Jose I. Illana

Charged lepton flavor violation from massive neutrinos in Z decays

Present evidences for neutrino masses and lepton flavour mixings allow to predict, in the Standard Model with light neutrinos, branching rates for the decays Z --> e mu, mu tau, e tau of less than 10^{-54}, while present experimental exclusion limits from LEP 1 are of order 10^{-5}. The GigaZ option of the TESLA Linear Collider project will extend the sensitivity down to about 10^{-8}. We study in a systematic way some minimal extensions of the Standard Model and show that GigaZ might well be sensitive to the rates predicted from these scenarios.

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.

Excursions into FeynArts and FormCalc

Programming techniques which extend the capabilities of FeynArts and FormCalc are introduced and explained using examples from real applications.

Atmospheric lepton fluxes at ultrahigh energies

In order to estimate the possibility to observe exotic physics in a neutrino telescope, it is essential to first understand the flux of atmospheric neutrinos, muons and dimuons. We study the production of these leptons by high-energy cosmic rays. We identify three main sources of muons of energy E ≥ 106 GeV: the weak decay of charm and bottom mesons and the electromagnetic decay of unflavored mesons. Contrary to the standard assumption, we find that η mesons, not the prompt decay of charm hadrons, are the dominant source of atmospheric muons at these energies. We show that, as a consequence, the ratio between the neutrino and muon fluxes is significantly reduced. For dimuons, which may be a background for long-lived staus produced near a neutrino telescope, we find that pairs of E ≈ 107 GeV forming an angle above 10−6 rad are produced through D (80%) or B (10%) meson decay and through Drell-Yan proceses (10%). The frequency of all these processes has been evaluated using the jet code PYTHIA.

The Possibility of using a large heavy ion collider for measuring the electromagnetic properties of the tau-lepton

Abstract We study the potential of a large heavy-ion collider for the measurement of the electromagnetic properties of the tau lepton. Measuring the anomalous magnetic and the electric dipole moments of the tau at q 2 ∼ 0 with a precision of ∼4 x 10 −5 and ∼4 x 10 −3 respectively, at the LHC and/or SSC should be no problem. Whereas the precision at RHIC should be a few percent, comparable to present limits and to the expected precision at LEP.

A realistic model for dark matter interactions in the neutrino portal paradigm

A bstractWe discuss a simple extension of the Standard Model (SM) that provides an explicit realization of the dark-matter (DM) neutrino-portal paradigm. The dark sector is composed of a scalar Φ and a Dirac fermion Ψ, with the latter assumed to be lighter than the former. These particles interact with the SM through the exchange of a set of heavy Dirac fermion mediators that are neutral under all local SM symmetries, and also under the dark-sector symmetry that stabilizes the Ψ against decay. We show that this model can accommodate all experimental and observational constraints provided the DM mass is below ∼ 35 GeV or is in a resonant region of the Higgs or Z boson. We also show that if the dark scalar and dark fermion are almost degenerate in mass, heavier DM fermions are not excluded. We note that in this scenario DM annihilation in the cores of astrophysical objects and the galactic halo produces a monochromatic neutrino beam of energy mΨ, which provides a clear signature for this paradigm. Other experimental signatures are also discussed.

A new physics interpretation of the IceCube data

Abstract IceCube has recently observed 37 events of TeV–PeV energies. The angular distribution, with a strong preference for downgoing directions, the spectrum, and the small muon to shower ratio in the data cannot be accommodated assuming standard interactions of atmospheric neutrinos. We obtain an excellent fit, however, if a diffuse flux of ultrahigh energy (cosmogenic) neutrinos experiences collisions where only a small fraction of the energy is transferred to the target nucleon. We show that consistent models of TeV gravity or other non-Wilsonian completions of the standard model provide cross sections with these precise features. An increased statistics could clearly distinguish our scenario from the one assumed by IceCube (a diffuse flux of astrophysical neutrinos with a ∝ E - 2 spectrum) and establish the need for new physics in the interpretation of the data.

Lepton flavor violation in the Simplest Little Higgs model

The flavor sector of Little Higgs models based on product groups, notably the Littlest Higgs with T parity (LHT), has been extensively studied and some amount of fine tuning was found to be required to meet the experimental constraints. However, no such attention has been paid to other classes of models. Here we analyze the phenomenology of flavor mixing in the lepton sector of a simple group model, the Simplest Little Higgs (SLH). We obtain the Feynman rules of the SLH in the ’t Hooft-Feynman gauge up to the necessary order and calculate the leading contributions to the rare processes μ → eγ,

μ − e conversion in the Littlest Higgs model with T-parity

\mu \to {\text{ee}\bar{\rm e}}

tau -pair production via photon-photon collisions at the CERN e+e- collider LEP.

and μ − e conversion in nuclei. We find results comparable to those of the LHT model, because in both cases they arise at the one-loop level. These require the flavor alignment of the Yukawa couplings of light and heavy leptons at the per cent level or an effective scale of around 10 TeV.