First evidence of hard scattering processes in single tagged gamma-gamma-collisions

For the first time, multihadronic production from single tagged gammagamma collisions has been studied, where one of the scattered leptons was tagged at very low virtual photon absolute mass squared ( = 0.06GeV/(c2))2 ). Data collected during 1991 and 1992 in the DELPHI experiment at LEP are shown to agree well with predictions which included the non-perturbative vector meson dominance model in which the interacting photons are assumed to have converted into a vector meson (rho, omega or pi), a quark-parton model which describes direct photon interactions and a QCD-based model which considers the photon to have quark and gluon structure functions. Five different parametrizations of these st…

Search for neutralinos, scalar leptons and scalar quarks in e(+)e(-) interactions at root s=130 GeV and 136 GeV

Using data accumulated by DELPHI during the November 1995 LEP run at 130¿136 GeV, searchs have been made for events with jets or leptons in conjunction with missing momentum. The results are interpreted in terms of limits on the production of neutralinos, scalar leptons, and scalar quarks.

A measurement of b+ and b-0 lifetimes using (d)over-bar-l(+) events

A measurement of B meson lifetimes is presented using data collected from 1991 to 1993 by the DELPHI detector at the LEP collider. Samples of events with a D meson and a lepton in the same jet are selected where D¯0ℓ+ and D*ℓ+ events originate mainly from the semileptonic decays of B+ and B0 mesons, respectively. From the reconstructed B decay length and the estimated B momentum, taking into account the dilution due to B decays into D¯∗∗ℓ+v , the following B meson lifetimes and lifetime ratio are measured: τ(B+)=1.61+0.16−0.16(stat.)±0.12(syst.)psτ(B0)=1.61+0.14−0.13(stat.)±0.08(syst.)psτ(B+)/τ(B0)=1.00+0.17−0.15(stat.)±0.10(syst.) and an average lifetime of B+ and B0 mesons is obtained: τ(…

Search for pair-produced heavy scalars in Z° decays

A search for charged Higgs bosons decaying into quarks is described, based on statistics of around 1.7 million hadronic Z0 decays detected in DELPHI. Despite the very high background from standard hadronic decays of the Z0, masses in the range up to 43.5 GeV/c2 are excluded at the 95% con dence level. After combination with a search for leptonic decays, this mass limit is extended to cover all branching ratios. A similar analysis sets new limits on the possible production of any pair-produced heavy scalar decaying into a pair of jets, such as neutral Higgs bosons in a two doublet scheme and diquarks.

An investigation of the very rare K+ → π+ vv¯ decay

The NA62 experiment reports an investigation of the K+→π+ν ̄ν mode from a sample of K+ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389±0.024)×10−10, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4±1.0)×10−11. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78×10−10 for the K+→π+ν ̄ν branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of (0.48+0.72−0.48)×10−10, are currently the most preci…

Search for neutral Higgs bosons in e(+)e(-) collisions at root s=183 GeV

Neutral Higgs bosons were searched for in the data collected by DELPHI at a centre-of-mass energy of 183 GeV, corresponding to an integrated luminosity of 54 pb TeX . The production of the lightest neutral Higgs boson with either an on-shell Z boson or a neutral pseudo-scalar Higgs boson was analysed. Lower limits at the 95% confidence level were obtained on the Higgs boson masses. The limits are 85.7 GeV/ TeX for the Standard Model Higgs boson and 74.4 GeV/ TeX for the scalar and 75.3 GeV/ TeX for the pseudo-scalar Higgs bosons as predicted by the minimal super-symmetric extension of the Standard Model with commonly used assumptions on the model parameters, plus the assumption that the mas…

Measurement of inclusive rho(0), f(0)(980), f(2)(1270), K-2(*0)(1430) and f '(2)(1525) production in Z(0) decays

