Isosinglet Fermions Research Articles

2HD plus light pseudoscalar model for a combined explanation of the possible excesses in the CDF MW measurement and (g−2)μ with dark matter

The new measurement of the $W$ boson mass performed by the CDF experiment at the Tevatron shows a significant deviation not only with the expectation in the Standard Model but also with other precision measurements performed at LEP, the Tevatron and the LHC. We nevertheless take this new measurement at face value and interpret it as an effect of new physics. We particularly try to link it with other possible anomalies such as the recent muon $g-2$ and consider a scenario that addresses some shortcomings of the Standard Model. We show that a model with two doublets and a light pseudoscalar Higgs fields, supplemented by a stable isosinglet fermion, can simultaneously explain the possible $M_W$ and $(g-2)_\mu$ anomalies and accounts for the weakly interacting massive particle that could be responsible of the dark matter in the universe.

New fermions in the light of the (g − 2)μ

The very precise measurement of the anomalous magnetic moment of the muon, recently released by the Muon g-2 experiment at Fermilab, can serve to set stringent constraints on new particles. If the observed 4σ discrepancy from the Standard Model value is indeed real, it will set a tight margin on the scale of the masses and couplings of these particles. Instead, if the discrepancy is simply a result of additional theoretical and experimental uncertainties to be included, strong constraints can be put on their parameters. In this mini-review, we summarize the impact of the latest muon g-2 measurement on new fermions that are predicted by a wide range of new physics models and with exotic quantum numbers and interactions. We will particularly discuss the case of vector-like leptons, excited leptons, and supersymmetric fermions, as well as spin-3/2 isosinglet fermions, which have been advocated recently.

Models with two Higgs doublets and a light pseudoscalar: A portal to dark matter and the possible (g−2)μ excess

In the context of a two-Higgs doublet model, supplemented by an additional light pseudoscalar Higgs boson and a stable isosinglet fermion, we consider the possibility of addressing simultaneously the discrepancy from the standard expectation of the anomalous magnetic moment of the muon recently measured at Fermilab and the longstanding problem of the dark matter in the universe which can be accounted for by a thermal weakly interacting massive particle. We show that it is indeed possible, for a range of masses and couplings of the new light pseudoscalar and the fermionic states, to explain at the same time the two features while satisfying all other constraints from astroparticle physics and collider searches, including the constraints from flavor physics.

A not so little Higgs?

Most recent models assuming the Higgs boson is a pseudo-Nambu–Goldstone boson (pNGb) are motivated by the indication from Standard Model fits that its mass is ⩽200 GeV. Starting from a modified SM of Forshaw et al. with a triplet boson added and a heavier Higgs boson, we consider a pNGb model. This differs in several ways from most little Higgs models: apart from using only one loop, the cutoff scale is reduced to 5 TeV, and consequently a linear sigma model is used to alleviate FCNC effects; no new vector bosons are required, but vector-like isosinglet fermions are needed, but play no part in determining the mass of the Higgs boson. The phenomenology of the isosinglet pNGb that arises from the SU(3)×SU(3)→SU(3) model we use is briefly discussed. Some potential theoretical and phenomenological problems are mentioned briefly.

Extra-dimensions, isosinglet charged leptons and neutrino factories

Isosinglet fermions naturally arise in a variety of extensions of the Standard Model, in particular in models with extra-dimensions. In this paper, we study the effect of the addition of a new isosinglet charged lepton to the standard spectrum, with special emphasis on implications for neutrino asymmetries to be measured at future neutrino factories. Lepton flavour violation in neutral current and lepton universality constraints are extensively discussed. We show that New Physics effects in ν e – ν μ CP asymmetries are significantly enhanced due to leptonic maximal mixings but still too small to give a signature at future neutrino factories. A signal for CP asymmetries in ν μ – ν τ channel due to New Physics could be observed at 1–3 σ if lepton-flavour-violating τ decays are seen in a very close future in B-factories like Belle experiment.

Inverse hierarchy approach to fermion masses

The first fermion family might play a special role in understanding the physics of flavour. This possibility is suggested by the observation that the up-down splitting within quark families increases with the family number: m u d , m c > m s , m t ⪢ m b . We constrruct a model that realizes this feature of the spectrum in a natural way. The inter-family hierarchy is first generated by radiative phenomena in a sector of heavy isosinglet fermions and then transferred to quarks by means of a universal seesaw. A crucial role is played by left-right parity and up-down isotopic symmetry. No family symmetry is introduced. The model implies m u/ md > 0.5 and the Cabibbo angle is forced to be ∼√ m d / m s . The top quark is naturally in the 100 GeV range, but not too heavy: m t <150 GeV. Inspired by the mass matrices obtained in the model for quarks, we suggest an ansatz also including charged leptons. The difference between u-, d- and e-type fermions are simply parametrized by three complex coefficients ϵ u , ϵ d and ϵ e . Additional comnsistent predictions are obtained: m s = 100–150 MeV and m u / m d <0.75.

Universal seesaw and radiative quark mass hierarchy

A new approach to the fermion mass problem is suggested. The mass hierarchy is first radiatively generated in a sector of heavy isosinglet fermions and then projected to the visible ones by means of a universal seesaw. In this way flavour-changing phenomena are naturally suppressed. A quantitatively correct picture of quark masses and mixing is obtained. In particular, in contrast to previous approaches, the correct value of the Cabibbo angle can be accommdated without troubles for the perturbation expansion. By fitting the values of the masses of the five known quarks, we find that the natural value of m t is in the 100 GeV range.

Two-body hadronic decays of exotic quarks in E6 theories.

We explore the possibility that the exotic Q=-(1/3), color-triplet, isosinglet fermion (D) present in ${\mathrm{E}}_{6}$ theories may have a sizable two-body hadronic decay mode D\ensuremath{\rightarrow}d+g, where d is the usual down quark and g is a gluon. This process proceeds via a class of one-loop penguin-type diagrams. If the mass of the D (${M}_{D}$) exceeds that of the W boson, then it is easy to see that the branching ratio for the above decay is negligible. A more interesting case occurs when 23\ensuremath{\le}${M}_{D}$\ensuremath{\le}82 GeV, for which we find branching ratios for this process of order a few percent. This two-jet decay mode may be a clear signal for D production.