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Experimental Physics Division Experimental Physics Division The LEP Programme ALEPH In 2003 the ALEPH Collaboration (Aachen, Annecy, Barcelona, Bari, Beijing, CERN, Clermont-Ferrand, Copenhagen, Demokritos, Ecole Polytechnique, Firenze, Frascati, Glasgow, Haverford College, Heidelberg, Imperial College, Innsbruck, Lancaster, Louvain-la-Neuve, Mainz, Marseille, Milano, MPI München, Orsay, Pisa, RAL, Royal Holloway, Saclay, Santa Cruz, Sheffield, Siegen, Trieste, Washington, Wisconsin, Zurich) completed and published several final physics analyses. The data collected during the life of the experiment and the software developed over many years have been archived to allow long-term analysis of ALEPH data. The final results on the search for the lightest supersymmetric particle (LSP), one of the main candidates for explaining the existence of the dark matter in the universe, have been complemented by a thorough search for stable hadronizing squarks and gluinos. The analysis of event-shape variables and inclusive particle spectra at the highest LEP energies was completed and the energy evolution of the strong coupling constant was studied in detail. The cross-section of the process e+e– → W+W– was measured with high precision. Many interesting results in two-photon physics were published. Overall, 15 papers were published or submitted for publication by ALEPH during 2003. Searches Charginos and Neutralinos were searched for in the data collected at centre-of-mass energies up to 209 GeV. In the many topologies considered, the number of candidates observed was consistent with the background expected from Standard Model processes. An absolute lower limit of 43.1 GeV/c2 was derived on the mass of the LSP, assumed to be the lightest neutralino, in the framework of MSSM and R-parity conservation. The effect of stau mixing was studied in great detail. It was shown that stau mixing would only slightly degrade the above limit. The final 95% CL limit on the LSP mass, as a function of tanβ, is shown in Fig. ALEPH–1. Another important achievement in the search for supersymmetry is the completion of the search for stable hadronizing squarks and gluinos. The final publication presents several searches at the Z pole and higher energies. The existence of LSP gluinos is excluded for masses below 26.9 GeV/c2, finally closing the long-standing low mass gluino window. Down-type (up-type) LSP squarks are excluded for masses below 92 (95) GeV/c2. Stop decaying to gluinos are excluded up to masses of 80 GeV/c2. Experimental Physics Division 5 Fig. ALEPH–1: The 95% CL lower limit on the mass of the lightest neutralino, as a function of tanβ. The dashed curve indicates the limit from chargino searches 2 β alone for m0 = 500 GeV/c . The grey area at small tan shows the improvement obtained if chargino and neutralino searches are combined. The other limits are obtained from (right to left) slepton searches in the corridor, Higgs boson searches in the corridor, chargino searches for large sfermion masses and Higgs boson searches. QCD and the Strong Interaction Coupling Improved tests of QCD have been completed using hadronic final-state observables with data collected at LEP1 and LEP2. The measurements include event shape variables, jet rates, and inclusive charged-particle distributions. Particular attention has been given to the study of the energy evolution of these variables. A large α part of the long paper dedicated to this subject discusses the measurement of s from event shape variables. α Non-perturbative aspects of the determination of s are studied by means of power law techniques, which are α corrections scaling with Q, the four-momentum transfer. The final result is s = 0.1214 ± 0.0048. The energy evolution of the coupling constant is shown in Fig. ALEPH–2. Inclusive and Two-Photon Physics In the area of inclusive physics the publication of a detailed investigation of Bose–Einstein correlations in hadronic Z decays is worth mentioning. The two-pion correlation function is measured in both one and two dimensions, using either unlike-sign or mixed-events reference samples. The results indicate that the correlation radii values depend on the chosen kind of reference sample and on the two-jet purity. 