arXiv subjects

P. Abreu

Publications and source records attributed to P. Abreu.

At least 145 records · Page 8Linked to original sources

arXiv2001

Determination of |Vub|/|Vcb| with DELPHI at LEP

The ratio of the CKM quark-mixing matrix elements |Vub|/|Vcb| has been measured using B hadron semileptonic decays. The analysis uses the reconstructed mass M_X of the secondary hadronic system produced in association with an identified lepton. Since B -> X_u lepton anti-nu transitions are characterised by hadronic masses below those of the D mesons produced in B -> X_c lepton anti-nu transitions, events with a reconstructed value of M_X significantly below the D mass are selected. Further signal enrichments are obtained using the topology of reconstructed decays and hadron identification. A fit to the numbers of decays in the b -> u enriched and depleted samples with M_X above and below 1.6 GeV/c^2 and to the shapes of the lepton energy distribution in the B rest frame gives |Vub|/|Vcb|= 0.103^{+0.011}_{-0.012} (stat.) +/- 0.016 (syst.) +/- 0.010 (model) and, correspondingly, a charmless semileptonic B decay branching fraction of BR(B -> X_u lepton anti-nu) = (1.57 +/- 0.35 (stat.) +/- 0.48 (syst.) +/- 0.27 (model)) x 10^{-3}.

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arXiv2001

Rapidity-Rank Structure of p pbar Pairs in Hadronic Z0 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.

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arXiv2001

Measurement of the Mass and Width of the W Boson in e+e- Collisions at sqrt(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 the accelerator. Combining this result with those previously published by the DELPHI collaboration gives the result M_W = 80.359 +/- 0.074 (stat) +/- 0.032 (syst) +/- 0.017 (LEP) +/- 0.033 (FSI) GeV/c2. The combined value for the W width is Γ_W = 2.266 +/- 0.176 (stat) +/- 0.056 (syst) +/- 0.052 (FSI) GeV/c2.

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arXiv2001

Measurement of V_{cb} from the decay process \bar{B^0} -> D^{*+} \ell^- \barν

A new precise measurement of |V_{cb}| and of the branching ratio BR(\bar{B^0} -> D^{*+} \ell^- \bar{ν_\ell}) has been performed using a sample of about 5000 semileptonic decays \bar{B^0} -> D^{*+} \ell^- \bar{ν_\ell}, selected by the DELPHI detector at LEP I by tagging the soft pion from D^{*+} -> D^0 π^+. 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{ν_\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 the experimental resolution.

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arXiv2001

Search for charginos in e+e- interactions at sqrt(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 (μ,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.

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arXiv2001

Search for a fermiophobic Higgs at LEP 2

Higgs bosons predicted by the fermiophobic scenario within Two Higgs Doublets Models were searched for in the data collected by the DELPHI detector at centre-of-mass energies between 189 GeV and 202 GeV, corresponding to a total integrated luminosity of 380 pb^{-1}. No signal was found and confidence limits were derived in the framework of possible extensions of the Standard Model Higgs sector.

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arXiv2001

Searches for Neutral Higgs Bosons in e+e- Collisions around sqrt(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.

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arXiv2001

Search for sleptons in e+e- collisions at sqrt(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 χ_1^0}), producing acoplanar lepton pairs plus missing energy, and the cascade decay ({\tilde \ell} -> \ell {\tilde χ_2^0} -> \ell γ{\tilde χ_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% CL excluded mass limits for selectrons, smuons and staus are m_{\tilde {e}} \leq 87 GeV/c^2, m_{\tilde μ} \leq 80 GeV/c^2 and m_{\tilde τ} \leq 75 GeV/c^2, respectively, for values of μ=-200 GeV/c^2 and tan(beta)=1.5.

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arXiv2001

Photon Events with Missing Energy at sqrt(s) = 183 to 189 GeV

The production of single photons has been studied in the reaction e+e- -> gamma + invisible particles at centre-of-mass energies of 183 GeV and 189 GeV. A previously published analysis of events with multi-photon final states accompanied by missing energy has been updated with 189 GeV data. The data were collected with the DELPHI detector and correspond to integrated luminosities of about 51 pb^{-1} and 158 pb^{-1} at the two energies. The number of light neutrino families is measured to be 2.84 +/- 0.15(stat) +/- 0.14(syst). The absence of an excess of events beyond that expected from Standard Model processes is used to set limits on new physics as described by supersymmetric and composite models. A limit on the gravitational scale is also determined.

