String threshold corrections and flipped SU (5)
Abstract
We revise previous calculations of the effective unification scale m_{SU} at which the extrapolated lowenergy gauge couplings should appear to become equal, and we show explicitly how to calculate m_{SU} in the fermionic construction of fourdimensional strings. In the case of the flipped SU (5) GUT derived from the string, the SU (5) and U (1) couplings defined in the DR scheme become equal to g_{SU} at m_{SU} ⋍ 1.76 × g_{SU} × 10^{18} GeV. This scale is significantly larger than m_{GUT}, the scale at which the lowenergy SU (3) and SU (2) couplings become equal if extrapolated using the renormalization group equations of the minimal supersymmetric extension of the standard model. The existence of an intermediate SU (5) × U (1) phase could have an observable effect on the calculated value of sin^{2}θ_{w}.
 Publication:

Physics Letters B
 Pub Date:
 October 1991
 DOI:
 10.1016/03702693(91)90802W
 Bibcode:
 1991PhLB..268..188A