6 Experimental Physics Division α Fig. ALEPH–2: The measurements of the strong coupling constant s between 91.2 and 206 GeV. The results using six different event-shape variables are combined and correlations are taken into account. The inner error bars exclude the perturbative uncertainty, which is expected to be highly correlated between the measurements. The outer error bars indicate the total error. A fit to the three- loop QCD evolution formula using the uncorrelated errors is shown. The year 2003 saw the publication of several results on two-photon physics. Charm production in γγ ∗ ∗ collisions has been measured by selecting events containing D + mesons. The differential D + production has been determined as a function of pseudorapidity and transverse momentum and found to be in agreement with NLO QCD calculations. The exclusive production of pion and kaon meson pairs in two-photon collisions has also been investigated and compared to predictions. The angular distributions of these simple exclusive processes are described by QCD, but the data are found to have a significantly higher normalization. Another important measurement is related to the hadronic structure of the photon, which has been studied in two regions of momentum transfer (〈Q2〉 = 17.3 GeV2, 〈Q2〉 = 63.2 GeV2) using γγ collisions where one of the scattered electrons is detected. Electroweak Physics A precise determination of the W+W– production cross-section, using all of the data collected by ALEPH at LEP2, is ready for publication. Individual cross-sections for the different topologies arising from W decays into leptons and hadrons, as well as the total W-pair cross-section, are measured at ten centre-of-mass energies. The results are found to be in agreement at the one per cent level with recently developed Standard Model calculations. The branching fraction of the W boson into hadrons is measured to be + → BR(W hadrons) = 67.13 ± 0.38 ± 013%, from which the CKM matrix element |Vcs| is determined to be Experimental Physics Division 7 0.958 ± 0.017 ± 0.007. The W+W– cross-section as a function of the centre-of-mass energy is shown in Fig. ALEPH–3. The study of anomalous gauge boson couplings is close to completion. Constraints on the existence of anomalous quartic gauge couplings have been obtained from acoplanar photon pairs, and the results are ready for publication. Finally the long, thorough studies related to the measurement of the W mass are essentially completed, and the final publication is in preparation. Fig. ALEPH–3: Measurement of the W-pair production cross-section as a function of the centre-of-mass energy, compared to Standard Model predictions. DELPHI The DELPHI Collaboration consists of teams from Ames, Amsterdam, Antwerp, Athens, Bergen, Bologna, Bratislava, Brussels, CERN, Cracow, Dubna, Genoa, Grenoble, Helsinki, Karlsruhe, Lisbon, Liverpool, Ljubljana, Lund, Lyon, Marseille, Milan, Mons, Orsay, Oslo, Oxford, Padua, Paris, Prague, Rio de Janeiro, Rome, Rutherford, Saclay, Santander, Serpukhov, Stockholm, Strasbourg, Torino, Trieste, Udine, Uppsala, Valencia, Vienna, Warsaw and Wuppertal. The barrel part of the DELPHI detector is being prepared as a permanent exhibit for CERN visitors. It has been moved to its final position in pit 8, compatible with the spatial arrangement of the LHCb counting houses. A visitor platform has been installed. Several subdetectors are being prepared to allow viewing of internal construction details. It is foreseen to make the detector exhibit available to the public from summer 2004 onwards. During 2003 the DELPHI Collaboration continued the analysis of data collected both at the Z peak (LEP1) and at centre-of-mass energies 161–209 GeV (LEP2). Many analyses were finalized. A total of 15 papers were published or accepted for publication in refereed journals in 2003 and an additional 12 papers were submitted for publication. Out of these, nine papers are based on LEP1 data and covered mainly B and τ physics topics. 8 Experimental Physics Division A further 16 papers are in preparation and it is expected that ongoing analyses will lead to another 25 papers in 2004–2005. A total of 47 contributions were submitted to the 2003 Europhysics Conference on High-Energy Physics in Aachen and the Lepton–Photon Symposium at Fermilab. DELPHI and other LEP results were presented by some 45 DELPHI members at conferences during 2003. Heavy Flavour and τ Physics Using high-performance neural network techniques, precise measurements of B+, B0 and mean b-hadron τ τ lifetimes have been obtained: B+ = 1.624 ± 0.014(stat) ± 0.018(syst) ps, B0 = 1.531 ± 0.021(stat) ± τ + 0.031(syst) ps and b = 1.570 ± 0.005(stat) ± 0.008(syst) ps. The B and average b-hadron lifetime are the most accurate to date. The measurement of the fraction of B+ mesons in a sample of weakly decaying b-hadrons from Zbb→ decays is so far the most precise one: f =±()40.99 0.82(stat) ± 1.11(syst) %. Bu 0 → ∗+l − ν The CKM matrix element |Vcb| has been measured from the decays BDd l using exclusively ∗ reconstructed D + decays. Combining this measurement with a more inclusive measurement, previously published by DELPHI, results in |Vcb| = 0.0414 ± 0.0012(stat) ± 0.0021(syst) ± 0.0018(theor). In the τ sector, the final results on the τ lifetime and on the exclusive and semi-exclusive hadronic branching ratios of final states with up to five (resp. six) hadrons have been submitted for publication. The results are in good agreement with current world averages and have slightly smaller or similar errors compared to measurements from other experiments.
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