The physics of the τ lepton

A comprehensive review of the status of τ-lepton physics is presented. We include the knowledge on the properties of the tau, the decay branching fractions and tests of the standard model. Discussions of possible puzzles and an indication of the future possibilities in this field are a lso presented.     

The τ lepton as a laboratory for quantum chromodynamics

Decays of the τ lepton provide a clean environment to study hadron dynamics in an energy regime dominated by resonances. Inclusive spectral functions are the basis for quantum chromodynamics (QCD) analyses, providing a most accurate determination of the strong coupling constant and quantitative information on nonperturbative contributions. The τ vector spectral function is used together with e⁺e⁻ data in order to compute vacuum polarization integrals arising in the calculations of the anomalous magnetic moment of the muon and the running of the electromagnetic coupling constant. To cite this article: M. Davier, A. Höcker, C. R. Physique 3 (2002) 1223–1233.     

Measurement of the leptonic branching ratios of the τ lepton

The leptonic branching ratios of the tau lepton have been determined from data collected by the OPAL detector in 1991 and 1992. From a sample of 27196 e⁺e^–+– candidates we find 7322 and 7941 candidates. Using efficiency and background estimates determined from a study of Monte Carlo events and control samples of data, the branching ratios =(18.14±0.20±0.28)% and =(17.48±0.18±0.23)% have been obtained. These new results have been combined with the published results for the 1990 OPAL data to yield the following branching ratios for data taken between 1990 and 1992:

Kinematics of τ two-body decay near τ threshold at BESⅢ

The kinematic properties of two-body decay near τ threshold are studied according to the special capacity of the BEPC accelerator and the BESⅢ detector.Explicitly presented are the transformations of energy and momentum of hadronic particles between different reference frames,and the corresponding distributions.A brand new method is proposed to obtain the energy spread of the accelerator by fitting the energy distribution of hadron from τ semi-leptonic decays.     

Kinematics of τ two-body decay near τ threshold at BESⅢ

The kinematic properties of two-body decay near τ threshold are studied according to the special capacity of the BEPC accelerator and the BESⅢ detector.Explicitly presented are the transformations of energy and momentum of hadronic particles between different reference frames,and the corresponding distributions.A brand new method is proposed to obtain the energy spread of the accelerator by fitting the energy distribution of hadron from τ semi-leptonic decays.     

T-odd momentum correlation in radiative β decay

The triple-product correlations observable in ordinary neutron or nuclear beta decay are all naively T violating and can connect, through an assumption of CPT invariance, to constraints on sources of CP violation beyond the Standard Model. They are also spin dependent. In this context the study of radiative beta decay opens a new possibility, in that a triple-product correlation can be constructed from momenta alone. Consequently its measurement would constrain new spin-independent sources of CP violation. We will describe these in light of the size of the triple momentum correlation in the decay rate arising from electromagnetic final-state interactions in the Standard Model. Our expression for the corresponding T-odd asymmetry is exact in ${\cal O}(\alpha)$ up to terms of recoil order, and we evaluate it numerically under various kinematic conditions. We consider the pattern of the asymmetries in nuclear β decays and show that the asymmetry can be suppressed in particular cases, facilitating searches for new sources of CP violation in such processes.     

Measurement of exclusive one-prong and inclusive three-prong branching ratios of the τ lepton

Using the ARGUS detector at thee ⁺ e ^– storage ring DORIS II at DESY, a study ofB ⁰ oscillations has been performed using three different techniques. Besides the standard dilepton method, charge correlations betweenD ^* mesons and one or two leptons have also been investigated. The mixing parameterr is determined to be (20.6±7.0)%.Supported by the German Bundesministerium für Forschung und Technologie, under contract number 054DO51P     

Exclusive decay of J/Ψ into a lepton pair combined with light hadrons

We study the exclusive decay of J/Ψ into a lepton pair combined with light hadrons in the kinematic region, specified by that the total energy of the light hadrons is much smaller than m_c, the mass of the c-quark. In this region, the nonperturbative effect related to J/Ψ and that related to the light hadrons can be separated, the former is represented by a NRQCD matrix element, while the later is represented by a matrix element of a correlator of electric chromofields. The results are obtained in a axial gauge by assuming that contributions from two-gluon emission are dominant. But we can show that these results can be obtained without the assumption in arbitrary gauges. A discussion of the results are presented.     

Status of the lattice and τ decay determinations of α_s

The two highest precision determinations of α s (M 2 Z ),that based on the analysis of short-distancesensitive lattice observables,and that based on an analysis of hadronic τ decay data,have,until very recently,given results which are not in good agreement.I review new versions of these analyses which bring the two determinations into excellent agreement,and discuss prospects for additional future improvements.     

