Octet Fermions Research Articles

Assessing perturbativity and vacuum stability in high-scale leptogenesis

We consider the requirements that all coupling constants remain perturbative and the electroweak vacuum metastable up to the Planck scale in high-scale thermal leptogenesis, in the context of a type-I seesaw mechanism. We find a large region of the model parameter space that satisfies these conditions in combination with producing the baryon asymmetry of the Universe. We demonstrate these conditions require Tr[YN†YN] ≲ 0.66 on the neutrino Yukawa matrix. We also investigate this scenario in the presence of a large number NF of coloured Majorana octet fermions in order to make quantum chromodynamics asymptotically safe in the ultraviolet.

Lepton number and lepton flavor violation through color octet states

We discuss neutrinoless double beta decay and lepton flavor violating decays such as $\mu-> e\gamma$ in the colored seesaw scenario. In this mechanism, neutrino masses are generated at one-loop via the exchange of TeV-scale fermionic and scalar color octets. The same particles mediate lepton number and flavor violating processes. We show that within this framework a dominant color octet contribution to neutrinoless double beta decay is possible without being in conflict with constraints from lepton flavor violating processes. We furthermore compare the "direct" color octet contribution to neutrinoless double beta decay with the "indirect" contribution, namely the usual standard light Majorana neutrino exchange. For degenerate color octet fermionic states both contributions are proportional to the usual effective mass, while for non-degenerate octet fermions this feature is not present. Depending on the model parameters, either of the contributions can be dominant.

Small steps towards grand unification and the electron/positron excesses in cosmic-ray experiments

We consider a small extension of the standard model by adding two Majorana fermions; those are adjoint representations of the SU(2)_L and SU(3)_c gauge groups of the standard model. In this extension, the gauge coupling unification at an energy scale higher than 10^{15} GeV is realized when the masses of the triplet and the octet fermions are smaller than 10^4 GeV and 10^{12} GeV, respectively. We also show that an appropriate symmetry ensures a long lifetime of the neutral component of the triplet fermion whose thermal relic density naturally explains the observed dark matter density. The electron/positron excesses observed in recent cosmic-ray experiments can be also explained by the decay of the triplet fermion.

Investigating gluino production at the LHC

Gluinos are expected to be one of the most massive sparticles (supersymmetric partners of usual particles) which constitute the Minimal Supersymmetric Standard Model (MSSM). The gluinos are the partners of the gluons and they are color octet fermions, due this fact they can not mix with the other particles. Therefore in several scenarios, given at SPS convention, they are the most massive particles and their nature is a Majorana fermion. Therefore their production is only feasible at a very energetic machine such as the Large Hadron Collider (LHC). Being the fermion partners of the gluons, their role and interactions are directly related with the properties of the supersymmetric QCD (sQCD). We review the mechanisms for producing gluinos at the LHC and investigate the total cross section and differential distributions, making an analysis of their uncertainties, such as the gluino and squark masses, as obtained in several scenarios, commenting on the possibilities of discriminating among them.

Multi-jet physics at hadron colliders

We discuss four-jet and six-jet events at the SSC, LHC and Tevatron. First, the QCD backgrounds to non-standard physics are presented. Then we investigate multi-jet signals for hadronically-decaying color-octet pseudoscalars and color-sextet or octet fermions. Finally, the discovery potential of each collider is estimated.

Potentials and chiral condensates for dynamical fermions in higher representations

We analyze SU(3) gauge theory with dynamical fermions in the fundamental and adjoint representation. Our results show that octet fermions in contrary to triplet fermions do not break the gluon string between static triplet charges. This is demonstrated by investigating the polarization cloud around static mesons. As a consequence the quark-antiquark potential is linear for octet fermions and screened for triplet fermions.

Quark confinement with dynamical fermions in higher representations

We study the static interquark potential on a hypercubic lattice in pure gauge theory and with dynamical fermions in triplet and octet representation. Our computation shows that octet fermions in contraryto triplet fermions have no effect on the linear confinement behavior of the quark-antiquark potential. This fact gives interesting insight into the confinement mechanism of supersymmetric Yang-Mills theories if octet fermions are interpreted as a simple model for gluinos.

Colored lepton mass bounds from p [formula omitted] collider data

If leptons are composite particles containing colored preons one expects the existence of color octet fermions with quantum numbers similar to those of ordinary leptons. Such “colored leptons” are expected to decay predominantly into a lepton and a gluon. Comparing the present CERN collider data with the calculated cross section for colored lepton pair production via strong interactions we derive a lower limit on the mass of neutral colored leptons of 55–80 GeV. The colored partner μ 8 of the muon must be heavier than about 110 GeV.

Chiral symmetry breaking and condensates in the rishon model

The rishon model is studied in the limit g c → 0, α → 0 when its global flavour symmetry is SU(6) × SU(6) × U(1) analogous to six massless flavour QCD. Recently it was shown that the ad hoc breaking SU(6) × SU(6) → SU(3) × SU(3) allows the anomaly constraint to be satisfied. In this paper this is shown to be but one of several successful patterns of chiral symmetry breaking. The condensates required to perform these breakings are fully discussed. A plausibility argument based on single gauge boson exchange is presented which determines the condensate uniquely to be 〈(v LV L) 3〉 corresponding to the original breaking above. The same argument applies to QCD, which is argued to differ in its chiral behaviour due to the large intrinsic masses of the quarks. The implications of the above condensate and pattern of chiral symmetry breaking for the rishon model include the prediction of integer charged colour octet fermions, a naive mass formula m e = 2 m u − m d, new insight into the parity-violating condensate 〈(v Lv L) 2(v Rv R)〉 and the prediction of 52 new pseudos whose masses are estimated.