Family unification within SO(15)

We present a model for the unification of fermion families based on the gauge symmetry SO(15). It is a minimal SO(n) model which can accommodate the known fermions within a single irreducible representation. The model predicts four ordinary fermion families and four families of mirror fermions. The latter have V + A weak interactions, and their mass scale is predicted to be 10² GeV/c². We argue that radiative corrections to the fermion masses can cause non-negligible mixing between ordinary and mirror fermions. The implications of these mixings for the weak interaction phenomenology and solar neutrinos are discussed.     

Radiatively induced lepton masses

The leptonic sector is studied in the context of a scheme proposed to explain fermion mass hierarchy from radiative corrections. In the charged leptonic sector, the masses obey a pattern m_τ:m_μm:m_e ⋍ 1:γ:γ², where γ is the loop expansion parameter. A pair of isosinglet heavy fermions plays an important role in generating mass hierarchy. In the neutrino sector, ϱ_τ acquires a small mass due to the seesaw mechanism induced from the heavy fermions while ϱ_μm and ϱ_e do so from radiative corrections.     

Effective Flavor-changing Weak Neutral Current in the Standard Theory and Z-Boson Decay

We have studied the effective Zf ₁f₂-vertex in the Glashow-Salam-Weinberg theory arising from 1-loop corrections, allowing for arbitrary fermion masses m_i in the loop. We use our results to study the branching ratio of the Z-boson decay into two different fermions. This is of phenomenological interest if there exist yet unknown fermions with large mass which could show up at present and designed accelerators only through radiative corrections.     

On fermion mass hirerachies in MSSM-like quiver models with stringy corrections

Using instanton effects, we discuss the problem of fermion mass hierarchies in an MSSM-like Type IIA orientifolded model with U(3) × Sp(1) × U(1) × U(1) gauge symmetry obtained from intersecting D6-branes. In the corresponding four-stack quiver, the different scales of the generated superpotential couplings offer a partial solution to fermion mass hierarchies. Using the known data with neutrino masses m_vt <~2 eVm_{v_\tau } \lesssim 2 eV, we give the magnitudes of the relevant scales.     

Parity Nonconserving (PNC) Electroweak Radiative Corrections for Highly Charged Ions (HCI)

We derive the electroweak radiative corrections to the basic PNC atomic transition amplitude for highly charged hydrogenlike ions. In the case of highly charged ions (HCI) effects of strong fields are reflected by the momentum transfer q ² involved. It is of the order q ²≈m _e ² inHCI, while q ²≈0 inneutral atoms. This may open the possibility to search for “new physics” beyond the Standard Model and to test the Standard Model in experiments with HCI. This revised version was published online in July 2006 with corrections to the Cover Date.     

New non-abelian couplings and radiative corrections to the main decay of the top quark

We calculate first order radiative corrections to the main decay of the top quark,tb+W, beyond the Standard Model, by grafting onto the standard lagrangian (using the terminology of de Rújula, Gavela, Hernandez and Massó), a new operator, which respects the gauge symmetry of the Standard Model and modifies theZ ⁰ W ⁺ W ^–,W ⁺ W ^– vertices In our renormalization scheme we use ,G _F,M _Z as input parameters. Them _t dependence is given where we use =1 TeV as a possible scale of the new physics. Form _t =150(200) GeV and =1 TeV, the maximal effect due to the new non-abelian couplings, is to change the relative size of the electroweak radiative corrections of the Standard Model by adding a value of –2.98(–6.96)%, i.e. from a value of 5.55(3.26)% to 2.57(–3.7)%.     

QCD corrections to b → se e decay

We give a more complete calculation of b → se⁺ e⁻ decay including leading log QCD corrections from m_top to M_W in addition to corrections from M_W to m_b. The differential decay rate is found to be slightly suppressed for a large invariant mass of the e⁺ e⁻ pair; while the integrated width is slightly enhanced comparing with the results without the QCD running from m_top to M_W.     

