Pair Production Of Heavy Leptons Research Articles

Search for type-III seesaw heavy leptons in leptonic final states in pp collisions at sqrt{s} = 13~text {TeV} with the ATLAS detector

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139,text {fb}^{-1} of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.

Search for type-III seesaw heavy leptons in dilepton final states in pp collisions at sqrt{s} = 13,{text {TeV}} with the ATLAS detector

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139,{text {fb}}^{-1} of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy-lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790 GeV at 95% confidence level.

Electromagnetic heavy-lepton pair production in relativistic heavy-ion collisions

We calculate the cross sections of electromagnetic productions of muon- and tauon-pair productions from the ultra-relativistic heavy ion collisions. Since the Compton wavelengths of muon and tauon are comparable to the radius of the colliding ions, nuclear form factors play important roles for calculating the cross sections. Recent measurement (Abrahamyan et al., Phys Rev Lett 108:112502, 2012) indicates that the neutrons are differently distributed from the protons; therefore this affects the cross section of the heavy-lepton pair production. In order to see the effects of the neutron distributions in the nucleus, we used analytical expression of the Fourier transforms of the Wood–Saxon distribution. Cross section calculations show that the Wood–Saxon distribution function is more sensitive to the parameter R compared to the parameter a.

Search for type-III seesaw heavy leptons inppcollisions ats=8 TeVwith the ATLAS detector

A search for the pair-production of heavy leptons ($N^0,L^{\pm}$) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels $N^0\rightarrow W^{\pm}l^{\mp}$ ($\ell = e, \mu, \tau$) and $L^{\pm}\rightarrow W^\pm \nu$ ($\nu=\nu_e, \nu_\mu, \nu_\tau$) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying $W$ boson, and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of $20.3\,fb^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair-production is observed. Heavy leptons with masses below 325--540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.

Heavy lepton pair production in nucleus–nucleus collisions at LHC energy – a case study

We present a study of $\tau^{+}\tau^{-}$ lepton pair production in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.5 TeV. The larger $\tau^{\pm}$ mass ($\sim$ 1.77 GeV) compared to $e^{\pm}$ and $\mu^{\pm}$ leads to considerably small hadronic contribution to the $\tau^{+}\tau^{-}$ pair invariant mass ($M$) distribution relative to the production from thermal partonic sources. The quark-anti-quark annihilation processes via intermediary virtual photon, Z and Higgs bosons have been considered for the production of $ \tau^{+}\tau^{-}$. We observe that the contribution from Drell-Yan process dominates over thermal yield for $\tau^{+}\tau^{-}$ pair mass from 4 to 20 GeV at the LHC energy. We also present the ratio of $\tau$ lepton pair yields for nucleus-nucleus collisions relative to yields from p+p collisions scaled by number of binary collisions at LHC energies as a function $\tau$ pair mass. The ratio is found to be significantly above unity for the mass range 4 to 6 GeV. This indicates the possibility of detecting $\tau^{+}\tau^{-}$ pair from quark gluon plasma (QGP) in the mass window $4\leq M$(GeV)$\leq 6$.

Heavy lepton pair production at LHC: Model discrimination with multi-lepton signals

If new leptons exist close to the electroweak scale, they can be produced in pairs at LHC through standard or new interactions. We study the production of heavy lepton pairs in SM extensions with: (i) a Majorana or Dirac lepton triplet, as those appearing in type-III seesaw; (ii) a lepton isodoublet ( N E ) L , R ; (iii) a charged isosinglet E L , R ; (iv) a Majorana or Dirac neutrino singlet N and an additional Z ′ gauge boson. It is shown that the trilepton final state ℓ ± ℓ ± ℓ ∓ , which has a small SM background, constitutes the golden channel for heavy neutrino searches, being very sensitive to Majorana or Dirac neutrinos in triplet, doublet or singlet SU ( 2 ) L representations. For higher luminosities, signals in this final state can also distinguish lepton triplets from doublets and singlets. The Majorana or Dirac nature of the heavy neutrinos is revealed by the presence or not of like-sign dilepton ℓ ± ℓ ± signals without significant missing energy. Notably, large ℓ ± ℓ ± signals but with large missing energy are characteristic of Dirac triplets, distinguishing them from the other two models with a heavy Dirac neutrino. Further discrimination is achieved with the analysis of the clean ℓ + ℓ + ℓ − ℓ − final state.

Bound-free heavy-lepton pair production for photon and ion impact on atomic nuclei

Finite nuclear size is found to cause reduction in photoproduction of $\ensuremath{\mu}$ and $\ensuremath{\tau}$ pairs with bound negative lepton. The reduction is less dramatic for muons than claimed previously but very severe for experimental production of tauonic atoms. For both types of leptons, $L$-shell production exceeds $K$-shell production in heavy targets, and light targets are more favorable for ground-state $\ensuremath{\tau}$-atom production. Implications for heavy-ion collisions are outlined.

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.

Heavy-quark and lepton-pair production on nuclei.

We present a description of the [ital A] dependence of heavy-quark and Drell-Yan pair production in hadron-nucleus collisions, based on the Reggeon approach to strong interactions at high energies and the parton model. The same method has been successfully applied to the production of light-quark states. The existence of a new energy scale, which depends on the mass and [ital x][sub [ital F]] of the heavy system, is emphasized. A phenomenological model, which includes both the shadowing corrections to the nucleon structure function and the rescattering of the heavy-quark states, is proposed. The model quantitatively describes the available experimental data on [ital J]/[psi] and heavy lepton-pair production in [ital hA] collisions as well as the European Muon Collaboration effect at low [ital x].

