Search

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. [ABSTRACT FROM AUTHOR]

The data taken by Delphi at centre-of-mass energies between 189 and 209 GeV are used to place limits on the CP-conserving trilinear gauge boson couplings $\Delta g^{Z}_{1}$, λ γ and Δ κ 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: 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. [ABSTRACT FROM AUTHOR]

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 $WW{\rightarrow}l\nu q\bar{q}$ ( 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, $\theta _{W^{-}}$, at an average reaction energy of 198.2 GeV. Averaged over all $\cos\theta_{W^{-}}$, the following joint probabilities are obtained: 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. [ABSTRACT FROM AUTHOR]

Single photons detected by the DELPHI experiment at LEP2 in the years 1997–2000 are reanalysed to investigate the existence of a single extra dimension in a modified ADD scenario with slightly warped large extra dimensions. The data collected at centre-of-mass energies between 180 and 209 GeV for an integrated luminosity of ∼650 pb−1 agree with the predictions of the Standard Model and allow a limit to be set on graviton emission in one large extra dimension. The limit obtained on the fundamental mass scale M D is 1.69 TeV/ c2 at 95% CL, with an expected limit of 1.71 TeV/ c2. [ABSTRACT FROM AUTHOR]

Measurements are presented of Rb, the ratio of the ${\mathrm{b}}\bar{\mathrm{b}}$ cross-section to the ${\mathrm{q}}\bar{\mathrm{q}}$ cross-section in e+e− collisions, and the forward-backward asymmetry A at twelve energy points in the range $\sqrt{s}=130$ –207 GeV. 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. [ABSTRACT FROM AUTHOR]

The production of two high- p T jets in the interactions of quasi-real photons in e + e − collisions at $\sqrt{s_{ee}}$ from 189 GeV to 209 GeV is studied with data corresponding to an integrated e + e − luminosity of 550 pb−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 3 GeV/c. The differential di-jet cross-section is measured as a function of the mean transverse momentum $\overline{p_{T}}$ of the jets and is compared to perturbative QCD calculations. [ABSTRACT FROM AUTHOR]

A correction to the article "Higgs boson searches in CP-conserving and CP-violating MSSM scenarios with the DELPHI detector," that was published in the previous issue is presented.

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: $m_b(M_Z) = 3.76 \pm0.32 ({\text{stat}}) \pm0.17 ({\text{syst}}) \pm0.22 ({\text{had}}) \pm0.90 ({\text{theo}})\,\text{GeV}/c^2\,.$ 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. [ABSTRACT FROM AUTHOR]

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 660 pb-1 and was collected over a range of centre-of-mass energies from 161 to 209 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-→ℓν̄ℓqq̄’ and W+W-→qq̄’q̄q’ channels. The W mass result for the combined data set is $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 ${\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. [ABSTRACT FROM AUTHOR]

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’ (τ,τ’=± 1 or 0), are determined as a function of the WW± (τ,τ’=± 1 or 0), are determined as a function of the W- beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Λ-, 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: $\frac{\sigma_{\text{L}}}{\sigma_{{\text{tot}}}} = 24.9 \pm4.5({\text{stat}}) \pm2.2({\text{syst}})\%$ at a mean energy of 198 GeV. The SDM elements are also used to determine the triple gauge couplings Δgττ’, 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: $\frac{\sigma_{\text{L}}}{\sigma_{{\text{tot}}}} = 24.9 \pm4.5({\text{stat}}) \pm2.2({\text{syst}})\%$ 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, $\tilde{\kappa}_{Z}$ and $\tilde{\lambda}_{Z}$ . For the CP-violating couplings the results of single parameter fits are: $g_4^{{Z}} = -0.39^{+0.19}_{-0.20}$ $\tilde{\kappa}_{{Z}} = -0.09^{+0.08}_{-0.05}$ $\tilde{\lambda}_{{Z}} = -0.08\pm0.07 .$ The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model. [ABSTRACT FROM AUTHOR]

Neutral triple-gauge-boson couplings ZZZ, ZZγ and Zγγ have been studied with the DELPHI detector using data at energies between 183 and 208 GeV. 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. [ABSTRACT FROM AUTHOR]

Measurements of Zγ* production are presented using data collected by the DELPHI detector at centre-of-mass energies ranging from 183 to 209 GeV, corresponding to an integrated luminosity of about 667 pb-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. [ABSTRACT FROM AUTHOR]

