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The DELPHI Collaboration

Publications and source records attributed to The DELPHI Collaboration.

At least 91 records · Page 5Linked to original sources
arXiv2001

Measurement of the B/s0 Lifetime and Study of B/s0 anti-B/s0 Oscillations using D/s l Events

Lifetime and oscillations of B0s mesons have been studied in events with a large transverse momentum lepton and a Ds of opposite electric charge in the same hemisphere, selected from about 3.6 million hadronic Z0 decays accumulated by DELPHI between 1992 and 1995. The B0s lifetime and the fractional width difference between the two physical B0s states have been found to be: τ_(B0s) = (1.42^{+0.14}_{-0.13}(stat.) +/- 0.03(syst.)) ps ΔΓ(B0s)/Γ(B0s) < 0.46 at the 95% C.L. In the latter result it has been assumed that τ_(B0s)=τ_(B0d). Using the same sample, a limit on the mass difference between the physical B0s states has been set: Delta(m_B0s) > 7.4 ps^{-1} at the 95% C.L. with a corresponding sensitivity equal to 8.1 ps^{-1}.

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arXiv2001

A Study of the Lorentz Structure in Tau Decays

This paper describes a measurement of the Michel parameters, η, ρ, ξ, ξδ, and the average ν_τ helicity, h_{ν_τ}, in tau lepton decays together with the first measurement of the tensor coupling in the weak charged current. The tau+tau- pairs were produced at the LEP e+e- collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universality in the decays of the tau the measured values of the parameters were: η= -0.005 +/- 0.036 +/- 0.037, ρ= 0.775 +/- 0.023 +/- 0.020, ξ= 0.929 +/- 0.070 +/- 0.030, ξδ= 0.779 +/- 0.070 +/- 0.028, h_{ν_τ} = -0.997 +/- 0.027 +/- 0.011. The strength of the tensor coupling was measured to be κ^W_τ= -0.029 +/- 0.036 +/- 0.018. The first error is statistical and the second error is systematic in all cases. The results are consistent with the V-A structure of the weak charged current in decays of the tau lepton.

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arXiv2001

Identified Charged Particles in Quark and Gluon Jets

A sample of 2.2 million hadronic Z decays, selected from the data recorded by the Delphi detector at LEP during 1994-1995 was used for an improved measurement of inclusive distributions of pi+, K+ and p and their antiparticles in gluon and quark jets. The production spectra of the individual identified particles were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, xi^*, of the xi-distributions for kaons in gluon and quark jets are observed to be different.

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arXiv2001

Study of Dimuon Production in Photon-Photon Collisions and Measurement of QED Photon Structure Functions at LEP

Muon pair production in the process e+e- -> e+e-mu+mu- is studied using the data taken at LEP1 (sqrt(s) \simeq m_Z) with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb^{-1}. The QED predictions have been tested over the whole Q^2 range accessible at LEP1 (from several GeV^2/c^4 to several hundred GeV^2/c^4) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function F_2^γ. Azimuthal correlations are used to obtain information on additional structure functions, F_A^γand F_B^γ, which originate from interference terms of the scattering amplitudes. The measured ratios F_A^γ/F_2^γand F_B^γ/F_2^γare significantly different from zero and consistent with QED predictions.

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arXiv2001

Study of B0_s anti-B0_s oscillations and B0_s lifetimes using hadronic decays of B0_s mesons

Oscillations of B0s mesons have been studied in samples selected from about 3.5 million hadronic Z decays detected by DELPHI between 1992 and 1995. One analysis uses events in the exclusive decay channels: B0s -> Ds- pi+ or Ds- a1+ and B0s -> anti-D0 K- pi+ or anti-D0 K- a1+, where the D decays are completely reconstructed. In addition, B0s anti-B0s oscillations have been studied in events with an exclusively reconstructed Ds accompanied in the same hemisphere by a high momentum hadron of opposite charge. Combining the two analyses, a limit on the mass difference between the physical B0s states has been obtained: Delta(m_B0s) > 4.0 ps^{-1} at the 95% C.L. with a sensitivity of Delta(m_B0s) = 3.2 ps^{-1}. Using the latter sample of events, the B0s lifetime has been measured and an upper limit on the decay width difference between the two physical B0s states has been obtained: tau(B0s) = 1.53^{+0.16}_{-0.15}(stat.) +/- {0.07}(syst.) ps ΔΓ(B0s)/Γ(B0s) < 0.69 at the 95% C.L. The combination of these results with those obtained using Ds+- lepton-+ sample gives: Delta(m_B0s) > 4.9 ps^{-1} at the 95% C.L. with a sensitivity of Delta(m_B0s) = 8.7 ps^{-1}. tau(B0s) = 1.46 +/- 0.11 ps and ΔΓ(B0s)/Γ(B0s) < 0.45 at the 95% C.L.

