αs is measured by an analysis of energy-energy correlations (EEC) and the global event shape variables thrust, C-parameter and oblateness. It is shown that the theoretical uncertainties can be significantly reduced if the final state particles are first combined in clusters using a minimum scaled invariant mass cut, Ycut, before these variables are computed. The combined result from all shape variables of pre-clustered events is αs(MZ2 = 0.117±0.005 for a renormalization scale μ= 1 2 M Z . For μ values between MZ and the b-quark mass, the res…
Measurement of the b hadron lifetime with the dipole method
A measurement of the average lifetime of b hadrons has been performed with the dipole method on a sample of 260000 hadronic Z decays recorded with the ALEPH detector during 1991. The dipole is the distance between the vertices built in the opposite hemispheres. The mean dipole is extracted from all the events without attempting b enrichment. Comparing the average of the data dipole distribution with a Monte Carlo calibration curve obtained with different b lifetimes, an average b hadron lifetime of 1.51 +/- 0.08 ps is extracted. RI ANTONELLI, ANTONELLA/C-6238-2011; Buttar, Craig/D-3706-2011; Stahl, Achim/E-8846-2011; Passalacqua, Luca/F-5127-2011; Murtas, Fabrizio/B-5729-2012; St.Denis, Ric…
Study of the four-fermion final state at the Z resonance
The process e+e−→, where l is a charged or a neutral lepton and f any charged fermion, is analyzed. The study uses the ALEPH data collected at LEP from 1989 to 1993 at centre-of-mass energies between 88 and 95 GeV, corresponding to almost two million hadronic Z decays and to a total integrated luminosity of 79 pb−1. For all channels, the data agree well with the standard model expectation both in shape and normalization. The indication of an excess in the\(e^ + e^ - \to \tau ^ + \tau ^ - f\bar f\) channel, reported by ALEPH in 1991, is not confirmed.
Search for the Standard Model Higgs boson at LEP in the year 2000
Searches for the Standard Model Higgs boson have been performed in the data collected by the DELPHI experiment at LEP in the year 2000 at centre-of-mass energies between 200 and 209 GeV corresponding to a total integrated luminosity of 224 pb^{-1}. No evidence for a Higgs signal is observed in the kinematically accessible mass range, and a 95% CL lower mass limit of 114.3 GeV/c^2 is set, to be compared with an expected median limit of 113.5 GeV/c^2 for these data.
Z production cross sections and lepton pair forward-backward asymmetries
Precise values of the Z resonance cross sections and lepton pair forward-backward asymmetries are measured from a sample of 1.3 million Z decays into hadrons and charged leptons taken with the ALEPH detector at LEP. A silicon-tungsten luminosity calorimeter was installed in ALEPH in 1992, permitting the determination of the absolute luminosity to a systematic precision of 0.15% (experimental) and 0.25% (theoretical). This is combined with a better understanding of the hadronic event selection and an improved tau pair selection. Including the ALEPH results of the past three years one obtains sigma(h)0 = (41.60 +/- 0.16) nb, and R(l) = 20.69 +/- 0.09. The corresponding number of light neutrin…
Updated measurement of the average b hadron lifetime
An improved measurement of the average lifetime of b hadrons has been performed with the ALEPH detector. From a sample of 260000 hadronic Z0 decays, recorded during the 1991 LEP run with the silicon vertex detector fully operational, a fit to the impact parameter distribution of lepton tracks coming from semileptonic decays yields an average b hadron lifetime of 1.49 +/- 0.03 +/- 0.06 ps.
Measurement of the charged particle multiplicity of weakly decaying B hadrons
From the Z decays recorded in 1994 and 1995 by the DELPHI detector at LEP, the charged particle multiplicity of weakly decaying B hadrons was measured to be: 4.97 +/- 0.03 +/- 0.06 excluding the K-o and Lambda decay products. (C) 1998 Published by Elsevier Science B.V. All rights reserved.
