Heavy Charged Leptons Research Articles

Remark on neutrino masses and oscillations in anSU(3)L×U(1)Nmodel with the radiative mechanism

We discussed how neutrino masses and oscillations are radiatively generated in an $SU(3)_L$ $\times$ $U(1)_N$ gauge model with a symmetry based on $L_e-L_\mu-L_\tau$ ($\equiv$$L^\prime$). The model is characterized by lepton triplets $\psi^i=(\nu^i,\ell^{-i},E^{-i})$, where $E^{-i}$ are negatively charged heavy leptons, an $SU(3)_L$ triplet Higgs scalar $\xi$ and a singlet Higgs scalar $k^{++}$. These Higgs scalars can be interpreted as a Zee's and Zee-Babu's scalar for radiative mechanisms. We demonstrated that the mass hierarchy of $\Delta m_{atm}^2$ $\gg$ $\Delta m_\odot^2$ arise as a consequence of the dynamical hierarchy between $L^\prime$-conserving one-loop effects and $L^\prime$-violating two-loop effects, and our model is relevant to yield quasivacuum solution for solar neutrino problem.

Search for heavy neutral and charged leptons in e+e− annihilation at LEP

A search for exotic unstable neutral and charged heavy leptons as well as for stable charged heavy leptons is performed with the L3 detector at LEP. Sequential, vector and mirror natures of heavy leptons are considered. No evidence for their existence is found and lower limits on their masses are set.

T-channel production of heavy charged leptons

We study the pair production of heavy charged exotic leptons at e+ e- colliders in the SU(2)_L x SU(2)_I x U(1)_Y model. This gauge group is a subgroup of the grand unification group E6; SU(2)_I commutes with the electric charge operator, and the three corresponding gauge bosons are electrically neutral. In addition to the standard photon and Z boson contributions, we also include the contributions from extra neutral gauge bosons. A t-channel contribution due to W_I-boson exchange, which is unsuppressed by mixing angles, is quite important. We calculate the left-right and forward-backward asymmetries, and discuss how to differentiate different models.

Search for unstable heavy and excited leptons at LEP2

Searches for unstable neutral and charged heavy leptons, N and ${\rm L^\pm}$ , and for excited states of neutral and charged leptons, $\nu^*$ , ${\rm e}^*$ , $\mu^*$ , and $\tau^*$ , have been performed in ${\rm e^+e^-}$ collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of about 58 pb $^{-1}$ at a centre-of-mass energy of 183 GeV, and about 10 pb $^{-1}$ 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, $f/\Lambda$ , 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 $f^\prime$ .

Search for unstable heavy and excited leptons at LEP2

Searches for unstable neutral and charged heavy leptons, N and L+-, and for excited states of neutral and charged leptons, nu*, e*, mu*, and tau*, have been performed in e+e- collisions using data collected by the OPAL detector at LEP. The data analysed correspond to an integrated luminosity of about 58pb-1 at a centre-of-mass energy of 183GeV, and about 10pb-1 each at 161GeV and 172GeV. 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, f/Lambda, 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 f'.

Radiatively generated neutrino masses in SU(3) L× U(1) N gauge models

In SU(3) L × U(1) N gauge models for electroweak interactions, we discuss how to implement a radiative mechanism of generating Majorana neutrino masses by considering the property that the Higgs scalar, which has a coupling to a charged lepton (ℓ)-neutrino ( ν) pair, is naturally included. The mechanism is shown to work in models with a heavy charged lepton, ω +, in a lepton triplet ( ν, ℓ, ω +) and with a heavy neutral lepton, N, in ( ν, ℓ, N). A minimal model with ℓ and ℓ c in ( ν, ℓ, ℓ c ) together with a sextet Higgs scalar suffers from a fine-tuning problem to suppress tree-level neutrino masses.

Search for heavy neutral and charged leptons in e +e − annihilation at [formula omitted] = 183 and 189 GeV

A search for unstable neutral and charged heavy leptons as well as for stable charged heavy leptons is performed at center-of-mass energies s = 183 and 189 GeV with the L3 detector at LEP. No evidence for their existence is found. We exclude neutral heavy leptons which couple to the electron, muon or tau family, of the Dirac type for masses below 92.4, 93.3 and 83.3 GeV, and of the Majorana type for masses below 81.8, 84.1 and 73.5 GeV, respectively. We exclude unstable charged heavy leptons for masses below 93.9 GeV for a wide range of the associated neutral heavy lepton mass. If the unstable charged heavy lepton decays to a light neutrino, we exclude masses below 92.4 GeV. The production of stable charged heavy leptons with mass less than 93.5 GeV is also excluded.

Imaginary part of the electromagnetic lepton form factors

The charge F 1(0) and the magnetic F 2(0) form factors of heavy charged leptons have been shown in the framework of the perturbation theory to have imaginary part. The imaginary parts of the form factors for muon and tau lepton have been calculated at the two-loop level in the Standard Model. The effects where these imaginary parts could manifest themselves are discussed.

