Heavy Lepton Pair Research Articles

On the spin correlations of final leptons produced in the high-energy annihilation and two-photon processes e+e− → μ+ μ−, τ+τ− and γγ → e+e−, μ+μ−, τ+τ−

The electromagnetic processes of annihilation of (e+e−) pairs, into heavy flavor lepton pairs are theoretically studied in the one-photon approximation, using the technique of helicity amplitudes . For the process e+e− − μ+μ−, it is shown that - in the case of the unpolarized electron and positron − the final muons are also unpolarized but their spins are strongly correlated; the structure of their triplet states is analyzed and explicit expressions for the correlation tensor components are derived. On the other hand, the theoretical study of spin structure for the processes of lepton pair production by photon pairs is also performed: for the two-photon process γγ → e+e−, it is found that – quite similarly - in the case of unpolarized photons the spins of final unpolarized particles prove to be strongly correlated, and explicit expressions for the correlation tensor components and relative fractions of singlet and triplet states of the final system are obtained. It is established that in both these processes the spin correlations of final leptons have the purely quantum character, since one of the Bell-type incoherence inequalities for the correlation tensor components is always violated. Analogous analysis can be wholly applied also to the annihilation process e+e− → τ+τ− and to the two-photon processes γγ → μ+μ−, γγ → τ+τ−, becoming possible at considerably higher energies.

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.

The effects of nucleus form factors on the electromagnetically produced heavy lepton pairs

In this paper, the effects of different nucleus form factors to the cross-sections of electromagnetically produced heavy lepton pairs such as muons and tauons from the ultra-relativistic heavy ion collisions have been calculated. Nucleus form factors play important roles in the calculation of the cross-sections, because the Compton wavelengths of muon and tauon are comparable to the radius of the colliding ions. Some realistic form factors in these calculations have been used and compared the results among them. It has been shown that nucleus form factors reduce the cross-section calculations for muon and tauon pair production very strongly especially at RHIC energies.

QCD corrections to pair production of Type III Seesaw leptons at hadron colliders

If kinematically accessible, hadron collider experiments provide an ideal laboratory for the direct production of heavy lepton partners in Seesaw models. In the context of the Type III Seesaw Mechanism, the $\mathcal{O}(\alpha_s)$ rate and shape corrections are presented for the pair production of hypothetical, heavy $SU(2)_L$ triplet leptons in $pp$ collisions at $\sqrt{s}=$ 13, 14, and 100 TeV. The next-to-leading order (NLO) $K$-factors span, approximately, $K^{NLO}=1.1 - 1.4$ for both charged current and neutral current processes over a triplet mass range $m_T = 100~\text{GeV}-2~\text{TeV}$. Total production cross sections exhibit a $^{+5\%}_{-6\%}$ scale dependence at 14 TeV and $\pm1\%$ at 100 TeV. The NLO differential $K$-factors for heavy lepton kinematics are largely flat, suggesting that na\"ive scaling by the total $K^{NLO}$ is reasonably justified. The resummed transverse momentum distribution of the dilepton system is presented at leading logarithmic (LL) accuracy. The effects of resummation are large in TeV-scale dilepton systems. Discovery potential to heavy lepton pairs at 14 and 100 TeV is briefly explored: At the High-Luminosity LHC, we estimate a $4.8-6.3\sigma$ discovery potential maximally for $m_T = 1.5-1.6~\text{TeV}$ after 3000 fb$^{-1}$. With 300 (3000) fb$^{-1}$, there is $2\sigma$ sensitivity up to $m_T = 1.3-1.4~\text{TeV}~(1.7-1.8~\text{TeV})$ in the individual channels. At 100 TeV and with 10 fb$^{-1}$, a $5\sigma$ discovery can be achieved for $m_T=1.4-1.6~\text{TeV}$. Due to the factorization properties of Drell-Yan-type systems, the fixed order and resummed calculations reduce to convolutions over tree-level quantities.

Neutrinos with a linear seesaw mechanism in a scenario of gaugedB−Lsymmetry

We consider a mechanism for neutrino mass generation, based on a local B-L extension of the standard model, which becomes a linear seesaw regime for light neutrinos after spontaneous symmetry breaking. The spectrum of extra particles includes heavy neutrinos with masses near the TeV scale and a heavy Z' boson, as well as three extra neutral scalars and a charged scalar pair. We study the production and decays of these heavy particles at the LHC. Z' will decay mainly into heavy neutrino pairs or charged lepton pairs, similar to other low scale seesaw scenarios with local B-L, while the phenomenology of the extra scalars is what distinguishes the linear seesaw from the previous models. One of the neutral scalars is produced by Z' Z' fusion and decays mainly into vector boson pairs, the other two neutral scalars are less visible as they decay only into heavy or light neutrino pairs, and finally the charged scalars will decay mainly into charged leptons and missing energy.