DELPHI results are presented on the inclusive production of the neutral mesons ρ0, f0(980), f2(1270), KView the MathML source and f′2(1525) in hadronic Z0 decays. They are based on about 2 million multihadronic events collected in 1994 and 1995, using the particle identification capabilities of the DELPHI Ring Imaging Cherenkov detectors and measured ionization losses in the Time Projection Chamber. The total production rates per hadronic Z0 decay have been determined to be: 1.19±0.10 for ρ0; 0.164±0.021 for f0(980); 0.214±0.038 for f2(1270); 0.073±0.023 for KView the MathML source; and 0.012±0.006 for f′2(1525). The total production rates for all mesons and differential cross-sections for …

Study of B-s(0)-(B-s(0))over-bar oscillations and B-s(0) lifetimes using hadronic decays of B-s(0) mesons

Oscillations of B0s mesons have been studied in samples selected from about 3.5 million hadronic Z decays detected by DELPHI between 1992 and 1995. One analysis uses events in the exclusive decay channels: B0s -> Ds- pi+ or Ds- a1+ and B0s -> anti-D0 K- pi+ or anti-D0 K- a1+, where the D decays are completely reconstructed. In addition, B0s anti-B0s oscillations have been studied in events with an exclusively reconstructed Ds accompanied in the same hemisphere by a high momentum hadron of opposite charge. Combining the two analyses, a limit on the mass difference between the physical B0s states has been obtained: Delta(m_B0s) > 4.0 ps^{-1} at the 95% C.L. with a sensitivity of Delta(m_B0s) …

Study of prompt photon production in hadronic Z° decays.

From a sample of 1.5 million Z0 ! qq decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated nal state photons is measured and is compared with the O( ; s) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to the Z0 boson. The measured values c2=3 = 0:91+0:25 0:36 and c1=3 = 1:62+0:24 0:17 are compatible with the Standard Model prediction, c2=3 = 1:145 and c1=3 = 1:477.

Measurement of the mass and width of the W boson in e(+)e(-) collisions at root s=189 GeV

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

A Measurement of B Meson Production and Lifetime Using D`− Events in Z0 Decays

A study of B meson decays into D l- X final states is presented. In these events, neutral and charged D mesons originate predominantly from B+ and B0 decays, respectively. The dilution of this correlation due to D** production has been taken into account. From 263700 hadronic Z0 decays collected in 1991 with the DELPHI detector at the LEP collider, 92 D0 --> K- pi+, 35 D+ --> K- pi+ pi+ and 61 D*+ --> D0 pi+ followed by D0 --> K- pi+ or D0 --> K- pi+ pi+ pi-, are found with an associated lepton of the same charge as the kaon. From the D0 l- and D*+ l-, the probability f(d) that a b quark hadronizes into a B- (or B0BAR),meson is found to be 0.44 +/-0.08 +/-0.09, corresponding to a total (B(s…

Charged and identified particles in the hadronic decay of W bosons and in e(+)e(-) -> q(q)over-bar from 130 to 200 GeV

Inclusive distributions of charged particles in hadronic W decays are experimentally investigated using the statistics collected by the DELPHI experiment at LEP during 1997, 1998 and 1999, at centre-of-mass energies from 183 to around 200 GeV. The possible effects of interconnection between the hadronic decays of two Ws are not observed. Measurements of the average multiplicity for charged and identified particles in q qbar and WW events at centre-of-mass energies from 130 to 200 GeV and in W decays are presented. The results on the average multiplicity of identified particles and on the position xi^* of the maximum of the xi_p = -log(2p/sqrt(s)) distribution are compared with predictions o…

Measurement of the B0 - anti-B0 mixing using the average electric charge of hadron jets in Z0 decays

From the data recorded with the DELPHI detector at LEP in the years 1991-1992, 46 497 events were selected having a high-momentum muon in hadron jets. A fit to the average electric charge sum of the jets recoiling against a b-quark jet tagged by a high-PT muon results in an average mixing parameter of chiBAR = 0.144 +/- 0.014(stat.)+0.017/-0.011(syst.).