Search for unstable heavy and excited leptons in e+e− collisions at ∝s = 170-172 GeV

We have searched for unstable neutral and charged heavy leptons, N and L±, and for excited states of neutral and charged leptons, ν* e*, μ* and τ*, in e+e− collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived. From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper limits on the ratio of the coupling to the compositeness scale, ϕ/λ, are determined for masses up to the kinematic limit.

Search for unstable heavy and excited leptons in e

We have searched for unstable neutral and charged heavy leptons, N and L±, and for excited states of neutral and charged leptons, ν* e*, μ* and τ*, in e+e− collisions at centre-of-mass energies of 170 and 172 GeV using the OPAL detector at LEP. No evidence for their existence was found. From the analysis of charged-current decays of pair-produced unstable heavy leptons, and of charged-current and photonic decays of pair-produced excited leptons, lower limits on their masses are derived. From the analysis of charged-current and photonic decays of singly-produced excited leptons, upper limits on the ratio of the coupling to the compositeness scale, ϕ/λ, are determined for masses up to the kinematic limit.

Search for heavy neutral and charged leptons in e+e− annihilation at =161 = 172

Abstract A search for unstable neutral and charged heavy leptons as well as for stable charged heavy leptons has been made at center-of-mass energies s = 161 GeV and s 172 GeV with the L3 detector at LEP. No evidence for their existence was found. We exclude unstable neutral leptons of Dirac (Majorana) type for masses below 78.0 (66.7), 78.0 (66.7) and 72.2 (58.2) GeV, if the heavy neutrino couples to the electron, muon or tau family, respectively. We exclude unstable charged heavy leptons for masses below 81.0 GeV for a wide mass range of the associated neutral heavy lepton. The production of stable charged heavy leptons with a mass less than 84.2 GeV is also excluded. If the unstable charged heavy lepton decays via mixing into a massless neutrino, we exclude masses below 78.7 GeV.

Search for unstable neutral and charged heavy leptons in e +e − collisions at [formula omitted

Searches for unstable neutral and charged heavy leptons (L 0, L ±) have been performed using a data sample of 10.0 pb −1 at a centre-of-mass energy of s =161.3 GeV collected with the OPAL detector at LEP during July and August 1996. Two candidate events were observed after the selection, which is consistent with the expected total background of 1.57 events, and lower limits have been derived on heavy lepton masses for various models. If an unstable Dirac neutral heavy lepton, L 0, decays only into eW ∗, μW ∗ or τW ∗, the lower limits on its mass at the 95% C.L. are 69.3 GeV, 72.0 GeV and 66.0 GeV, respectively. The limits are modified for a Majorana L 0 to 59.5 GeV, 60.5 GeV and 55.7 GeV, respectively. If an unstable heavy charged lepton, L ±, decays into a stable heavy neutrino, and if m l ± - m ν l > 13 GeV, a lower limit on its mass of 73.5 GeV is obtained. If the L ± decays through lepton flavour mixing into a massless neutrino ν i and W ∗±, the lower limit on m l ± was determined to be 76.7 GeV at 95% C.L.

Charged heavy lepton production in superstring-inspired E6models

The possibility of studying superstring-inspired ${\mathrm{E}}_{6}$ phenomenology at high energy hadron colliders is investigated. A very simple low energy rank-5 supersymmetric $(N=1)$ model is considered, which consists of three Higgs scalars, ${H}_{i=1,2,3}^{0}$, two charged Higgs bosons, ${H}^{\ifmmode\pm\else\textpm\fi{}}$, one Higgs pseudoscalar, ${P}^{0}$, and an extra vector boson, the ${Z}^{\ensuremath{'}}$. The production of charged heavy lepton pairs ${L}^{+}{L}^{\ensuremath{-}}$ by gluon-gluon fusion and the Drell-Yan mechanism is discussed. For gluon-gluon fusion an enhancement in the parton level cross section is expected due to the heavy (s)fermion loops which couple to the gluons. This mechanism is expected to dominate over the Drell-Yan process for ${L}^{+}{L}^{\ensuremath{-}}$ invariant masses above the ${Z}^{\ensuremath{'}}$ mass.

STUDY OF e+e− INTERACTIONS AT CENTER-OF-MASS ENERGIES OF 130 AND 136 GeV

At the end of 1995, the LEP collider at CERN was operated at center-of-mass energies of 130 and 136 GeV and data corresponding to about 6 pb−1 were collected by each of the four LEP experiments. The cross-sections for fermion-pair production processes and the forward-backward asymmetries for charged lepton pairs were measured and compared to the standard model predictions. Events containing only energetic photons in the final state were used to look for effects arising from new physics. Direct searches for new particles predicted by various models beyond the standard model were performed. Searches for pair or singly produced excited leptons, for unstable charged and neutral heavy leptons, and for supersymmetric particles (chargino, neutralino, scalar leptons and scalar top quark) resulted in new exclusion limits. The ALEPH collaboration reported an excess of four-jet events in its data, which was not confirmed by the other LEP experiments.

