0000000000900627
AUTHOR
Reyes Alemany Fernández
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.
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) …
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…
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…
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…
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…
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.
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.
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 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[([…
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…
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 …