Search for SUSY with R-parity violating LL(E)over-bar couplings at root s=189 GeV

Searches for pair production of supersymmetric particles under the assumption that R-parity is not conserved are presented, based on data recorded by the DELPHI detector in 1998 from e+e- collisions at a centre-of-mass energy of 189 GeV. Only one R-parity violating LL\bar{E} term (i.e. one lambda coupling), which couples scalar leptons to leptons, is considered to be dominant at a time. Moreover, it is assumed that the strength of the R-parity violating couplings is such that the lifetimes can be neglected. The search for pair production of neutralinos, charginos and sleptons has been performed for both direct R-parity violating decays and indirect cascade decays. The results are in agreeme…

Search for heavy stable and long-lived particles in e(+)e(-) collisions at root s=189 GeV

A search for stable and long-lived heavy charged particles was performed using the data taken by the DELPHI experiment at an energy of 189 GeV. The Cherenkov light detected in the Ring Imaging Cherenkov Detector and the ionisation loss measured in the Time Projection Chamber were used to identify heavy particles passing through the detector. No evidence for the production of such particles has been found, therefore exclusion limits at 95% confidence level were derived on the masses of left and right handed smuons and staus. The results were combined with previous DELPHI searches in this channel. Including previous DELPHI results, masses of left (right) handed stable smuons and staus can be …

A study of the reaction e(+)e(-)->mu(+)mu(-)gamma(ISR) at LEP and search for new physics at annihilation energies near 80 GeV

A study of the channel e+e− → μ+μ−γ scISR, where γ\scISR is an initial state radiation photon, is presented using data collected by the DELPHI experiment from 1991 to 1994. The total cross-section at effective annihilation energies (s′−−√) below the Z0 peak is obtained by using the events with relatively hard initial state radiative photon(s) (Eγ > ~1~GeV). The differential cross-section as a function of the muon polar production angle is also determined in order to extract the forward-backward asymmetries for the reaction e+e−→μ+μ− at energies s′−−√ between 20 and 87~GeV. The ratio of the helicity cross-sections σLL+σRRσRL+σLR, where the two subscripts stand for the helicities of the incom…

Search for leptoquarks and FCNC in e+e- annihilations at √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 134GeV/c2 to 171GeV/c2 for electromagnetic type couplings) and couplings of the leptoquark states.A search for top-charm flavour changing neutral currents(e+e−→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…

Observation of short-range 3-particle correlations in e(+)e(-) annihilations at lep energies

\def\tpc{three-particle correlation} \def\twopc{two-particle correlation} Measurements are presented of short range three-particle correlations in e^+ e^- annihilations at LEP using data collected by the DELPHI detector. %The jet structure is studied using three-particle correlation functions. At small values of the four-momentum difference, strong three-particle correlations are observed for like-sign (+++ and ---) and for unlike-sign (++- and +--) pion combinations which are not a consequence of two-particle correlations. A possible explanation of the observed effects in like-sign combinations is the existence of higher order Bose-Einstein interference, which significantly changes the par…

Search for stable heavy charged particles in e(+)e(-) collisions at root s=130-136, 161 and 172 GeV

A search for stable or long-lived heavy charged particles in e+e− interactions at energies of 130¿136, 161 and 172 GeV has been performed using the data taken by the DELPHI experiment at LEP. The search is based on particle identification provided by the Time Projection Chamber and the Ring Imaging Cherenkov detector. Upper limits at 95% confidence level are derived on the cross-section for heavy long-lived pair-produced charge ±e and View the MathML source particles in the range of 0.4¿2.3 pb for masses from 45 to 84 GeV/c2. Within supersymmetric extensions of the Standard Model, long-lived charginos with masses from 45 to 84 (80) GeV/c2 for high (low) sneutrino masses can be excluded at 9…

