Publications (803)101.72 Total impact
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Article: A study of production in e+e− collisions at –207 GeV
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ABSTRACT: Measurements are presented of R b, the ratio of the b[`(b)]{\mathrm{b}}\bar{\mathrm{b}} cross-section to the q[`(q)]{\mathrm{q}}\bar{\mathrm{q}} cross-section in e+e− collisions, and the forward-backward asymmetry A FBb at twelve energy points in the range Ös=130\sqrt{s}=130 –207GeV. These results are found to be consistent with the Standard Model expectations. The measurements are used to set limits on new physics scenarios involving contact interactions.European Physical Journal C 04/2012; 60(1):1-15. · 3.63 Impact Factor -
Article: Higgs boson searches in CP-conserving and CP-violating MSSM scenarios with the DELPHI detector
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ABSTRACT: This paper presents the final interpretation of the results from DELPHI on the searches for Higgs bosons in the minimal supersymmetric extension of the Standard Model (MSSM). A few representative scenarios are considered, that include CP conservation and explicit CP violation in the Higgs sector. The experimental results encompass the searches for neutral Higgs bosons at LEP1 and LEP2 in final states as expected in the MSSM, as well as LEP2 searches for charged Higgs bosons and for neutral Higgs bosons decaying into hadrons independent of the quark flavour. The data reveal no significant excess with respect to background expectations. The results are translated into excluded regions of the parameter space in the various scenarios. In the CP-conserving case, these lead to limits on the masses of the lightest scalar and pseudoscalar Higgs bosons, h and A, and on tanβ. The dependence of these limits on the top quark mass is discussed. Allowing for CP violation reduces the experimental sensitivity to Higgs bosons. It is shown that this effect depends strongly on the values of the parameters responsible for CP violation in the Higgs sector.European Physical Journal C 04/2012; 54(1):1-35. · 3.63 Impact Factor -
Article: Search for neutral MSSM Higgs bosons at LEP
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ABSTRACT: The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.European Physical Journal C 04/2012; 47(3):547-587. · 3.63 Impact Factor -
Article: Search for single top quark production via contact interactions at LEP2
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ABSTRACT: Single top quark production via four-fermion contact interactions associated to flavour-changing neutral currents was searched for in data taken by the DELPHI detector at LEP2. The data were accumulated at centre-of-mass energies ranging from 189 to 209 GeV, with an integrated luminosity of 598.1pb−1. No evidence for a signal was found. Limits on the energy scale Λ, were set for scalar-, vector- and tensor-like coupling scenarios.European Physical Journal C 04/2012; 71(2):1-13. · 3.63 Impact Factor -
Article: Measurements of CP-conserving trilinear gauge boson couplings WWV (V≡γ,Z) in e+e− collisions at LEP2
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ABSTRACT: The data taken by Delphi at centre-of-mass energies between 189and 209GeV are used to place limits on the CP-conserving trilinear gauge boson couplings DgZ1\Delta g^{Z}_{1}, λ γ and Δκ γ associated to W + W − and single W production at Lep2. Using data from the jj ℓ ν, jjjj, jjX and ℓ X final states, where j, ℓ and X represent a jet, a lepton and missing four-momentum, respectively, the following limits are set on the couplings when one parameter is allowed to vary and the others are set to their Standard Model values of zero: ${l}\Delta g^Z_1=-0.025^{+0.033}_{-0.030},\\\noalign{\vskip6pt}\lambda_\gamma =0.002^{+0.035}_{-0.035}\qquad\mbox{and}\\\noalign{\vskip6pt}\Delta\kappa_\gamma =0.024^{+0.077}_{-0.081}.$\begin{array}{l}\Delta g^Z_1=-0.025^{+0.033}_{-0.030},\\\noalign{\vskip6pt}\lambda_\gamma =0.002^{+0.035}_{-0.035}\qquad\mbox{and}\\\noalign{\vskip6pt}\Delta\kappa_\gamma =0.024^{+0.077}_{-0.081}.\end{array} Results are also presented when two or three parameters are allowed to vary. All observations are consistent with the predictions of the Standard Model and supersede the previous results on these gauge coupling parameters published by Delphi.European Physical Journal C 04/2012; 66(1):35-56. · 3.63 Impact Factor -
