Precise determination of the Z resonance parameters at LEP: “Zedometry”

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in collisions with the OPAL detector makes use of the full LEP 1 data sample comprising of integrated luminosity and selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: GeV, GeV, nb, , and . Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, . Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, . Interpreting the data within the context of the Standard Model allows the mass of the top quark, GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: and GeV. Received: 1 December 2000 / Published online: 6 April 2001     

Are there anomalous Z fermion couplings?

The couplings of the fermions to the Z boson are of great importance in establishing the validity of the Standard Model and in looking for physics beyond it. The couplings of the b-quark to the Z boson have been the subject of much experimental study and theoretical interpretation. The apparent excess in the value of , the ratio of the partial width of the Z boson to to its total hadronic width, above the Standard Model expectation reported a few years ago has now become much less significant. However, the measurements of the pole forward-backward asymmetry for b-quarks at the Z pole and of the polarisation parameter , obtained using a polarised electron beam, have improved considerably in accuracy. The latest data are examined and values of the vector and axial-vector b-quark and c-quark couplings to the Z are extracted. The left and right handed couplings are also extracted. It is found that whereas the c-quark couplings are compatible with the Standard Model, those of the b-quark data are only compatible with the Standard Model at about the 1% level. In addition, the individual lepton couplings are extracted and the degree to which the data support the hypothesis of lepton universality is discussed. The sensitivity of the limits from electroweak fits to the Higgs boson mass to these data is examined. Received: 21 December 1998 / Revised version: 12 February 1999 / Published online: 15 April 1999     

Measurement of the Tau Polarisation at LEP

The polarisation of 's produced in Z decay is measured using 160 pb of data accumulated at LEP by the ALEPH detector between 1990 and 1995. The variation of the polarisation with polar angle yields the two parameters and which are consistent with the hypothesis of e- universality. Assuming universality, the value is obtained from which the effective weak mixing angle is derived. Received: 2 March 2001 / Published online: 8 June 2001     

Measurements of cross sections and forward-backward asymmetries at the Z resonance and determination of electroweak parameters

Abstract. We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993–95. A total luminosity of was collected at centre-of-mass energies and which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990–92, we obtain the final results: An invisible width of is derived which in the Standard Model yields for the number of light neutrino species . Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be and . Including our measurements of the forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of GeV is set at 95% confidence level. Received: 4 February 2000 / Published online: 18 May 2000     

Current algebra derivation of temperature dependence of hadron couplings with currents

The vector and the axial-vector meson couplings with the vector and the axial-vector currents respectively at finite temperature were obtained by calculating all the relevant one-loop Feynman graphs with vertices obtained from the effective chiral Lagrangian. On the other hand, the same couplings were also derived by applying the method of current algebras and the hypothesis of partial conservation of axial-vector currents. The latter method appears to miss certain terms; in the case of the vector meson coupling with the vector current, for example, a term containing the ρωπ coupling is missed. A similar situation would also appear for the nucleon coupling with the nucleon current. In this note we resolve these differences.     

Kl4 decays

An effective theory of large-N_C QCD of pseudoscalar, vector, and axial-vector mesons has been used to study six K_l4 decay modes. It has been found that the matrix elements of the axial-vector current dominate the K_l4 decays. PCAC is satisfied. A relationship between three form factors of the axial-vector current has been predicted. Partial-wave analysis has been done. Non-zero phase shifts are originated in ρ→ππ. The decay rates are calculated in the chiral limit. In this study there is no adjustable parameter. Received: 17 October 2000 / Accepted: 20 February 2001     

Neutral current effects in the parity violating nuclear force

The measurements of the ΔI = 1 part of the parity violating nuclear force when combined with information on neutral current couplings from neutrino scattering and pion production experiments allow an estimate of the isoscalar admixture in the neutral current if it has a vector axial-vector structure has been shown.     

