Charged Lepton Pairs Research Articles

Heavy Majorana neutrinos in dilepton production in deep-inelastic lepton-proton scattering

The cross section for the production of pairs of likely charged leptons in the deep-inelastic processes \(e^ + p \to \bar v_e \ell ^ + \ell '^ + X(\ell ,\ell ' = e,\mu ,\tau )\) induced by the exchange of heavy Majorana neutrinos is calculated. The effect of interference between different neutrino mass eigenstates is studied. The possibilities for observing this effect at future lepton-proton colliders are considered.

Search for anomalous production of di-lepton events with missing transverse momentum in e + e - collisions at $\sqrt{s} = {}$ 183-209 GeV

Events containing two charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb\(^{-1}\) at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from \(\mathrm{W}^+\mathrm{W}^-\) production with each W decaying l eptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented.

Exotic heavy leptons predicted by extended models at the CERN LHC

We investigate the production, signatures, and backgrounds of a pair of heavy-exotic charged leptons and a heavy-exotic neutrino in association with the exotic charged lepton in the framework of the vector singlet model, vector doublet model, and fermion-mirror-fermion model, at the CERN Large Hadron Collider. We show that the cross sections for the production of a pair of heavy exotic leptons are competitive with the ones for the single production of exotic leptons.

Decay of Pseudoscalars into Lepton Pairs and Large-NcQCD

The counterterm combination that describes the decay of pseudoscalar mesons into charged lepton pairs at lowest order in chiral perturbation theory is considered within the framework of QCD in the limit of a large number of colors ${N}_{c}$. When further restricted to the lowest meson dominance approximation to large- ${N}_{c}$ QCD, our results agree well with the available experimental data.

Study of neutral-current four-fermion and ZZ production in e +e − collisions at [formula omitted] 183 GeV

A study of neutral-current four-fermion processes is performed using a data sample corresponding to 55.3 pb −1 of integrated luminosity collected by the L3 detector at LEP at an average centre-of-mass energy of 183 GeV. The neutral-current four-fermion cross sections for final states with a pair of charged leptons plus jets and with four charged leptons are measured to be consistent with the Standard Model predictions. Events with fermion pair masses close to the Z boson mass are selected in all observable final states and the ZZ production cross section is measured to be σ ZZ =0.30 +0.22 +0.07 −0.16 −0.03 pb, in agreement with the Standard Model expectation. No evidence for the existence of anomalous triple gauge boson ZZZ and ZZ γ couplings is found and limits on these couplings are set.

Search for anomalous production of di-lepton events with missing transverse momentum in e

Events containing two charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb $^{-1}$ at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from $\mathrm{W}^+\mathrm{W}^-$ production with each W decaying l eptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented.

QCD corrections to decay distributions of neutral Higgs bosons with (in)definiteCPparity

We compute the order ${\ensuremath{\alpha}}_{s}$ QCD corrections to the density matrix for the decay of a neutral Higgs boson $\ensuremath{\varphi}$ with (in)definite $\mathrm{CP}$ parity into a quark-antiquark pair and the QED corrections for the decay into a pair of charged leptons. We classify and calculate single spin asymmetries and spin-spin correlations which are generated by the scalar and pseudoscalar Yukawa couplings. These spin effects can be traced in $\ensuremath{\varphi}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{\ensuremath{-}}{\ensuremath{\tau}}^{+}$ and, for heavy Higgs bosons, in $\ensuremath{\varphi}\ensuremath{\rightarrow}t\overline{t}$. We also calculate resulting correlations among the final states and estimate, for the respective decay modes, the number of events needed to measure the Yukawa couplings with these correlations at the 3$\ensuremath{\sigma}$ level.

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.

Forward-backward asymmetries in hadronically produced lepton pairs.

It has now become possible to observe appreciable numbers of hadronically produced lepton pairs in mass ranges where the contributions of the photon and $Z^0$ are comparable. Consequently, in the reaction $p \bar p \to \ell^- \ell^+ + \ldots$, substantial forward-backward asymmetries can be seen. These asymmetries provide a test of the electroweak theory in a new regime of energies, and can serve as diagnostics for any new neutral vector bosons coupling both to quarks and to charged lepton pairs.

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.

PQCD analysis of exclusive charmless B meson decay spectra

We apply the perturbative QCD formalism, including Sudakov effects, to exclusive charmless semi-leptonic B meson decays. Predictions for both the charged lepton and lepton pair invariant mass spectra of the B → πℓ ν and B → ϱℓ ν decays in the large recoil region are presented. After combining soft pionn results, we extract | V ub | ≤ 4.5 × 10 −3 from curren data. With our predictions, future data will not only provide a more precise extraction of | V ub |, but also test perturbative QCD in B meson decays.

Search for dilepton gauge bosons in hadron colliders

Recently proposed SU(3) c×SU(3) L×U(1) gauge model predicts doubly charged dilepton gauge bosons coupling to two leptons as well as to two quarks. These dilepton gauge bosons can be produced at the Large Hadron Collider (LHC) and (and also to a lesser extent at the Fermilab Tevatron) with observable cross sections. Their decays to a pair of same sign charged leptons ( l + l + or l − l −, l= e, μ, τ) give rise to very clean signals for physics beyond the Standard Model.