Article: Correlations between polarisation states of W particles in the reaction e−e+→W−W+ at LEP2 energies 189–209 GeV
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ABSTRACT: In a study of the reaction e − e +→W − W + with the DELPHI detector, the probabilities of the two W particles occurring in the joint polarisation states transverse–transverse (TT), longitudinal–transverse plus transverse–longitudinal (LT) and longitudinal–longitudinal (LL) have been determined using the final states WW®lnq[`(q)]WW{\rightarrow}l\nu q\bar{q} (l=e,μ). The two-particle joint polarisation probabilities, i.e. the spin density matrix elements ρ TT , ρ LT , ρ LL , are measured as functions of the W − production angle, qW-\theta _{W^{-}}, at an average reaction energy of 198.2GeV. Averaged over all cosqW-\cos\theta_{W^{-}}, the following joint probabilities are obtained: [`(r)]TT=(67±8)%,\bar{\rho}_{TT}=(67\pm8)\%, [`(r)]LT=(30±8)%,\bar{\rho}_{LT}=(30\pm8)\%, [`(r)]LL=(3±7)%.\bar{\rho}_{LL}=(3\pm7)\%. These results are in agreement with the Standard Model predictions of 63.0%, 28.9% and 8.1%, respectively. The related polarisation cross-sections σ TT , σ LT and σ LL are also presented.European Physical Journal C 04/2012; 63(4):611-623. · 3.63 Impact Factor -
Article: Investigation of Colour Reconnection in WW events with the DELPHI detector at LEP-2
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ABSTRACT: In the reaction e+e-→WW→(q1q̄2)(q3q̄4) the usual hadronization models treat the colour singlets q1q̄2 and q3q̄4 coming from two W bosons independently. However, since the final state partons may coexist in space and time, cross-talk between the two evolving hadronic systems may be possible during fragmentation through soft gluon exchange. This effect is known as colour reconnection. In this article the results of the investigation of colour reconnection effects in fully hadronic decays of W pairs in DELPHI at LEP are presented. Two complementary analyses were performed, studying the particle flow between jets and W mass estimators, with negligible correlation between them, and the results were combined and compared to models. In the framework of the SK-I model, the value for its κ parameter most compatible with the data was found to be: κSK-I=2.2+2.5 -1.3 corresponding to the probability of reconnection P\textreco\mathcal{P}_{\text{reco}} to be in the range 0.31 < P\textreco < 0.680.31European Physical Journal C 04/2012; 51(2):249-269. · 3.63 Impact Factor -
Article: A study of the b-quark fragmentation function with the DELPHI detector at LEP I and an averaged distribution obtained at the Z Pole
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ABSTRACT: The nature of b-quark jet hadronisation has been investigated using data taken at the Zpeak by the DELPHI detector at LEP. Two complementary methods are used to reconstruct the energy of weakly decaying b-hadrons, EBweakE_{\mathrm{B}}^{\mathrm{weak}}. The average value of xweakB = EBweak/Ebeamx^{\mathrm{weak}}_{\mathrm{B}} = E_{\mathrm{B}}^{\mathrm{weak}}/E_{\mathrm{beam}} is measured to be 0.699±0.011. The resulting xweakBx^{\mathrm{weak}}_{\mathrm{B}} distribution is then analysed in the framework of two choices for the perturbative contribution (parton shower and Next to Leading Log QCD calculation) in order to extract measurements of the non-perturbative contribution to be used in studies of b-hadron production in other experimental environments than LEP. In the parton shower framework, data favour the Lund model ansatz and corresponding values of its parameters have been determined within PYTHIA6.156 from DELPHI data: a = 1.84+0.23-0.21and b=0.642+0.073-0.063GeV-2,a= 1.84^{+0.23}_{-0.21}\quad\mbox{and}\quad b=0.642^{+0.073}_{-0.063}~\mathrm{GeV}^{-2}, with a correlation factor ρ=92.2%. Combining the data on the b-quark fragmentation distributions with those obtained at the Zpeak by ALEPH, OPAL and SLD, the average value of xweakBx^{\mathrm{weak}}_{\mathrm{B}} is found to be 0.7092±0.0025 and the non-perturbative fragmentation component is extracted. Using the combined distribution, a better determination of the Lund parameters is also obtained: a = 1.48+0.11-0.10and b=0.509+0.024-0.023GeV-2,a= 1.48^{+0.11}_{-0.10}\quad\mbox{and}\quad b=0.509^{+0.024}_{-0.023}~\mathrm{GeV}^{-2}, with a correlation factor ρ=92.6%.European Physical Journal C 04/2012; 71(2):1-29. · 3.63 Impact Factor -
