Heavy Leptons Research Articles

Next-to-leading order cross sections for new heavy fermion production at hadron colliders

We evaluate the cross sections for new heavy fermion production at three Large Hadron Collider energies accurate to next-to-leading order in perturbative quantum chromodynamics. We treat the cases of pair production of heavy quarks via strong interactions, single heavy quark production via electroweak interactions, and the production of heavy leptons. Theoretical uncertainties associated with the choice of the renormalization scale and the parton distribution functions are specified. We derive a simple and useful parameterization of our results which should facilitate phenomenological studies of new physics models that predict new heavy quarks and/or leptons.

Heavy Charged Leptons from Type-III Seesaw and Pair Production of the Higgs Boson H at the International Linear e+e− Collider

Type-III seesaw scenario predicts the existence of the heavy charged leptons, which might generate observable signals in future collider experiments. We consider the contributions of these new particles to the process e+e− → HH. We find that, for the large mixing between the light and heavy leptons, these new particles can indeed generate significant contributions to this process.

Higgs boson decay into heavy quarks and heavy leptons: higher order corrections

Theoretical predictions for the decay width of Standard Model Higgs boson into bottom quarks and tau-leptons, in the case when M_H< 2M_W, are briefly reviewed. The effects of higher order perturbative QCD (up to alpha_s^4-level) and QED corrections are considered. The uncertainties of the decay width of Higgs boson into bb and tau+tau- are discussed.

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.

Neutrino masses and heavy triplet leptons at the LHC: Testability of the type III seesaw mechanism

We study LHC signatures of Type III seesaw in which SU(2)_L triplet leptons are introduced to supply the heavy seesaw masses. To detect the signals of these heavy triplet leptons, one needs to understand their decays to standard model particles which depend on how light and heavy leptons mix with each other. We concentrate on the usual solutions with small light and heavy lepton mixing of order the square root of the ratio of light and heavy masses, (m_\nu/M_{\nu_R})^{1/2}. This class of solutions can lead to a visible displaced vertex detectable at the LHC which can be used to distinguish small mixing and large mixing between light and heavy leptons. We show that, in this case, the couplings of light and heavy triplet leptons to gauge and Higgs bosons, which determine the decay widths and branching ratios, can be expressed in terms of light neutrino masses and their mixing. Using these relations, we study heavy triplet lepton decay patterns and production cross section at the LHC. If these heavy triplet leptons are below a TeV or so, they can be easily produced at the LHC due to their gauge interactions from being non-trivial representations of SU(2)_L. We consider two ideal production channels, 1) E^+E^- \to \ell^+\ell^+ \ell^-\ell^- jj (\ell=e,\mu,\tau) and 2) E^\pm N \to \ell^\pm \ell^\pm jjjj in detail. For case 1), we find that with one or two of the light leptons being \tau it can also be effectively studied. With judicious cuts at the LHC, the discovery of the heavy triplet leptons as high as a TeV can be achieved with 100 fb^{-1} integrated luminosity.

Does dark matter consist of baryons of new stable family quarks?

We investigate the possibility that the dark matter consists of clusters of the heavy family quarks and leptons with zero Yukawa couplings to the lower families. Such a family is predicted by the approach unifying spins and charges as the fifth family. We make a rough estimation of properties of baryons of this new family members and study possible limitations on the family properties due to the direct experimental and the cosmological evidences, studying the cosmological evolution of the fifth family clusters.

Large mixing of light and heavy neutrinos in seesaw models and the LHC

In the type-I seesaw model the size of mixing between light and heavy neutrinos, nu and N, respectively, is of order the square root of their mass ratio, (m_nu/m_N)^(1/2), with only one generation of the neutrinos. Since the light-neutrino mass must be less than an eV or so, the mixing would be very small, even for a heavy-neutrino mass of order a few hundred GeV. This would make it unlikely to test the model directly at the LHC, as the amplitude for producing the heavy neutrino is proportional to the mixing size. However, it has been realized for some time that, with more than one generation of light and heavy neutrinos, the mixing can be significantly larger in certain situations. In this paper we explore this possibility further and consider specific examples in detail in the context of type-I seesaw. We study its implications for the single production of the heavy neutrinos at the LHC via the main channel q qbar' -> W^* -> l N involving an ordinary charged lepton l. We then extend the discussion to the type-III seesaw model, which has richer phenomenology due to presence of the charged partners of the heavy neutrinos, and examine the implications for the single production of these heavy leptons at the LHC. In the latter model the new kinds of solutions that we find also make it possible to have sizable flavor-changing neutral-current effects in processes involving ordinary charged leptons.