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 $\mathcal{P}_{\text{reco}}$ to be in the range $0.31 <\mathcal{P}_{{\text{reco}}} < 0.68$ at 68% confidence level with its best value at 0.52. [ABSTRACT FROM AUTHOR]

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 209 GeV, corresponding to an integrated luminosity of 420 pb-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 103 GeV/ c 2. These limits, together with the theoretical branching ratios predicted by a sequential four generations model, were used to constrain the value of $R_{\text{CKM}}=|\frac{V_{cb\prime}}{V_{\text{tb}\prime}V_{tb}}|$ , where Vcb′, Vtb′ and Vtb are elements of the extended CKM matrix. [ABSTRACT FROM AUTHOR]

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 0.2γ<1 GeV and transverse momentum with respect to the closest jet direction pT<80 MeV/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)×10-3 γ/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)×10-3 γ/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. [ABSTRACT FROM AUTHOR]

An experimental study of the normalized three-jet rate of b quark events with respect to light quarks events (light=ℓ≡u,d,s) has been performed using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are found to agree with theoretical predictions treating mass corrections at next-to-leading order. Measurements of the b quark mass have also been performed for both the b pole mass: Mb and the b running mass: mb(MZ). Data are found to be better described when using the running mass. The measurement yields: $m_b(M_Z)=2.85\pm0.18 (\text{stat}) \pm0.13 (\text{exp}) \pm0.19 (\text{had}) \pm0.12 (\text{theo}) \text{GeV}/c^2.$ for the CAMBRIDGE algorithm. This result is the most precise measurement of the b mass derived from a high energy process. When compared to other b mass determinations by experiments at lower energy scales, this value agrees with the prediction of quantum chromodynamics for the energy evolution of the running mass. The mass measurement is equivalent to a test of the flavour independence of the strong coupling constant with an accuracy of 7 ‰. [ABSTRACT FROM AUTHOR]

Double-tagged interactions of photons with virtualities Q2 between 10 GeV2 and 200 GeV2 are studied with the data collected by DELPHI at LEPII from 1998 to 2000, corresponding to an integrated luminosity of 550 pb-1. The γ*γ*→μ+μ- data agree with QED predictions. The cross-section of the reaction γ*γ*→ hadrons is measured and compared to the LO and NLO BFKL calculations. [ABSTRACT FROM AUTHOR]

The charged particle multiplicity in hadronic three-jet events from Z decays is investigated. The topology dependence of the event multiplicity is found to be well described by a modified leading logarithmic prediction. A parameter fit of the prediction to the data yields a measurement of the colour factor ratio C A / C F with the result in agreement with the SU(3) expectation of QCD. The quark-related contribution to the event multiplicity is subtracted from the three-jet event multiplicity resulting in a measurement of the multiplicity of two-gluon colour-singlet states over a wide energy range. The ratios $r = N_{gg}(s)/N_{q\bar q}(s)$ of the gluon and quark multiplicities and $r^{(1)} = N_{gg}'(s)/N_{q\bar q}'(s)$ of their derivatives are compared with perturbative calculations. While a good agreement between calculations and data is observed for r (1), larger deviations are found for r indicating that non-perturbative effects are more important for r than for r (1). [ABSTRACT FROM AUTHOR]

The charmed strange baryon $\Xi{_c^0}$ was searched for in the decay channel $\Xi{_c^0} \rightarrow \Xi^- \pi^ + $ , and the beauty strange baryon $\Xi{_b}$ in the inclusive channel $\Xi_b \rightarrow \Xi^{-} \ell^{-} \bar{\nu} X$ , using the 3.5 million hadronic Z events collected by the DELPHI experiment in the years 1992-1995. The $\Xi^-$ was reconstructed through the decay $\Xi^- \rightarrow \Lambda \pi^-$ , using a constrained fit method for cascade decays. An iterative discriminant analysis was used for the $\Xi{_c^0}$ and $\Xi{_b}$ selection. The production rates were measured to be $f_{\Xi{_c^0}} \times$ BR $(\Xi{_c^0} \rightarrow \Xi^- \pi^ + ) = (4.7 \pm 1.4 ({\rm stat.}) \pm 1.1 ({\rm syst.}))\times 10^{-4}$ per hadronic Z decay, and BR $(b \rightarrow \Xi{_b}) \times$ BR $(\Xi{_b} \rightarrow \Xi^- \ell^- X) = (3.0 \pm 1.0({\rm stat.}) \pm 0.3({\rm syst.}))\times 10^{-4}$ for each lepton species (electron or muon). The lifetime of the $\Xi{_b}$ baryon was measured to be $ \tau_{\Xi{_b}} = 1.45{^{ + 0.55}_{-0.43}} ({\rm stat.}) \pm 0.13 ({\rm syst.})$ ps. A combination with the previous DELPHI lifetime measurement gives $ \tau_{\Xi{_b}} = 1.48{^{ + 0.40}_{-0.31}} ({\rm stat.}) \pm 0.12 ({\rm syst.})$ ps. [ABSTRACT FROM AUTHOR]

