Measurement of inclusive π0 production in hadronic Z 0 decays

An analysis is presented of inclusive π⁰ production in Z ⁰ decays measured with the DELPHI detector. At low energies, π⁰ decays are reconstructed by using pairs of converted photons and combinations of converted photons and photons reconstructed in the barrel electromagnetic calorimeter (HPC). At high energies (up to $x_p={2cdot p?/{sqrt s}=0.75}$) the excellent granularity of the HPC is exploited to search for two-photon substructures in single showers. The inclusive differential cross section is measured as a function of energy for qq? and bb? events. The number of π⁰’s per hadronic Z ⁰ event is $N(?^0)/Z_{had} ^0=9.2pm 0.2({? stat})pm 1.0 ({? syst})$ and for bb? events the number of π⁰’s is ${? N}(?^0)/{? b?r b}=10.1pm 0.4({? stat})pm 1.1 ({? syst})$. The ratio of the number of π⁰’s in bb? events to hadronic Z ⁰ events is less affected by the systematic errors and is found to be 1.09 ±0.05 ±0.01. The measured π⁰ cross sections are compared with the predictions of different parton shower models. For hadronic events, the peak position in the $xi_{? p}={? ln}(1/{? x_p})$ distribution is $xi_p^{?ar}=3.90_{-0.14}^{+0.24}.$ The average number of π⁰’s from the decay of primary B hadrons is found to be N(B → π⁰ X)/B hadron = 2.78 ± 0.15(stat) ± 0.60(syst).     

Tau leptonic branching ratios ALEPH Collaboration

A sample of 62249 τ-pair events is selected from data taken with the ALEPH detector in 1991, 1992 and 1993. The measurement of the branching fractions for τ decays into electrons and muons is presented with emphasis on the study of systematic effects from selection, particle identification and decay classification. The results obtained are: B _e = 17.79±0.12(stat)±0.06(syst)(%) and B_μ = 17.31±0.11(stat)±0.05(syst)(%). Combined with the most recent ALEPH determination of the τ lifetime, these results provide a relative measurement of the leptonic couplings in the weak charged current for transverse W bosons: ${cal G}_{?}/{cal G}_{e}=1.0002pm 0.0051$ and ${cal G}_{?u}/{cal G}_{u}=0.9943pm 0.0065$.     

Radiative pion decay: Determination of FA(0) from τ-lepton decay data

The form factor F_A(0) in π→eνγ is calculated, at the soft pion point, directly from data on the semileptonic τ-lepton decays τ→ν_τ + nπ, which determine the relevant vector and axial-vector hadronic spectral functions. The reliability of this calculation is assessed by using these fitted spectral functions to check the saturation of the first two Weinberg sum rules. The results are: F_A(0)=0.017±0.004, and γ(0)F_A(0)/F_V(0)=0.67±0.04±0.16.     

Study of the CP-violating effects with <Emphasis Type="Italic">gg</Emphasis> → <Emphasis Type="Italic">Η</Emphasis> → τ<Superscript>+</Superscript>τ<Superscript>–</Superscript> process

Study of the gg → Η → τ⁺τ^– process was performed at Monte Carlo level within the framework of searching for CP-violating effects. The sensitivity of chosen observables to CP-parity of the Higgs boson was demonstrated for hadronic 1-prong τ decays (τ^± → π^±, ρ^±). Monte Carlo samples for the gg → Η → τ⁺τ^- process were generated including the parton hadronisation to final state particles. This generation was performed for the Standard Model Higgs boson, the pseudoscalar Higgs boson, the Z → τ⁺τ^– background, and mixed CP-states of the Higgs boson.     

Tau decays into four pions

We compute branching ratios and invariant mass distributions of the tau decays into four pions. The hadronic matrix elements are obtained by starting from the structure of the hadronic current in the chiral limit and then implementing low-lying resonances in the different channels. Reasonable agreement with experiment is obtained both for the τ → v_τ+(4π) decay rates and the e⁺ e^? → (4π) cross sections. Furthermore we supply an interface to use our matrix elements within the Tauola Monte-Carlo program.     