Article: Study of the dependence of direct soft photon production on the jet characteristics in hadronic Z0 decays
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ABSTRACT: An analysis of the direct soft photon production rate as a function of the parent jet characteristics is presented, based on hadronic events collected by the DELPHI experiment at LEP1. The dependences of the photon rates on the jet kinematic characteristics (momentum, mass, etc.) and on the jet charged, neutral and total hadron multiplicities are reported. Up to a scale factor of about four, which characterizes the overall value of the soft photon excess, a similarity of the observed soft photon behavior to that of the inner hadronic bremsstrahlung predictions is found for the momentum, mass, and jet charged multiplicity dependences. However for the dependence of the soft photon rate on the jet neutral and total hadron multiplicities a prominent difference is found for the observed soft photon signal as compared to the expected bremsstrahlung from final state hadrons. The observed linear increase of the soft photon production rate with the jet total hadron multiplicity and its strong dependence on the jet neutral multiplicity suggest that the rate is proportional to the number of quark pairs produced in the fragmentation process, with the neutral pairs being more effectively radiating than the charged ones.European Physical Journal C 04/2012; 67(3):343-366. · 3.63 Impact Factor -
Article: Measurement of the mass and width of the W boson in e+e- collisions at = 161–209 GeV
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ABSTRACT: A measurement of the W boson mass and width has been performed by the DELPHI collaboration using the data collected during the full LEP2 programme (1996–2000). The data sample has an integrated luminosity of 660pb-1 and was collected over a range of centre-of-mass energies from 161 to 209GeV. 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-→ℓν̄ℓqq̄’ and W+W-→qq̄’q̄q’ channels. The W mass result for the combined data set is MW = 80.336 ±0.055 (\textStat.) ±0.028 (\textSyst.) ±0.025 (\textFSI) ±0.009 (\textLEP) \textGeV/c2, M_W = 80.336 \pm0.055 ({\text{Stat.}}) \pm0.028 ({\text{Syst.}}) \pm0.025 ({\text{FSI}}) \pm0.009 ({\text{LEP}}) \text{GeV}/c^2, where FSI represents the uncertainty due to final state interaction effects in the qq̄’q̄q’ channel, and LEP represents that arising from the knowledge of the collision energy of the accelerator. The combined value for the W width is GW = 2.404 ±0.140 (\textStat.) ±0.077 (\textSyst.) ±0.065 (\textFSI) \textGeV/c2. {\Gamma_{W}} = 2.404 \pm0.140 ({\text{Stat.}}) \pm0.077 ({\text{Syst.}}) \pm0.065 ({\text{FSI}}) \text{GeV}/c^2. These results supersede all values previously published by the DELPHI collaboration.European Physical Journal C 04/2012; 55(1):1-38. · 3.63 Impact Factor -
Article: Di-jet production in γ γ collisions at LEP2
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ABSTRACT: The production of two high-p T jets in the interactions of quasi-real photons in e + e − collisions at Ö{see}\sqrt{s_{ee}} from 189GeV to 209GeV is studied with data corresponding to an integrated e + e − luminosity of 550pb−1. The jets reconstructed by the k ⊥ -cluster algorithm are defined within the pseudo-rapidity range −1<η<1 and with jet transverse momentum, p T , above 3GeV/c. The differential di-jet cross-section is measured as a function of the mean transverse momentum [`(pT)]\overline{p_{T}} of the jets and is compared to perturbative QCD calculations.European Physical Journal C 11/2008; 58(4):531-541. · 3.63 Impact Factor -
Article: Observation of the muon inner bremsstrahlung at LEP1
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ABSTRACT: Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2<E γ ≤1GeV and transverse momentum with respect to the parent muon p T <40MeV/c, and 1<E γ ≤10GeV and p T <80MeV/c. A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z 0 decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06±0.12±0.07 in the photon energy range 0.2<E γ ≤1GeV and 1.04±0.09±0.12 in the photon energy range 1<E γ ≤10GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP.European Physical Journal C 09/2008; 57(3):499-514. · 3.63 Impact Factor -
Article: Study of W-boson polarisations and triple gauge boson couplings in the reaction e+e-→W+W- at LEP 2