Bose-Einstein correlations (BEC) between final state particles in the reaction $\rm e^ + e^- \to W^ + W^- \to q_1\overline{q_2}q_3\overline{q_4}$ have been studied. Data corresponding to a total integrated luminosity of 550 pb-1, recorded by the DELPHI detector at centre-of-mass energies ranging from 189 to 209 GeV, were analysed. An indication for inter-W BEC between like-sign particles has been found at the level of 2.4 standard deviations of the combined statistical and systematic uncertainties. [ABSTRACT FROM AUTHOR]

This paper describes flavour independent searches for hadronically decaying neutral Higgs bosons in the data collected by the DELPHI experiment at LEP, at centre-of-mass energies between 189 and 209 GeV. The collected data-set corresponds to an integrated luminosity of around 610 pb-1. The and processes are considered, with direct Higgs boson decays into hadrons. No evidence for Higgs boson production is found, and cross-section limits are set as a function of the Higgs boson masses. No explicit assumptions are made on the underlying physics beyond the Standard Model, allowing interpretation of the data in a large class of models. [ABSTRACT FROM AUTHOR]

The nature of ultrahigh-energy cosmic rays (UHECRs) at energies>1020?eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within~50?Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or?-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing. [ABSTRACT FROM AUTHOR]

A novel high precision method measures the b-quark forward-backward asymmetry at the Z pole on a sample of 3,560,890 hadronic events collected with the DELPHI detector in 1992 to 2000. An enhanced impact parameter tag provides a high purity b sample. For event hemispheres with a reconstructed secondary vertex the charge of the corresponding quark or anti-quark is determined using a neural network which combines in an optimal way the full available charge information from the vertex charge, the jet charge and from identified leptons and hadrons. The probability of correctly identifying b-quarks and anti-quarks is measured on the data themselves comparing the rates of double hemisphere tagged like-sign and unlike-sign events. The b-quark forward-backward asymmetry is determined from the differential asymmetry, taking small corrections due to hemisphere correlations and background contributions into account. The results for different centre-of-mass energies are:.Combining these results yields the b-quark pole asymmetry [ABSTRACT FROM AUTHOR]

A test of the benchmark QED processis 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 processwas 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. [ABSTRACT FROM AUTHOR]

Hadronic event shape distributions are determined from data in e+e- collisions between 183 and 207 GeV. From these the strong coupling α3 is extracted in O(αs²), NLLA and matched O(αs²)+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 Mz allows a combined measurement of αs from all DELPHI data and a test of the energy dependence of the strong coupling. [ABSTRACT FROM AUTHOR]

The tau lepton lifetime has been measured with the e+e-→ T+T- 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 T decay channels. Two measurements have been made using events in which both taus decay to a single charged particle. Combining these measurements gave TT-(l prong) 291.8 ± 2.3stat ± 1.5sys fs. A third measurement using taus which decayed to three charged particles yielded TT(3 prong) 288.6 ± 2.4stat ± l.3sys fs. These were combined with previous DELPHI results to measure the tau lifetime, using the full LEP1 data sample, to be TT = 290.9 ± 1.4stat ± 1.0sys fs. [ABSTRACT FROM AUTHOR]

Searches for pair-production of supersymmetric particles under the assumption of non-conservation of R-parity with a dominant LLĒ or Ū&Dmacr;&Dmacr; 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Tau-pair production in the process e+e- → e+e-Τ+τ- 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. [ABSTRACT FROM AUTHOR]