Leptonic signals from off-shellZ boson pairs at hadron colliders

We study the gluon fusion into pairs of off-shellZ bosons and their subsequent decay into charged lepton pairs at hadron colliders:gg→ZZ→4l ± (l ±: charged lepton). Throughout this paper we do not restrict the intermediate stateZ bosons to the narrow width approximation but allow for arbitrary invariant masses. We compare the strength of this process with the known leading order results for\(q\bar q \to ZZ \to 4\ell ^ \pm\) and forgg→H→ZZ→4l ± At LHC energies (\(\sqrt s\)=14 TeV) the contribution from the gluon fusion background is around 20% of the contribution from quark-antiquark annihilation. These two processes do not form a severe irreducible background to the Higgs signal. At Higgs masses below 120 GeV the final state interference for the decay channelH→ZZ→4μ± is increasingly constructive. This has no effect on the Higgs search as in this mass region the signal remains too small. One can extend the intermediate mass Higgs search via off-shellZ boson pairs at the LHC down to about 130 GeV Higgs mass. However a careful study of the reducible background is needed for definitive conclusions.

Electroweak corrections to the supersymmetric charged Higgs boson fermionic decay.

One-loop radiative corrections to the charged Higgs coupling to two fermions and to the charged Higgs fermionic decay are studied. A renormalization scheme is proposed including the definition of the parameter $\tan\beta$ through the coupling of the CP-odd Higgs boson to a charged lepton pair. Loops including top and bottom quarks and squarks are considered using the unitary gauge. Large corrections to the coupling are found when there is a substantial mixing in the squark mass matrix, but these corrections are suprress in the decay rate. Interesting applications to rare processes are discussed.

The anomalous sector of the strong interaction effective lagrangian

The effective action in the abnormal intrinsic parity sector is derived to next-to-leading order in the chiral perturbation theory expansion in the chiral limit under the assumption that chiral symmetry breaking is well described by the addition of a constituent quark mass term to the QCD action. The result is combined with the loop calculation done previously for the case of pseudoscalar decays into a photon and a charged lepton pair. This allows for a physical determination of the parameter in the expansion that can be interpreted as the constituent quark mass. An expression for the abnormal intrinsic parity effective action is derived to all orders in terms of only one set of Seeley-DeWitt coefficients.

Production of right-handed gauge bosons and heavy Majorana neutrinos at the Superconducting Super Collider.

The processes {ital p}+{ital p}{r arrow}2{ital l}{sup {plus minus}}+{ital X}, {ital p}+{ital p}{r arrow}{ital W}{sub {ital R}}{r arrow}{ital N}+{ital l}+{ital X}, and {ital p}+{ital p}{r arrow}{ital Z}{sub {ital R}}{r arrow}2{ital N}+{ital l}+{ital X} (where {ital N} is a heavy Majorana neutrino) are discussed and estimated at the energy scale of the Superconducting Super Collider (SSC). These reactions may serve as the indications of right-handed gauge bosons and heavy Majorana neutrinos that propagate in the intermediate state or are produced directly. The observation of same-sign charged-lepton pairs in final states is a possible clean signal for production of {ital W}{sub {ital R}}, {ital Z}{sub {ital R}}, and the heavy Majorana neutrino {ital N}. A detailed calculation shows that with suitable cuts the background can be overwhelmed by the signals for a wide range of right-handed gauge-boson and heavy-neutrino masses (from hundreds GeV to several TeV), and this signal is observable at the SSC. With the completion of the SSC, such experiments could be carried out.

Cojets 5.12: A Monte Carlo simulation program for [formula omitted]-p and p-p collisions

An updated version of the Monte Carlo program COJETS is presented. The program simulates p -p and p-p interactions at high energy. It covers: (i) purely hadronic production, including particles with charm, bottom and top; (ii) weak Drell-Yan production of the W and Z vector bosons; (iii) electromagnetic Drell-Yan production of charged lepton pairs. The simulation is based, at the parton level, on perturbative QCD and the Weinberg-Salam model. Partons are independently fragmented into jets of hadrons according to the Field-Feynman model. Multiple QCD radiation of initial and final partons is included. Multiple QED radiation effects of charged leptons in Drell-Yan processes and top particle decays are taken into account.

Generation of Leptonia and Charged Fourth Lepton

Charged leptons belonging up to fourth generation are assigned to the fundamental representa­ tion of Su. group. The generation of leptonia is shortly discussed. The nature of the charged fourth lepton is examined in connection with the properties of W-boson effect specifically in its leptonic decay. In a previous paper/) we suggested that the mass eigenvalue problem of quarkonia made from inequivalent quark and antiquark may be reduced to the unitary transformation among them in terms of generators of the associated group. The purpose. of this paper is to show that a similar idea will be applied to the leptonia for equivalent and inequivalent charged lepton pairs and to examine the properties of the charged fourth lepton. Although this approach may in principle be extended to the unlimited number of lepton pairs in the sense of Kac-Moody alge-bra/) we shall confine ourselyes up to the fourth generation. We start our discussion by assigning i!1equivalent N charged leptons of N generations to the fundamental repre­ sentation of SUN and then treat SU4 as an example. The group theoretical approach to leptonia do not have much power except for their classification, because we do not expect more than the electroweak effects, so it constitutes an introductory part of this work. Our main concern is the nature of the charged fourth lepton, denoted as (J, in relevance to the W-boson effect in its leptonic decay. The search of charged fourth lepton would soon become of interest in the coming high-energy e+ e- collider at the simultaneous search of t quark, since we do not have any definitive information on them from the presently operating machine. 3 ) The wave function for leptonia made from N unidentical charged leptons may collectively be given by

SINGAL OF E6 VECTOR LEPTONS AT THE SSC

Some versions of superstring inspired models have vector leptons, which decay only through their suppressed mixing with ordinary leptons. The striking signals of an opposite sign dilepton and a pair of weak gauge bosons (ll-bar WZ, ll-bar WW and ll-bar ZZ) from such vector leptons produced at the SSC are examined. The authors find that the mass degenerate pair of charged and neutral leptons will be observable at the SSC if they are lighter than a few TeV.