A search for heavy leptons in cosmic radiation underground

An experiment to search massive long-lived, weakly interacting particles (leptons) in cosmic radiation has been conducted at Kolar Gold Fields at a depth of 7.6 hg cm⁻² (1 hg cm⁻²=100 g cm⁻²) below surface. The apparatus was senstive to sub-relativistic (velocity<0.75 c) charged leptons of mass greater than that of a proton and life times greater than a microsecond. The method consists of selecting charged particles using a scintillator counter telescope and vetoing relativistic particles (velocity >0.75 c) by using a water Čerenkov detector. The range of the particle is observed in arrays of neon flash tubes interspersed with iron absorbers. During 3000 hours of observation 28 events were recorded satisfying the trigger and event selection criteria. Bulk of these events were interpreted as due to recoil protons (low energy) from the inelastic scattering of high energy muons in the overhead absorber. The remaining events were interpreted as either atmospheric stopping protons or stopping muons that failed to generate a Čerenkov signal. The observed events are thus consistent with the background and no heavy leptons were seen. From our observations an upper limit of 2.12×10⁻⁷ (with 90% confidence level) is set on the ratio of the flux of heavy leptons to that of all muons at this depth.     

New bounds for dilepton production from heavy neutral leptons

Bounds on 〈E_?ⁿ〉/〈E₊ⁿ〈, 〉E₊E_?〈/〉E₂²〈 and 〈E₊E_?〉/〈E₊〉〈E_?〉 are direved for the processes ν_μN → μ^?μ⁺(e⁺) + X and μN → μ^?μ⁺ + X if dileptons are mediated by a $\text{spin}\text{-}\text{1}\text{2}$ heavy neutral lepton L₀. The bounds are shown to be independent of the production mechanism and mass of L₀. Useful conditional bounds are obtained relating the bounded quantities, which give information about the structure of the weak current responsible for L₀ decay.     

Search for neutral heavy leptons produced in Z decays

Weak isosinglet Neutral Heavy Leptons (v _m) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 10⁶ hadronic Z⁰ decays at LEP1. Four separate searches have been performed, for short-lived v _m production giving monojet or acollinear jet topologies, and for long-lived v _m giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z⁰ → v _m ?) of about 1.3 × 10^?6 at 95% confidence level for v _m masses between 3.5 and 50 GeV/c². Outside this range the limit weakens rapidly with the v _m mass. The results are also interpreted in terms of limits for the single production of excited neutrinos.     

Signatures of neutral heavy leptons in neutrino-induced dilepton events

The tentative indication in some experiments of μ^?e⁺ or π^?μ⁺ events with unusual characteristics revives the speculation that neutrino-induced dileptons may contain a weak signal of neutral heavy lepton (L⁰) production. We study the characteristics of dileptons expected from the production of an L⁰ through a current of the form $\text{L}{}^0\text{γ}{}_{\mathrm{\alpha }}\text{(1+γ}{}_5\text{)ν}{}_{\mathrm{\mu }}$. Both quasi-elastic and deep inelastic excitation are investigated and a comparison is made with dileptons of charm origin. As an illustration of a model with a ν_μ→L⁰ transition, we discuss the 2-2 version of the SU(2) × U(1) model and comment on the role of the Higgs particle in such transitions.     

Search for heavy quasistable leptons

Negative results of a search for heavy quasistable charged leptons at the IHEP accelerator are reported. For the selection of heavy leptons a system of scintillation and gas ?erenkov counters was used. The upper limit estimations of the differential cross sections for heavy lepton production in pN collisions at E_p = 70 GeV are, for example, d²σ/dpω (p = 30 GeV/c, θ = 2 mrad, 1 ? M_λ ? 4.8 GeV) = 4.10^?38 cm²/sr · GeV. The results of the present work, together with the data on muon pair production in nucleon-nucleon interactions, show that there are no heavy charged leptons with masses from 0.55 GeV (τ_λ > 7 · 10^?10 sec) up to 4.5 GeV (τ_λ > 3 · 10^?8 sec).     

Search for heavy neutral leptons in e+e− annihilations at 29 GeV

A search was made in 29 GeV e⁺e^? annihilations for heavy, neutral leptons decaying to e^±X^?(ν), where X is a muon or charged meson. Six events with isolated e^±X^? pairs were found for an intergrated luminosity of 106 pb^?1. The expected background is 5.5 ± 2.2 events. Limits on $\text{σ}\text{s}\text{?}\text{B}$ depend on mass and range from 8 to 20 × 10^?5 nb.     

Strongly interacting heavy leptons

A simple model of heavy leptons with strong interactions is proposed for ψ particles and di-muon production in ν interactions.     

Anomalous moments of heavy charged leptons

If leptons and quarks are composite particles we expect them to have anomalous moments: an anomalous magnetic moment and an analogue of this with respect to the weak neutral current. If these moments are suppressed by chiral symmetry they will have sizeable values only for heavy fermions, e.g. for fermions of the fourth generation. The forward-backward asymmetry of e⁺e⁻ → f̄f on the Z⁰ resonance is particularly suited for detecting the anomalous “weak magnetic” moment of f if f is the charged lepton of the fourth generation. Under favourable circumstances this moment may lead to a 10% effect.     

A search for heavy stable particles

A search is made for cosmic ray produced heavy stable particles of unit positive charge in terrestrial water. By electrolysis of 6000 litres of heavy water, small samples are produced with a heavy isotope enrichment factor 3 · 10¹¹ relative to natural water. Comparison of enriched and unenriched samples in a sensitive mass spectrometer showed no evidence for new particles in the mass range 6 to 350 proton masses, giving upper concentration limits in natural water between 2 · 10^?22 and 10^?21 over most of the mass range. An experiment to improve this limit by a factor of 10⁶ is in progress.     

Lifetimes and branching ratios of heavy leptons

The decay properties of charged heavy leptons are discussed under the assumption that the conjectured leptons (and their associated neutrinos) participate in the weak interactions like the known leptons.     

Search for unstable heavy and excited leptons at LEP2

Searches for unstable neutral and charged heavy leptons, N and , and for excited states of neutral and charged leptons, , , , and , have been performed in collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of about 58 pb at a centre-of-mass energy of 183 GeV, and about 10 pb each at 161 GeV and 172 GeV. No evidence for new particles was found. Lower limits on the masses of unstable heavy and excited leptons are derived. From the analysis of charged-current, neutral-current, and photonic decays of singly produced excited leptons, upper limits are determined for the ratio of the coupling to the compositeness scale, , for masses up to the kinematic limit. For excited leptons, the limits are established independently of the relative values of the coupling constants f and . Received: 29 November 1999 / Published online: 6 March 2000     

Weak neutral currents under photogeneration of hadrons on leptons

In this article we investigate the process of photogeneration of hadrons on leptons in the region of large imparted momenta (q² 1000 GeV²). Contributions of weak neutral currents and polarizations of initial and final electrons are taken into account, as well as the circular polarization of the photon. The asymmetry parameters A(^e) and A() and the spin correlation A(^e) are defined. The structure functions in the quark-parton model are found, and their dependence on x for q²=1000 GeV² is investigated. Graphs of the dependence of the asymmetry parameters on the square of imparted momentum q² and x are shown.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–14, October, 1987.