Induced Higgs couplings to neutral bosons in e+e− collisions

We calculate induced couplings of the type HV_γ in the standard model, where H is a Higgs meson and V is a virtual or real neutral gauge boson (Z⁰ or photon). Numerous applications are given for e⁺e^? collisions and various Higgs meson decays. The calculated rates are in general somewhat too low to make these processed an attractive way to search for the Higgs boson. However, once it has been found, it is argued that these processes should be studied experimentally since the induced couplings probe the structure of the gauge theory in an interesting way. In particular, it may be possible to infer the existence of one or more heavy fermion generations (of mass ?m_Z) by observing their virtual effects in radiative decays into Higgs particles. We also briefly treat the related coupling HV_γ with V a heavy quarkonium vector state.     

Aspects of the super-unification of strong,electroweak and gravitational interactions

We develop a strategy for extracting low-energy phenomenological four-dimensional physics from the superstring. We discuss supersymmetry and gauge symmetry breaking, emphasizing key ingredients in the construction of a realistic model based on Calabi-Yau compactification. The incorporation of a no-scale mechanism for the dynamical generation of the electroweak gauge hierarchy imposes a unique choice of the gauge group SU(3) × SU(2) × U(1)², an almost unique set of matter fields and of Yukawa couplings. Our phenomenological analysis of this model includes the derivation of bounds on the mass of the new neutral gauge boson from the Z⁰ boson mass, low-energy neutral currents, and cosmology. We calculate the ratios of sparticle masses and give estimates of their magnitudes. These are based on detailed dynamical calculations demonstrating the feasibility of weak gauge symmetry breaking, made possible by radiative corrections to supersymmetry breaking initiated by a gaugino mass.     

Generalization of the radiative corrections to β and μ decays in the SU(2)L × U(1) gauge model

Previous results for the radiative corrections to the ratio of decay probabilities of β and μ decays are generalized to the case of arbitrary symmetry breaking in the framework of the SU(2)_L × U(1) gauge model. In this more general case, the results remain very simple: the corrections are given by the corresponding expressions of the local V-A theory with m_W substituted for the cutoff Λ plus a positive definite contribution which depends on θ_W and R = m_W²/m_Z².Thus, the corrections in the general case are still constrained to have large values as required by Cabibbo universality and the present experimental values on β and μ decays. For reasonable values of m_Z and m_W the results in the general case are numerically close to those previously obtained in the simplest version of the theory. To illustrate this point the corrections are compared for two different choices of the parameters of the theory. In an appendix we discuss, rather briefly, the theoretical arguments that allow us to generalize pprevious discussions the case of fractionally charged quarks.     

Radiative corrections to neutral pion-pair production

We calculate the one-photon loop radiative corrections to the neutral pion-pair photoproduction process p^-g ?p^-p⁰p⁰\pi^-\gamma \ensuremath \rightarrow\pi^-\pi^0\pi^0 . At leading order this reaction is governed by the chiral pion-pion interaction. Since the chiral p⁺ \pi^{+}_{} p^- \pi^{-}_{} ? \rightarrow p⁰ \pi^{0}_{} p⁰ \pi^{0}_{} contact vertex depends only on the final-state invariant mass it factors out of all photon loop diagrams. We give analytical expressions for the multiplicative correction factor R ~ a/2p\ensuremath R\sim \alpha/2\pi arising from eight classes of contributing one-photon loop diagrams. An electromagnetic counterterm has to be included in order to cancel the ultraviolet divergences generated by the photon loops. Infrared finiteness of the virtual radiative corrections is achieved (in the standard way) by including soft photon radiation below an energy cut-off l \lambda . The radiative corrections to the total cross-section vary between +2% and -2% for center-of-mass energies from threshold up to 7m_p\ensuremath 7m_{\pi} . We study also the radiative corrections to the p⁰p⁰\ensuremath \pi^0\pi^0 mass spectrum.     

