Limits on electron compositeness from Bhabha scattering

We have studied the reactione ⁺ e ^–e ⁺ e ^– using data taken by the CELLO detector at 35 GeV centre of mass energy with an integrated luminosity of 86 pb^–1. The differential cross section is found to agree with the Standard Model and is used to set limits on possible deviations from the pointlike structure of the electron. The lower limits obtained for the compositeness scale range from 0.8 to 6.0 TeV (95% C.L.), depending on the structure of the current.     

Physics prospects at a linear e + e − collider

The talk described the prospects of studying standard model parameters as well as scenarios beyond the standard model, like the minimal supersymmetric standard model, theories with extra dimensions and theories with extra neutral gauge bosons, at a future linear e ⁺ e ⁻ collider.     

Measurement of the WWγ cross section and direct limits on anomalous quartic gauge boson couplings at LEP

The process e⁺e⁻→W⁺W⁻γ is analysed using the data collected with the L3 detector at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.8 pb⁻¹. Based on a sample of 42 selected W⁺W⁻ candidates containing an isolated hard photon, the W⁺W⁻γ cross section, defined within phase-space cuts, is measured to be: σ_WWγ=290±80±16 fb, consistent with the Standard Model expectation. Including the process , limits are derived on anomalous contributions to the Standard Model quartic vertices W⁺W⁻γγ and W⁺W⁻Zγ at 95% CL: −0.043 GeV⁻²<a₀/Λ²<0.043 GeV⁻², −0.08 GeV⁻²<a_c/Λ²<0.13 GeV⁻², −0.41 GeV⁻²<a_n/Λ²<0.37 GeV⁻².     

Effects of superstrings in collisions

Superstring theory in d = 10 dimensions after Calabi—Yau compactification yields a minimum low-energy gauge group SU(3)_C × SU(2)_L × U(1)_Y × U(1)_E. The low-energy theory includes particles with the quantum numbers of 27 representations of E₆, each of which contains an extra neutrino ν^c conventionally called a “right-handed neutrino”. The contributions of ν and ν^c to through Z⁰ and Z_E mixing is calculated. Small contributions are found of the new right-handed neutrino and of the superstring boson Z_E to σ(e⁺e⁻ → γ + nothing).     

Electron heating in a submicron-size n GaAs wire

We have studied electron heating in a submicron-size GaAs wire from 4.2 K to 50 K. We find that the energy relaxation rate for the electrons is of the form τ_E⁻¹ = α + βT_eⁿ where α, β are constants and T_e is the electron temperature. We associate the temperature-independent term with a quasi-elastic surface scattering process in which an electron losses 1% of its energy at each collision. The temperature dependent term may be due to electron-phonon scattering. It is possible to fit the data to 2 < n < 3.     

Spin analysis of supersymmetric particles

The spin of supersymmetric particles can be determined at e⁺e^- colliders unambiguously. This is demonstrated for a characteristic set of non-colored supersymmetric particles – smuons, selectrons, and charginos/neutralinos. The analysis is based on the threshold behavior of the excitation curves for pair production in e⁺e^- collisions, the angular distribution in the production process and decay angular distributions. In the first step we present the observables in the helicity formalism for the supersymmetric particles. Subsequently we confront the results with corresponding analyses of Kaluza–Klein particles in theories of universal extra space dimensions which behave distinctly different from supersymmetric theories. It is shown in the third step that a set of observables can be designed which signal the spin of supersymmetric particles unambiguously without any model assumptions. Finally in the fourth step it is demonstrated that the determination of the spin of supersymmetric particles can be performed experimentally in practice at an e⁺e^- collider.     

Study of the process ee→ππγ in c.m. energy range 600–970 MeV at CMD-2

The cross section of the process e⁺e⁻→π⁰π⁰γ has been measured in the c.m. energy range 600–970 MeV with the CMD-2 detector. The following branching ratios have been determined:

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and

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. Evidence for the ρ⁰→f₀(600)γ decay has been obtained:

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. From a search for the process e⁺e⁻→ηπ⁰γ the following upper limit has been obtained: at 90% CL.     