An effective chiral Lagrangian model for the τ-decay into three charged pions

An effective chiral Lagrangian for the π-ρ-A ₁-system is used to study the decayτ ^±→ν _τπ⁺π^?π^±. Demanding that the hadronic current 〈0|J _μ|3π〉 agrees at the low energy limit with the one obtained from the corresponding pseudoscalar meson Lagrangian ?(π), an ambiguity linked to free parameters in ?(π,ρ,A ₁) is eliminated. The decay spectrum and branching ratio of τ^±→ν _τπ⁺π^?π^± are calculated as functions of theA ₁-mass and compared with the experimental results. So conclusions can be drawn concerning theA ₁-parameters, which are found to be $$m_{A_1 } = (1180 \pm 50)MeV;\Gamma _{A_1 } = (450 \pm 100)MeV.$$ . The mass-value is smaller than recently published ones. This is attributed to the effect of suppressed decay channels not seen in the experiments up to now. A contribution from direct, non-(ρ ⁰π^±) resonanceA ₁-decay is obtained, being about 15%.     

Status of the Chorus experiment

The CHORUS experiment at CERN searches for ν_μ ? ν_τ oscillations by looking for τ decays from charged-current ν_τ interactions. The emulsion target of the detector, having a resolution of about a micron, enables the detection of the decay topology of the τ. After having analyzed a sample of 126000 events containing an identified muon and 7500 purely hadronic events, no ν_τ candidate has been found. This result translates in a limit on the mixing angle sin²2θ_μτ < 8 × 10^?4 at 90% C.L. for large Δm _μτ ² .     

Production of stop,sbottom, and stau at LEP2

We present a comprehensive study of pair production and decay of stops, sbottoms, and staus in e⁺e^? annihilation at LEP2. We give numerical predictions within the Minimal Supersymmetric Standard Model for cross sections and decay rates, and discuss the important signatures. In the case of stau production we also study the polarization of the τ in the decays ${\tilde \tau}_{1}\rightarrow \tau \ {\tilde \chi}_{1,2}^{0}$ .     

A study of the generalized Schwinger model in the Hamiltonian formulation

Semileptonic τ decays to two and three pions are considered. Based on recent data for the pion form factor a new prediction for Γ(τ→vπ^{- 0})/Γ(τ→vev_e) of 1.32±0.05 is derived. The chiral model-supplemented by vector dominance-is used to predict rate and differential distributions for the three pion mode in good agreement with present data. The parity violating asymmetry which has been predicted theoretically and observed experimentally is studied and found to be fairly insensitive towards the details of the model for the hadronic decays.     

Tau neutrino mass from semileptonic decays

The semileptonic three particle decays of the tau provide determinations of the tau neutrino mass. The shift of the maximal energy of the observable final state particles islinear in the neutrino mass. The endpoint energy of the pion and a partially integrated decay rate in τ → πωv _τ and τ → πρV _τ are sensitive to a neutrino mass smaller than 100 MeV. Thus, the present bound on \(m_{v_\tau } \) can significantly be improved.     

Hadronic decay of τ

We present predictions for invariant mass distributions of the pions in τ→ν+2π and τ→ν+4π as well as improved predictions for hadronic decay rates of τ. We discuss a number of predictions which, if contradicted by τ decays, will imply the existence of a new weak interaction.     

Hadronic quantum numbers beyond charm as a possible source of v induced trimuons

Characteristics of trimuons emerging from cascade decay of two new hadronic quantum numbers are compared with experiments. The possibility that the lower (spinless) member decays preferentially into Perl's sequential heavy lepton τ via τ + ν_τ is incorporated.     

Isospin violation in τ → 3πντ

Isospin violating signals in the τ^? → (3π)^?ν_τ decay mode are discussed. For the τ^? → π^?π^?π⁺ν_τ decay mode, isospin violation arises from the vector current contribution in the τ^? → ωπ^?ν_τ decay with the subsequent isospin violating ω decay into π⁺π^?. We demonstrate that such effects may be observed in presently available data through the measurement of the interference effects of these vector current contributions with the dominating axial vector current,i. e. through a measurement of the structure functionsW _F, WG, WH andW _I. In the case of the τ^? → π⁰π⁰π^?ν_τ decay mode, a vector current contribution is generated by ηπ⁰ mixing in the decay chain τ^? → η?^?ν_τ → π⁰π⁰π^?ν_τ. We find that this effect is rather small, the magnitude of the associated interference terms being too low for present statistics.     

Inclusive production of neutral pions in hadronic Z decays

A measurement of the inclusive production of π⁰ mesons in hadronic Z decays is presented and compared to Monte Carlo model predictions. The analysis is based on approximately 2 million hadronic events recorded with the ALEPH detector at LEP at a centre-of-mass energy of ?s = 91.2 GeV. Neutral pions are reconstructed using photons measured in the electromagnetic calorimeter and photons from conversion pairs. The inclusive π⁰ momentum spectrum is measured in the range 0.025 < x _p = p/p _beam < 1. In this range the number of π⁰ per hadronic Z is found to be 4.80 ± 0.07(stat) ± 0.31(sys). The differential inclusive π⁰ cross section is also measured as a function of transverse momentum with respect to the event plane (p _T ⁱⁿ and p _T ^out ).     

