Lepton charge and model of weak interactions with μ-e transitions

The gauge model of weak interactions is proposed and μ^±?e^? transitions predicted by the model are discussed. Estimates are given for μ^±→e^? transitions on hadrons like K⁺→π^?μ⁺e⁺ with Γ (K⁺→π^?μ⁺e⁺) ? 10^?11Γ(K⁺ and μ^? capture on nuclei with W(μ^?→e⁺) ? 10^?12W(μ^? → ν). Single μ^± production in e^? beams with a cross section δ ～ 10^?42?10^?41 cm² is predicted.     

Can one test technicolour?

We estimate the couplings to ordinary particles of the lightest bound states in technicolour theories and discuss the resulting phenomenology. We compute their couplings to light gauge bosons through axial anomalies and also estimate their non-anomalous couplings at low energies. We estimate their couplings to fermions under the general simplifying assumption that each fermion acquires its mass from a unique technifermion condensate (“monophagy”), in which case they are naturally flavour conserving and relatively well-defined. We find that the classic Higgs search experiments (ttoponium → H⁰ + γ, e⁺e^? → H⁺H^?, e⁺e^? → Z⁰ + H⁰) enable one to make a decisive discrimination between elementary and composite models of spontaneous symmetry breaking. We also emphasize the interest of improving experimental limits on K_L⁰ → μe in the context of dynamical symmetry breaking models.     

Experimental limits on the strength of weak neutral currents in lepton pair production at PETRA energies

The processes e⁺e^? → e⁺e^? and μ⁺μ^? have been studied at PETRA using the JADE detector. The data, which were collected at s-values of up to 1300 GeV² have been analysed in terms of an electro-weak extension of QED to obtain values for the weak vector and axial vector couplings in the lepton sector. The values obtained agree with the predictions of the standard Salam-Weinberg model and the data are further analysed in terms of this model to obtain the limits 0.10 < sin²?_w < 0.40 (68% CL). The mass of the neutral weak gauge boson is deduced to be greater than 51 GeV/c².     

Hard bremsstrahlung corrections for the process e+e− → μ+μ−

An analytical approach is developed to the calculation of the hard bremsstrahlung corrections for the process e⁺e^? → μ⁺μ^? in the context of the standard SU(2) × U(1) model for unified weak and e.m. interactions. Numerical results are shown for different values of the c.m. energy and scattering angle. They are strongly dependent on the experimental setup and indicate that hard effects become extremely important in the region of the neutral vector boson peak. Total corrections to the process are also analyzed. Finally hard photon corrections are reexamined for pure QED giving agreement with previous numerical calculations.     

Neutral heavy lepton candidate in e+e− interactions

An event of the form e⁺e^? → μ⁺μ^? + (2 jets) recently observed at √s = 43.45 GeV is interpreted as production of a pair of neutral heavy leptons N, each with mass 20.5 ± 1.0 Gev/c². Two possibilities are explored: (i) the lepton is a wak isodoublet neutrino, produced in pairs by virtual Z⁰ decay. In this case, one expects $\text{B(}\text{Z}{}^0\text{→}\text{N}\text{N}\text{) ≈ 5\%}$; (ii) the lepton is a “right-handed neutrino”, produced in pairs via a new vector boson Z_χ. In this case, in one model, the Z_χ must lie between about 50 and 67 GeV/c². More generally, it must be very weakly coupled to ordinary quarks and leptons in order not to conflict with low-q² neutral-current data. Suggestions are made for further observation of N$\text{N}$ pairs and other effects of Z_χ in forthcoming e⁺e^? and p$\text{p}$ collisions.     

Quarks and leptons

We review the physics of quarks and leptons within the framework of gauge theories for the weak and electromagnetic interactions. The Weinberg-Salam SU(2) × U(1) theory is used as a “reference point” but models based on larger gauge groups, especially SU(2)_L × SU(2)_R × U(1), are discussed. We distinguish among thre “generations” of fundamental fermions: The first generation (e^?, ν_e, u, d), the second generation (μ^?, ν_μ, c, s) and the third generation (τ^?, ν_τ, t, b). For each generation we discuss the classification of all fermions, the charged and neutral weak currents, possible right-handed currents, parity and CP-violation, fermion masses and Cabibbo-like angles and related problems. We review theoretical ideas as well as experimental evidence, emphasizing open theoretical problems and possible experimental tests. The possibility of unifying the weak, electromagnetic and strong interactions in a grand unification scheme is reviewed. The problems and their possible solutions are presented, generation by generation, but a brief subject-index (following the table of contents) enables the interested reader to follow any specific topic throughout the three generations.     

