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ALEPH Collaboration

Publications and source records attributed to ALEPH Collaboration.

71 records · Page 4Linked to original sources
arXiv2000

Measurement of W-pair production in e+e- collisions at 189GeV

The production of W-pairs is analysed in a data sample collected by ALEPH at a mean centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 174.2 pb^-1. Cross sections are given for different topologies of W decays into leptons or hadrons. Combining all final states and assuming Standard Model branching fractions, the total W-pair cross section is measured to be 15.71 +- 0.34 (stat) +- 0.18 (syst) pb. Using also the W-pair data samples collected by ALEPH at lower centre-of-mass energies, the decay branching fraction of the W boson into hadrons is measured to be BR (W > hadrons) = 66.97 +- 0.65 (stat) +- 0.32 (syst) %, allowing a determination of the CKM matrix element |V(cs)|= 0.951 +- 0.030 (stat) +- 0.015 (syst).

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arXiv1999

Search for the glueball candidates f0(1500) and fJ(1710) in gamma gamma collisions

Data taken with the ALEPH detector at LEP1 have been used to search for gamma gamma production of the glueball candidates f0(1500) and fJ(1710) via their decay to pi+pi-. No signal is observed and upper limits to the product of gamma gamma width and pi+pi- branching ratio of the f0(1500) and the fJ(1710) have been measured to be Gamma_(gamma gamma -> f0(1500)). BR(f0(1500)->pi+pi-) < 0.31 keV and Gamma_(gamma gamma -> fJ(1710)). BR(fJ(1710)->pi+pi-) < 0.55 keV at 95% confidence level.

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arXiv1999

Study of Charm Production in Z Decays

The production rates of D*+-, Ds*+-, D+-, D0 / D0bar, Ds+, and Lambda_c in Z to ccbar decays are measured using the LEP I data sample recorded by the ALEPH detector. The fractional energy spectrum of the D*+- 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 D*+ is found to be f(c to D*+) = 0.233 +- 0.010 (stat.) +- 0.011 (syst.). The average fraction of the beam energy carried by D*+- mesons in Z to cc events is measured to be < X_E (D*+-) >_cc = 0.4878 +- 0.0046 (stat.) +- 0.0061 (syst.). The D*+- 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 cc pair: n_{gluon to cc} = (3.23 +- 0.48 (stat.) +- 0.53 (syst.) %. The ratio of the Vector/(Vector+Pseudoscalar) production rates in charmed mesons is found to be P_V = 0.595 +- 0.045. The fractional decay width of the Z into cc pairs is determined from the sum of the production rates for various weakly decaying charmed states to be Rc = 0.1738 +- 0.0047 (stat.) +- 0.0116 (syst.).

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arXiv1999

Search for neutral Higgs bosons in e+e- collisions at sqrt{s} <= 196 GeV

A preliminary search for neutral Higgs bosons is performed in the data collected by ALEPH in 1999, corresponding to integrated luminosities of 29.0 and 69.5 pb^-1 at centre-of-mass energies of 191.6 and 195.6 GeV respectively. No evidence for a signal is found. Combined with the lower energy ALEPH data, this observation leads to a 95% confidence level lower mass limit of 98.8 GeV/c^2 for the Standard Model Higgs boson. In the MSSM, the lower limit for the mass of the lightest CP-even Higgs boson h, is found to be 85.2 GeV/c^2 for all values of tan(beta)>=1.

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arXiv1999

Determination of the LEP centre-of-mass energy from Zgamma events

Radiative returns to the Z resonance (Zgamma events) are used to determine the LEP2 centre-of-mass energy from the data collected with the ALEPH detector in 1997. The average centre-of-mass energy is measured to be: E_CM = 182.50 +- 0.19(stat) +- 0.08(syst) GeV in good agreement with the precise determination by the LEP energy working group of 182.652 +- 0.050 GeV. If applied to the measurement of the W mass, its precision translates into a systematic error on M_W which is smaller than the statistical error achieved from the corresponding dataset.