Search for unstable neutral and charged heavy leptons in e +e − collisions at [formula omitted] and 136 GeV

Searches for unstable neutral and charged heavy leptons (L 0, L ±) have been performed using a data sample of 2.6 pb −1 at a centre-of-mass energy of s =130 GeV and 2.6 pb −1 at 136 GeV collected with the OPAL detector at LEP during November 1995. No candidate event was observed. If an unstable Dirac neutral heavy lepton L 0 decays only into eW ∗, μW ∗ or τW ∗, the lower limits on its mass at 95% C.L. are 62.5 GeV, 63.0 GeV and 57.4 GeV, respectively. The limits are modified for a Majorana L 0 to 51.4 GeV, 52.2 GeV and 44.2 GeV, respectively. For charged heavy leptons, a lower mass limit of 64.5 GeV at 95% C.L. was obtained, if L ± decays into a stable heavy neutrino ν l and W ∗±, and m l ± − m ν l > 10 GeV. If L ± decays through lepton flavour mixing into a massless neutrino ν ℓ and W ∗±, the lower limit on m l ± was determined to be 63.9 GeV at 95% C.L.

Search for heavy lepton pair production in e+e− collisions at centre-of-mass energies of 130 and 136 GeV

A search for pair production of new heavy leptons has been performed assuming different scenarios for the mixing of the new particles with Standard Model leptons. No candidate events were found in a data sample corresponding to an integrated luminosity of 5.6 pb−1 collected by the ALEPH detector at centre-of-mass energies of 130 and 136 GeV. New limits on production cross-sections and on masses of sequential leptons were obtained which significantly extend the mass regions excluded at LEP1. For instance, charged heavy leptons with masses below 63.5 GeV/c2 are excluded at 95% C.L. for mass differences to the associated neutral lepton of more than 7 GeV/c2.

Search for unstable sequential neutral and charged heavy leptons in e +e − annihilation at [formula omitted] and 136 GeV

A search for unstable sequential neutral and charged heavy leptons has been made at center-of-mass energies 130 and 136 GeV with the L3 detector at LEP. The neutral leptons are assumed to decay via mixing to electrons and muons. No evidence for their existence was found. We exclude unstable Dirac neutrinos for masses below 59.3 (57.9) GeV and unstable Majorana neutrinos below 48.6 (47.2) GeV if the neutrino couples to the electron(muon) family. We exclude unstable charged heavy leptons for masses below 61 GeV for a wide range of the associated neutral lepton mass.

Photon- and Z-induced heavy charged lepton pair production at a hadron supercollider.

We investigate the pair production of charged heavy leptons via photon-induced processes at the proposed CERN Large Hadron Collider (LHC). Using effective photon and $Z$ approximations, rates are given for ${L}^{+}{L}^{\ensuremath{-}}$ production due to $\ensuremath{\gamma}\ensuremath{\gamma}$ fusion and $Z\ensuremath{\gamma}$ fusion for the cases of inelastic, elastic, and semielastic $\mathrm{pp}$ collisions. These are compared with the corresponding rates for production via the gluon fusion and Drell-Yan mechanisms. Various $\ensuremath{\gamma}\ensuremath{\gamma}$ and $Z\ensuremath{\gamma}$ differential luminosities for $\mathrm{pp}$ collisions are also presented.

Charged leptons with nanosecond lifetimes.

Some extensions of the standard model contain additional leptons which are vectorlike under weak isospin. A class of models is considered in which these leptons do not appreciably mix with the known leptons. In such models, the heavy charged lepton and the heavy neutrino are degenerate in mass, and the degeneracy is broken by radiative corrections. The mass splitting is calculated and found to be very weakly dependent on the lepton mass, varying from 250 to 330 MeV as the mass varies from 100 to 800 GeV. This result is {\it not} affected significantly by inclusion in a supersymmetric model in spite of the additional loops involving the superpartners. As a result, this fairly general class of models has a charged lepton whose lifetime varies in the narrow range from 0.5 to 2.0 nanoseconds, and which decays into neutrals plus a very low energy electron or muon.

Production, collection, and utilization of very-long-lived heavy charged leptons.

If a fourth generation of leptons exists, both the neutrino and its charged partner must be heavier than 45 GeV. We suppose that the neutrino is the heavier of the two, and that a global or discrete symmetry prohibits intergenerational mixing. In that case, non-renormalizable Planck scale interactions will induce a very small mixing; dimension five interactions will lead to a lifetime for the heavy charged lepton of $O(1-100)$ years. Production of such particles is discussed, and it is shown that a few thousands can be produced and collected at a linear collider. The possible uses of these heavy leptons is also briefly discussed.