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J. Abdallah

Publications and source records attributed to J. Abdallah.

At least 73 records · Page 4Linked to original sources

Search for SUSY in the AMSB scenario with the DELPHI detector

The DELPHI experiment at the LEP e+e- collider collected almost 700 pb^-1 at centre-of-mass energies above the Z0 mass pole and up to 208 GeV. Those data were used to search for SUSY in the Anomaly Mediated SUSY Breaking (AMSB) scenario with a flavour independent common sfermion mass parameter. The searches covered several possible signatures experimentally accessible at LEP, with either the neutralino, the sneutrino or the stau being the Lightest Supersymmetric Particle (LSP). They included: the search for nearly mass-degenerate chargino and neutralino, which is a typical feature of AMSB; the search for Standard-Model-like or invisibly decaying Higgs boson; the search for stable staus; the search for cascade decays of SUSY particles resulting in the LSP and a low multiplicity final state containing neutrinos. No evidence of a signal was found, and thus constraints were set in the space of the parameters of the model.

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Measurement of the Lambda_b^0 Decay Form Factor

The form factor of Lambda_b^0 baryons is estimated using 3.46 10^6 hadronic Z decays collected by the DELPHI experiment between 1992 and 1995. Charmed Lambda_c^+ baryons fully reconstructed in the pK-pi+, pK0_S, and Lambda pi+pi+pi- modes, are associated to a lepton with opposite charge in order to select Lambda_b^0 -> Lambda_c^+ l^- anti-nu_l decays. From a combined likelihood and event rate fit to the distribution of the Isgur-Wise variable w, and using the Heavy Quark Effective Theory (HQET), the slope of the b-baryon form factor is measured to be: rho-hat^2 = 2.03 +/- 0.46 (stat) ^{+0.72}_{-1.00} (syst). The exclusive semileptonic branching fraction Br(Lambda_b^0 -> Lambda_c^+ l^- anti-nu_l) can be derived from rho-hat^2 and is found to be (5.0^{+1.1}_{-0.8} (stat) ^{+1.6}_{-1.2} (syst))%. Limits on other branching fractions are also obtained.

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Measurement of the Forward-Backward Asymmetries of e+e- -> Z -> b bbar and e+e- -> Z -> c cbar using prompt leptons

The forward-backward asymmetries of the processes e+e- -> Z -> b bbar and e+e- -> Z -> c cbar were measured from a sample of hadronic Z decays collected by the DELPHI experiment between 1993 and 1995. Enriched samples of b bbar and c cbar events were obtained using lifetime information. The tagging of b and c quarks in these samples was based on the semileptonic decay channels b/c -> X + mu and b/c -> X + e combined with charge flow information from the hemisphere opposite to the lepton. Combining the A_{FB}^{b bbar} and A_{FB}^{c cbar} measurements presented in this paper with published results based on 1991 and 1992 DELPHI data samples, the following pole asymmetries were obtained: A_{FB}^{0,b} = 0.1021 +/- 0.0052 (stat) +/- 0.0024 (syst) A_{FB}^{0,c} = 0.0728 +/- 0.0086 (stat) +/- 0.0063 (syst) The effective value of the weak mixing angle derived from these measurements is sin^2 theta_{W,eff}^{lept} = 0.23170 +/- 0.00097 .

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Measurement of the W-pair Production Cross-section and W Branching Ratios in e+e- Collisions at sqrt(s) = 161-209 GeV

These final results on e+e- -> W+W- production cross-section measurements at LEP2 use data collected by the DELPHI detector at centre-of-mass energies up to 209 GeV. Measurements of total cross-sections, W angular differential distributions and decay branching fractions, and the value of the CKM element |V_{cs}| are compared to the expectations of the Standard Model. These results supersede all values previously published by DELPHI.

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Final results from DELPHI on the searches for SM and MSSM Neutral Higgs bosons

These final results from DELPHI searches for the Standard Model (SM) Higgs boson, together with benchmark scans of the Minimal Supersymmetric Standard Model (MSSM) neutral Higgs bosons, used data taken at centre-of-mass energies between 200 and 209 GeV with a total integrated luminosity of 224 pb^-1. The data from 192 to 202 GeV are reanalysed with improved b-tagging for MSSM final states decaying to four b-quarks. The 95% confidence level lower mass bound on the Standard Model Higgs boson is 114.1 GeV/c2. Limits are also given on the lightest scalar and pseudo-scalar Higgs bosons of the MSSM.

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Searches for invisibly decaying Higgs bosons with the DELPHI detector at LEP

Searches for HZ production with the Higgs boson decaying into an invisible final state were performed using the data collected by the DELPHI experiment at centre-of-mass energies between 188 GeV and 209 GeV. Both hadronic and leptonic final states of the Z boson were analysed. In addition to the search for a heavy Higgs boson, a dedicated search for a light Higgs boson down to 40 GeV/c^2 was performed. No signal was found. Assuming the Standard Model HZ production cross-section, the mass limit for invisibly decaying Higgs bosons is 112.1 GeV/c^2 at 95% confidence level. An interpretation in the Minimal Supersymmetric extension of the Standard Model (MSSM) and in a Majoron model is also given.