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arXiv1999

Determination of the upper limit on m_nu_tau from LE

A review of the direct determinations of the upper limit on the tau-neutrino mass from the LEP experiments is given. The experimental methods, the results and the comparison with non LEP measurements are also discussed. The study of the systematic errors shows that the LEP results are statistically limited so that their combination will improve the sensitivity to a massive tau-neutrino. An unofficial combination of the ALEPH and OPAL measurements gives a 95% confidence level upper limit of 15 MeV on the tau-neutrino mass.

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arXiv1999

Measurement of W-pair production in e+e- collisions at 183 GeV

The production of W+W- pairs is analysed in a data sample collected by ALEPH at a mean centre-of-mass energy of 182.7 GeV, corresponding to an integrated luminosity of 57 pb-1. Cross sections are given for different topologies of W decays into leptons or hadrons. Under Standard Model assumptions for the W-pair production and decay, the W-pair cross section is measured to be 15.57+-0.62(stat.)+-0.29(syst.) pb. Using also the W-pair data samples collected by ALEPH at lower centre-of-mass energies, the decay branching ratio of the W boson into hadrons is measured to be B(W->hadrons)= 68.93+-1.21(stat.)+-0.51(syst.)%, allowing a determination of the CKM matrix element |Vcs|= 1.043 +- 0.058(stat.) +- 0.026(syst.). The agreement of the cross sections with the Standard Model prediction allows a limit to be set on the W decay rate to undetectable final states.

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arXiv1999

Study of tau decays involving kaons, spectral functions and determination of the strange quark mass

All ALEPH measurements of branching ratios of tau decays involving kaons are summarized including a combination of results obtained with K^0_S and K^0_L detection. The decay dynamics are studied, leading to the determination of contributions from vector K^*(892) and K^{*}(1410), and axial-vector K_1(1270) and K_1(1400) resonances. Agreement with isospin symmetry is observed among the different final states. Under the hypothesis of the conserved vector current, the spectral function for the K\bar{K}πmode is compared with the corresponding cross section for low energy e^+e^- annihilation, yielding an axial-vector fraction of (94^{+6}_{-8})% for this mode. The branching ratio for tau decay into all strange final states is determined to be B(τ^-\to X^-(S=-1)ν_τ)=(28.7\pm1.2)\times 10^{-3}. The measured mass spectra of the strange tau decay modes are exploited to derive the S=-1 spectral function. A combination of strange and nonstrange spectral functions is used to determine the strange quark mass and nonperturbative contributions to the strange hadronic width. A method is developed to avoid the bad convergence of the spin zero hadronic component, with the result m_s(M_τ^2)=(176^{+46}_{-57}) MeV/c^2. The evolution down to 1 GeV gives m_s(1 {GeV}^2) = (234^{+61}_{-76}) MeV/c^2.

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arXiv1999

One-prong tau decays with kaons

One-prong tau decays into final states involving kaons are studied with about 161k tau+tau- events collected by the ALEPH detector from 1991 to 1995. Charged kaons are identified by dE/dx measurement, while K0L's are detected through their interaction in calorimeters. Branching ratios are measured for the inclusive mode, B(tau- -> K-Xnu_tau)=(1.52 +- 0.04 +- 0.04)%, where X can be any system of neutral particles, and for the exclusive modes B(tau- to K- nu_tau) = (6.96 =- 0.25 +- 0.14)x 10-3, B(tau- to K- pi0 nu_tau) = (4.44 +- 0.26 +- 0.24)x 10-3, B(tau-to K- pi0 pi0 nu_tau)= (0.56 +- 0.20 +- 0.15) x 10-3, B(tau- to K- pi0 pi0 pi0 nu_tau = (0.37 +- 0.21 +- 0.11) x 10-3, B(tau- to K-K0 nu_tau) = (1.62 +- 0.21 +- 0.11) x 10-3, B(tau- to K-K0 pi0 nu_tau) = (1.43 +- 0.25 +- 0.21)x 10-3, B(tau- to \overline{K0} pi- nu_tau) = (9.28 +- 0.45 +- 0.34)x 10-3, B(tau- to \overline{K0} pi-pi0 nu_tau) = (3.47 +- 0.53 +- 0.37) x 10-3. Upper limits for B(tau- to \overline{K0} pi- pi0 pi0 nu_tau) and B(tau- to K- K0 pi0 pi0 nu_tau are also obtained. Mass spectra in the final states are addressed in order to study the relevant dynamics.