Measurement of and the dipion contribution to the muon anomaly with the KLOE detector

We have measured the cross section σ(e⁺e⁻→π⁺π⁻γ(γ)) at DAΦNE, the Frascati -factory, using events with initial state radiation photons emitted at small angle and inclusive of final state radiation. We present the analysis of a new data set corresponding to an integrated luminosity of 240 pb⁻¹. We have achieved a reduced systematic uncertainty with respect to previously published KLOE results. From the cross section we obtain the pion form factor and the contribution to the muon magnetic anomaly from two-pion states in the mass range 0.592<M_ππ<0.975 GeV. For the latter we find Δ^ππa_μ=(387.2±0.5_stat±2.4_exp±2.3_th)×10⁻¹⁰.     

Fourier spectrometry: Rovibrational analysis of an infrared Π → Σ system of yttrium monoiodide

The infrared spectrum of yttrium monoiodide has been excited in an electrodeless microwave discharge and explored between 2500 and 12 000cm⁻¹ with a high-resolution Fourier transform spectrometer. A unique system is observed (ν₀₀ = 9905.520 cm⁻¹), which we attribute to a ¹Π → ¹Σ transition and an extensive analysis is made. Rovibrational constants are obtained for both states mainly from a simultaneous multiband fitting. This procedure is applied to the whole set of 2231 observed line wavenumbers in the 1-0, 0-0, and 0–1 bands, yielding a final weighted standard deviation of 0.0038 cm⁻¹. Furthermore, a partial analysis of the 2-0 and 3-1 bands is performed. The following equilibrium constants are derived (cm⁻¹): ω′_e=192.210 ω′_ex′_e=0.463B′_e=0.0399133 α′_e=0.0001150ω″_e=215.815 ω″_ex″_e=0.514B″_e=0.0422163 α″_e=0.0001125 High-order constants D_v and H_v are also calculated for the various vibrational levels (v′ = 0, 1, 2, 3; v″ = 0, 1).     

Update on the search for μ→ e oscillations by the 2 experiment

The neutrino experiment sets the most stringent exclusion limits for _μ→ _e oscillations. Analyzing the data set recorded from Feb.1997 up to March 2000 with the upgraded experimental configuration, the search of _e appearance via the p( _e, e⁺)n reaction yields no hints for neutrino oscillations. Applying a likelihood method to the measured event sample of 11 events (background expectation 12.3 events), we deduce an upper limit of sin² (2Θ) < 1.3 · 10⁻³ for large Δm² > 100 eV² and Δm² < 0.049 eV² for sin² (2Θ)=1.     

Search for associated production of charginos and neutralinos in the trilepton final state using 2.3 fb of data

We report the results of a search for associated production of charginos and neutralinos using a data set corresponding to an integrated luminosity of 2.3 fb⁻¹ collected with the DØ experiment during Run II of the Tevatron proton–antiproton collider. Final states containing three charged leptons and missing transverse energy are probed for a signal from supersymmetry with four dedicated trilepton event selections. No evidence for a signal is observed, and we set limits on the product of production cross section and leptonic branching fraction. Within minimal supergravity, these limits translate into bounds on m₀ and m_1/2 that are well beyond existing limits.     

A reassessment of the O(αs) three-jet cross section for ee annihilation

Results of a new evaluation of the O(α_s²) dressed three-jet cross section for e⁺e⁻ annihilation are summarized. For vanishing cutoffs, the different results in published three-jet cross section formulae are due to differing definitions of resolvable four-jet events. Subleading terms which were ignored in previous calculations are evaluated and found to be sizeable in the phase space region where three-jet events become more two-jet-like. A simple calculation demonstrates that α_s values determined according to the three-jet cross section formulae of Gutbrod, Kramer and Schierholz are systematically too large.     

Observation of χb production in the exclusive reaction ′→γγχb→γγ→γγ(eeorμμ−)

We report on the observation of 1 ³P_J (χ_b) production in the reaction ′→γχ_b→γγ→γγ(e⁺e⁻ or μ⁺μ⁻). The data were recorded with the nonmagnetic CUSB detector at the Cornell Electron Storage Ring, CESR. We observe 124 γγ events with either an electron or muon pair in the final state. In the γγ correlation plot about 40% of the events cluster around (120, 430) MeV.     

Higher dimensional cosmologies

Einstein equations are derived for D-dimensional space-time that spontaneously compactify to the product M⁴ × Π_{i = 1}^α M^d_i in which the metric is taken to be of the generalized Robertson-Walker form. Cosmological solutions for these equations are studied with power law, oscillatory and exponential behaviour for the D-dimensional Einstein-Maxwell, N = 2, D = 10 and N = 1, D = 11 supergravity models. In the Einstein-Maxwell case the presence of a cosmological constant forces the extra dimensions to be static. Nevertheless, it is possible to find solutions with vanishing effective 4 dimensional cosmological constant with an expanding 4-dimensional space-time. In the supergravity models the requirement of having compact extra dimensions restricts the solutions to have expansion only in the 4-dimensional space-time. Matter contribution is added to the energy-momentum tensor in an attempt to find new solutions.     

