String-derivedD4flavor symmetry and phenomenological implications

Abstract

In this paper we show how some flavor symmetries may be derived from the heterotic string, when compactified on a 6D orbifold. In the body of the paper we focus on the D 4 family symmetry, recently obtained in Z 3 Z 2 orbifold constructions. We show how this flavor symmetry constrains fermion masses, as well as the soft SUSY breaking mass terms. Flavor symmetry breaking can generate the hierarchy of fermion masses and at the same time the flavor symmetry suppresses large flavor changing neutral current processes.

A zip-code for quarks, leptons and Higgs bosons

“…Since the Higgs bosons H u , H d live in the untwisted sector there are no allowed trilinear Yukawa couplings and thus quark-and lepton masses are suppressed. The specific location of the two families at the points c = 1 and c = 3 in the third torus gives rise to a D 4 family symmetry [23,24] and thus avoids the problem of flavour changing neutral currents. This is another example of a discrete symmetry which are rather common in the Mini-Landscape and exhibit the rich symmetry structure of a successful MSSM model.…”

“…We find that the geographic location of the fields (in the extra dimensions) is of utmost importance, most notably for a solution to the µ-problem [18,21,22], the Yukawa-coupling of the top-quark [16,18], the existence of discrete family symmetries [23,24] as well as the pattern of soft supersymmetry breaking terms [25][26][27][28]. Our analysis shows that the Higgs bosons and the top-quark preferably live in the bulk (D = 10) while quarks and leptons of the first two families are localized at (different) fixed points/tori in the extra dimensions.…”

Stringy origin of non-Abelian discrete flavor symmetries

“…For example, degeneracy due to nonAbelian flavor symmetry would be useful to suppress dangerous flavor changing neutral currents (see, e.g., [36,21]). It appears possible to arrive at a situation where in the Kähler potential, and therefore in the soft terms, the non-Abelian discrete symmetries survive while the Yukawa couplings receive important modifications from spontaneous symmetry breakdown.…”

“…In these models, the three generations are comprised of a singlet and a doublet under the D 4 symmetry. This D 4 flavor symmetry has important phenomenological implications [18,21].…”

Three-generation asymmetric orbifold models from heterotic string theory

“…Since strings on an orbifold can be described by a solvable worldsheet conformal field theory [16,17], it is possible to calculate Yukawa couplings and selection rules [18][19][20][21][22]. Furthermore, the geometrical structure of orbifold fixed points can be an origin of a discrete symmetry [23,24], which may lead to a hierarchical structure of masses/mixings of quarks and leptons.…”