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DELPHI Collaboration

Publications and source records attributed to DELPHI Collaboration.

34 records · Page 2Linked to original sources
arXiv2001

Single Intermediate Vector Boson Production in e+e- collisions at sqrt(s) = 183 and 189 GeV

The cross-sections for the production of single charged and neutral intermediate vector bosons were measured using integrated luminosities of 52 pb^{-1} and 154 pb^{-1} collected by the DELPHI experiment at centre-of-mass energies of 182.6 GeV and 188.6 GeV, respectively. The cross-sections for the reactions were determined in limited kinematic regions. The results found are in agreement with the Standard Model predictions for these channels.

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arXiv2001

Search for the Standard Model Higgs Boson at LEP

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have collected 2465 pb-1 of e+e- collision data at energies between 189 and 209 GeV, of which 542 pb-1 were collected above 206 GeV. Searches for the Standard Model Higgs boson have been performed by each of the LEP collaborations. Their data have been combined and examined for their consistency with the Standard Model background and various Standard Model Higgs boson mass hypotheses. A lower bound of 114.1 GeV has been obtained at the 95% confidence level for the mass of the Higgs boson. The likelihood analysis shows a preference for a Higgs boson with a mass of 115.6 GeV. At this mass, the probability for the background to generate the observed effect is 3.4%.

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arXiv2001

Searches for the Neutral Higgs Bosons of the MSSM: Preliminary Combined Results Using LEP Data Collected at Energies up to 209 GeV

In the year 2000 the four LEP experiments collected data at centre-of-mass energies between 200 and 209 GeV, integrating approximately 870 pb-1 of luminosity, with about 510 pb-1 above 206 GeV. The LEP working group for Higgs boson searches has combined these data with data sets collected previously at lower energies. In representative scans of the parameters of the Minimal Supersymmetric Standard Model (MSSM), the mass limits mh>91.0 GeV and mA>91.9 GeV are obtained for the light CP-even and the CP-odd neutral Higgs boson, respectively. For a top quark mass less than or equal to 174.3 GeV, assuming that the stop quark mixing is maximal, and choosing conservative values for other SUSY parameters affecting the Higgs sector, the range 0.5<tanbeta<2.4 is excluded. Additionally, the results of flavour-independent searches for hadronically decaying Higgs bosons are included, allowing exclusion of MSSM models with suppressed decays of the Higgs bosons to pairs of b quarks.

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arXiv2001

Search for Charged Higgs bosons: Preliminary Combined Results Using LEP data Collected at Energies up to 209 GeV

In the year 2000 the four LEP experiments have collected 870 pb-1 of data at energies between 200 and 209 GeV, with about 510 pb-1 above 206 GeV. These data have been combined with data sets collected earlier at lower energies. %The following 95% confidence level bounds have been obtained. For charged Higgs bosons predicted by two-doublet extensions of the Standard Model and decaying only into the channels H+->csbar and tau+nutau, a lower mass bound of 78.6 GeV is obtained, at the 95% confidence level.

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arXiv2001

Flavour Independent Search for Hadronically Decaying Neutral Higgs Bosons at LEP

Hadronic decays of Higgs bosons, not necessarily into b-quarks, have been searched for using data collected at LEP-2. Such searches are complementary to the usual Standard Model Higgs searches and lead to more model-independence. Preliminary results obtained by the four LEP collaborations are presented for the hZ production mechanism (using the q-qbarq-qbar, q-qbarnu-nubar and q-qbarl+l- channels) and combined for the first time.

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arXiv2001

Combination procedure for the precise determination of Z boson parameters from results of the LEP experiments

The precise determination of the Z boson parameters from the measurements at the Z resonance by the four collaborations ALEPH, DELPHI, L3 and OPAL in e+e- collisions at the large electron positron collider LEP at CERN is a landmark for precision tests of the electroweak theory. The four experiments measured quantities which were used to extract the mass and width of the Z boson, the hadronic cross-section at the pole of the resonance, the ratio of hadronic and leptonic decay widths, and the leptonic forward-backward asymmetries at the pole. The combination procedure based on these parameters is presented in this paper.