Cosmic ray positrons from annihilations into a new, heavy lepton

Recent results from the PAMELA experiment indicate an excess in the positron spectrum above 10 GeV, but antiproton data are consistent with the expected astrophysical backgrounds. We propose a scenario that reproduces these features. Dark matter annihilates through channels involving a new heavy vectorlike lepton which then decays by mixing with standard model leptons. If charged, this heavy lepton might be produced at the LHC, and could lead to multilepton final states or to long-lived charged tracks. Large neutrino detectors such as ANTARES or IceCube might be sensitive to a monochromatic neutrino line. This scenario may be simply embedded in various models, including an extension to the next-to-minimal supersymmetric standard model.

LEPTON FLAVOR VIOLATING HIGGS DECAYS AND UNPARTICLE PHYSICS

We predict the branching ratios of the lepton flavor violating Higgs decays H0 → e± μ±, H0 → e± τ± and H0 → μ± τ± in the case that the lepton flavor violation is carried by the scalar unparticle mediation. We observe that their branching ratios are strongly sensitive to the unparticle scaling dimension and they can reach the values of the order of 10-4, for the heavy lepton flavor case and for the small values of the scaling dimension.

Radiative neutrino mass in type III seesaw model

The simplest type III seesaw model as originally proposed introduces one lepton triplet. It thus contains four active neutrinos, two massive and two massless at tree level. We determine the radiative masses that the latter receive first at two loops. The masses are generally so tiny that they are definitely excluded by the oscillation data, if the heavy leptons are not very heavy, say, within the reach of the CERN LHC. To accommodate the data on masses, the seesaw scale must be as large as the scale of grand unification. This indicates that the most economical type III model would entail no new physics at low energies beyond the tiny neutrino masses.

Fourth generation leptons and muong−2

We consider the contributions to ${g}_{\ensuremath{\mu}}\ensuremath{-}2$ from fourth generation heavy neutral and charged leptons $N$ and $E$, respectively, at the one-loop level. Diagrammatically, there are two types of contributions: boson-boson-$N$ and $E\ensuremath{-}E$-boson in the loop diagram. In general, the effect from $N$ is suppressed by off-diagonal lepton mixing matrix elements. For $E$, we consider flavor changing neutral couplings arising from various new physics models, which are stringently constrained by $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$. We assess how the existence of a fourth generation would affect these new physics models.

Two dark matter components in dark matter extension of the minimal supersymmetric standard model and the high energy positron spectrum in PAMELA/HEAT data

We present a dark matter extension of the minimal supersymmetric standard model to give the recent trend of the high energy positron spectrum of the PAMELA/HEAT experiments. If the trend is caused indeed by dark matter, the minimal supersymmetric standard model needs to be extended. Here, we minimally extend the minimal supersymmetric standard model with one more dark matter component N together with a heavy lepton E and introduce the coupling e{sub R}E{sub R}{sup c}N{sub R}. This coupling naturally appears in the flipped SU(5) grand unification models. We also present the needed parameter ranges of these additional particles.

LEPTON FLAVOR CONSERVING Z BOSON DECAYS AND SCALAR UNPARTICLE

We predict the contribution of scalar unparticle to the branching ratios of the lepton flavor conserving Z → l+l- decays and we study the discrepancy between the experimental and the QED corrected standard model branching ratios. We observe that these decays are sensitive to the unparticle scaling dimension du for its small values, especially for heavy lepton flavor output.

Precise limits from lepton flavour violating processes on the littlest Higgs model with T-parity

We recalculate the leading one-loop contributions to μ → eγ and μ → eeē in the Littlest Higgs model with T-parity, recovering previous results for the former. When all the Goldstone interactions are taken into account, the latter is also ultraviolet finite. The present experimental limits on these processes require a somewhat heavy effective scale ~ 2.5 TeV, or the flavour alignment of the Yukawa couplings of light and heavy leptons at the ~ 10% level, or the splitting of heavy lepton masses to a similar precision. Present limits on τ decays set no bounds on the corresponding parameters involving the τ lepton.