A Study of radiactive muon-pair events at Z° energies and limits on an additional Z' gauge boson

An analysis is reported on the channele + e −→μ+μ−(nγ), n=1,2..., using data taken with the DELPHI detector at LEP from 1990 to 1992. Differential cross sections of the radiative photons as a function of photon energy and of the angle between the photon and the muon are presented. No significant deviations from expectations are observed. The data are also used to extract the muon-pair cross section and asymmetry below the Z0 peak by using those events with relatively hard initial state radiative photon(s). The measured cross section and asymmetry show no significant deviation from the Standard Model expectations. These results together with the DELPHI cross section and asymmetry measurement…

Measurement of A(FB)(b anti-b) in hadronic Z decays using a jet charge technique

The b {b} forward-backward asymmetry has been determined from the average charge flow measured in a sample of 3,500,000 hadronic Z decays collected with the DELPHI detector in 1992-1995. The measurement is performed in an enriched b {b} sample selected using an impact parameter tag and results in the following values for the b {b} forward-backward asymmetry: AFBb\overline{b} (89.55 GeV) = 0.068 ± 0.018 (stat.)± 0.0013(syst.) AFBb\overline{b} (91.26 GeV) = 0.0982±0.0047(stat.)±0.0016(syst.) AFBb\overline{b} (92.94 GeV) = 0.123 ± 0.016 (stat.)± 0.0027(syst.) The b {b} charge separation required for this analysis is directly measured in the b tagged sample, while the other charge separations a…

Measurement of $\mbox{B}^0_d$ - $\overline{\mbox{B}^0_d}$ oscillations

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

Data taken by the DELPHI experiment at centre-of-mass energies of 183 GeV and 189 GeV with a total integrated luminosity of 212 pb^{-1} have been used to search for the supersymmetric partners of the electrons, muons, and taus in the context of the Minimal Supersymmetric Standard Model (MSSM). The decay topologies searched for were the direct decay ({\tilde \ell} -> \ell {\tilde \chi_1^0}), producing acoplanar lepton pairs plus missing energy, and the cascade decay ({\tilde \ell} -> \ell {\tilde \chi_2^0} -> \ell \gamma {\tilde \chi_1^0}), producing acoplanar lepton and photon pairs plus missing energy. The observed number of events is in agreement with Standard Model predictions. The 95% C…

Hadronization properties of b quarks compared to light quarks in e(+)e(-)-> q(q)over-bar from 183 to 200 GeV

The DELPHI detector at LEP has collected 54 pb^{-1} of data at a centre-of-mass energy around 183 GeV during 1997, 158 pb^{-1} around 189 GeV during 1998, and 187 pb^{-1} between 192 and 200 GeV during 1999. These data were used to measure the average charged particle multiplicity in e+e- -> b bbar events, _{bb}, and the difference delta_{bl} between _{bb} and the multiplicity, _{ll}, in generic light quark (u,d,s) events: delta_{bl}(183 GeV) = 4.55 +/- 1.31 (stat) +/- 0.73 (syst) delta_{bl}(189 GeV) = 4.43 +/- 0.85 (stat) +/- 0.61 (syst) delta_{bl}(200 GeV) = 3.39 +/- 0.89 (stat) +/- 1.01 (syst). This result is consistent with QCD predictions, while it is inconsistent with calculations ass…

Measurement of the B-s(0) lifetime and study of B-s(0)-(B-s(0))over-bar oscillations using D(s)l events

Lifetime and oscillations of B0s mesons have been studied in events with a large transverse momentum lepton and a Ds of opposite electric charge in the same hemisphere, selected from about 3.6 million hadronic Z0 decays accumulated by DELPHI between 1992 and 1995. The B0s lifetime and the fractional width difference between the two physical B0s states have been found to be: \tau_(B0s) = (1.42^{+0.14}_{-0.13}(stat.) +/- 0.03(syst.)) ps \Delta\Gamma(B0s)/\Gamma(B0s) 7.4 ps^{-1} at the 95% C.L. with a corresponding sensitivity equal to 8.1 ps^{-1}.