Z boson effects in electron positron scattering at epic energies at 14 GeV

We discuss the problem of testing the intermediate vector boson effects in the reactions e^?e⁺ → e^?e⁺ + hadrons, e^?e⁺ → e^?e⁺ + μ^?μ⁺, e^?p → e^?x with the next generation of accelerators, and conclude that these reactions must be considered. The differential cross sections for the one lepton at large four-momentum transfer are obtained, and a significant contribution of Z effects is found.     

Muon-electron universality at high energies and hadronic corrections to e+e− → μ+μ−

It is argued that unitarity and muon-electron universality are sufficient to determine the contribution of hadrons to the imaginary part of the forward amplitude for the process e⁺e^? → μ⁺μ^?. With these assumptions Im T₁^h(e⁺e^? → μ⁺μ^?) = sσ(e⁺e^? → hadrons) to all orders in α. Some consequences of this formula are discussed.     

Leptonic coupling-types in K+ → π++- decays and the question of strangeness-changing semileptonic neutral currents

A study of the leptonic coupling types in K⁺→π⁺?⁺?^? (? = e, μ) provides come model-independent distinction between the various underlying mechanism: lowest order weak cum electromagnetic vs higher order weak interactions vs genuine strangeness-changing semileptonic neutral currents. This distinction is necessary in order to know the existence of these neutral currents. Pion energy spectrum for ? = e and the ratio of the total rate for ? = e to that for ? = μ is useful for this purpose, and the pion energy spectrum for ? = μ is of further help. Available data (? = eonly) are inadequate to provide the above distinction. Estimates of various coupling strengths are made from the observed ? = e rate.     

Lepton number nonconservation in supersymmetric theories

We consider a class of supersymmetric models lepton number nonconservation due to mixing of scalar leptons. It is shown that a range of parameters exists such that the processes μ→eγ, μ→eγγ, μ→3e, μ⁺ e ^?→μ^? e ⁺ are strongly suppressed, whereas the decay \(\mu \to e\tilde \gamma \tilde \gamma \) can proceed with a rate \(BR(\mu \to e\tilde \gamma \tilde \gamma ) \lesssim 0.01\) . The signatures of such decay can be the production of right-handed electrons with the energy aroundm _μ/3 and modification of the energy spectrum of the electrons in that region as compared with the standard model prediction.     

Testing weak interaction models after the null bismuth result

The theories advanced to explain the absence of parity violation in bismuth vapour need further experiments to discriminate between them. We show that any observed asymmetry in ep scattering with polarized electrons must have a specific y-dependence if it is to be consistent with the bismuth experiments. Different charge asymmetries in e⁺e^? → μ⁺μ^? and μ^±p → μ^±X are also predicted in the different types of theory.     

Study of electron-positron collisions at the highest PETRA energy

We report on the results of the study of e⁺e^? collisions at the highest PETRA energy of √s = 31.57 GeV, using the 4π sr, electromagnetic and calorimetric detector Mark J. Based on 88 hadron events, and an integrated luminosity of 243 nb^?1 we obtain R = σ(e⁺e^? → hadrons)/σ(e⁺e^? → μ⁺μ^?) = 4.0 ± 0.5 (statistical) ± 6 (systematic). The R value, the measured thrust distribution and average spherocity show no evidence for the production of new quark flavors.     

Flavour-violating charged lepton decays in seesaw-type models

Analytic expressions of lepton-flavour- and lepton-number-violating decays of charged leptons are derived in the context of general SU(2)_L ⊗ U(1)_Y seesaw scenarios that are motivated by grand unified theories (GUT's) or superstring models, in which left-handed and/or right-handed neutral singlets are present. Possible constraints imposed by cosmology and low-energy data are briefly discussed. The violation of the decoupling theorem in flavour-dependent graphs due to the presence of heavy neutral leptons of Dirac or Majorana nature is emphasized. Numerical estimates reveal that the decays τ⁻ → e⁻ e⁻ e⁺or τ⁻ → e⁻ μ⁻ μ⁺ can be as large as ∼ 10⁻⁶, which may be observed in LEP experiments or other τ factories.     