Electroweak scale seesaw and heavy Dirac neutrino signals at LHC

Models of type I seesaw can be implemented at the electroweak scale in a natural way provided that the heavy neutrino singlets are quasi-Dirac particles. In such case, their contribution to light neutrino masses has the suppression of a small lepton number violating parameter, so that light neutrino masses can arise naturally even if the seesaw scale is low and the heavy neutrino mixing is large. We implement the same mechanism with fermionic triplets in type III seesaw, deriving the interactions of the new quasi-Dirac neutrinos and heavy charged leptons with the SM fermions. We then study the observability of heavy Dirac neutrino singlets (seesaw I) and triplets (seesaw III) at LHC. Contrarily to common wisdom, we find that heavy Dirac neutrino singlets with a mass around 100 GeV are observable at the 5σ level with a luminosity of 13 fb−1. Indeed, in the final state with three charged leptons ℓ±ℓ±ℓ∓, not previously considered, Dirac neutrino signals can be relatively large and backgrounds are small. In the triplet case, heavy neutrinos can be discovered with a luminosity of 1.5 fb−1 for a mass of 300 GeV in the same channel.

Determining the heavy seesaw neutrino mass matrix from low-energy parameters

We explore how the seesaw sector in neutrino mass models may be constrained through symmetries to be completely determined in terms of low-energy mass, mixing angle and CP-violating phase observables. The key ingredients are intra-family symmetries to determine the neutrino Dirac mass matrix in terms of the charged-lepton or quark mass matrices, together with inter-family or flavor symmetries to determine diagonalization matrices. Implications for leptogenesis and collider detection of heavy neutral leptons are discussed. We show that leptogenesis can succeed in small regions of parameter space for the case where the neutrino Dirac mass matrix equals the up-quark mass matrix. The model where the neutrino Dirac mass matrix equals the charged-lepton mass matrix can yield a heavy neutral lepton as light as about 1 TeV, but detecting such a particle will be difficult.

Highlights from PHENIX: I

This contribution highlights recent results from the PHENIX Collaboration at RHIC. It covers global variables, flow and 2-particle correlations. A second contribution in this issue, by T C Awes (2008 J. Phys. G: Nucl. Part. Phys. 35 104007), covers PHENIX results on heavy quarks, leptons and photons.

Two-loop QED hadronic corrections to Bhabha scattering

Theoretical predictions for Bhabha scattering at the two-loop level require the inclusion of hadronic vacuum polarization in the photon propagator. We present predictions for the contributions from reducible amplitudes which are proportional to the vacuum polarization π ( q 2 ) and from irreducible ones where the vacuum polarization appears in a loop representing vertex or box diagrams. The second case can be treated by using dispersion relations with a weight function proportional to the R-ratio as measured in electron–positron annihilation into hadrons and kernels that can be calculated perturbatively. We present simple analytical forms for the kernels and, using two convenient parametrizations for the function R ( s ) , numerical results for the quantities of interest. As a cross check we evaluate the corresponding corrections resulting from light and heavy lepton loops and we find perfect agreement with previous calculations. For the hadronic correction our result are in good agreement with a previous evaluation.

Unparticle physics and lepton flavor violating radion decays in the Randall–Sundrum scenario

We predict the branching ratios of the lepton flavor violating radion decays r -> e^{\pm} \mu^{\pm}, r -> e^{\pm} \tau^{\pm} and r ->\mu^{\pm} \tau^{\pm} in the framework of the Randall-Sundrum scenario that the lepton flavor violation is carried by the scalar unparticle mediation. We observe that their BRs are strongly sensitive to the unparticle scaling dimension and, for its small values, the branching ratios can reach to the values of the order of 10^{-8}, for the heavy lepton flavor case.

Lepton flavor violating muon decays in a model of electroweak-scale right-handed neutrinos

The small neutrino mass observed in neutrino oscillations is nicely explained by the seesaw mechanism. Rich phenomenology is generally expected if the heavy neutrinos are not much heavier than the electroweak scale. A model with this feature built in has been suggested recently by Hung. The model keeps the standard gauge group but introduces chirality-flipped partners for the fermions. In particular, a right-handed neutrino forms a weak doublet with a charged heavy lepton, and is thus active. We analyze the lepton flavor structure in gauge interactions. The mixing matrices in charged currents (CC) are generally non-unitary, and their deviation from unitarity induces flavor changing neutral currents (FCNC). We calculate the branching ratios for the rare decays μ→eγ and μ→eee¯ due to the gauge interactions. Although the former is generally smaller than the latter by three orders of magnitude, parameter regions exist in which μ→eγ is reachable in the next generation of experiments even if the current stringent bound on μ→eee¯ is taken into account. If light neutrinos dominate for μ→eγ, the latter cannot set a meaningful bound on unitarity violation in the mixing matrix of light leptons due to significant cancelation between CC and FCNC contributions. Instead, the role is taken over by the decay μ→eee¯.