arXiv subjects

The DELPHI Collaboration

Publications and source records attributed to The DELPHI Collaboration.

At least 73 records · Page 4Linked to original sources

arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

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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arXiv2003

Inclusive b Decays to Wrong Sign Charmed Mesons

The production of wrong sign charmed mesons b -> \bar D_{(s)} X, D_{(s)}=(D0,D+,D_s), is studied using the data collected by the DELPHI experiment in the years 1994 and 1995. Charmed mesons in Z -> b bbar events are exclusively reconstructed by searching for the decays D0 -> K- pi+, D+ -> K- pi+ pi+ and D_s^+ -> phi pi+ -> K+ K- pi+. The wrong sign contribution is extracted by using two discriminant variables: the charge of the b-quark at decay time, estimated from the charges of identified particles, and the momentum of the charmed meson in the rest frame of the b-hadron. The inclusive branching fractions of b-hadrons into wrong sign charm mesons are measured to be: Br(b -> \bar D0 X) + Br(b -> D- X) = (9.3 +/- 1.7(stat) +/- 1.3(syst) +/- 0.4(Br))%, Br(b -> D_s^- X) = (10.1 +/- 1.0(stat) +/- 0.6(syst) +/- 2.8(Br))% where the first error is statistical, the second and third errors are systematic.

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arXiv2003

Search for resonant sneutrino production at sqrt(s) = 183 to 208 GeV

Searches for resonant sneutrino production in e+e- collisions under the assumption that R-parity is not conserved and that the dominant R-parity violating coupling is lambda_{121} or lambda_{131} used data recorded by DELPHI in 1997 to 2000 at centre-of-mass energies of 183 to 208 GeV. No deviation from the Standard Model was observed. Upper limits are given for the lambda_{121} and lambda_{131} couplings as a function of the sneutrino mass and total width. The limits are especially stringent for sneutrino masses equal to the centre-of-mass energies with the highest integrated luminosities recorded.

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arXiv2003

Search for Doubly Charged Higgs Bosons at LEP2

A search for pair-produced doubly charged Higgs bosons has been performed using the data collected by the DELPHI detector at LEP at centre-of-mass energies between 189 and 209 GeV. No excess is observed in the data with respect to the Standard Model background. A lower limit for the mass of 97.3 GeV/c^2 at the 95% confidence level has been set for doubly charged Higgs bosons in left-right symmetric models for any value of the Yukawa coupling between the Higgs bosons and the tau leptons.

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arXiv2003

Search for supersymmetric particles in light gravitino scenarios and sleptons NLSP

A search for sleptons, neutralinos, charginos, sgoldstinos and heavy stable charged sleptons in the context of scenarios where the lightest supersymmetric particle is the gravitino, is presented. Data collected during 2000 with the DELPHI detector at centre-of-mass energies from 204 to 208 GeV were analysed and combined with all the data collected from 1995 to 1999 at lower energies. No evidence for the production of sleptons, neutralinos and charginos has been found, therefore new limits on the mass of these supersymmetric particles and on the model parameter space are set. The search for heavy stable charged sleptons also updates the stable sleptons mass limit. The absence of evidence for sgoldstino production allows limits to be set on its mass and on the scale of supersymmetry breaking.