Radiative decays of charged leptons in arbitrary order

In radiative decays of charged leptons induced by non-degenerate neutrino masses and consequent lepton flavour-mixing, the dominant suppression factor at the 1-loop level is (Σ m _a ² )/m _W ² ,m _a being the mass of theath neutrino. We show that this suppression factor is presentin all orders in a class of models including the standard model.     

Universal higher order singlet QED corrections to unpolarized lepton scattering

We calculate the universal flavor singlet radiative QED corrections to unpolarized lepton scattering applicable to general differential scattering cross sections, involving charged fermions or photons in the initial or final states. The radiators are derived to O((αln(Q²/m_f ²))⁵) in analytic form. Numerical illustrations are given.     

Electroweak radiative corrections toe + e −→HZ

The cross section fore ⁺ e ^–HZ with arbitrary lepton andZ-boson polarizations is calculated within the electroweak standard model including the completeO() virtual and soft photonic corrections. These were obtained using the computer-algebra packagesFeynArts andFeynCalc. We list the relevant analytical results and discuss the influence of the weak radiative corrections on the unpolarized as well as the polarized cross sections. For LEP200 energies the weak radiative corrections amount to several per cent. For higher energies the corrections are usually of the same order for the total unpolarized cross section. However, for large values of the Higgs mass or for the suppressed cross section of transversely polarizedZ-bosons they reach +30% and –40%, respectively. The differential cross section in the forward and backward directions gets even larger corrections for high energies, where the box diagrams constitute the dominant part of the radiative corrections.     

Gauge hierarchies and mass bounds

We consider the effects of weak symmetry breaking by radiative corrections in a scheme of gauge hierarchies. We obtain new mass bounds on heavy fermions and show how discovery of a Higgs boson may distinguish between two different approaches to hierarchial spontaneous gauge symmetry breakdown. If there are no intrinsic mass scales below ～10¹⁵ GeV then discovery of a Higgs boson at ～9 GeV implies the existence of heavy fermions.     

Empirical lepton-quark mass formula: Four dimensional state space for fermion structure?

It is observed that a simple mass formula of the formm =¯mQ ²(exp) is wholly consistent with experimental measurements and quark model estimates for all 12 fundamental fermions. Here¯m = 433.3 MeV is an input (mean fermion mass) constant,Q is the charge number of the lepton or quark, and is a real root of a quartic equation that brings in a principal quantum numbern (= 0, 1, 2, 3). The charged lepton masses are given accurately to within 0.3 of 1%, all neutrino masses are zero, and the top mass is predicted by the formula to be close tom _t = 163.6 GeV.     

Uncertainties from fermion masses ine^ + e^ - \to v\bar v\gamma

In the reaction \(e^ + e^ - \to v\bar v\gamma \) , the influence of the unknown masses of heavy fermions for three and four fermion generations is studied, which arises via virtual 1-loop corrections in the framework of the standard model. These masses are the top quark massm _t and in the case of 4 generations the lepton massm t, wherel ^? is theSU(2) partner of the fourth generation neutrino. Increasing the fermion masses from 50 GeV to 250 GeV, the maximum of the total cross section is found to decrease by a few percent.     

Hierarchy of quark masses in the isotopic doublets in N=1 supergravity models

In the framework of some generic softly broken SUSY models we study the SU (2) × U(1) breaking by radiative corrections, starting with the Yukawa couplings which at the Planck scale M_P satisfy h^t = h^b. Physically acceptable solutions exist with the heirarchy of VEVs: ν₂/ν₁ ≈ m_t/m_b provided m_t ⩾ 50 GeV. When the SUSY breaking is driven only by the gaugino mass the solutions uniquely predict ν₂=O(10)ν₁ and the top mass in the range 50–65 GeV. Also, the mass ratio in the second quark generation can be accounted for.