## Journal Club

Seminar Room, Mondays at 14:00

### Wednesday 20th of February, 2013

**Presented by Pérez-Victoria **

# Improving Electro-Weak Fits with TeV-scale Sterile Neutrinos

(Submitted on 7 Feb 2013)

We study the impact of TeV-scale sterile neutrinos on electro-weak precision observables and lepton number and flavour violating decays in the framework of a type-I see-saw extension of the Standard Model. At tree level sterile neutrinos manifest themselves via non-unitarity of the PMNS matrix and at one-loop level they modify the oblique radiative corrections. We derive explicit formulae for the S,T,U parameters in terms of the neutrino masses and mixings and perform a numerical fit to the electro-weak observables. We find regions of parameter space with a sizable active-sterile mixing which provide a better over-all fit compared to the case where the mixing is negligible. Specifically we find improvements of the invisible Z-decay width, the charged-to-neutral-current ratio for neutrino scattering experiments and of the deviation of the W boson mass from the theoretical expectation.

**Presented by Chala **

# Custodial Leptons and Higgs Decays

(Submitted on 24 Jan 2013)

Abstract: We study the effects of extended fermion sectors, respecting custodial symmetry, on Higgs production and decay. The resulting protection for the Z->b_L b_L and Z->tau_Rtau_R decays allows for potentially interesting signals in Higgs physics, while maintaining the good agreement of the Standard Model with precision tests, without significant fine-tuning. Although being viable setups on their own, the models we study can particularly be motivated as the low energy effective theories of the composite Higgs models MCHM_5 and MCHM_10 or the corresponding gauge-Higgs unification models. The spectra can be identified with the light custodians present in these theories. These describe the relevant physics in the fermion sectors of the models in a simplified and transparent way. In contrast to previous studies of composite models, we consider the impact of a realistic lepton sector on the Higgs decays. We find significant modifications in the decays to tau leptons and photons due to the new leptonic resonances. While from a pure low energy perspective an enhancement of the channel pp->h->gammagamma turns out to be possible, if one considers constraints on the parameters from the full structure of the composite models, the decay mode into photons is always reduced. We also demonstrate that taking into account the non-linearity of the Higgs sector does not change the qualitative picture for the decays into tau-leptons or photons in the case of the dominant Higgs production mechanism.

**Presented by Donati **

# New physics in top decay

(Submitted on 5 Feb 2013)

Abstract: After a short introduction to effective field theories, most of their features are illustrated using the top decay. The effects of heavy new physics on the top decay are computed and the constraints on the coefficients of the dimension-six operators are derived from the available measurements.

**Presented by Illana **

# SUSY vs LHC

(Submitted on 6 Feb 2013)

Abstract: In light of the discovery of the new particle at 125GeV and the strong lower limits on the masses of superparticles from LHC, we discuss a possible picture of weak scale supersymmetry.

**Presented by Setzer **

# Probing exotic Higgs sectors from the precise measurement of Higgs boson couplings

(Submitted on 30 Jan 2013)

Abstract: We study coupling constants of the standard model like Higgs boson with the gauge bosons $hZZ$ and $hWW$ and fermions $hfbar{f}$ in the general Higgs sector which contains higher isospin representations with arbitrary hypercharge. In Higgs sectors with exotic Higgs representations, the $hZZ$ and $hWW$ coupling constants can be larger than those in the standard model. We calculate deviations in the Higgs boson couplings from standard model values in the model with a real or complex triplet field, the Georgi-Machacek model and the model with a septet scalar field. We also study deviations in the event rates of $hto ZZ^*$, $hto WW^*$, $hto gammagamma$, $hto bbar{b}$ and $hto tautau$ channels.