Soft and collinear factorization and transverse momentum dependent parton distribution functions

In this work we consider how a parton distribution function, with an explicit transverse momentum dependence can be properly defined in a regularization-scheme independent manner. We argue that by considering a factorized form of the transverse momentum dependent spectrum for the production of a heavy lepton pair in Drell–Yan reaction, one should first split the relevant soft function into two boost invariant contributions. When those soft contributions are added to the pure collinear contributions, well-defined hadronic matrix elements emerge, i.e., the transverse momentum dependent distributions. We also perform a comparison with Collinsʼ definition.

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)$\leq 6$.

Electromagnetic interactions of heavy ions at small impact parameters and lepton pair productions

In peripheral collisions, the strong electromagnetic fields are produced since the fully stripped heavy ions moves at ultra-relativistic velocities. Lepton pairs, especially the electron-positron pairs are the result of this collisions. At the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), virtual photons produce many particles such as electron-positron pairs, heavy lepton (muons and tauons) pairs, magnetic monopoles, W-pairs, b-quark pairs and possibly the Higgs. At small impact parameters, the colliding nuclei make peripheral collisions, photon fluxes are very large and these are responsible for the multiple photonuclear interactions. Free pair productions, bound free pair productions, and nuclear Coulomb excitations are important examples of such interactions, and these processes play important roles in the beam luminosity at RHIC and LHC. Here we obtained bound free electron-positron pair production with nuclear breakup for heavy ion collisions at RHIC and LHC.

Spontaneous R-parity violating type III seesaw

We present a model where neutrino masses are generated by a combination of spontaneous R-parity violation and Type III seesaw. In addition to the usual MSSM particle content, our model consists of one extra triplet matter chiral superfield containing heavy SU(2) triplet fermions and its superpartners. R-parity is broken spontaneously when the sneutrinos associated with the one heavy neutrino as well as the three light neutrinos get vacuum expectation values, giving rise to the mixed $8\times 8$ neutralino-neutrino mass matrix. We show that our model can comfortably explain all the existing neutrino oscillation data. Due to the presence of the triplet fermion, we have a pair of additional heavy charged leptons which mix with the standard model charged leptons and the charginos. This gives rise to a $6\times 6$ chargino-charged lepton mass matrix, with 6 massive eigenstates. Finally we discuss about the different R-parity violating possible decay modes and the distinctive collider signatures which our model offers.

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.

NLO QCD CORRECTIONS TO THE PRODUCTION OF HEAVY CHARGED LEPTONS BY GLUON FUSION AT THE LHC

The search for heavy charged leptons predicted by generalizations of the standard model is an objective of the experiments at future colliders, in particular at the LHC. We compute the Next-to-Leading Order (NLO) QCD virtual corrections to the production of pairs of heavy charged leptons by gluon fusion. The radiative corrections increase the production rate by about 30% and reduce its dependence on the renormalization and the factorization scales. The effect of a fourth family of quarks on the production cross-section is briefly discussed.

Angular and energy distributions of heavy lepton pairs produced through photon–photon fusion at large hadron collider

We examine the inclusive lepton pairs production in hadron–hadron collisions at LHC energy via two-photon exchange mechanism. We build a threefold and a twofold differential cross sections in order to investigate angular and energy distributions of the lepton pairs.

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.

Pairs of charged heavy-leptons from an SU(3) L⊗U(1) N model at CERN LHC

One of the versions of the SU(3) L ⊗U(1) N electroweak model predicts charged heavy-leptons which do not belong to any class of heavy-leptons proposed up to now. We investigate the production and signatures of pairs of these heavy-leptons via the Drell–Yan process and the gluon–gluon fusion at the CERN Large Hadron Collider (LHC). As an example we study the decay of the exotic leptons into another ones. We see that the lifetime of the exotic leptons can be short.

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.

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.

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.

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.

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].

Higher order QCD analysis of the photon structure

A new factorization scheme for the higher order QCD analysis of the photon's hadronic structure, avoiding the large-χ instabilities occuring in the usual MS treatment, is discussed. The parton content of real photons is generated in leading and higher order from a pure valence-like VMD input at a low resolution scale adopted from the pion, allowing for a prediction of the rather unknown gluon density. The agreement of the resulting quark distributions with presently available data is demonstrated. The inclusive production of heavy quarks and lepton pairs at ep colliders, which can provide separate information on the photon's gluon and quark content, respectively, is investigated.