Inclusive Sigma(-) and Lambda(1520) production in hadronic Z decays

Production of Sigma- and Lambda(1520) in hadronic Z decays has been measured using the DELPHI detector at LEP. The Sigma- is directly reconstructed as a charged track in the DELPHI microvertex detector and is identified by its Sigma -> n pi decay leading to a kink between the Sigma- and pi-track. The reconstruction of the Lambda(1520) resonance relies strongly on the particle identification capabilities of the barrel Ring Imaging Cherenkov detector and on the ionisation loss measurement of the TPC. Inclusive production spectra are measured for both particles. The production rates are measured to be = 0.081 +/- 0.002 +/- 0.010, = 0.029 +/- 0.005 +/- 0.005. The production rate of the Lambda(1…

Measurement of the multiplicity of gluons splitting to bottom quark in hadronic Z° decays

An inclusive measurement of the average multiplicity of bb pairs from gluons, gbb, in hadronic Z0 events collected by the DELPHI experiment at LEP, is presented. A counting technique, based on jet b-tagging in 4-jet events, has been used. Looking for secondary bottom production in events with production of any primary flavour, by requiring two b-tagged jets in well defined topological configurations, gave gbb = (0.21 +/- 0.11 (stat) +/- 0.09(syst)) %. This result was checked with a different method designed to select events with four b quarks in the final state. Agreement within the errors was found.

Production of strange b-baryons decaying into xi-/+-l-/+ pairs at LEP

An excess of events containing, in a jet, a same-sign Xi(-/+) - l(-/+) pair as compared to those with an opposite-sign Xi(-/+) - l(-/+) pair has been observed in an analysis of 1.7 million hadronic Z(o) 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 x 10(-4). The measured production fraction corresponds to: P(b --> B - baryon) x BR(B - baryon --> Xi(-) l(-) X) = (5.9 +/- 2.1 +/- 1.0) x 10(-4), per lepton species, averaged for electrons and muons and assuming the two channels have an equal contribution. Semileptonic decays of Lambda(b) baryons can account for less than 10% of thes…

Measurement of the B¯-- >D(*)πlν¯l branching fraction

A study of b semileptonic decays into D, D pi+- and D* pi+- final states is presented. The D0, D+ and D*+ mesons are exclusively reconstructed in Z decay data recorded from 1992 to 1995 in the DELPHI experiment at LEP. The overall branching fractions are measured to be: BR(b -> D^0 \ell^- \bar{\nu}_{\ell} X) = (7.04 +/- 0.34 (stat) +/- 0.36 (syst.exp) +/- 0.17 (BR_D))% BR(b -> D^+ \ell^- \bar{\nu}_{\ell} X) = (2.72 +/- 0.19 (stat) +/- 0.16 (syst.exp) +/- 0.18 (BR_D))% BR(b -> D^{*+} \ell^- \bar{\nu}_{\ell} X) = (2.75 +/- 0.17 (stat) +/- 0.13 (syst.exp) +/- 0.09 (BR_D))% where the D0 and D+ results include also contributions from D*0 and D*+ decays. A fit to the distribution of the pi+- impa…

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 a…

Lambda(b) polarization in Z(0) decays at LEP

The longitudinal polarization of the Λb baryon is measured at the LEP e+e− collider by DELPHI. It is determined from the charged lepton and neutrino energy spectra in 249±19 Λb semileptonic decays reconstructed in ≈3.5 million hadronic Z0 decays using Λ0-lepton correlations. The measured polarization is: PΛb=−0.49−0.30+0.32(stat.)±0.17(syst.)