Preliminary results and status of the MUNU experiment

We built a low background detector to measure the _ee⁻ elastic cross section at low energy. The detector has been installed close to a nuclear reactor in Bugey and it is running since almost one year. After having reduced the electron background by more than 2 orders of magnitude we are now taking data to be sensitive to a neutrino magnetic moment in the region below 10⁻¹⁰ Bohr magnetons.     

Study of radiative leptonic events with hard photons and search for excited charged leptons at 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 m_e^* > 62.5 GeV/c², m_μ^* > 62.6 GeV/c² and m_τ^* > 62.2 GeV/c² at 95% confidence level were established. For single excited lepton production, upper limits on the ratio λ/m_l^* of the coupling of the excited charged lepton to its mass were derived.     

Supersymmetry parameter analysis: SPA convention and project

High-precision analyses of supersymmetry parameters aim at reconstructing the fundamental supersymmetric theory and its breaking mechanism. A well defined theoretical framework is needed when higher-order corrections are included. We propose such a scheme, Supersymmetry Parameter Analysis SPA, based on a consistent set of conventions and input parameters. A repository for computer programs is provided which connect parameters in different schemes and relate the Lagrangian parameters to physical observables at LHC and high energy e ⁺ e^- linear collider experiments, i.e., masses, mixings, decay widths and production cross sections for supersymmetric particles. In addition, programs for calculating high-precision low energy observables, the density of cold dark matter (CDM) in the universe as well as the cross sections for CDM search experiments are included. The SPA scheme still requires extended efforts on both the theoretical and experimental side before data can be evaluated in the future at the level of the desired precision. We take here an initial step of testing the SPA scheme by applying the techniques involved to a specific supersymmetry reference point.     

LEP Higgs boson searches beyond the standard model and minimum supersymmetric standard model

Ever since the center-of-mass energy was increased in 1995 above the Zℴ resonance, the four LEP experiments (ALEPH, DELPHI, OPAL and L3) have renewed their effort to search for the Higgs boson. Data taking ended in the year 2000 with about 130 pb⁻¹ of data collected per experiment above 206 GeV ine ⁺ e ⁻¹ collisions but the data analysis is still very active. Most recently, the wealth of theoretical models and predictions has stimulated new analyses and model interpretations which go beyond the standard model and minimal supersymmetric standard model. These include the searches for charged Higgs bosons, models with two Higgs field doublets, searches for ‘fermiophobic’ Higgs decay, invisible Higgs boson decays, decay-mode independent searches, and limits on Yukawa and anomalous Higgs couplings. I review the searches done by the four LEP experiments and present the LEP combined results when they exist.     

Search for γγ→ηb in ee collisions at LEP 2

A search for the pseudoscalar meson η_b is performed in two-photon interactions at LEP 2 with an integrated luminosity of 699 pb⁻¹ collected at e⁺e⁻ centre-of-mass energies from 181 GeV to 209 GeV. One candidate event is found in the six-charged-particle final state and none in the four-charged-particle final state, in agreement with the total expected background of about one event. Upper limits of Γ_γγ(η_b)×BR(η_b→4 charged particles)<48 eV, Γ_γγ(η_b)×BR(η_b→6 charged particles)<132 eV are obtained at 95% confidence level, which correspond to upper limits of 9.0% and 25% on these branching ratios.     

Search for squark production in events with jets, hadronically decaying tau leptons and missing transverse energy at

A search for supersymmetric partners of quarks is performed in the topology of multijet events accompanied by at least one tau lepton decaying hadronically and large missing transverse energy. Approximately 1 fb⁻¹ of collision data from the Fermilab Tevatron Collider at a center of mass energy of 1.96 TeV recorded by the DØ detector is analyzed. Results are combined with the previously published DØ inclusive search for squarks and gluinos. No evidence of physics beyond the Standard Model is found and lower limits on the squark mass up to 410 GeV are derived in the framework of minimal supergravity with tanβ=15, A₀=−2m₀ and μ<0, in the region where decays to tau leptons dominate. Gaugino masses m_1/2 are excluded up to 172 GeV.