Search for Scalar Quarks in e+e- Collisions at sqrt(s) up to 209 GeV Searches for scalar top, scalar bottom and mass-degenerate scalar quarks are performed in the data collected by the ALEPH detector ...at LEP, at centre-of-mass energies up to 209 GeV, corresponding to an integrated luminosity of 675 pb-1. No evidence for the production of such particles is found in the decay channels stop->c/u chi, stop->b l snu, sbottom-> b chi, squark-> q chi or in the stop four-body decay channel stop-> b chi f f' studied for the first time at LEP. The results of these searches yield improved mass lower limits. In particular, an absolute lower limit of 63GeV/c2 is obtained for the stop mass, at 95% confidence level, irrespective of stop lifetime and decay branching ratios.
A search for charged Higgs bosons produced in pairs is performed with data collected at centre-of-mass energies ranging from 189 to 209 GeV by ALEPH at LEP, corresponding to a total luminosity of 629 ...invpb. The three final states taunutaunu, taunucs and cscs are considered. No evidence for a signal is found and lower limits are set on the mass M_H+ as a function of the branching fraction B(H to taunu). In the framework of a two-Higgs-doublet model, and assuming B(H+ to taunu + B(H+ to cs) = 1 charged Higgs bosons with masses below 79.3 Gev/c2 are excluded at 95% confidence level independently of the branching ratios.
A search for the Higgsstrahlung process e+e- -> HZ is carried out, covering decays of the Higgs boson into any quark pair, a gluon pair or a tau pair. The analysis is based on the 630pb-1 of data ...collected by the ALEPH detector at LEP at centre-of-mass energies from 189 to 209GeV. A 95% C.L. lower mass limit of 109.1GeV/c2 is obtained for a Higgs boson cross section equal to that expected from the Standard Model if the Higgs boson decays exclusively into hadrons and/or taus, irrespective of the relative branching fractions.
Single top production via the flavour changing neutral current reactions e+e- -> \bar{t}c , \bar{t}u is searched for within the 214~pb-1 of data collected by ALEPH at centre-of-mass energies between ...204 and 209~GeV. No deviation from the Standard Model expectation is observed and upper limits on the single top production cross sections are derived. The combination with data collected at lower centre-of-mass energies yields an upper limit on the branching ratio BR(t -> Zc)+BR(t -> Zu) 14%, for BR(t -> \gamma c)+BR(t -> \gamma u)= 0 and mt=174 GeV/c2$.
The sample of hadronic Z decays collected by the ALEPH detector at LEP in the years 1991-1995 is analysed in order to measure the forward-backward asymmetry in $Z \to b\bar{b}$ and $Z \to c\bar{c}$ ...events and the $B^0-\bar{B^0}$ average mixing parameter $\bar{\chi}$. Quark charges are tagged by the charges of electrons and muons produced in b and c semileptonic decays. Multivariate analyses are used to separate the event flavours and $b \to \ell$/$b \to c \to ell$ processes. The b and c quark asymmetries are measured simultaneously; the average mixing parameter and the pole asymmetries are determined to be \begin{eqnarray*} \bar{\chi} &=& 0.1196 \pm 0.0049 (stat) {}^{+0.0043}_{-0.0050} (syst) \\ \A^{0,b}_{FB} &=& 0.0998 \pm 0.0040 (stat) \pm 0.0017 (syst),\\ \A^{0.c}_{FB} &=& 0.0732 \pm 0.0053 (stat) \pm 0.0037 (syst).\\ \end{eqnarray*} These asymmetries, combined with the ALEPH measurements of the b asymmetry using inclusive b hadron decays and of the c asymmetry using reconstructed D mesons, correspond to a value of the effective electroweak mixing angle of $\sin^2\theta^{eff}_{W} = 0.23188 \pm 0.00046$.
Searches for pair production of squarks, sleptons, charginos and neutralinos are performed with the data collected by the ALEPH detector at LEP at centre-of-mass energies from 188.6 to 201.6 GeV. No ...evidence for any such signals is observed in a total integrated luminosity of about 410 pb**-1. The negative results of the searches are translated into exclusion domains in the space of the relevant MSSM parameters, which improve significantly on the constraints set previously. Under the assumptions of gaugino and sfermion mass unification, these results allow a 95% C.L. lower limit of 37 GeV/c**2 to be set on the mass of the lightest neutralino for any tan(beta) and sfermion mass. Additional constraints in the MSSM parameter space are derived from the negative results of ALEPH searches for Higgs bosons. The results are also interpreted in the framework of minimal supergrav
DIRC – A particle identification system for BaBar Höcker, Andreas
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
05/2000, Letnik:
446, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The Detection of Internally Reflected Cherenkov light (DIRC) is a novel type of Cherenkov imaging device that has been developed, built and installed as part of the BaBar detector at the asymmetric ...B-factory PEP-II at SLAC. The DIRC is based on total internal reflection of Cherenkov photons produced and guided within thin, rectangular quartz bars covering the barrel region of BaBar. The photon detector is an array of photomultiplier tubes covering the photon phase space at the backward end of the bars. In its first few months of operation the DIRC performance has been found to achieve the design requirements. This note presents results from cosmic-ray data and an analysis of the first beam collision runs.
The single top production via flavour changing neutral currents in the reactions ee -> t c/u is searched for in approximately 411 pb-1 of data collected by ALEPH at centre-of-mass energies in the ...range between 189 and 202 GeV. In total, 58 events are selected in the data to be compared with 50.3 expected from Standard Model backgrounds. No deviation from the Standard Model expectation is observed. A 95\%~CL upper limit of 0.72 pb on the single top production cross section at 202 GeV is derived assuming a top mass of 174 GeV/c^2 and a 100% branching ratio of the top decay into bW. A model dependent limit on the flavour-changing couplings kZ and kg is obtained by combining all centre-of-mass energies.
Data collected at centre-of-mass energies from 189 GeV to 202 GeV by the ALEPH detector at LEP, corresponding to an integrated luminosity of 411 pb-1, are analysed in a search for the scalar top in ...the decay channels 'stop -> c/u neutralino' for small mass differences between the stop and the lightest neutralino. No evidence for deviations from the Standard Model expectation is found and a lower limit of 59 GeV/c**2 is set for the stop mass, independent of the stop to neutralino mass difference and of the stop lifetime.
Searches for topologies characteristic of Gauge Mediated SUSY Breaking models (GMSB) are performed by analysing 173.6 pb^-1 of data collected at Ecm = 188.6~GeV with the ALEPH detector.These ...topologies include acoplanar photons, non-pointing single photon, acoplanar leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and four leptons plus missing energy final states.No evidence for these new phenomena is observed and limits on production cross sections and sparticle masses are derived. A scan of a minimal GMSB parameter space is performed and model dependent lower limits of about 45 GeV/c^2 on the next-to-lightest supersymmetric particle (NLSP) mass and of about 9 TeV on the mass scale parameter Lambda are derived, independently of the NLSP lifetime.
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