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arXiv2001

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 (syst)^{+0.14}_{-0.20}(model))% BR(b -> cbar -> lepton-) = (1.61 +/- 0.20 (stat) +/- 0.17 (syst)^{+0.30}_{-0.44}(model))% chi-bar = 0.127 +/- 0.013 (stat) +/- 0.005 (syst) +/- 0.004(model)

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arXiv2001

A Measurement of the Tau Topological Branching Ratios

Using data collected in the DELPHI detector at LEP-1, measurements of the inclusive tau branching ratios for decay modes containing one, three, or five charged particles have been performed, giving the following results: B_1 = B(τ^- -> (particle)^- \geq 0pi^0 \geq 0K^0 ν_τ(\barν)) = (85.316 +/- 0.093 +/- 0.049)%; B_3 = B(τ^- -> 2h^-h^+ \geq 0pi^0 \geq 0K^0 ν_τ) = (14.569 +/- 0.093 +/- 0.048)%; B_5 = B(τ^- -> 3h^-2h^+ \geq 0pi^0 \geq 0K^0 ν_τ) = (0.115 +/- 0.013 +/- 0.006)%, where h is either a charged pi or K meson. The first quoted uncertainties are statistical and the second systematic.

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arXiv2001

Measurement of the anti-B ---> D* pi lepton anti-neutrino(lepton) Branching Fraction

A study of b semileptonic decays into D, D pi+- and D* pi+- final states is presented. The D0, D+ and D*+ mesons are exclusively reconstructed in Z decay data recorded from 1992 to 1995 in the DELPHI experiment at LEP. The overall branching fractions are measured to be: BR(b -> D^0 \ell^- \barν_{\ell} X) = (7.04 +/- 0.34 (stat) +/- 0.36 (syst.exp) +/- 0.17 (BR_D))% BR(b -> D^+ \ell^- \barν_{\ell} X) = (2.72 +/- 0.19 (stat) +/- 0.16 (syst.exp) +/- 0.18 (BR_D))% BR(b -> D^{*+} \ell^- \barν_{\ell} X) = (2.75 +/- 0.17 (stat) +/- 0.13 (syst.exp) +/- 0.09 (BR_D))% where the D0 and D+ results include also contributions from D*0 and D*+ decays. A fit to the distribution of the pi+- impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D0 pi+- X, D+ pi+- X and D*+ pi+- X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for \bar{B} -> D π\ell^- \bar{ν_{\ell}} and \bar{B} -> D^{*} π\ell^- \bar{ν_{\ell}} have been obtained, corresponding to a total branching fraction: BR(\bar{B} -> D π\ell^- \bar{ν_{\ell}}) + BR(\bar{B} -> D^{*} π\ell^- \bar{ν_{\ell}}) = (3.40 +/- 0.52 (stat) +/- 0.32 (syst))% . This result agrees well with the observed difference between the total B semileptonic branching fraction and the sum of the \bar{B} -> D \ell^- \bar{ν_{\ell}} and D^* \ell^- \bar{ν_{\ell}} branching fractions.

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arXiv2001

Determination of |Vub|/|Vcb| with DELPHI at LEP

The ratio of the CKM quark-mixing matrix elements |Vub|/|Vcb| has been measured using B hadron semileptonic decays. The analysis uses the reconstructed mass M_X of the secondary hadronic system produced in association with an identified lepton. Since B -> X_u lepton anti-nu transitions are characterised by hadronic masses below those of the D mesons produced in B -> X_c lepton anti-nu transitions, events with a reconstructed value of M_X significantly below the D mass are selected. Further signal enrichments are obtained using the topology of reconstructed decays and hadron identification. A fit to the numbers of decays in the b -> u enriched and depleted samples with M_X above and below 1.6 GeV/c^2 and to the shapes of the lepton energy distribution in the B rest frame gives |Vub|/|Vcb|= 0.103^{+0.011}_{-0.012} (stat.) +/- 0.016 (syst.) +/- 0.010 (model) and, correspondingly, a charmless semileptonic B decay branching fraction of BR(B -> X_u lepton anti-nu) = (1.57 +/- 0.35 (stat.) +/- 0.48 (syst.) +/- 0.27 (model)) x 10^{-3}.

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arXiv2001

Rapidity-Rank Structure of p pbar Pairs in Hadronic Z0 Decays

The rapidity-rank structure of p pbar pairs is used to analyze the mechanism of baryon production in hadronic Z0 decay. The relative occurrence of the rapidity-ordered configuration p M pbar, where M is a meson, and that of p pbar adjacent pairs is compared. The data are found to be consistent with predictions from a mechanism producing adjacent-rank p pbar pairs, without requiring `string-ordered' p M pbar configurations. An upper limit of 15% at 90% confidence is determined for the p M pbar contribution.