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ABSTRACT: A determination of the single W spin density matrix (SDM) elements in the reaction e+e-→W+W-→lνqq̄(l=e/μ) is reported at centre-of-mass energies between 189 and 209 GeV. The data sample used corresponds to an integrated luminosity of 520 pb-1 taken by DELPHI between 1998 and 2000. The single W SDM elements, ρττ’ W± (τ,τ’=± 1 or 0), are determined as a function of the W- production angle with respect to the e- beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Λττ’, which assume the V-A coupling of the W-boson to fermions. The measured SDM elements are used to obtain the fraction of longitudinally polarised Ws, with the result: \fracs\textLs\texttot = 24.9 ±4.5(\textstat) ±2.2(\textsyst)%\frac{\sigma_{\text{L}}}{\sigma_{{\text{tot}}}} = 24.9 \pm4.5({\text{stat}}) \pm2.2({\text{syst}})\% at a mean energy of 198 GeV. The SDM elements are also used to determine the triple gauge couplings Δg1 Z,Δκγ,λγ and g4 Z, [(k)\tilde]Z\tilde{\kappa}_{Z} and [(l)\tilde]Z\tilde{\lambda}_{Z}. For the CP-violating couplings the results of single parameter fits are: g4Z = -0.39+0.19-0.20g_4^{{Z}} = -0.39^{+0.19}_{-0.20} [(k)\tilde]Z = -0.09+0.08-0.05\tilde{\kappa}_{{Z}} = -0.09^{+0.08}_{-0.05} [(l)\tilde]Z = -0.08±0.07 .\tilde{\lambda}_{{Z}} = -0.08\pm0.07 . The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.European Physical Journal C 03/2008; 54(3):345-364. · 3.63 Impact Factor -
Article: Study of b-quark mass effects in multijet topologies with the DELPHI detector at LEP
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ABSTRACT: The effect of the heavy b-quark mass on the two, three and four-jet rates is studied using LEP data collected by the DELPHI experiment at the Z peak in 1994 and 1995. The rates of b-quark jets and light quark jets (ℓ=uds) in events with n=2, 3, and 4 jets, together with the ratio of two and four-jet rates of b-quarks with respect to light-quarks, Rn bℓ, have been measured with a double-tag technique using the CAMBRIDGE jet-clustering algorithm. A comparison between experimental results and theory (matrix element or Monte Carlo event generators such as PYTHIA, HERWIG and ARIADNE) is done after the hadronisation phase. Using the four-jet observable R4 bℓ, a measurement of the b-quark mass using massive leading-order calculations gives: mb(MZ) = 3.76 ±0.32 (\textstat) ±0.17 (\textsyst) ±0.22 (\texthad) ±0.90 (\texttheo)\textGeV/c2.m_b(M_Z) = 3.76 \pm0.32 ({\text{stat}}) \pm0.17 ({\text{syst}}) \pm0.22 ({\text{had}}) \pm0.90 ({\text{theo}})\,\text{GeV}/c^2\,. This result is compatible with previous three-jet determinations at the MZ energy scale and with low energy mass measurements evolved to the MZ scale using QCD renormalisation group equations.European Physical Journal C 01/2008; 55(4):525-538. · 3.63 Impact Factor -
Article: Study of triple-gauge-boson couplings ZZZ, ZZγ and Zγγ at LEP
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ABSTRACT: Neutral triple-gauge-boson couplings ZZZ, ZZγ and Zγγ have been studied with the DELPHI detector using data at energies between 183 and 208GeV. Limits are derived on these couplings from an analysis of the reactions e+e-→Zγ, using data from the final states γff̄, with f=q or ν, from e+e-→ZZ, using data from the four-fermion final states qq̄qq̄, qq̄μ+μ-, qq̄e+e-, qq̄νν̄, μ+μ-νν̄ and e+e-νν̄, and from e+e-→Zγ*, in which the final state γ is off mass-shell, using data from the four-fermion final states qq̄e+e- and qq̄μ+μ-. No evidence for the presence of such couplings is observed, in agreement with the predictions of the Standard Model.European Physical Journal C 07/2007; 51(3):525-542. · 3.63 Impact Factor -
Article: Zγ* production in e+e- interactions at
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ABSTRACT: Measurements of Zγ* production are presented using data collected by the DELPHI detector at centre-of-mass energies ranging from 183 to 209GeV, corresponding to an integrated luminosity of about 667pb-1. The measurements cover a wide range of the possible final state four-fermion configurations: hadronic and leptonic (e+e-qq̄, μ+μ-qq̄, qq̄νν̄), fully leptonic (l+l-l’+l’-) and fully hadronic final states (qq̄qq̄, with a low mass qq̄ pair). Measurements of the Zγ* cross-section for the various final states have been compared with the Standard Model expectations and found to be consistent within the errors. In addition, a total cross-section measurement of the l+l-l’+l’- cross-section is reported, and found to be in agreement with the prediction of the Standard Model.European Physical Journal C 07/2007; 51(3):503-523. · 3.63 Impact Factor -
