arXiv subjects

The DELPHI Collaboration

Publications and source records attributed to The DELPHI Collaboration.

At least 55 records · Page 3Linked to original sources

arXiv2004

A Precise Measurement of the Tau Lifetime

The tau lepton lifetime has been measured with the e+e- -> tau+tau- events collected by the DELPHI detector at LEP in the years 1991-1995. Three different methods have been exploited, using both one-prong and three-prong tau decay channels. Two measurements have been made using events in which both taus decay to a single charged particle. Combining these measurements gave tau_tau (1 prong) = 291.8 +/- 2.3 (stat) +/- 1.5 (sys) fs. A third measurement using taus which decayed to three charged particles yielded tau_tau (3 prong) = 288.6 +/- 2.4 (stat) +/- 1.3 (sys) fs. These were combined with previous DELPHI results to measure the tau lifetime, using the full LEP1 data sample, to be tau_tau = 290.9 +/- 1.4 (stat) +/- 1.0 (sys) fs.

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arXiv2004

Determination of the e+e- -> gamma gamma (gamma) cross-section at LEP 2

A test of the benchmark QED process e+e- -> gamma gamma (gamma) is reported, using the data collected with the DELPHI detector at LEP 2. The data analysed were recorded at centre-of-mass energies ranging from 161 GeV to 208 GeV and correspond to a total integrated luminosity of 656.4 pb^{-1}. The Born cross-section for the process e+e- -> gamma gamma (gamma) was determined, confirming the validity of QED at the highest energies ever attained in electron-positron collisions. Lower limits on the parameters of a number of possible deviations from QED, predicted within theoretical frameworks expressing physics beyond the Standard Model, were derived.

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arXiv2004

Search for supersymmetric particles assuming R-parity non-conservation in e+e- collisions at sqrt{s} = 192 to 208 GeV

Searches for pair-production of supersymmetric particles under the assumption of non-conservation of R-parity with a dominant LLEbar or UbarDbarDbar term have been performed using the data collected by the DELPHI experiment at LEP in e+e- collisions at centre-of-mass energies from 192 up to 208 GeV. No excess of data above Standard Model expectations was observed. The results were used to constrain the MSSM parameter space and to derive limits on the masses of supersymmetric particles.

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arXiv2004

Search for fermiophobic Higgs bosons in final states with photons at LEP 2

Higgs boson production with subsequent decay to photons was searched for in the data collected by the DELPHI detector at centre-of-mass energies between 183 GeV and 209 GeV, corresponding to a total integrated luminosity of nearly 650 pb^{-1}. No evidence for a signal was found, and limits were set on h0Z0 and h0A0 production with h0 decay to photons. These results were used to exclude regions in the parameter space of fermiophobic scenarios of Two Higgs Doublet Models.

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arXiv2004

Study of Tau-pair Production in Photon-Photon Collisions at LEP and Limits on the Anomalous Electromagnetic Moments of the Tau Lepton

Tau-pair production in the process e+e- -> e+e-tau+tau- was studied using data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb^{-1}. The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.

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arXiv2004

The measurement of alpha_s from event shapes with the DELPHI detector at the highest LEP energies

Hadronic event shape distributions are determined from data in e+e- collisions between 183 and 207 GeV. From these the strong coupling alpha_s is extracted in O(alpha_s^2), NLLA and matched O(alpha_s^2)+NLLA theory. Hadronisation corrections evaluated with fragmentation model generators as well as an analytical power ansatz are applied. Comparing these measurements to those obtained at and around M_Z allows a combined measurement of alpha_s from all DELPHI data and a test of the energy dependence of the strong coupling.

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arXiv2004

Search for single top production via FCNC at LEP at sqrt(s) = 189-208 GeV

A search for single top production (e+e- -> t cbar) via Flavour Changing Neutral Currents (FCNC) was performed using the data taken by the DELPHI detector at LEP2. The data analysed have been accumulated at centre-of-mass energies ranging from 189 to 208 GeV. Limits at 95% confidence level were obtained on the anomalous coupling parameters kappa_gamma and kappa_Z.

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arXiv2004

Search for Charged Higgs Bosons at LEP in General Two Higgs Doublet Models

A search for pair-produced charged Higgs bosons was performed in the data collected by the DELPHI detector at LEP II at centre-of-mass energies from 189 GeV to 209 GeV. Five different final states, tau+ nu_tau tau- anti-nu_tau, c sbar cbar s, c sbar tau- anti-nu_tau, W* A W* A and W* A tau- anti-nu_tau were considered, accounting for the major expected decays in type I and type II Two Higgs Doublet Models. No significant excess of data compared to the expected Standard Model processes was observed. The existence of a charged Higgs boson with mass lower than 76.7 GeV/c^2 (type I) or 74.4 GeV/c^2 (type II) is excluded at the 95% confidence level, for a wide range of the model parameters. Model independent cross-section limits have also been calculated.

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arXiv2004

Search for B0_s - anti-B0_s oscillations in DELPHI using high-p_t leptons

Oscillations in the B0_s - anti-B0_s system were studied in events selected from about 4.3 million hadronic Z^0 decays registered by DELPHI between 1992 and 2000. This paper presents updates of two published analyses ([11,12]). The first analysis, which utilizes leptons emitted with large momentum transverse to a jet, was improved by means of a better algorithm for the vertex reconstuction and a new algorithm for flavour-tagging at production time. The second analysis, which utilizes D_s-lepton events, was improved by optimizing the treatment of proper time resolution. No signal of B0_s oscillations was observed and limits on the mass difference between the physical B0_s states were obtained to be: Δm_s > 8.0 ps^{-1} at the 95% C.L. with a sensitivity of Δm_s = 9.1 ps^{-1} in the high p_t lepton analysis and Δm_s > 4.9 ps^{-1} at the 95% C.L. with a sensitivity of Δm_s = 8.6 ps^{-1} in the D_s-lepton analysis. Previously published results on these analyses are superseed. The combination of these results with those obtained in other independent analyses previously performed in DELPHI (D_s-hadron, exclusive B0_s, inclusive vertex) gives: Δm_s > 8.5 ps^{-1} at the 95% C.L. with a sensitivity of Δm_s = 12.0 ps^{-1}.

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arXiv2004

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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arXiv2004

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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arXiv2004

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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arXiv2004

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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arXiv2004

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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arXiv2004

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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arXiv2004

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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arXiv2004

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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arXiv2003

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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