Measurement of the ZZ cross-section in e(+)e(-) interactions at 183-189 GeV
Measurements of on-shell ZZ production are described, using data collected by DELPHI in 1997 and 1998, at centre-of-mass energies sqrt(s) = 182.6 GeV and 188.6 GeV respectively. Results obtained in each of the final states q qbar q qbar, mu+mu- q qbar, e+e- q qbar, nu nubar q qbar, l+l-l+l-, and nu nubar l+l- are presented. The measured cross-sections for on-shell ZZ production via the tree-level doubly-resonant graphs (NC02) are: sigma_{NC02}(182.6 GeV) = 0.38 +- 0.18 (stat) +- 0.04 (syst) pb, sigma_{NC02}(188.6 GeV) = 0.60 +- 0.13 (stat) +- 0.07 (syst) pb. They are consistent with the Standard Model expectations of 0.25 pb and 0.65 pb at each energy.
Measurement of the charged particle multiplicity distribution in hadronic Z decays
The charged particle multiplicity distribution of hadronic Z decays was measured on the peak of the Z resonance using the ALEPH detector at LEP. Using a model independent unfolding procedure the distribution was found to have a mean = 20.85 +/- 0.24 and a dispersion D = 6.34 +/- 0.12. Comparison with lower energy data supports the KNO scaling hypothesis in the energy range square-root s = 29-91.25 GeV. At square-root s = 91.25 GeV the shape of the multiplicity distribution is well described by a log-normal distribution, as predicted from a cascading model for multi-particle production. The same model also successfully describes the energy dependence of the mean and width of the multiplicity…
Multiplicity fluctuations in one- And two-dimensional angular intervals compared with analytic QCD calculations
Multiplicity fluctuations in rings around the jet axis and in off-axis cones have been measured by the DELPHI collaboration in $e^+e^-$ annihilations into hadrons at LEP energies. The measurements are compared with analytical perturbative QCD calculations for the corresponding multiparton system, using the concept of Local Parton Hadron Duality. Some qualitative features are confirmed by the data but substantial quantitative deviations are observed.
Measurement of the semileptonic b branching fractions and average b mixing parameter in Z decays
The semileptonic branching fractions for primary and cascade b decays BR(b -> lepton-), BR(b -> c -> lepton+) and BR(b -> cbar -> lepton-) were measured in hadronic Z decays collected by the DELPHI experiment at LEP. The sample was enriched in b decays using the lifetime information and various techniques were used to separate leptons from direct or cascade b decays. By fitting the momentum spectra of di-leptons in opposite jets, the average b mixing parameter chi-bar was also extracted. The following results have been obtained: BR(b -> lepton-) = (10.70 +/- 0.08 (stat) +/- 0.21 (syst)_{+0.44}^{-0.30}(model))% BR(b -> c -> lepton+) = (7.98 +/- 0.22 (stat) +/- 0.21 (s…
Measurement and Interpretation of Fermion-Pair Production at LEP Energies of 183 and 189 GeV
An analysis of the data collected in 1997 and 1998 with the DELPHI detector at e+e- collision energies close to 183 and 189 GeV was performed in order to extract the hadronic and leptonic fermion-pair cross-sections, as well as the leptonic forward-backward asymmetries and angular distributions. The data are used to put limit on contact interactions between fermions, the exchange of R-parity violating SUSY sneutrinos, Z' bosons and the existence of gravity in extra dimensions.
Search for supersymmetric particles with R-parity violation in Z decays
Searches for supersymmetric particles produced in e(+)e(-) interactions at the Z peak have been performed under the assumptions that R-parity is not conserved, that the dominant R-parity violating coupling involves only leptonic fields, and that the lifetime of the lightest supersymmetric particle can be neglected. In a data sample collected by the ALEPH detector at LEP up to 1993, and corresponding to almost two million hadronic Z decays, no signal was observed. As a result, supersymmetric particle masses and couplings are at least as well constrained as under the usual assumption of R-parity conservation.