Article: Search for a fourth generation b’-quark at LEP-II at 196–209 GeV
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ABSTRACT: A search for the pair production of fourth generation b’-quarks was performed using data taken by the DELPHI detector at LEP-II. The analysed data were collected at centre-of-mass energies ranging from 196 to 209GeV, corresponding to an integrated luminosity of 420pb-1. No evidence for a signal was found. Upper limits on BR(b’→bZ) and BR(b’→cW) were obtained for b’ masses ranging from 96 to 103GeV/c 2. These limits, together with the theoretical branching ratios predicted by a sequential four generations model, were used to constrain the value of R\textCKM=|\fracVcb¢V\texttb¢Vtb|R_{\text{CKM}}=|\frac{V_{cb\prime}}{V_{\text{tb}\prime}V_{tb}}|, where Vcb′, Vtb′ and Vtb are elements of the extended CKM matrix.European Physical Journal C 03/2007; 50(3):507-518. · 3.63 Impact Factor -
Article: Evidence for an excess of soft photons in hadronic decays of Z0
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ABSTRACT: Soft photons inside hadronic jets converted in front of the DELPHI main tracker (TPC) in events of qq̄ disintegrations of the Z0 were studied in the kinematic range 0.2<Eγ<1GeV and transverse momentum with respect to the closest jet direction pT<80MeV/c. A clear excess of photons in the experimental data as compared to the Monte Carlo predictions is observed. This excess (uncorrected for the photon detection efficiency) is (1.17±0.06±0.27)×10-3γ/jet in the specified kinematic region, while the expected level of the inner hadronic bremsstrahlung (which is not included in the Monte Carlo) is (0.340±0.001±0.038)×10-3γ/jet. The ratio of the excess to the predicted bremsstrahlung rate is then (3.4±0.2±0.8), which is similar in strength to the anomalous soft photon signal observed in fixed target experiments with hadronic beams.European Physical Journal C 07/2006; 47(2):273-294. · 3.63 Impact Factor -
Article: Rapidity-rank structure of p
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ABSTRACT: The rapidity-rank structure of pp¯ pairs is used to analyze the mechanism of baryon production in hadronic Z0 decay. The relative occurrence of the rapidity-ordered configuration pMp¯, where /M is a meson, and that of pp¯ adjacent pairs is compared. The data are found to be consistent with predictions from a mechanism producing adjacent-rank pp¯ pairs, without requiring `string-ordered' pMp¯ configurations. An upper limit of 15% at 90% confidence is determined for the pMp¯ contribution.Physics Letters B 08/2000; 490:61-70. · 3.95 Impact Factor -
Article: Measurement of the B¯-- >D(*)πlν¯l branching fraction
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ABSTRACT: A study of b semileptonic decays into D, Dπ+/- and D*π+/- 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-->D0l-ν¯lX)=(7.04+/- 0.34(stat)+/-0.36(syst.exp)+/-0.17(BRD))%,BR(b-- >D+l-ν¯lX)=(2.72+/- 0.19(stat)+/-0.16(syst.exp)+/-0.18(BRD))%,BR(b-- >D*+l-ν¯lX)=(2.75+/- 0.17(stat)+/-0.13(syst.exp)+/-0.09(BRD))%,where the D0 and D+ results include also contributions from D*0 and D*+ decays. A fit to the distribution of the π+/- impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D0π+/-X, D+π+/-X and D*+π+/-X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for B¯-->Dπl-νl¯ and B¯-->D*πl-νl¯ have been obtained, corresponding to a total branching fraction:BR(B¯-->Dπl-νl¯)+BR(B¯-- >D*πl-νl¯)=(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 B¯-->Dl-νl¯ and D*l-νl¯ branching fractions.Physics Letters B 02/2000; 475:407-428. · 3.95 Impact Factor
Top co-authors
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Institutions
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2007–2012
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University of Ljubljana
- Department of Physics
Ljubljana, Ljubljana, Slovenia
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1992–1994
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Università degli studi di Udine
Udine, Friuli Venezia Giulia, Italy
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1991–1993
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Joint Institute for Nuclear Research
Dubna, Moskovskaya, Russia
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