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arXiv2001

Search for Heavy Stable and Long-Lived Particles in e+e- Collisions at sqrt(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 excluded between 2 GeV/c^2 and 88 (87.5) GeV/c^2 at 95% CL.

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arXiv2001

Update of the search for charginos nearly mass-degenerate with the lightest neutralino

The data collected by DELPHI in 1998 at the centre-of-mass energy of 189 GeV have been used to update the search for charginos nearly mass-degenerate with the lightest supersymmetric particle, which is assumed to be the lightest neutralino. Mass differences below ΔM = 3 GeV/c^2 are considered. No excess of events with respect to the Standard Model expectation has been observed, and exclusions in the plane of ΔM versus chargino mass are given. The new ΔM independent lower limit on the mass of the chargino is 62.4 GeV/c^2 in the higgsino scenario (which includes the gaugino mass unification scenario), if all sfermions are heavier than the lightest chargino. In the approximation of large sfermion masses the limit is 59.8 GeV/c^2, independently of the field content.

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arXiv2001

Search for supersymmetric partners of top and bottom quarks at sqrt(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.

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arXiv2001

Search for supersymmetric particles in scenarios with a gravitino LSP and stau NLSP

Sleptons, neutralinos and charginos were searched for in the context of scenarios where the lightest supersymmetric particle is the gravitino. It was assumed that the stau is the next-to-lightest supersymmetric particle. Data collected with the DELPHI detector at a centre-of-mass energy near 189 GeV were analysed combining the methods developed in previous searches at lower energies. No evidence for the production of these supersymmetric particles was found. Hence, limits were derived at 95% confidence level.

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arXiv2001

Measurement and Interpretation of Fermion-Pair Production at LEP Energies of 183 and 189 GeV

An analysis of the data collected in 1997 and 1998 with the DELPHI detector at e+e- collision energies close to 183 and 189 GeV was performed in order to extract the hadronic and leptonic fermion-pair cross-sections, as well as the leptonic forward-backward asymmetries and angular distributions. The data are used to put limit on contact interactions between fermions, the exchange of R-parity violating SUSY sneutrinos, Z' bosons and the existence of gravity in extra dimensions.

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arXiv2001

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(1520) suggests that a large fraction of the stable baryons descend from orbitally excited baryonic states. It is shown that the baryon production rates in Z decays follow a universal phenomenological law related to isospin, strangeness and mass of the particles.

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arXiv2001

Hadronization properties of b quarks compared to light quarks in e+e- -> q qbar 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 assuming that the multiplicity accompanying the decay of a heavy quark is independent of the mass of the quark itself.

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arXiv2001

W pair production cross-section and W branching fractions in e+e- interactions at 189 GeV

The cross-section for the process e+e- -> W+W- has been measured with the data sample collected by DELPHI at an average centre-of-mass energy of 189 GeV and corresponding to an integrated luminosity of 155 pb^{-1}. Based on the 2392 events selected as WW candidates, the cross-section for the doubly resonant process e+e- -> W+W- has been measured to be 15.83 +- 0.38 (stat) +- 0.20 (syst) pb. The branching fractions of the W decay were also measured and found to be in good agreement with the Standard Model expectation. From these a value of the CKM mixing matrix element |V_{cs}| = 1.001 +- 0.040 (stat) +- 0.020 (syst) was derived.

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arXiv2001

Limits on the Masses of Supersymmetric Particles at sqrt(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 not depend on m_0 or M_2 and are valid for 1 \leq tan(beta) \leq 40, in the mu region where the lightest neutralino is the LSP. If the sneutrino is heavier than the chargino the lightest neutralino has to be heavier than 32.4 GeV/c^2. The effects of mixings in the third family of sfermions on these limits are discussed. The confidence level of all limits given is 95%.

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