Electron-hydrogen scattering just above the ionization threshold

The scattering of electrons by hydrogen atoms is studied for an incident energy of 15 eV, by means of a close coupling calculation using a pseudostate basis. Results are presented for elastic scattering and n = 2 excitation, including angular correlation parameters. The ionization cross section is extracted by an improved projection technique.     

Gauge coupling unification in the exceptional supersymmetric Standard Model

We consider the renormalisation group flow of gauge couplings within the so-called exceptional supersymmetric Standard Model (E₆SSM) based on the low-energy matter content of 27-dimensional representations of the gauge group E₆$E_6$, together with two additional non-Higgs doublets. The two-loop beta functions are computed, and the threshold corrections are studied in the E₆SSM. Our results show that gauge coupling unification in the E₆SSM can be achieved for phenomenologically acceptable values of α₃(MZ)${\alpha }_3(M_Z)$, consistent with the central measured low-energy value, unlike in the minimal supersymmetric Standard Model (MSSM) which, ignoring the effects of high energy threshold corrections, requires significantly higher values of α₃(MZ)${\alpha }_3(M_Z)$, well above the experimentally measured central value.     

Composite quarks and leptons in supersymmetric grand unification

Chirality under SUSY-GUT is suggested as a sufficient condition for composites to be massless, in addition to the necessary anomaly matching á la 't Hooft. This results in a very stringent set of self-consistency requirements, which dictates also the number and structure of the generations. As an illustration I present an [SU(3)]³×[SUSY] model. It involves three generations (2 light + 1 heavy) and some other realistic features.     

One-loop threshold effects in string unification

Like grand unification of old, string unification predicts simple tree-level relations between the couplings of all unbroken gauge groups such as SU(3)_C or SU(2)_W. I show here how to compute one-loop corrections to these relations for any four-dimensional model based on a classical vacuum of the heterotic string. The result can be used to calculate both sin²θ_Wand Λ_QCD in terms of α_QCD and M_Planck.