Study of radiative leptonic events with hard photons and search for excited charged leptons at S**(1/2) = 130-GeV - 136-GeV

During the last 1995 data acquisition period at LEP, the DELPHI experiment collected an integrated luminosity of 5.9 pb−1 at centre-of-mass energies of 130 GeV and 136 GeV. Radiative leptonic events (e,μ,τ) with high energy photons were studied and compared to Standard Model predictions. The data were used to search for charged excited leptons decaying through an electromagnetic transition. No significant signal was found. From the search for pair produced excited leptons, the limits me∗>62.5 GeV/c2, mμ∗>62.6 GeV/c2 and mτ∗>62.2 GeV/c2 at 95\% confidence level were established. For single excited lepton production, upper limits on the ratio λ/mℓ∗ of the coupling of the excited charged lepto…

Limits on the masses of supersymmetric particles at root s=189 GeV

Searches for charginos, neutralinos and sleptons at LEP2 centre-of-mass energies from 130 GeV to 189 GeV have been used to set lower limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles within the MSSM framework. R-parity conservation has been assumed. The lightest neutralino was found to be heavier than 32.3 GeV/c^2 independent of the m_0 value. The lightest chargino, the second-to-lightest neutralino, the next-to-heaviest neutralino, the heaviest neutralino, the sneutrino and the right-handed selectron were found to be heavier than 62.4 GeV/c^2, 62.4 GeV/c^2, 99.9 GeV/c^2, 116.0 GeV/c^2, 61.0 GeV/c^2, and 87.0 GeV/c^2, respectively. These limits do…

Search for supersymmetric partners of top and bottom quarks at root s=189 GeV

Searches for supersymmetric partners of top and bottom quarks are presented using data taken by the DELPHI experiment at LEP in 1997 and 1998. No deviations from standard model expectations are observed in these data sets, which are taken at centre-of-mass energies of 183 GeV and 189 GeV and correspond to integrated luminosities of 54 pb^{-1} and 158 pb^{-1}. These results are used in combination with those obtained by DELPHI at lower centre-of-mass energies to exclude regions in the squark-neutralino mass plane at 95% confidence level.

Measurement of the gluon fragmentation function and a comparison of the scaling violation in gluon and quarks jets

The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the Delphi detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy e+e− annihilation experiments. For gluon jets it appears to be signifi…

Search for R-parity violation with a (U)over-bar(D)over-bar(D)over-bar coupling at root s approximate to 189 GeV

Searches for pair production of gauginos and squarks in e+e- collisions at a centre-of-mass energy of 189 GeV have been performed on data corresponding to an integrated luminosity of 158 pb^{-1} collected by the DELPHI detector at LEP. The data were analyzed under the assumption of non-conservation of R-parity through a single dominant Ubar Dbar Dbar coupling between squarks and quarks. Typical final states contain between 4 and 10 jets with or without additional leptons. No excess of data above Standard Model expectations was observed. The results were used to constrain domains of the MSSM parameter space and derive limits on the masses of supersymmetric particles. The following mass limit…

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

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

Measurement of V-cb from the decay process (B)over-bar(0)-> D(*+)l(-)(nu)over-bar

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

An upper limit for Br(Z°→ggg) from symmetric 3-jet hadronic decays.

An upper limit for BR(Z0 → 3g) is obtained from a correlation method, which distinguishes statistically between quark and gluon jets by using the difference in their charged particle multiplicity distributions. From the sample of threefold symmetric three-jet events collected by the DELPHI experiment at LEP during 1991¿1994, the 95% confidence level upper limit is deduced to be: BR(Z0 → 3g) ≤ 1.6 × 10−2 for the JADE and BR(Z0 → 3g) ≤ 1.5 × 10−2 for the DURHAM jet-finder.

Production of Lambda and Lambda anti-Lambda correlations in the hadronic decays of the Z (0)

An analysis of the production of the LAMBDA baryon in the hadronic decays of the Z0 is presented, based on about 993K multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. The differential cross section of the LAMBDA and the correlations between LAMBDA and LAMBDABAR produced in the same event are compared to current models, based both on string fragmentation and on cluster decay. The predictions of the string fragmentation model are found to give satisfactory agreements with the data, clearly better than those of the cluster model.

