4,418 research outputs found
Top quark mass coupling and classification of weakly-coupled heterotic superstring vacua
The quest for the Standard Model among the huge number of string vacua is
usually based on a set of phenomenological criteria related to the massless
spectrum of string models. In this work we study criteria associated with
interactions in the effective low energy theory and in particular with the
presence of the coupling that provides mass to the top quark. Working in the
context of the Free Fermionic Formulation of the heterotic superstring, we
demonstrate that, in a big class of phenomenologically promising compactifications, these criteria can be expressed entirely in terms of
the generalised GSO projection coefficients entering the definition of the
models. They are shown to be very efficient in identifying phenomenologically
viable vacua, especially in the framework of computer-based search, as they are
met by approximately one every models. We apply our results in the
investigation of a class of supersymmetric Pati-Salam vacua, comprising
configurations, and show that when combined with other
phenomenological requirements they lead to a relatively small set of about
Standard Model compatible models that can be fully classified.Comment: 13 pages, 1 figure, minor changes, to be published in The European
Physical Journal
The "landscape" of Pati--Salam heterotic superstring vacua
The main aspects of a recently developed method for classification of
heterotic superstring vacua in the Free Fermionic Formulation are presented.
Using these techniques we classify a big number of approximately
heterotic string vacua with Pati--Salam, SU(4)x{SU(2)}_Lx{SU(2)}_R gauge
symmetry with respect to their main phenomenological features as the number of
families, Pati--Salam breaking Higgs, Standard Model Higgs doublets, additional
triplets and exotic charge states. We identify an interesting subclass of these
vacua, approximately one to one million, whose massless spectrum is completely
free of fractionally charge states.Comment: 4 pages, 2 figues, Presented at "School and Workshops on Cosmology
and Strings: Theory-Cosmology-Phenomenology" (CORFU2009), September 6-13,
Corfu, Greec
Nonequational Stable Groups
We introduce a combinatorial criterion for verifying whether a formula is not
the conjunction of an equation and a co-equation. Using this, we give a
transparent proof for the nonequationality of the free group, which was
originally proved by Sela. Furthermore, we extend this result to arbitrary free
products of groups (except ), providing an abundance
of new stable nonequational theories.Comment: 10 pages, 1 figur
A Solution to the Decompactification Problem in Chiral Heterotic Strings
We present a solution to the decompactification problem of gauge thresholds
in chiral heterotic string theories with two large extra dimensions, where
supersymmetry is spontaneously broken by the Scherk-Schwarz mechanism. Whenever
the Kaluza-Klein scale is much lower than the string scale, the infinite towers
of heavy states contribute non-trivially to the renormalisation of gauge
couplings, which typically grow linearly with the large volume of the internal
space and invalidate perturbation theory. We trace the origin of the
decompactification problem to properties of the six dimensional theory obtained
in the infinite volume limit and show that thresholds may instead exhibit
logarithmic volume dependence and we provide the conditions for this to occur.
We illustrate this mechanism with explicit string constructions where the
decompactification problem does not occur.Comment: 26 pages, 1 figur
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