A study of ultraviolet renormalon ambiguities in the determination of α s from τ decay

The divergent large-order behaviour of the perturbative series relevant for the determination of _s from decay is controlled by the leading ultraviolet (UV) renormalon. Even in the absence of the first infrared (IR) renormalon, an ambiguity of order ²/m ² is introduced. We make a quantitative study of the practical implications of this ambiguity. We discuss the magnitude of UV renormalon corrections obtained in the large-N _f limit, which, although unrealistic, is nevertheless interesting to some extent. We then study a number of improved approximants for the perturbative series, based on a change of variable in the Borel representation, such as to displace the leading UV renormalon singularity at a larger distance from the origin than the first IR renormalon. The spread of the resulting values of _s (m ²) obtained by different approximants, at different renormalization scales, is exhibited as a measure of the underlying ambiguities. Finally, on the basis of mathematical models, we discuss the prospects of an actual improvement, given the signs and magnitudes of the computed coefficients, the size of _s (m ²) and what is known of the asymptotic properties of the series. Our conclusion is that a realistic estimate of the theoretical error cannot go below _s (m ²)±0.060, or _s (m _Z ²)±0.006.On leave of absence from INFN, Sezione di Milano, Milan, Italy     

New Measurement of the π0 radiative decay width

High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst) eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.     

Status of the lattice and τ decay determinations of αs

The two highest precision determinations of α s （M 2 Z ）,that based on the analysis of short-distancesensitive lattice observables,and that based on an analysis of hadronic τ decay data,have,until very recently,given results which are not in good agreement.I review new versions of these analyses which bring the two determinations into excellent agreement,and discuss prospects for additional future improvements.     

Observation of D(1285)→φγ radiative decay

The D(1285) radiative decay has been observed, 19±5 events of this type have been detected. The branching ratio Br[D(1285)→φγ] = (0.9±0.2±0.4) × 10⁻³ and the partial width γ [D(1285)→φγ] = 23 ±5±10) keV have been determined. The conclusion has been made that, most probably, the E(1420) meson does not belong to the axial nonet.     

Azimuthal distributions of final-state particles and fragments and transverse structure of emission source in high-energy nucleus-nucleus collisions

The azimuthal distributions of final-state particles and fragments produced in high-energy nucleus--nucleus collisions are described by a modified multisource ideal gas model which contains the expansions and movements of the emission sources. The transverse structures of the sources are given in the transverse plane by momentum components p_x and p_y,and described by parameters in the model. The results of the azimuthal distributions, calculated by the Monte Carlo method, are in good agreement with the experimental data in nucleus--nucleus collisions at high energies.     

Investigation of the antineutrino angular distribution in experiments on the β decay of polarized neutrons

Since the emission of γ grays unavoidably accompanies β decay, the final state after the β decay of a neutron includes a photon along with a proton, an electron, and an antineutrino, i.e., four particles, rather than three. Therefore, when only the electron and proton momenta are detected and the γ-ray momentum is not detected in an experiment, the antineutrino momentum cannot be uniquely reconstructed, and only its mean value over a γ-ray momentum distribution determined from corresponding calculations can be considered. The γ grays are significant for finding the asymmetry parameter B of the antineutrino angular distribution from experiments on the β decay of polarized neutrons, where the electron momentum p directed along the x axis and the projection of the proton momentum P _x onto the x axis are detected, and the neutron polarization vector ξ is parallel or antiparallel to x. Since the γ rays are not detected in such experiments, the antineutrino kinematics are not uniquely specified by the observables p and P _x and can be reconstructed only on the average, so that the antineutrino momentum distribution averaged over a γ-ray momentum distribution is considered. Thus, the exact value of B cannot be obtained from these experiments, but the true value of B can be estimated on the average by considering the mean (most likely) value 〈B〉 and the dispersion (rms deviation) ΔB. The unavoidable uncertainty in the estimate of B amounts to several percent and is thus significant for present-day experiments, which are intended to obtain the value of B to a very high accuracy of ∼ (0.1–1)%. If electromagnetic interactions are taken into account, measurements of the electron and proton momentum distributions can also be used to obtain g _A, i.e., the axial β-decay amplitude, to high accuracy. Zh. éksp. Teor. Fiz. 116, 1505–1522 (November 1999)     

Search for hadronic decays of a light Higgs boson in the radiative decay Υ→γA0

We search for hadronic decays of a light Higgs boson (A(0)) produced in radiative decays of an Υ(2S) or Υ(3S) meson, Υ→γA(0). The data have been recorded by the BABAR experiment at the Υ(3S) and Υ(2S) center-of-mass energies and include (121.3±1.2)×10(6) Υ(3S) and (98.3±0.9)×10(6) Υ(2S) mesons. No significant signal is observed. We set 90% confidence level upper limits on the product branching fractions B(Υ(nS)→γA(0))B(A(0)→hadrons) (n=2 or 3) that range from 1×10(-6) for an A(0) mass of 0.3 GeV/c(2) to 8×10(-5) at 7 GeV/c(2).     

The radiative decay ϕ → γππ in a coupled-channel model and the structure of f0(980)

A coupled-channel model is used to study the nature of the scalar mesons produced in the decay ϕ → γππ. The K molecular picture of f₀(980) is found to be in a good agreement with the recent experimental data from SND and CMD-2. The structure of the light scalar mesons is elucidated by investigating the S-matrix poles and the q spectral density.     

125 GeV Higgs decay with lepton flavor violation in the μνSSM

Recently,the CMS and ATLAS collaborations have reported direct searches for the 125 GeV Higgs decay with lepton flavor violation,h→μτ.In this work,we analyze the signal of the lepton flavour violating(LFV) Higgs decay h→μτ in the μ.from γ Supersyninietric Standard Model(μνSSM) with slepton flavor mixing.Simultaneously,we consider the constraints from the LFV decay τ→μγ,the muon anomalous magnetic dipole moment and the lightest Higgs mass around 125 GeV.