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arXiv2000

Consistent Measurements of alpha_s from Precise Oriented Event Shape Distributions

An updated analysis using about 1.5 million events recorded at $\sqrt{s} = M_Z$ with the DELPHI detector in 1994 is presented. Eighteen infrared and collinear safe event shape observables are measured as a function of the polar angle of the thrust axis. The data are compared to theoretical calculations in ${\cal O} (α_s^2)$ including the event orientation. A combined fit of $α_s$ and of the renormalization scale $x_μ$ in $\cal O(α_s^2$) yields an excellent description of the high statistics data. The weighted average from 18 observables including quark mass effects and correlations is $α_s(M_Z^2) = 0.1174 \pm 0.0026$. The final result, derived from the jet cone energy fraction, the observable with the smallest theoretical and experimental uncertainty, is $α_s(M_Z^2) = 0.1180 \pm 0.0006 (exp.) \pm 0.0013 (hadr.) \pm 0.0008 (scale) \pm 0.0007 (mass)$. Further studies include an $α_s$ determination using theoretical predictions in the next-to-leading log approximation (NLLA), matched NLLA and $\cal O(α_s^2$) predictions as well as theoretically motivated optimized scale setting methods. The influence of higher order contributions was also investigated by using the method of Padé approximants. Average $α_s$ values derived from the different approaches are in good agreement.

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arXiv1999

Measurement of the forward backward asymmetry of c and b quarks at the Z pole using reconstructed D mesons

A measurement of the forward--backward asymmetry of $e^{+}e^{-} \to c\bar{c}$ and $e^{+}e^{-} \to b\bar{b}$ on the $Z$ resonance is performed using about 3.5 million hadronic $Z$ decays collected by the DELPHI detector at LEP in the years 1992 to 1995. The heavy quark is tagged by the exclusive reconstruction of several $D$ meson decay modes. The forward--backward asymmetries for $c$ and $b$ quarks at the $Z$ resonance are determined to be: \[ \renewcommand{\arraystretch}{1.6} \begin{array}{rcr@{}l} \Afbc(\sqrt{s} = 91.235 {\rm GeV}) &=& &0.0659 \pm 0.0094 (stat) \pm 0.0035 (syst) \Afbb (\sqrt{s} = 91.235 {\rm GeV}) &=& &0.0762 \pm 0.0194 (stat) \pm 0.0085 (syst) \Afbc(\sqrt{s} = 89.434 {\rm GeV}) &=&-&0.0496 \pm 0.0368 (stat) \pm 0.0053 (syst) \Afbb(\sqrt{s} = 89.434 {\rm GeV}) &=& &0.0567 \pm 0.0756 (stat) \pm 0.0117 (syst) \Afbc(\sqrt{s} = 92.990 {\rm GeV}) &=& &0.1180 \pm 0.0318 (stat) \pm 0.0062 (syst) \Afbb(\sqrt{s} = 92.990 {\rm GeV}) &=& &0.0882 \pm 0.0633 (stat) \pm 0.0122 (syst) \end{array} \] The combination of these results leads to an effective electroweak mixing angle of: SINEFF = 0.2332 \pm 0.0016

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arXiv1999

Search for charginos nearly mass-degenerate with the lightest neutralino

A search for charginos with masses close to the mass of the lightest neutralino is reported, based on the data collected with the DELPHI detector at LEP from 1995 to 1997 at centre-of-mass energies between 130 and 183 GeV. The signature of a photon at high transverse momentum radiated from the initial state reduces the two-photon background to acceptable rates, thus making the mass differences between a few hundred MeV/c^2 and 3 GeV/c^2 detectable. In very nearly degenerate scenarios, the lifetime of the chargino can be large enough to produce either visible secondary vertices or decays outside the detector; therefore, quasi-stable heavy charged particles and displaced decay vertices were also searched for. No excess of events with respect to the Standard Model expectations was observed, and limits in the plane of chargino-neutralino mass difference versus chargino mass are given.