Cross-sections and leptonic forward-backward asymmetries from the Z^0 running of LEP

During 1993 and 1995 LEP was run at 3 energies near the Z peak in order to give improved measurements of the mass and width of the resonance. During 1994, LEP operated only at the Z peak. In total DELPHI accumulated data corresponding to an integrated luminosity of approximately 116 pb. Analyses of the hadronic cross-sections and of the cross-sections and forward-backward asymmetries in the leptonic channels used the most precise evaluations of the LEP energies. In the dimuon channel, events with a photon radiated from the initial state have been used to probe the cross-sections and asymmetries down to PETRA energies. Model independent fits to all DELPHI lineshape and asymmetry data from 1990 to 1995 have been carried out giving values of the resonance parameters: These values are significantly more precise than those previously published. The results are interpreted in terms of the Standard Model. Received: 4 February 2000 / Revised version: 7 March 2000 / Published online: 8 June 2000     

A determination of the leptonic neutral current couplings

An analysis is presented of the recent data which are sensitive to thee, μ and τ neutral current couplings. A fit combining all results (e ⁺ e ^?, μC,ve, eD, atoms) selects a unique solution in agreement with the standard-model expectation. Assuming lepton universality, the vector and axial-vector couplings are determined to bev=?0.013±0.048 anda=?0.520±0.014. Similarly we find (sin² θ=0.213±0.012,ρ=0.015±0.038) or (sin² θ=0.211α0.012, ρ≡1 which, combined with all other values, gives an average of sin² θ=0.216±0.006.     

Vectorial versus axial Goldstone bosons

The Yukawa interactions of fermions with Goldstone bosons are given in closed form for an arbitrary renormalizable field theory to all orders of perturbation theory or for a general effective Lagrangian. Although the diagonal couplings are always pseudoscalar there is an important difference between spontaneously broken vector and axial-vector global symmetries. Compared to the axial case, the diagonal couplings of “vectorial” Goldstone bosons to charged fermions are suppressed by mixing angles or appear only via radiative corrections involving gauge fields. This general result may be relevant for the problem of flavour symmetry breaking in composite models.     

A study of the Lorentz structure in tau decays

This paper describes a measurement of the Michel parameters, , , , , and the average helicity, , in lepton decays together with the first measurement of the tensor coupling in the weak charged current. The pairs were produced at the LEP collider at CERN from 1992 through 1995 in the DELPHI detector. Assuming lepton universality in the decays of the the measured values of the parameters were: , , , , . The strength of the tensor coupling was measured to be . The first error is statistical and the second error is systematic in all cases. The results are consistent with the structure of the weak charged current in decays of the lepton. Received: 14 February 2000 / Revised version: 7 March / Published online: 8 May 2000     

Ward-Takahashi Identities and Mass Spectra in (2+1) Dimensional SU(2) Four-fermi Model

Ward-Takahashi identities with composite fields are utilized to inverstigate (2+1) dimensional model with SU(2) four-fermion couplings.When SU(2) chiral symmetry is both explicitly and dynamically broken,fermion mass is dynamically generated and mass spectra of the bound states are obtained The properties of vector and axial-vector currents are discussed.It turns out that the bound state π_α acquires a mass and the axial-vector current is partially conserved,and the Goldberger-Treimab relation is approximately valid in the case of small fermion current masses.     

\mathrm{W^+W^-} production and triple gauge boson couplings at LEP energies up to 183 GeV

A study of W-pair production in annihilations at LEP is presented, based on candidates corresponding to an integrated luminosity of 57 pb at GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be %. The number of W-pair candidates and the angular distributions for each final state (, are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain , and , where the errors include both statistical and systematic uncertainties and each coupling is determined by setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be . All these measurements are consistent with the Standard Model expectations. Received: 23 October 1998 / Published online: 1 March 1999     

Leptoquark signal from global analysis

Data from HERA, LEP and the Tevatron as well as from low energy experiments are used to constrain the Yukawa couplings for scalar and vector leptoquarks in the Buchmüller–Rückl–Wyler effective model. In the limit of very high leptoquark masses constraints on the coupling to the mass ratio are derived using the contact-interaction approximation. For finite masses the coupling limits are studied as a function of the leptoquark mass. Some leptoquark models are found to describe the existing experimental data much better than the standard model. The increase in the global probability observed for models including or leptoquark production/exchange corresponds to an effect of more than . Assuming that a real leptoquark signal is observed, the allowed region in the –M plane is calculated. The leptoquark signal mostly results from the new data on the atomic parity violation in cesium, but is also supported by recent LEP2 measurements, unitarity violation in the CKM matrix and HERA high- results. Received: 21 June 2000 / Revised version: 31 August 2000 / Published online: 23 October 2000     