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Measurement of |V_cb| using the semileptonic decay anti-B0d -> D*+ l- anti-nu_l

Data from Z decays in DELPHI have been searched for anti-B0d -> D*+ l- anti-nu_l with the D*+ decaying to D0 pi+ and D0 -> K- pi+, K- pi+ pi+ pi- or K- pi+ (pi0). These events are used to measure the CKM matrix element |V_cb| and the form factor slope, rho_{A_1}^2: F_D*(1)|Vcb| = 0.0392 +/- 0.0018 +/- 0.0023; rho_{A_1}^2 = 1.32 +/- 0.15 +/- 0.33 corresponding to a branching fraction: BR(anti-B0d -> D*+ l- anti-nu_l) = (5.90 +/- 0.22 +/- 0.50)%. Combining these and previous DELPHI measurements gives: F_D*(1)|Vcb| = 0.0377 +/- 0.0011 +/- 0.0019, rho_{A_1}^2 = 1.39 +/- 0.10 +/- 0.33 and BR(anti-B0d -> D*+ l- anti-nu_l) = (5.39 +/- 0.11 +/- 0.34)%. Using F_{D*}(1) = 0.91 +/- 0.04, yields: |Vcb| = 0.0414 +/- 0.0012(stat.) +/- 0.0021(syst.) +/- 0.0018(theory). The b-quark semileptonic branching fraction into a D*+ emitted from higher mass charmed excited states has also been measured to be: BR(b -> D*+ X l- anti-nu_l) = (0.67 +/- 0.08 +/- 0.10)%.

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A Precise Measurement of the B+, B0 and Mean b-hadron Lifetime with the DELPHI Detector at LEP I

Final results from the DELPHI Collaboration on the lifetime of B+ and B0 mesons and the mean b-hadron lifetime, are presented using the data collected at the Z0 peak in 1994 and 1995. Elaborate, inclusive, secondary vertexing methods have been employed to ensure a b-hadron reconstruction with good efficiency. To separate samples of B+ and B0 mesons, high performance neural network techniques are used that achieve very high purity signals. The results obtained are: tau_B+ = 1.624 +/- 0.014 (stat) +/- 0.018 (syst) ps tau_B0 = 1.531 +/- 0.021 (stat) +/- 0.031 (syst) ps tau_B+/tau_B0 = 1.060 +/- 0.021 (stat) +/- 0.024 (syst) and for the average b-hadron lifetime: tau_b = 1.570 +/- 0.005 (stat) +/- 0.008 (syst) ps.

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Searches for supersymmetric particles in e+e- collisions up to 208 GeV and interpretation of the results within the MSSM

DELPHI data collected at centre-of-mass energies up to 208 GeV have been analysed to search for charginos, neutralinos and sfermions in the framework of the Minimal Supersymmetric Standard Model (MSSM) with R-parity conservation. No evidence for a signal was found in any of the channels. The results of each search were used to derive limits on production cross-sections and particle masses. In addition, the combined result of all searches excludes regions in the parameter space of the constrained MSSM, leading to limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles.

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A measurement of the branching fractions of the b-quark into charged and neutral b-hadrons

The production fractions of charged and neutral b-hadrons in b-quark events from Z0 decays have been measured with the DELPHI detector at LEP. An algorithm has been developed, based on a neural network, to estimate the charge of the weakly-decaying b-hadron by distinguishing its decay products from particles produced at the primary vertex. From the data taken in the years 1994 and 1995, the fraction of bbar-quarks fragmenting into positively charged weakly-decaying b-hadrons has been measured to be: f^+ = (42.09 +/- 0.82 (stat.) +/- 0.89 (syst.))%. Subtracting the rates for charged Xibar_b^+ and Omegabar_b^+ baryons gives the production fraction of B^+ mesons: f_Bu = (40.99 +/- 0.82 (stat.) +/- 1.11 (syst.))%.

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Measurement of the e+e- -> W+W-gamma Cross-section and Limits on Anomalous Quartic Gauge Couplings with DELPHI

W+W-gamma production at LEP2 is studied using data collected with the DELPHI detector at centre-of-mass energies between 189 GeV and 209 GeV, corresponding to an integrated luminosity of about 600 pb^-1. Cross-sections are measured for the production of W+W- with a hard, central and isolated photon in the final state, and are found to be compatible with the Standard Model prediction. The photon energy spectra are used to derive limits on anomalous contributions to the W+W-Z0gamma and W+W-gammagamma vertices.

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The eta_c(2980) formation in two-photon collisions at LEP energies

eta_c(2980) production in gamma-gamma interactions has been detected via its decays into K0_s K+/- pi-/+, K+ K- K+ K- and K+ K- pi+ pi- in the data taken with the DELPHI detector at LEP1 and LEP2 energies. The two-photon radiative width averaged over all observed decay channels is Γ_{γγ} = 13.9 +/- 2.0 (stat.) +/- 1.4 (syst.) +/- 2.7 (BR) keV. No direct decay channel eta_c -> pi+ pi- pi+ pi- has been observed. An upper limit $Γ_{γγ}$ < 5.5 keV at 95% confidence level has been evaluated for this decay mode.