HEAVY LEPTON PAIR PRODUCTION FROM NEW Z’ RESONANCES

We examine the possibility that heavy lepton pair production can be significantly enhanced by the existence of a new neutral gauge boson, Z’, at SSC energies. We find that in some models, the heavy lepton production cross section resulting from this mechanism exceeds that from production via other scenarios.

Helicity amplitudes for heavy lepton production in e +e − annihilation

Signatures of new heavy lepton pair production in e +e − annihilation at TRISTAN/SLC/LEP energies are studied in detail. Complete helicity amplitudes for 2 → 6 process e + e − → L L → (ν L f 1 f 2)( ν L f 3 f 4 ) are given for arbitrary masses of final fermions and for arbitrary vector and axial vector couplings. Methods to measure that L and ν L masses, and the neutral- and charged-current couplings of L in terms of four-jet and one-lepton-dijet final state distributions are exemplified. Signatures of heavy neutrino-pair production are discussed briefly. A straightforward method for calculating arbitrary tree amplitudes with external fermions and vector bosons of arbitrary masses is presented for completeness.

Production of heavy leptons from gluon fusion

We show the production of heavy lepton pairs in pp and p p collisions at multi-TeV energies is dominated by gluon fusion for lepton masses above 100–200 GeV. We also find that the decay of the Higgs boson into heavy lepton pairs may provide a clear signal of Higgs production.

Is heavy lepton pair production on nuclei really independent of nuclear size?

We investigate the nuclear size dependence of the distribution of massive lepton pairs produced by hadronic beams. Our principle conclusion is that <p 2 T > of the lepton pair should have a substantial and readily observable component from nuclear rescattering even though the nuclear enhancement exponent α(p T ) lies within present experimental limits. We indicate the relevance of these effects to the study of quark propagation through nuclear matter.

Determination of quark-nucleon inclusive cross section from leptoproduction of hadrons in nuclear targets

The $A$ dependence of hadron leptoproduction from nuclear targets is interpreted as a measurement of inclusive inelastic and possibly also quasielastic quark-nucleon cross sections. The formulas which allow us to extract these cross sections from the data are derived and discussed. Numerical estimates indicate that the measurement of the total inelastic quark-nucleon cross section is perfectly feasible. Measurement of differential inclusive cross sections of the reaction quark + nucleon\ensuremath{\rightarrow}quark + anything is possible as well but requires experiments with rather high statistics. Implications of our calculations for the process of heavy-lepton pair production from nuclear targets are also briefly discussed.

Neutrino-induced heavy-lepton pair production at high energies and multimuon events

Total cross sections for the process ..nu../sub ..mu../ + /sup 56/Fe ..-->.. ..mu../sup -/ + L/sup +/ + L/sup 0/ + /sup 56/Fe have been calculated in conventional V - A weak-interaction theory. For multi-GeV/c/sup 2/ mass assignments for L/sup +/, the cross section is approx.10/sup -6/ of the observed rate of charged-current events production by neutrinos at Fermilab. However, for m/sub L/+ = m/sub ..mu../+, and m/sub L/0 approx. 1 to 2 GeV/c/sup 2/, the cross section is approx.10/sup -1/ of the reported trimuon rates. Allowing L/sup 0/ to decay into muons + anything, we find that the leptons in the final states may be very energetic.

Evidence for heavy leptons from anomalous μe production in e +e − annihilation

Events of the type e + e −→μ ±+e ∓ + “nothing” have been observed in the magnetic detector PLUTO at the storage ring DORIS at DESY. The data support the hypothesis of the pair production of heavy leptons, and allow to determine decay parameters. Decay rates into muons and electrons are equal within error, and argue for a new lepton number of the heavy lepton.

Parity violation in heavy-lepton production by longitudinally polarizede+e−beams

The production of heavy-lepton pairs by longitudinally polarized ${e}^{+}{e}^{\ensuremath{-}}$ colliding beams induces a parity-violating correlation in the angular distribution of the daughter leptons. The dependence of the parity asymmetry on the beam and lepton energy is studied in $V\ensuremath{-}A$ and $V+A$ current models for the heavy lepton. Large asymmetries are expected from the threshold of heavy-lepton production to very high energies. The energy-integrated angular distribution of the detected leptons becomes parity-symmetric at very high energies.

Measuring form factors and decay parameters of heavy spin-1/2 particles produced bye−e+annihilations

The cross section for e/sup -/e/sup +/ ..-->.. U/sup -/U/sup +/ ..-->.. decay products is given for a spin-1/2 U, assuming arbitrary beam polarizations, general electromagnetic form factors, and general nonderivative, four-fermion interactions. Measurement of the momentum of one of the particles emitted in the decay specifies form factors and decay parameters of the U. In the case of heavy-lepton pair production, measurements of the form factors test the assumed absence of strong interactions, and measurements of decay parameters test the assumed decay with V +- A currents and electron-muon universality. (AIP)

The Drell-Yan process in a non-scaling parton model

The Drell-Yan process of heavy lepton pair production in hadronic collisions in discussed in a parton model which gives logarithmic corrections to Bjorken scaling. It is found that the moments of the Drell-Yan structure function exhibit a simple scale breaking behaviour closely related to the behaviour of moments of the deep inelastic structure function of the model. The extent to which analogous results can be expected in an asymptotically free gauge theory is discussed.

Energy bounds for charm and for heavy Lepton production in e +e − annihilation

Bounds sre derived for the mean charged to mean neutral pion (and kaon) energy ratios, valid in arbitrary phase space domains, for charm and for heavy lepton pair production. Absolute bounds for the mean neutrino and/or π 0 energy in terms of the beam energy are also given. For charm production, D ∗ effects are included.