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arXiv1999

Search for charged Higgs bosons in e+e- collisions at sqrt(s) = 181-184 GeV

Data collected at centre-of-mass energies of 181-184 GeV by ALEPH at LEP corresponding to an integrated luminosity of 56.9 pb-1, are analysed in a search for pair-produced charged Higgs bosons H+-. Three analyses are employed to select the tau^+ nu_tau tau^- nubar_tau, c sbar tau^- nubar_tau/ cbar s tau^+ nu_tau and c sbar s cbar final states. No evidence for a signal is found. Mass limits are set as a function of the branching fraction B(H+ -> tau^+ nu_tau). Under the assumption that the decay modes considered cover the totality of the possible final states, charged Higgs bosons with masses below 59 GeV/c^2 are excluded at 95% C.L. independently of B(H+ -> tau^+ nu_tau).

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arXiv1999

Measurement of triple gauge WWgamma couplings at LEP2 using photonic events

A study of events with photons and missing energy has been performed with the data sample obtained with the ALEPH detector at centre-of-mass energies from 161 to 184 GeV,corresponding to a total integrated luminosity of about 80 pb**-1. The measured distributions are in agreement with Standard Model predictions, leading to constraints on W W gamma gauge coupling parameters Delta(kappa_gamma) and lambda_gamma. The results from the fit to the cross sections and to the energy and angular distributions of the photons are: Delta(kappa_gamma) = 0.05 +1.15/-1.10 (stat) +/- 0.25 (syst) lambda_gamma = 0.05 +1.55/-1.45 (stat) +/- 0.30 (syst)

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arXiv1998

Search for the Standard Model Higgs boson at the LEP2 collider near sqrt(S) = 183GeV

During 1997 the ALEPH experiment at LEP gathered 57pb^-1 of data at centre-of- mass energies near 183 GeV. These data are used to look for possible signals from the production of the Standard Model Higgs boson in the reaction e+e- -> HZ. No evidence of a signal is found in the data; seven events are selected, in agreement with the expectation of 7.2 events from background processes. This observation results in an improved lower limit on the mass of the Higgs boson: m_H > 87.9GeV/c^2 at 95% confidence level.

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arXiv1998

Determination of the LEP center-of-mass energy from Zgamma events

Radiative returns to the Z resonance (Zgamma events) provide a method to determine the centre-of-mass energy of the high energy e+e- colliders and already at LEP2. The method is applied for the data collected with the ALEPH detector in 1997 at 181-184 GeV, using e+e- -> gammaZ -> hadrons events.

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arXiv1997

Searches for Supersymmetry in the photon(s) plus missing energy channels at $\sqrt{s}$ = 161 GeV and 172 GeV

Searches for supersymmetric particles in channels with one or more photons and missing energy have been performed with data collected by the ALEPH detector at LEP. The data consist of 11.1 pb-1 at $\sqrt{s} = 161 GeV$, 1.1 pb-1 at 170 GeV and 9.5 pb-1 at 172 GeV. The e+e- -> nunu+photon cross section is measured. The data are in good agreement with predictions based on the Standard Model, and are used to set upper limits on the cross sections for anomalous photon production. These limits are compared to two different SUSY models and used to set limits on the neutralino mass. A limit of 71 GeV/c^2 at 95% C.L. is set on the mass of the lightest neutralino ($τ_{χ_{1}^{0}} \leq $ 3 ns) for the gauge-mediated supersymmetry breaking and LNZ models.

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arXiv1997

Search for pair-production of long-lived heavy charged particles in e+e- annihilation

A search for pair-production of long-lived, heavy, singly-charged particles has been performed with data collected by the ALEPH detector at a centre-of-mass energy of 172 GeV. Data at \sqrt{s} = 161, 136, and 130 GeV are also included to improve the sensitivity to lower masses. No candidate is found in the data. A model-independent 95% confidence level upper limit on the production cross section at 172 GeV of 0.2-0.4pb is derived for masses between 45 and 86 GeV/c^2. This cross section limit implies, assuming the MSSM, a lower limit of 67 (69) GeV/c^2 on the mass of right- (left-) handed long-lived scalar taus or scalar muons and of 86 GeV/c^2 on the mass of long-lived charginos.

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