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b-tagging in DELPHI at LEP

The standard method used for tagging b-hadrons in the DELPHI experiment at the CERN LEP Collider is discussed in detail. The main ingredient of b-tagging is the impact parameters of tracks, which relies mostly on the vertex detector. Additional information, such as the mass of particles associated to a secondary vertex, significantly improves the selection efficiency and the background suppression. The paper describes various discriminating variables used for the tagging and the procedure of their combination. In addition, applications of b-tagging to some physics analyses, which depend crucially on the performance and reliability of b-tagging, are described briefly.

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Measurement of Inclusive f_1(1285) and f_1(1420) Production in Z Decays with the DELPHI Detector

DELPHI results are presented on the inclusive production of two (K Kbar pi)^0 states in the mass region 1.2-1.6 GeV/c^2 in hadronic Z decays at LEP I. The measured masses (widths) are 1274 +/- 6 MeV/c^2 (29 +/- 12 MeV/c^2) and 1426 +/- 6 MeV/c^2 (51 +/- 14 MeV/c^2) respectively. A partial-wave analysis of the (K Kbar pi)^0 system shows that the first peak is consistent with the I^G(J^{PC})=0^+(1^{++})/(0^{-+}) a_0(980)pi and the second with the I^G(J^{PC})=0^+(1^{++}) K^*(892)Kbar + c.c. assignments. The total hadronic production rates per hadronic Z decay are (0.165 +/- 0.051) and (0.056 +/- 0.012) respectively. These measurements are consistent with the two states being the f_1(1285) and f_1(1420) mesons.

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Study of Inclusive J/psi Production in Two-Photon Collisions at LEP II with the DELPHI Detector

Inclusive J/psi production in photon-photon collisions has been observed at LEP II beam energies. A clear signal from the reaction gamma gamma -> J/psi+X is seen. The number of observed N(J/psi -> mu+mu-) events is 36 +/- 7 for an integrated luminosity of 617 pb^{-1}, yielding a cross-section of sigma(J/psi+X) = 45 +/- 9 (stat) +/- 17 (syst) pb. Based on a study of the event shapes of different types of gamma gamma processes in the PYTHIA program, we conclude that (74 +/- 22)% of the observed J/psi events are due to `resolved' photons, the dominant contribution of which is most probably due to the gluon content of the photon.

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A study of the energy evolution of event shape distributions and their means with the DELPHI detector at LEP

Infrared and collinear safe event shape distributions and their mean values are determined in e+e- collisions at centre-of-mass energies between 45 and 202 GeV. A phenomenological analysis based on power correction models including hadron mass effects for both differential distributions and mean values is presented. Using power corrections, alpha_s is extracted from the mean values and shapes. In an alternative approach, renormalisation group invariance (RGI) is used as an explicit constraint, leading to a consistent description of mean values without the need for sizeable power corrections. The QCD beta-function is precisely measured using this approach. From the DELPHI data on Thrust, including data from low energy experiments, one finds beta_0 = 7.86 +/- 0.32 for the one loop coefficient of the beta-function or, assuming QCD, n_f = 4.75 +/- 0.44 for the number of active flavours. These values agree well with the QCD expectation of beta_0=7.67 and n_f=5. A direct measurement of the full logarithmic energy slope excludes light gluinos with a mass below 5 GeV.

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ZZ production in e+e- interactions at sqrt(s) = 183 - 209 GeV

Measurements of on-shell ZZ production are described, using data from the DELPHI experiment at LEP in e+e- collisions at centre-of-mass energies between 183 and 209 GeV, corresponding to an integrated luminosity of about 665 pb^{-1}. Results obtained in each of the final states q anti-q q anti-q, nu anti-nu q anti-q, mu+ mu- q anti-q, e+ e- q anti-q, tau+ tau- q anti-q, l+ l- l+ l-, and nu anti-nu l+ l- (with l=e,mu) are presented. The measured production cross-sections are consistent with the Standard Model expectations. These results update and supersede those already published at 183 and 189 GeV.

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Search for B0_s - anti-B0_s oscillations and a measurement of B0_d - anti-B0_d oscillations using events with an inclusively reconstructed vertex

Neutral B meson oscillations in the B0_s - anti-B0_s and B0_d - anti-B0_d systems were studied using a sample of about 4.0 million hadronic Z decays recorded by the DELPHI detector between 1992 and 2000. Events with a high transverse momentum lepton were removed and a sample of 770 k events with an inclusively reconstructed vertex was selected. The mass difference between the two physical states in the B0_d - anti-B0_d system was measured to be: Δm_d = (0.531 +/- 0.025 (stat.) +/- 0.007 (syst.)) ps^{-1}. The following limit on the width difference of these states was also obtained: |ΔΓ_{B_d}| / Γ_{B_d} < 0.18 at 95% CL. As no evidence for B0_s - anti-B0_s oscillations was found, a limit on the mass difference of the two physical states was given: Δm_s > 5.0 ps^{-1} at 95% CL. The corresponding sensitivity of this analysis is equal to 6.6 ps^{-1}.

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