Measurement of the triple-gluon vertex from 4-JET events at LEP

From the combined data of 1990 and 1991 of the DELPHI experiment at LEP, 13057 4-jet events are obtained and used for determining the contribution of the triple-gluon vertex. The relevant variables are the generalized Nachtmann Reiter angle theta(NR)* and the opening angle of the two least energetic jets. A fit to their two-dimensional distribution yields C(A)/C(F)=2.12+/-0.35 and N(C)/N(A)=0.46+/-0.19, where C(A)/C(F) is the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung from quarks, and N(C)/N(A), the ratio of the number of quark colours to the number of gluons. This constitutes a convincing model-independent proof of the existence of the triple-…

Charged particle multiplicity in e(+)e(-) interactions at root s=130 GeV

From the data collected by DELPHI at LEP in autumn 1995, the multiplicity of charged particles at a hadronic energy of 130 GeV has been measured to be = 23:84 0:51(stat) 0:52(syst). When compared to lower energy data, the value measured is consistent with the evolution predicted by QCD with corrections at next-to-leading order, for a value s(130GeV) = 0:105 0:003(stat) 0:008(syst).

Search for charged Higgs bosons in e(+)e(-) collisions at root s = 172 GeV

This paper presents results on charged Higgs boson production, based on LEP data collected at s√=172 GeV, that complement the previous DELPHI results obtained at centre of mass energies up to 161 GeV. %The analysis is based on LEP data collected at s√=172 GeV. The charged Higgs bosons are assumed to be pair produced and to decay either into a quark pair %, identified as cs, or into τντ. The three different possible final states are included in the analysis. Data from ring imaging Cherenkov and microvertex detectors are used to identify the quarks as a cs pair. %No significant signal was found. The number of candidates found is compatible with the background expected from standard processes.…

Determination of the e(+)e(-)->gamma gamma(gamma) cross-section at centre-of-mass energies ranging from 189 GeV to 202 GeV

A test of the QED process e+e- -> gamma gamma (gamma) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb^{-1}. The differential and total cross-sections for the process e+e- -> gamma gamma were measured, and found to be in agreement with the QED prediction. 95% Confidence Level (C.L.) lower limits on the QED cut-off parameters of Lambda+ > 330 GeV and Lambda- > 320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/c^2 (for lambda_gamma = 1) was obtained. s-channel virtual graviton exchan…

Measurement of vertical bar V-cs vertical bar using W decays at LEP2

Decays of W± bosons, produced at LEP2, have been used to measure the |Vcs| element of the Cabibbo-Kobayashi-Maskawa matrix. Values for |Vcs| were extracted both from the measured hadronic branching ratio of W± decays and by tagging the flavour of hadronic jets produced in W± decays. Applying the two methods to the data collected during 1996 at energies of 161 and 172 GeV, DELPHI obtains |Vcs|=0.91+0.15−0.14(stat)±0.05(syst).

Upper limits on the branching ratios tau-]mu-gamma and tau-]e-gamma

The DELPHI collaboration has searched for lepton flavour violating decays tau -> mugamma and tau -> egamma using a data sample of about 70 pb-1 of integrated luminosity corresponding to 81 000 produced tau+tau- events. No candidates were found for either of the two modes. This yields branching ratio upper limits of B(tau -> egamma) mugamma) < 6.2 × 10-5 at 90% confidence level.