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arXiv2001

Measurement of the Mass and Width of the W Boson in e+e- Collisions at sqrt(s) = 189 GeV

A measurement of the W mass and width has been performed by the DELPHI collaboration using the data collected during 1998. The data sample has an integrated luminosity of 155 pb^{-1} and an average centre-of-mass energy of 188.6 GeV. Results are obtained by applying the method of direct reconstruction of the mass of the W from its decay products in both the W+W- -> lvqq and W+W- -> qqqq channels. The W mass result for the 1998 data set is M_W = 80.387 +/- 0.087 (stat) +/- 0.034 (syst) +/- 0.017 (LEP) +/- 0.035 (FSI) GeV/c2, where FSI represents the uncertainty due to final state interaction effects in the qqqq channel, and LEP represents that arising from the knowledge of the beam energy of the accelerator. Combining this result with those previously published by the DELPHI collaboration gives the result M_W = 80.359 +/- 0.074 (stat) +/- 0.032 (syst) +/- 0.017 (LEP) +/- 0.033 (FSI) GeV/c2. The combined value for the W width is Γ_W = 2.266 +/- 0.176 (stat) +/- 0.056 (syst) +/- 0.052 (FSI) GeV/c2.

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arXiv2001

Measurement of V_{cb} from the decay process \bar{B^0} -> D^{*+} \ell^- \barν

A new precise measurement of |V_{cb}| and of the branching ratio BR(\bar{B^0} -> D^{*+} \ell^- \bar{ν_\ell}) has been performed using a sample of about 5000 semileptonic decays \bar{B^0} -> D^{*+} \ell^- \bar{ν_\ell}, selected by the DELPHI detector at LEP I by tagging the soft pion from D^{*+} -> D^0 π^+. The results are: V_{cb}=(39.0 +/- 1.5 (stat.) ^{+2.5}_{-2.6} (syst. exp.) +/- 1.3 (syst. th.)) x 10^{-3} BR(\bar{B^0} -> D^{*+} \ell^- \bar{ν_\ell})=(4.70 +/- 0.13 (stat.) ^{+0.36}_{-0.31} (syst. exp.))% The analytic dependences of the differential cross-section and of the Isgur Wise form factor as functions of the variable w = v_{B^0}.v_{D^*} have also been obtained by unfolding the experimental resolution.

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arXiv2001

Search for charginos in e+e- interactions at sqrt(s) = 189 GeV

An update of the searches for charginos and gravitinos is presented, based on a data sample corresponding to the 158 pb^{-1} recorded by the DELPHI detector in 1998, at a centre-of-mass energy of 189 GeV. No evidence for a signal was found. The lower mass limits are 4-5 GeV/c^2 higher than those obtained at a centre-of-mass energy of 183 GeV. The (μ,M_2) MSSM domain excluded by combining the chargino searches with neutralino searches at the Z resonance implies a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 31.0 GeV/c^2 for tan(beta) \geq 1.

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arXiv2001

Search for a fermiophobic Higgs at LEP 2

Higgs bosons predicted by the fermiophobic scenario within Two Higgs Doublets Models were searched for in the data collected by the DELPHI detector at centre-of-mass energies between 189 GeV and 202 GeV, corresponding to a total integrated luminosity of 380 pb^{-1}. No signal was found and confidence limits were derived in the framework of possible extensions of the Standard Model Higgs sector.

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arXiv2001

Searches for Neutral Higgs Bosons in e+e- Collisions around sqrt(s) = 189 GeV

Searches for neutral Higgs bosons in the Standard Model and the MSSM have been performed using data collected by the DELPHI experiment at a centre-of-mass energy of 188.7 GeV, corresponding to an integrated luminosity of 158 pb^{-1}. These analyses are used, in combination with our results from lower energies, to set new 95% confidence level lower mass bounds on the Standard Model Higgs boson (94.6 GeV/c^2) and on the lightest neutral scalar (82.6 GeV/c^2) and neutral pseudoscalar (84.1 GeV/c^2) Higgs bosons in a representative scan of the MSSM parameters. The results are also interpreted in the framework of a general two-Higgs doublet model.

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arXiv2001

Search for sleptons in e+e- collisions at sqrt(s) = 183 to 189 GeV

Data taken by the DELPHI experiment at centre-of-mass energies of 183 GeV and 189 GeV with a total integrated luminosity of 212 pb^{-1} have been used to search for the supersymmetric partners of the electrons, muons, and taus in the context of the Minimal Supersymmetric Standard Model (MSSM). The decay topologies searched for were the direct decay ({\tilde \ell} -> \ell {\tilde χ_1^0}), producing acoplanar lepton pairs plus missing energy, and the cascade decay ({\tilde \ell} -> \ell {\tilde χ_2^0} -> \ell γ{\tilde χ_1^0}), producing acoplanar lepton and photon pairs plus missing energy. The observed number of events is in agreement with Standard Model predictions. The 95% CL excluded mass limits for selectrons, smuons and staus are m_{\tilde {e}} \leq 87 GeV/c^2, m_{\tilde μ} \leq 80 GeV/c^2 and m_{\tilde τ} \leq 75 GeV/c^2, respectively, for values of μ=-200 GeV/c^2 and tan(beta)=1.5.

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arXiv2001

Photon Events with Missing Energy at sqrt(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 composite models. A limit on the gravitational scale is also determined.

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