Oscillations in the Bs0-&Bs0macr; system were studied in events selected from about 4.3 million hadronic Z0 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 reconstruction and a new algorithm for flavour-tagging at production time. The second analysis, which utilizes Ds-lepton events, was improved by optimizing the treatment of proper time resolution. No signal of Bs0 oscillations was observed and limits on the mass difference between the physical Bs0 states were obtained to be: Δms > 8.0 ps-1 at the 95% C.L. with a sensitivity of Δms = 9.1 ps-1 in the high pt lepton analysis and Δms > 4.9 ps-1 at the 95% C.L. with a sensitivity of Δms = 8.6 ps-1 in the Ds-lepton analysis. Previously published results on these analyses are superseded. The combination of these results with those obtained in other independent analyses previously performed in DELPHI (Ds-hadron, exclusive Bs0, inclusive vertex) gives: Δms > 8.5 ps-1 at the 95% C.L. with a sensitivity of Δms = 12.0 ps-1. [ABSTRACT FROM AUTHOR]

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, τ+Vττ-Vτ, c&scmacr;s, c&smacr;τ&Vmacr;τ¯DτW*AW*A and W*Aτ¯?τ 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/c2 (type I) or 74.4 GeV/c2 (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. [ABSTRACT FROM AUTHOR]

The forward-backward asymmetries of the processes e+e- → Z→ b&bmacr; and e+e-&→Z →c&cmacr; were measured from a sample of hadronic Z decays collected by the DELPHI experiment between 1993 and 1995. Enriched samples of b&bmacr; and c&cmacr; events were obtained using lifetime information. The tagging of band c quarks in these samples was based on the semileptonic decay channels b/c →X +μ and b/c ? X +e combined with charge flow information from the hemisphere opposite to the lepton. [ABSTRACT FROM AUTHOR]

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 /VcsM/ are compared to the expectations of the Standard Model. These results supersede all values previously published by DELPHI. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Z° 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: τB+= 1.624±0.014 (stat)±0.018 (syst)ps τB0 = 1.531±0.021 (stat)±0.031 (syst)ps τB+/τB0= 1.060±0.021 (stat)±0.024 (syst) and for the average b-hadron lifetime: τb= 1.570±0.005 (stat)±0.008 (syst)ps. [ABSTRACT FROM AUTHOR]

Discusses a study conducted by researchers from Europe, which benefits from the reprocessing of DELPHI data taken between 1992 and 1995 through improved versions of the event reconstruction algorithms. Observation of some improvements in the mass and b-meson energy reconstruction; Identification of electrons using a neural network algorithm; Definition of variables which allow different decay mechanisms producing D mesons in the final state to be separated.

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/c2 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/c2 at 95% confidence level. An interpretation in the Minimal Supersymmetric extension of the Standard Model (MSSM) and in a Majoron model is also given. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Searches for resonant &νmacr; production in e+e- collisions under the assumption that R-parity is not conserved and that the dominant R-parity violating coupling is λ121 or λ131 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 A121 and A131 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In some supersymmetric models, the gluino (&gtilde;) 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 &gtilde; → qq&Xtilde;10. Data collected by the DELPHI detector in 1994 at 91.2 GeV have been analysed to search for qq&gtilde;&gtilde; events. No deviation from Standard Model predictions is observed and a gluino mass between 2 and 18 GeV/c² 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 (mq1,mg): mt1 > 90 GeV/c², and mb1 > 96 GeV/c² for purely left squarks. mt1 > 87 GeV/c², and mb1 > 82 GeV/c² independent of the mixing angle. [ABSTRACT FROM AUTHOR]

A search for high mass photon pairs from the processes $$e^ + e^ - \to \ell ^ + \ell ^ - \gamma \gamma $$ , $$e^ + e^ - \to q\bar q\gamma \gamma $$ and $$e^ + e^ - \to \nu \bar \nu \gamma \gamma $$ with the DELPHI detector at LEP I is reported. From a data sample containing 3.5 million hadronic Z decays, collected by DELPHI during the years 1991 to 1994, 79 events with two charged leptons and two isolated photons were selected with photon pair masses above 10 GeV/c, where 76±6 events were predicted from standard sources. In the same data sample, no $$\nu \bar \nu \gamma \gamma $$ candidates were found and no accumulation of events was visible for γγ masses above 10 GeV/c in the $$q\bar q\gamma \gamma $$ channel. Upper limits at 95% confidence level on the Z branching ratios for the three different channels were extracted from the data. In the mass region m >30 GeV/c the limits obtained are between 3×10 and 4×10. [ABSTRACT FROM AUTHOR]

