arXiv subjects

Nick Evans

Publications and source records attributed to Nick Evans.

113 records · Page 7Linked to original sources

arXiv1994

Dynamical Generation of the CKM Matrix

We argue that in dynamical models of fermion masses, which explain the three mass scales of the generations of fermions with three separate heavy scales above the electroweak symmetry breaking scale, the off diagonal CKM matrix elements must be generated at a fourth scale. The simplest form for the additional mass generation is a mechanism that has an $SU(3)_L \otimes SU(3)_R$ family symmetry. We show that such an ansatz can produce the up down mass inversion and the observed Cabibbo angle. If the mass terms for the third family quarks are enhanced by additional interactions the full CKM matrix may be realized. We present a toy model which realizes vacua with these masses.}

hep-ph↗

arXiv1994

Additional Fermion Families and Precision Electroweak Data

The S,T, and U formalism for studying electroweak precision data has recently been updated to include the effects of new light physics and within this analysis the latest LEP and SLC data tightly constrain models of physics beyond the Standard Model. We re-examine the constraints on additional strong and weakly interacting fermion families in the light of these developments. We conclude that the precision data favour models with at least one fermion with a mass below 150GeV.

hep-ph↗

arXiv1994

Fermion Mass Postdictions in a Generalized Extended Technicolour Scenario

We review the recent discussion in the literature of one family extended technicolour models with techni-fermion mass spectra compatible with the experimental data for the precision parameters S,T,U,V, W and X and ETC interactions compatible with the LEP measurements of the $Z\to b{\bar b}$ vertex. To investigate whether these scenarios are consistent with the third family fermion masses we develop a generalized ETC model in which ETC interactions are represented by four Fermi interactions. We discuss in detail the reliability of the gap equation approximation to the non-perturbative dynamics. Two generic scenarios of couplings fit the precision data and third family masses; one is an unpredicitive existence proof, the other, which generates the large top mass by direct top condensation, has a minimal number of interactions that break the global symmetry of the light fermions in the observed manner. This latter scenario makes surprisingly good predictions of the charm, strange and up quark masses.

hep-ph↗

arXiv1994

Strong Extended Technicolour Interactions and the $Zb\bar{b}$ Vertex

LEP precision measurements of the $Zb\bar{b}$ vertex coupling are sufficiently accurate to see the non-oblique corrections from the heavy gauge boson responsible for the large top mass in standard extended tecnicolour models. If the ETC couplings are strong the techni-fermion condensate may be enhanced by several orders of magnitude allowing a top mass of order 150GeV to be obtained for large values of the ETC breaking scale sufficient to supress the ETC contribution to the $Zb{\bar b}$ vertex. In this letter we investigate the degree of fine tuning of the ETC coupling required in a realistic model. We conclude that the fine tuning need only be of order $10\%$.

hep-ph↗

arXiv1993

V,W and X in Technicolour Models

Light techni-fermions and pseudo Goldstone bosons that contribute to the electroweak radiative correction parameters V,W and X may relax the constraints on technicolour models from the experimental values of the parameters S and T. Order of magnitude estimates of the contributions to V,W and X from light techni-leptons are made when the the techni-neutrino has a small Dirac mass or a large Majorana mass. The contributions to V,W and X from pseudo Goldstone bosons are calculated in a gauged chiral Lagrangian. Estimates of V,W and X in one family technicolour models suggest that the upper bounds on S and T should be relaxed by between 0.1 and 1 depending upon the precise particle spectrum.

hep-ph↗