Low-energy limit of chiral meson theory

Based on a phenomenologically successful effective chiral theory of pseudoscalar, vector, and axial-vector mesons, all the coefficients of the chiral perturbation theory are predicted. There is no new adjustable parameter in these predictions. Up to O(m ² _q) the formulas of the masses of the pseudoscalar mesons are the same as the ones obtained by ChPT. Received: 31 August 2000 / Accepted: 16 March 2001     

Properties of vector and axial-vector mesons from a generalized Nambu-Jona-Lasinio model

We construct a generalized Nambu-Jona-Lasinio lagrangian including scalar, pseudoscalar, vector and axial-vector mesons. We specialize to the two-flavor case. The properties of the structured vacuum as well as meson masses and coupling constants are calculated giving an overall agreement within 20% of the experimental data. We investigate the meson properties at finite density. In contrast to the mass of the scalar σ-meson, which decreases sharply with increasing density, the vector meson masses are almost independent of density. Furthermore, the vector-meson-quark coupling constants are also stable against density changes. We point out that these results imply a softening of the nuclear equation of state at high densities. Furthermore, we discuss the breakdown of the KFSR relation on the quark level as well as other deviations from phenomenological concepts such as universality and vector meson dominance.     

Study of \tau decays involving kaons, spectral functions and determination of the strange quark mass

All ALEPH measurements of branching ratios of decays involving kaons are summarized including a combination of results obtained with and detection. The decay dynamics are studied, leading to the determination of contributions from vector and , and axial-vector and resonances. Agreement with isospin symmetry is observed among the different final states. Under the hypothesis of the conserved vector current, the spectral function for the mode is compared with the corresponding cross section for low energy annihilation, yielding an axial-vector fraction of for this mode. The branching ratio for decay into all strange final states is determined to be . The measured mass spectra of the strange decay modes are exploited to derive the spectral function. A combination of strange and nonstrange spectral functions is used to determine the strange quark mass and nonperturbative contributions to the strange hadronic width. A method is developed to avoid the bad convergence of the spin zero hadronic component, with the result MeV/. The evolution down to 1 GeV gives . Received: 24 February 1999 / Published online: 14 October 1999     

Triple gauge couplings in polarised e^-e^+ \rightarrow W^-W^+ and their measurement using optimal observables

The sensitivity of optimal integrated observables to electroweak triple gauge couplings is investigated for the process fermions at future linear colliders. By a suitable reparameterisation of the couplings we achieve that all 28 coupling parameters have uncorrelated statistical errors and are naturally normalised for this process. Discrete symmetry properties simplify the analysis and allow checks on the stability of numerical results. We investigate the sensitivity to the couplings of the normalised event distribution and the additional constraints that can be obtained from the total rate. Particular emphasis is put on the gain in sensitivity one can achieve with longitudinal beam polarisation. We also point out questions that may best be settled with transversely polarised beams. In particular we find that with purely longitudinal polarisation one linear combination of coupling parameters is hardly measurable by means of the normalised event distribution. Received: 25 September 2002 / Published online: 19 February 2003     

Study of dimuon production in photon-photon collisions and measurement of QED photon structure functions at LEP

Muon pair production in the process is studied using the data taken at LEP1 () with the DELPHI detector during the years 1992-1995. The corresponding integrated luminosity is 138.5 pb. The QED predictions have been tested over the whole range accessible at LEP1 (from several GeV to several hundred GeV) by comparing experimental distributions with distributions resulting from Monte Carlo simulations using various generators. Selected events are used to extract the leptonic photon structure function . Azimuthal correlations are used to obtain information on additional structure functions, and , which originate from interference terms of the scattering amplitudes. The measured ratios and are significantly different from zero and consistent with QED predictions. Received: 23 February 2000 / Revised version: 7 December 2000 / Published online: 23 February 2001     

In-medium chiral perturbation theory

This talk will report about a systematical implementation of a chiral effective field theory in nuclear matter with explicit pion fields and in the presence of external sources[1]. Within the generating functional approach of Ref.[2] the so-called standard power counting rules for the calculation of in-medium pion properties are developed that apply if the residual nucleon energies are of the order of the pion mass. In addition, for the case of vanishing residual nucleon energies, a modified scheme (non-standard counting) is introduced. For both schemes the pertinent scales where the chiral expansions have to break down are established as well. We report about a systematic analysis of n-point in-medium Green functions up to and including next-to-leading order when the standard rules apply. These include the in-medium contributions to quark condensates, pion propagators, pion masses and couplings of the axial-vector, vector and pseudoscalar currents to pions.