Rare decays of beauty particles were studied in several charmless modes using the data collected with the DELPHI detector at LEP from 1991 to 1994. These decays are mediated by both tree level b→ u and one-loop penguin b→ s, d transitions. Evidence for charmless B decays was obtained in two body hadronic modes. The branching ratios of B to π π or K π and B to ρ π or K* π were found to be (2.8±0.2)×10 and (1.7±0.2)×10 respectively. The fraction of these decays with a charged kaon in the final state that is not from the spectator s quark, was measured to be 0.58±0.18. Upper limits were set at 90% confidence level on the branching ratios for three and four body charmless hadronic decays in the range of (1-3)×10, for inclusive radiative b→ sγ decays at 5.4×10, for the exclusive radiative decays B → K*(892) γ and B → φ(1020) γ at 2.1×10 and 7.0×10 respectively, and for dineutrino decays $$b \to s\nu \bar \nu $$ , in the exclusive channels $$B_d^0 \to K^* (892)^0 \nu \bar \nu $$ and $$B_s^0 \to \phi (1020)\nu \bar \nu $$ at 1.0×10 and 5.4×10 respectively. The limits on dineutrino decays constrain theories with a new U(1) gauge boson coupling predominantly to the third family of fermions. [ABSTRACT FROM AUTHOR]

The interference between initial and final state radiation in the process e e → μ μ at √ s≈M has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons. The interference is expected to be sensitive to the space-time separation of the initial and final state radiation. The measured asymmetry distribution has been compared to theoretical predictions using the koralz generator, with and without $${\mathcal{O}}(\alpha )$$ interference. The magnitude of the interference between initial and final state radiation was found to be of the order predicted and to follow the expected distribution. Using the theoretical predictions, a value of has been extracted. The interpretation of this result is discussed. There is an additional uncertainty in the estimate of Γ from as yet uncalculated higher order interference terms. By assuming a value of Γ consistent with the world average, the data were used to estimate the size of these uncalculated corrections. [ABSTRACT FROM AUTHOR]

A measurement of the mass difference, Δm , between the two physical B states has been obtained from the analysis of the impact parameter distribution of a lepton emitted at large transverse momentum ( p ) relative to the jet axis and from the analysis of the flight distance distribution of secondary vertices tagged by either a high p lepton or an identified kaon. In the opposite hemisphere of the event, the charge of the initial quark has been evaluated using a high p lepton, a charged kaon or the mean jet charge. With 1.7 million hadronic Z decays recorded by DELPHI between 1991 and 1993, Δm is found to be:. [ABSTRACT FROM AUTHOR]

Semileptonic decays B→ D* ℓ νX were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D* π states was performed and the ratio of the branching fractions: was determined. Taking into account this contribution, the differential decay width of B→D*ℓ ℓ ν was measured as a function of the momentum transfer from the B to the D* in two separate analyses, using exclusive and inclusive methods of D* reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of |V| times the normalization of the decay form factor F( q ):. The value of |V| was computed using theoretical calculations of F( q ), giving:. The total branching fraction Br(B →D* ℓ gn) was determined to be:. [ABSTRACT FROM AUTHOR]

The average lifetime of b-baryons has been studied using 3×10 hadronic Z decays collected by the DELPHI detector at LEP. Three methods have been used, based on the measurement of different observables: the proper decay time distribution of 206 vertices reconstructed with a Λ, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by a Λ in the same jet; and the proper decay time distribution of 125 Λ -lepton decay vertices with the Λ exclusively reconstructed through its pKπ, pK and Λ3 π decay modes. The combined result is: where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of the b-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the average b-baryon lifetime is:. [ABSTRACT FROM AUTHOR]

An excess of events containing, in a jet, a samesign Ξ−ℓ pair as compared to those with an oppositesign Ξ−ℓ pair has been observed in an analysis of 1.7 million hadronic Z decays collected by the DELPHI detector at LEP between 1991 and 1993 inclusive. The probability for this signal to come from non B-baryon decays is less than 5×10. The measured production fraction corresponds to: per lepton species, averaged for electrons and muons and assuming that the two channels have an equal contribution. Semileptonic decays of Λ baryons can account for less than 10% of these events and the major part of the signal has to originate from Ξ semileptonic decays. Using the subsample of these events where the Ξ trajectory has been measured in the Vertex Detector, the lifetime of B-baryons producing a Ξ in their semileptonic decay final state is found to be: [ABSTRACT FROM AUTHOR]