Determination of alpha(s) using the next-to-leading-log approximation of QCD

A new measurement of alpha(s), is obtained from the distributions in thrust, heavy jet mass, energy-energy correlation and two recently introduced jet broadening variables following a method proposed by Catani, Trentadue, Turnock and Webber. This method includes the full calculation of O(alpha(s)2) terms and leading and next-to-leading logarithms resummed to all orders of alpha(s). The analysis is based on data taken with the DELPHI detector at LEP during 1991. I its found that the inclusion of the resummed leading and next-to-leading logarithms reduces the scale dependence of alpha(s) and allows an extension of the fit range towards the infrared limit of the kinematical range. The combined…

Upper limit for the decay B- -> tau(-)(v)over-bar(tau) the b -> tau(v)over-bar(tau)X branching ratio

The branching ratio for the leptonic decay of charged B mesons (B- → tau (-)[([nu])over bar](tau)) has been measured using selected leptonic tau (-) → l(-)nu (tau)[([nu])over bar](l) and hadronic tau (-) → nu X-tau decays in Z → b (b) over bar decays recorded by DELPHI at LEP1 in 1992-1995. The result, BR(B- → tau (-)[([nu])over bar](tau)) lt 1.1 x 10(-3) at the 90% confidence level, is consistent with standard model expectations and puts a constraint on the ratio tan [beta]/MH+/- lt 0.46 (GeV/c(2))(-1) in the framework of models with two Higgs doublets (type II Higgs doublet model). From the missing energy distribution in Z → b (b) over bar decays without identified leptons, the b → tau[([…

Search for spontaneous R-parity violation at √s = 183 and 189 GeV

Searches for spontaneous R-parity violating signals at sqrt(s) = 183 GeV and sqrt(s) = 189 GeV have been performed using the 1997 and 1998 DELPHI data, under the assumption of R-parity breaking in the third lepton family. The expected topology for the decay of a pair of charginos into two acoplanar taus plus missing energy was investigated and no evidence for a signal was found. The results were used to derive a limit on the chargino mass and to constrain the allowed domains of the MSSM parameter space.

Rapidity-rank structure of p(p)over-bar pairs in hadronic Z(0) decays

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

Search for pair produced neutralinos in events with photons and missing energy from e+e− collisions at S(1/2) = 130-GeV to 183-GeV

The events with two photons and missing (transverse) energy collected by the DELPHI detector at centre-of-mass energies between 130~GeV and 183~GeV have been studied to search for processes of the type e+e−→YY with the subsequent decay Y→Xγ, where X is an undetectable neutral particle. Reactions of this kind are expected in supersymmetric models, where the Y particle can be either the lightest neutralino, decaying to a photon and a gravitino, or the next-to-lightest neutralino, decaying to a photon and the lightest neutralino. To study the case of long-lived Y particles, a search for single-photon events with the reconstructed photon axis pointing far from the beam interaction region has al…

Measurement of gamma(b(b)over-bar)/gamma(had) using impact parameter measurements and lepton identification

The partial decay width of theZ to bb¯quark pairs has been measured by the DELPHI detector at LEP.b-hadrons, containingb-quarks, were tagged by leptons with high transverse momentum relative to the hadron or by tracks with large impact parameters to the primary vertex.The ratio of the numbers of events with a single such tag to those with two tags was used to estimate the efficiency of the method and to reduce the systematic uncertainty. Combining all methods, the value: Γbb¯Γhad=0.2210±0.0033±0.0003(model)±0.0014(Γcc¯) was found, where the third error corresponds to a±8% uncertainty on the cc production width. A maximum likelihood in to the single and di-lepton distributions gave the branc…

Searches for neutral Higgs bosons in e(+)e(-) collisions around root s=189 GeV

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

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−→cc¯ and e+e−→bb¯ 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}) &=& &#;0659 \pm 0.0094 (stat) \pm 0.0035 (syst) \Afbb (\sqrt{s} = 91.235 {\rm GeV}) &=& &#;0762 \pm 0.0194 (stat) \pm 0.0085 (syst) \Afbc(\sqrt{s} = 89.434 {\rm GeV}) &=&-&#;0496 \pm 0.0368 (stat) \pm 0.0053 …