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arXiv1999

Search for Leptoquarks and FCNC in e^+ e^- annihilations at $\sqrt{s}=183 GeV$

A search for events with one jet and at most one isolated lepton used data taken at LEP-2 by the DELPHI detector.These data were accumulated at a center-of-mass energy of 183 GeV and correspond to an integrated luminosity of 47.7 1/pb. Production of single scalar and vector leptoquarks was searched for. Limits at 95% confidence level were derived on the masses (ranging from $134 GeV/c^2$ to $171 GeV/c^2$ for electromagnetic type couplings) and couplings of the leptoquark states.A search for top-charm flavour changing neutral currents($e^+ e^- \to \bar t c$ or charge conjugate) used the semileptonic decay channel. A limit on the flavour changing cross-section via neutral currents was set at 0.55pb (95% confidence level).

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arXiv1999

The Scale Dependence of the Hadron Multiplicity in Quark and Gluon Jets and a Precise Determination of $C_{A}/C_{F}$

Data collected at the Z resonance using the DELPHI detector at LEP are used to determine the charged hadron multiplicity in gluon and quark jets as a function of a transverse momentum-like scale. The colour factor ratio, \cacf, is directly observed in the increase of multiplicities with that scale. The smaller than expected multiplicity ratio in gluon to quark jets is understood by differences in the hadronization of the leading quark or gluon. From the dependence of the charged hadron multiplicity on the opening angle in symmetric three-jet events the colour factor ratio is measured to be: C_A/C_F = 2.246 \pm 0.062 (stat.) \pm 0.080 (syst.) \pm 0.095 (theo.)

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arXiv1999

Search for composite and exotic fermions at LEP 2

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb^{-1} at an e^+e^- centre-of-mass energy of 183 GeV and about 20 pb^{-1} equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/c^2 and 90 GeV/c^2, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermion

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arXiv1999

Search for lightest neutralino and stau pair production in light gravitino scenarios with stau NLSP

Promptly decaying lightest neutralinos and long-lived staus are searched for in the context of light gravitino scenarios. It is assumed that the stau is the next to lightest supersymmetric particle (NLSP) and that the lightest neutralino is the next to NLSP (NNLSP). Data collected with the Delphi detector at centre-of-mass energies from 161 to 183 \GeV are analysed. No evidence of the production of these particles is found. Hence, lower mass limits for both kinds of particles are set at 95% C.L.. The mass of gaugino-like neutralinos is found to be greater than 71.5 GeV/c^2. In the search for long-lived stau, masses less than 70.0 to 77.5 \GeVcc are excluded for gravitino masses from 10 to 150 \eVcc . Combining this search with the searches for stable heavy leptons and Minimal Supersymmetric Standard Model staus a lower limit of 68.5 \GeVcc may be set for the stau mass

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arXiv1999

A Search for Heavy Stable and Long-Lived Squarks and Sleptons in e^+ e^- Collisions at Energies from 130 to 183 GeV

A search for stable and long-lived heavy charged particles used the data taken by the DELPHI experiment at energies from 130 to 183 GeV. The Cherenkov light detected in the Ring Imaging Cherenkov Detector and the ionization loss measured in the Time Projection Chamber identify heavy particles from masses of 2 to nearly 89 GeV/c^2. Upper limits are given on the production cross-section and masses of sleptons, free squarks with a charge of $q = \pm 2/3e$ and hadronizing squarks.

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arXiv1998

The Strong Coupling: Measurements and Running

Measurements of α_s from event shapes in e^{+} e^{-} annihilation are discussed including recent determinations using experimentally optimized scales, studies of theoretically motivated scale setting prescriptions, and recently observed problems with predictions in Next to Leading Logarithmic Approximation. Other recent precision measurements of α_s are briefly discussed. The relevance of power terms for the energy evolution of event shape means and distributions is demonstrated. Finally a summary on the current results on α_s(M_Z) and its running is given.

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