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arXiv2003

Search for an LSP gluino at LEP with the DELPHI detector

In some supersymmetric models, the gluino is predicted to be light and stable. In that case, it would hadronize to form R-hadrons. In these models, the missing energy signature of the lightest supersymmetric particle is no longer valid, even if R-parity is conserved. Therefore, such a gluino is not constrained by hadron collider results, which looked for the decay gluino -> q qbar \tildeχ_1^0. Data collected by the DELPHI detector in 1994 at 91.2 GeV have been analysed to search for (q qbar gluino gluino) events. No deviation from Standard Model predictions is observed and a gluino mass between 2 and 18 GeV/c^2 is excluded at the 95% confidence level in these models. Then, R-hadrons produced in the squark decays were searched for in the data collected by DELPHI at the centre-of-mass energies of 189 to 208 GeV, corresponding to an overall integrated luminosity of 609 pb^-1. The observed number of events is in agreement with the Standard Model predictions. Limits at 95% confidence level are derived on the squark masses from the excluded regions in the plane (m_\tilde{q_1},m_gluino): m_\tilde{t_1} > 90 GeV/c^2, and m_\tilde{b_1} > 96 GeV/c^2 for purely left squarks. m_\tilde{t_1} > 87 GeV/c^2, and m_\tilde{b_1} > 82 GeV/c^2 independent of the mixing angle.

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arXiv2002

Rapidity-Alignment and p_T Compensation of Particle Pairs in Hadronic Z0 Decays

Observation is made of rapidity-alignment of K+K- and ppbar pairs which results from their asymmetric orientation in rapidity, with respect to the direction from primary quark to antiquark. The K+K- and ppbar data are consistent with predictions from the fragmentation string model. However, the ppbar data strongly disagree with the conventional implementation of the cluster model. The non-perturbative process of `gluon splitting to diquarks' has to be incorporated into the cluster model for it to agree with the data. Local conservation of p_T between particles nearby in rapidity (i.e., p_T compensation) is analysed with respect to the thrust direction for pi+pi-, K+K-, and ppbar pairs. In this case, the string model provides fair agreement with the data. The cluster model is incompatible with the data for all three particle pairs. The model with its central premiss of isotropically-decaying clusters predicts a p_T correlation not seen in the data.

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arXiv2002

Search for Charged Higgs Bosons in e+e- Collisions at sqrt(s)=189-202 GeV

A search for pair-produced charged Higgs bosons was performed in the high energy data collected by the DELPHI detector at LEP II at centre-of-mass energies from 189 GeV to 202 GeV. The three different final states, τντν, c \bar s \bar c s and c \bar s τνwere considered. New methods were applied to reject wrong hadronic jet pairings and for the tau identification, where a discriminator based on tau polarisation and polar angles was used. No excess of data compared to the expected Standard Model processes was observed and the existence of a charged Higgs boson with mass lower than 71.5 GeV/c^2 is excluded at the 95% confidence level.

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arXiv2002

Searches for neutral Higgs bosons in e+e- collisions from sqrt(s) = 191.6 to 201.7 GeV

Neutral Higgs bosons of the Standard Model (SM) and the Minimal Supersymmetric Standard Model (MSSM) were searched for in the data collected in 1999 by the DELPHI experiment at centre-of-mass energies between 191.6 and 201.7 GeV with a total integrated luminosity of 228 pb^{-1}. These analyses, in combination with our results at lower energies, set 95% confidence level lower mass bounds on the Standard Model Higgs boson (107.3 GeV/c^2) and on the lightest neutral scalar (85.9 GeV/c^2) and neutral pseudoscalar (86.5 GeV/c^2) Higgs bosons in representative scans of the MSSM parameter space. An extended scan of the MSSM parameter space was also performed to test the robustness of these limits.

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arXiv2001

Search for Technicolor with DELPHI

Technicolor represents a viable alternative to the Higgs mechanism for generating gauge boson masses. Searches for technicolor particles rho_T and pi_T have been performed in the data collected by the DELPHI experiment at LEP at centre-of-mass energies between 192 and 208 GeV corresponding to an integrated luminosity of 452 pb^{-1}. Good agreement is observed with the SM expectation in all channels studied. This is translated into an excluded region in the (M_{pi_T},M_{rho_T}) plane. The rho_T production is excluded for all 90 < M_{rho_T} < 206.7 GeV/c^2. Assuming a point-like interaction of the pi_T with gauge bosons, an absolute lower limit on the charged pi_T mass at 95% CL is set at 79.8 GeV/c^2, independently of other parameters of the technicolor model.

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