Ivica Picek

TeV-scale seesaw mechanism with quintuplet fermions

We propose a new seesaw model based on a fermionic hypercharge-zero weak quintuplet in conjunction with an additional scalar quadruplet which attains

Lorentz non-invariance

Various ideas on how Lorentz invariance might be broken are reviewed, but we mainly study a model characterized by weak interactions having a special metric. Such a model does not seem to fit together so well the three-standard deviations from Lorentz invariance that have been found for π± and K± lifetimes, and the recent K0 − K0 regeneration parameter analysis. Still, the indication of a C-even Lorentz non-invariant interaction from the K0 − K0 analysis agrees with our model. However, the most important blow to the hypothesis of Lorentz non-invariance in weak interactions comes from matching the resulting non-Lorentz invariant renormalization of the kinetic term in the Dirac lagrangian with the Phillips experiment on the diurnal variation of the torsion moment of a ferromagnet. Some additional experimental tests at the weak interaction scale are proposed, the test of a model at a more distant (e.g. proton-decay) scale being left to future phenomenology.

The Rédei-like model and testing Lorentz invariance

Abstract We present an argument in support of a very simple and predictive model for noncovariant laws of nature. Provided the deviation from Lorentz invariance is still rotational invariant, we end up with a very predictive one parameter model. It results in the Redei-like behaviour of particle lifetimes, and selection rules are obtained which determine the most promising weak-decay-experiment tests of Lorentz invariance. In this respect more accurate measurements of dominant charged pion and kaon decay modes are strongly encouraged.

Understanding Fine Structure Constants and Three Generations

Abstract We put forward a model inspired by random dynamics that relates the smallness of the gauge coupling constants to the number of generations being “large”. The new element in the present version of our model is the appearance of a free parameter x that is a measure of the (presumably relatively minor) importance of a term in the plaquette action proportional to the trace in the ( 1 6 , 2,3 ) representation of the standard model. Calling N gen the number of generations, the sets of allowed ( N gen , xN gen ) -pairs obtained by imposing the three measured coupling constant values of the standard model form three lines. In addition to finding that these lines cross at a single point (as needed for a consistent fit), the intersection occurs with surprising accuracy at the integer N gen =3 (thereby predicting exactly three generations). It is also encouraging that the parameter x turns out to be small and positive as expected.

Radiative neutrino mass with scotogenic scalar triplet

We present a radiative one-loop neutrino mass model with hypercharge zero scalar triplet in conjunction with another charged singlet scalar and an additional vectorlike lepton doublet. We study three variants of this mass model: the first one without additional beyond-SM symmetry, the second with imposed DM-stabilizing discrete Z2 symmetry, and the third in which this Z2 symmetry is promoted to the gauge symmetry U(1)D. The two latter cases are scotogenic, with a neutral component of the scalar triplet as a dark matter candidate. In first scotogenic model the Z2-odd dark matter candidate is at the multi-TeV mass scale, so that all new degrees of freedom are beyond the direct reach of the LHC. In second scotogenic setup, with broken U(1)D symmetry the model may have LHC signatures or be relevant to astrophysical observations, depending on the scale of U(1)D breaking.

New scotogenic model of neutrino mass with U(1)D gauge interaction

We propose a new realization of the one-loop radiative model of neutrino mass generated by dark matter (scotogenic), where the particles in the loop have an additional U(1)D gauge symmetry, which may be exact or broken to Z2. This model is relevant to a number of astrophysical observations, including AMS-02 and the dark matter distribution in dwarf galactic halos.

Novel TeV-scale seesaw mechanism with Dirac mediators

Abstract We propose novel tree level seesaw mechanism with TeV-scale vectorlike Dirac mediators that produce Majorana masses of the known neutrinos. The gauge quantum number assignment to the Dirac mediators allows them to belong to a weak triplet and a five-plet of non-zero hypercharge. The latter leads to new seesaw formula m ν ∼ v 6 / M 5 , so that the empirical masses m ν ∼ 10 − 1 eV can be achieved by M ∼ TeV new states. There is a limited range of the parameter space with M ⩽ a few 100 GeV where the tree level contribution dominates over the respective loop contributions and the proposed mechanism is testable at the LHC. We discuss specific signatures for Dirac type heavy leptons produced by Drell–Yan fusion at the LHC.

Enhancement of h → γγ by seesaw-motivated exotic scalars

We examine the role of seesaw-motivated exotic scalars in loopmediated Higgs decays. We consider a simple TeV-scale seesaw model built upon the fermionic quintuplet mediator in conjunction with the scalar quadruplet, where we examine portions of the model parameter space for which the contributions of charged components of the scalar quadruplet significantly increase the h → decay rate. The most significant change in the diphoton width comes from a dou

Critique of Fermionic RνMDM and its Scalar Variants

A bstractWe examine the stability of minimal dark matter (MDM) particle-candidates in the setup in which they participate in radiative neutrino (Rν) masses. We first point out the existence of an additional renormalizable term in recently proposed RνMDM Lagrangian, which violates the claimed accidental Z2 symmetry and spoils the stability of the fermionic MDM quintuplet component. We then explore the viability of RνMDM variants based on scalar MDM multiplets. There are ubiquitous super-renormalizable terms in the scalar potential which make these scalar multiplets unstable.

Exotic Seesaw-Motivated Heavy Leptons at the LHC

We study the LHC potential for discovering TeV-scale SU(2)L 5plet fermions introduced recently to explain small neutrino masses. We show that the Drell-Yan production and the decays of new exotic � leptons are testable at the LHC. Their production is abundant due to nontrivial electroweak gauge charges. For 1 fb −1 of integrated luminosity at the present LHC √ s = 7 TeV, there can be 270 �-� pairs produced for M� = 400 GeV. Besides producing same-sign dilepton events, they could lead, due to a chosen small mixing between heavy and light leptons, to ∼10 golden decays � +++ (� +++ ) → W ± W ± l ± with a specific decay signature.