Electroweak tests with τ leptons at LEP

We report on two sensitive tests of lepton universality carried out by the 4 LEP experiments at the Z⁰ pole. From measurements of the τ polarization in e⁺ e^?→τ ⁺ τ ^?, the ratios of the vector and axial vector coupling constants of the electron and the tau lepton to the weak neutral current are obtained to beg _{v e} /g _{a e} =0.066±0.015 andg _{V τ} /g _{A τ} =0.070±0.009 respectively. From measurement of the τ lifetime and the τ leptonic branching ratios, the ratio of the coupling constants describing weak leptonic decays of the τ and the μ is measured to beG _τ /G _μ =0.996±0.008.     

Measurement of inclusive K*0(892), ϕ(1020) and ${⤪ K}_{2}^{⇒t 0}(1430)$ production in hadronic Z decays DELPHI Collaboration

The inclusive production of the neutral vector mesons K*⁰(892) and ?(1020), and of the tensor meson ${? K}_{2}^{?t 0}(1430)$, in hadronic decays of the Z has been measured by the DELPHI detector at LEP. The average production rates per hadronic Z decay have been determined to be 0.77 ± 0.08 K*⁰(892), 0.104 ± 0.008 ?(1020) and ${? K}_{2}^{?t 0}(1430)$. The ratio of the tensor-to-vector meson production yields, $«ngle {? K}_{2}^{?t 0}(1430)»ngle$, is smaller than the 〈f₂(1270)〉/〈ρ⁰(770)〉 and $«ngle f_{2}^{?ime}(1525)»ngle$ ratios measured by DELPHI. The production rates and differential cross sections are compared with the predictions of JETSET 7.4 tuned to the DELPHI data and of HERWIG 5.8. The K*⁰(892) and ?(1020) data are compatible with model predictions, but a large disagreement is observed for the ${? K}_{2}^{?t 0}(1430)$.     

“Anomalous” eta production in tau decay

The decay modes of the τ lepton involving η particles in the final state are analyzed. Some of these channels are related to the non-abelian chiral anomaly of QCD. It is found that the τ⁻→ν_τπ⁻ηπ⁰ mode dominates the inclusive production of η mesons in τ decay.     

Decays of intermediate vector bosons,radiative corrections and QCD jets

We investigate decay properties of the intermediate vector bosons W^± and Z⁰. QED and QCD radiative corrections to leptonic and hadronic decay modes are calculated. Implications of the results for decay widths, branching ratios, determination of the number of neutrino species, e-μ-τ universality and properties of hadronic jets produced in W^± and Z⁰ decays are examined.     

Mass corrections to the tau decay rate

In this note radiative corrections to the total hadronic decay rate of the τ-lepton are studied employing perturbative QCD and the operator product expansion. We calculate quadratic quark mass corrections to the decay rate rationR _τ to the order0(α _s ² m²) and find that they contribute appreciably to the Cabbibo supressed decay modes of the τ-lepton. We also discuss corrections of mass dimensionD=4, where we emphasize the need of a suitable choice of the renormalization scale of the quark and gluon condensates.     

Azimuthal distributions in radiative decay of a polarized τ lepton

We investigate various distributions over emitted photon angles, especially over the azimuthal angle, in the one-meson radiative decay of the polarized τ lepton, τ^– → π^–γν_τ. In connection with this, the photon phase space is discussed in more detail because it is nontrivial in the case of a polarized τ lepton. The decay matrix element contains both the inner bremsstrahlung and the resonance (structural) contributions. The azimuthal dependence of some observables are calculated. They are the asymmetry of the differential decay width caused by the τ-lepton polarization, the Stokes parameters of the emitted photon itself, and the correlation parameters describing the influence of τ-lepton polarization on the photon Stokes parameters. A numerical estimation is done in the τ-lepton rest frame for an arbitrary direction of the τ-lepton polarization 3-vector. The vector and axial-vector form factors describing the structure-dependent part of the decay amplitude are determined using the chiral effective theory with resonances (RχT). It is found that the features of the azimuthal distributions allows separating various terms in the spin-dependent contribution. The so-called up–down and right–left asymmetries are also calculated.