Observation of a charge asymmetry in e+e− → μ+μ−

The angular distribution and the s dependence of the total cross section for the process e⁺e^? → μ⁺μ^? have been measured using the JADE detector at PETRA. After radiative corrections, a forward-backward asymmetry of ?(11.8±3.8) % was observed at an average centre of mass energy of 33.5 GeV. For comparison, an asymmetry of ?7.8 % is expected on the basis of the standard Glashow-Salam-Weinberg model.     

Pionic contribution to the muon magnetic moment

A procedure is developed to derive an optimal lower bounds for the pionic contribution to the muon magnetic moment from analyticity of the pion form factor F(t), its normalization F(0)=1 and from experimental information from both the processes e^?p → e^?π⁺n and e⁺e^? → π⁺π^?. It represents essentially the solution of a certain kind of optimization problem in Hilbert space. Numerical results are presented and compared to the recent data for the muon magnetic moment; we find a_μ(π⁺π^?) ? 42 × 10^?9.     

Majorana neutrinos in rare meson decays

The rare meson decays K ⁺ → π ^??⁺?′⁺ and D ⁺ → K ^??⁺?′⁺ (?, ?′ = e, μ), which are induced by Majorana neutrino exchange and which are accompanied by lepton-number nonconservation, are considered. The effects of the meson structure are taken into account on the basis of the Gaussian model for the respective Bethe-Salpeter amplitudes. It is shown that existing direct experimental constraints on the decay branching ratios are overly lenient and therefore give no way to set realistic limits on effective Majorana masses. On the basis of the constraints on the lepton-mixing parameters and neutrino masses from precision measurements of electroweak processes, neutrino-oscillation experiments, searches for neutrinoless double-beta decay of nuclei, and cosmological data, indirect constraints on the branching ratios for the decays in question are obtained and found to be much more stringent than the above direct constraints.     

Two-photon annihilation into pion pair

The first experimental study of the interaction γγ → π⁺π^t- has been carried out in the region of ?(660) through the reaction e⁺e^? → e⁺e^?π⁺π^?. Based on two observed events, the γγ width of ?(660) is obtained to be Γ_?γγ = 9.6_?8.0^+12.3 keV. Applied on this, theory of P.C.D.C. anomaly predicts R_∞ = 5.8_?3.5^+3.2 for the asymptotic value of σ(e⁺e^? → hadrons)/σ(e⁺e^? → μ⁺μ^?).     

Production and signature of intermediate vector bosons

The front-back asymmetry of outgoing muons in the inclusive reaction $\text{p}\text{p}\text{→ Z(→ μ}{}^{\mathrm{+}}\text{μ}{}^{\mathrm{?}}\text{)+}$ anything is calculated in the framework of the Drell-Yan model. We get very general expressions for the front-back asymmetry and the differential cross section in terms of the weak coupling constants and the parton distributions. Results for W^± production and proton-proton collisions are also given. Numerical estimates are made using various parton distributions and the Weinberg-Salam model for definiteness. We comment on the predictions of gauge theories that accommodate the absence of parity violating effects in atomic physics. We also give a new estimate of the inclusive cross sections $\text{p}\text{p}$ or pp → Z(→ μ⁺μ^?) or W^±(→ μ^±ν_μ) + anything. The effects of asymptotic freedom in the frontback asymmetries and the total cross sectons are investigated. We reanalyze results for the purely leptonic reaction e⁺e^? → γ, Z → μ⁺μ^?.     

The measurement ofR ine + e −-annihilation at center-of-mass energies between 7.25 and 10.34 GeV

The total cross section of the processe ⁺ e ^? →hadrons has been measured in the center-of-mass energy range between 7.25 and 10.34 GeV using the MD-1 detector at the VEPP-4 collider. The ratioR=σ(e ⁺ e ^? →hadrons)/σ(e ⁺ e ^? →μ ⁺ μ ^?) was found to be constant in this energy range with the average value of 3.58±0.02±0.14.     

Initial-state radiation at LEP energies and the corrections to Higgs boson production

It is shown that for e⁺e^? scattering processes described by the exchange of one photon or vector boson the O(α) radiative corrections to the initial state completely factorize the lowest-order cross section. This factorization holds for both hard and soft (or virtual) photons. We present a formalism for applying these radiative corrections in a process-independent way, and described the corresponding computer program. As an application we study the initial-state radiative corrections to the Higgs production processes e⁺e^? → μ⁺μ^?H and e⁺e^? → e⁺e^?H.